@article{chaudhury_sahoo_vinod_fisher_ekkad_narayanaswamy_fang_2024, title={Characteristics of Premixed Ammonia/Methane/Air Blends as an Alternative Fuel in a Swirl-Stabilized Gas Turbine Combustor Under Varying Pilot Percentage}, volume={146}, ISSN={["1528-8919"]}, DOI={10.1115/1.4065923}, abstractNote={Abstract Alternative low carbon fuel blends are a promising way towards clean energy transition in the transportation and power generation sectors. In this work, the objective was to study the combustion characteristics of one such low carbon fuel blend (premixed Ammonia, Methane and Air) in a swirl stabilized Gas Turbine Can Combustor under varying % of pilot fuel flow (= 8 % to 10 % of the main fuel flow rate) at atmospheric pressure conditions. Pure Methane was used as the pilot flame which helped in the ignition and stabilization of the main flame and was kept on throughout the experiment. Different volume % of Ammonia and Methane blends were analyzed (starting from 10 to 50 % Ammonia in the fuel blend and the rest being Methane) at Reynolds number of the incoming air ~ 50000, and at equivalence ratio = 0.6 and 0.7. Characteristics such as Combustor liner wall heat load and flame stability were studied using the Infrared Thermography technique and High-Speed flame imaging respectively. Additionally, both carbon and NOx emission trends were estimated for selected cases using the CONVERGE CFD software under steady state conditions incorporating the RANS RNG k-ε and SAGE modeling techniques. Among all cases, wall heat load was observed to be the least for the 50 % Ammonia-50 % Methane case and for cases under reduced pilot %. Also, under reduced pilot %, flames were mostly unstable wherein the manifestation of instabilities at equivalence ratio = 0.6 and 0.7 were markedly different from one another.}, number={11}, journal={JOURNAL OF ENGINEERING FOR GAS TURBINES AND POWER-TRANSACTIONS OF THE ASME}, author={Chaudhury, Meghna Das and Sahoo, Abinash and Vinod, Kaushik Nonavinakere and Fisher, Wesley and Ekkad, Srinath V. and Narayanaswamy, Venkateswaran and Fang, Tiegang}, year={2024}, month={Nov} }
 @article{bonds jr_iyer_ekkad_2023, title={Effects of Variable Pressure Outlets for Array Jet Impingement Cooling With a Bidirectional Exit Air Scheme}, volume={145}, ISSN={["2832-8469"]}, DOI={10.1115/1.4063106}, abstractNote={Abstract
               Array jet impingement in conjunction with other cooling methods such as effusion cooling is used in gas turbine combustion zones to provide optimized cooling in the form of double wall cooling around a combustion chamber. Utilizing a transient liquid crystal (TLC) technique an experimental investigation into the effects of pressure gradients and single versus multiple exits for array jet impingement crossflow is evaluated in the form of a detailed heat transfer analysis. In this study, four pressure gradients to bias mass flow ratios as (1:1), (2:1), (3:1), and (1:0), two jet array configurations either inline or staggered with jet to jet spacings (x/D = y/D) of 1.4, 1.9, and 2.2, three jet to target distances (z/D) ranging from 2 to 4, and three Reynolds number from 5000 to 15,000 are considered. In total, a test matrix of 72 different performance conditions was evaluated. Results are presented as local and area averaged Nusselt number plots along with local heat transfer coefficient contours. Overall, Nusselt number decreases with increased (z/D) and increased pressure gradient bias toward a single exit from (1:1) to (1:0). There is also slightly better performance from inline jet array configurations compared to staggered configurations.}, number={11}, journal={ASME JOURNAL OF HEAT AND MASS TRANSFER}, author={Bonds Jr, Michael and Iyer, Ganesh Subbarama and Ekkad, Srinath V.}, year={2023}, month={Nov} }
 @article{bhansali_ekkad_2023, title={Numerical study of heat transfer enhancement over a rotating surface. Part I: Effect of pin-fin shapes}, volume={5}, ISSN={1040-7782 1521-0634}, url={http://dx.doi.org/10.1080/10407782.2023.2209696}, DOI={10.1080/10407782.2023.2209696}, abstractNote={AbstractAbstractRotating machinery like gas turbine engines consist of rotating components like gas turbine disks that are exposed to high thermal loads and stresses due to centrifugal force acting on them, and thus effective cooling is essential. The disks are additionally exposed to hot turbine gas ingress as an adverse pressure gradient is created in the stator-rotor cavity due to outward pumping of near-disk flow. Impingement of a coolant jet on such a surface can effectively provide cooling and also counter this adverse gradient. There also exists a circular relative velocity between the disk and wall-jet formed by impinging air as well as the air in the disk vicinity. Thus, the study of heat transfer under rotating conditions becomes important. With the advent of advanced manufacturing techniques, it is possible to produce disks with novel heat transfer enhancement features such as pin-fins which have not been previously explored. They create unique flow features in addition to increasing the surface area. The present study numerically investigates the heat transfer over rotating surfaces under jet impingement with square, triangular, cylindrical and dome-shaped pin-fins. This will inform the optimization of pin geometry for better heat transfer effectiveness and uniformity. Three different jet Reynolds numbers between 5000 and 18000 based on the jet diameter have been studied for three different Rotational Reynolds numbers based on the disk diameter. Transient non-conjugate heat transfer analysis was conducted, and the technique was compared with experimental results on square pin-fins and a smooth circular disk. The heat transfer enhancement greatly depends on the flow separation and pin area normal to the flow. A heat transfer enhancement of close to 2.2 was observed for triangular fins for the lowest rotation speed, for the highest rotation speed, the highest enhancement of close to 1.7 was observed in the cylindrical pins.Keywords: Coolingjet impingementpin finsrotating diskturbine diskView correction statement:Correction}, journal={Numerical Heat Transfer, Part A: Applications}, publisher={Informa UK Limited}, author={Bhansali, Pratik S. and Ekkad, Srinath V.}, year={2023}, month={May}, pages={1–20} }
 @inbook{ekkad_singh_2023, title={Recent advancements in single-phase liquid-based heat transfer in microchannels}, volume={55}, ISBN={9780443157882}, ISSN={0065-2717}, url={http://dx.doi.org/10.1016/bs.aiht.2022.12.001}, DOI={10.1016/bs.aiht.2022.12.001}, abstractNote={The continued push to miniaturize electronic devices requires highly efficient heat dissipation concepts to ensure high system performance and reliability. Single-phase liquid-based cooling through microchannels have received industry acceptance over the years. This chapter covers the basics of microchannel heat transfer and frictional losses, and historical account of methods employed for enhanced thermal-hydraulic performance, with emphasis on advancements in the past decade. The investigations in microchannel enhanced heat transfer have been primarily focused on modifying the channel shapes, introduction of flow disturbances, inlet and outlet flow conditioning, multi-layer microchannel heat sinks, novel fluids with superior thermal conductivity and many others. The above concepts successfully enhance the heat transfer with some penalty on pressure drop, and the research efforts are aimed toward achieving reduced overall thermal resistance at a certain pumping power. Inclusion of porous structures (as a replacement to solid fins) and wall superhydrophobicity methods have also been evaluated for their ability to reduce the pressure drop while maintain enhanced cooling levels. The growing importance of multi-objective optimization with the above cooling concepts is outlined, along with the increasing role of additive manufacturing in the realization of next generation thermal management technologies in electronic cooling.}, booktitle={Advances in Heat Transfer}, publisher={Elsevier}, author={Ekkad, Srinath V. and Singh, Prashant}, editor={Abraham, John Patrick and Gorman, John M. and Minkowycz, Wally J.Editors}, year={2023}, pages={239–293} }
 @article{wahls_ekkad_2022, title={A new technique using background oriented schlieren for temperature reconstruction of an axisymmetric open reactive flow}, volume={33}, ISSN={["1361-6501"]}, DOI={10.1088/1361-6501/ac51a5}, abstractNote={Abstract
               A new technique, called 3D ray tracing, for refractive index field reconstruction of axisymmetric flows from displacement fields measured from background oriented schlieren (BOS) experiments is developed and applied to a lean premixed methane/air reactive flow at Reynolds number of 4000 on a 12 mm diameter circular burner. The temperature distribution is then calculated using a species independent direct relationship between refractive index, temperature, and ambient conditions. The error introduced by the approximation to reach this relationship is quantified using simulated flow fields and is found to be 8% within the inner unburnt region of the flow field, decreasing to 2% through the reaction zone, and then quickly reducing to 0% outside the flow field. The effect of random noise and reconstruction resolution on the accuracy of the method is assessed via application to synthetically generated data sets that mimic the characteristics of a heated air jet expelled into ambient. The novel 3D ray tracing allows for accurate temperature reconstructions of open axisymmetric reactive flows where 2D displacement fields are measured, which is shown to be a shortcoming of current direct methods in literature. Additionally, this is done without the need for any prior knowledge of flow field parameters; only ambient conditions to the system must be known. The simple experimental setup and low computational cost make this approach with BOS a good option for application into existing experimental combustion systems with minimal effort.}, number={5}, journal={MEASUREMENT SCIENCE AND TECHNOLOGY}, author={Wahls, Benjamin H. and Ekkad, Srinath V}, year={2022}, month={May} }
 @article{ahmed_wahls_ekkad_lee_ho_2022, title={Effect of Spanwise Hole-to-Hole Spacing on Overall Cooling Effectiveness of Effusion Cooled Combustor Liners for a Swirl-Stabilized Can Combustor}, volume={144}, ISSN={0889-504X 1528-8900}, url={http://dx.doi.org/10.1115/1.4054442}, DOI={10.1115/1.4054442}, abstractNote={Abstract
               One of the most effective ways to cool the combustor liner is through effusion cooling. Effusion cooling (also known as full-coverage effusion cooling) involves uniformly spaced holes distributed throughout the combustor liner wall. Effusion cooling configurations are preferred for their high effectiveness, low-pressure penalty, and ease of manufacturing. In this article, experimental results are presented for effusion cooling configurations for a realistic swirl driven can combustor under reacting (flame) conditions. The can combustor was equipped with an industrial engine swirler and gaseous fuel (methane), subjecting the liner walls to engine representative flow and combustion conditions. In this study, three different effusion cooling liners with spanwise spacings of r/d = 6, 8, and 10 and streamwise spacing of z/d = 10 were tested for four coolant-to-main airflow ratios. The experiments were carried out at a constant main flow Reynolds number (based on combustor diameter) of 12,500 at a total equivalence ratio of 0.65. Infrared thermography (IRT) was used to measure the liner outer surface temperature, and detailed overall effectiveness values were determined under steady-state conditions. The results indicate that decreasing the spanwise hole-to-hole spacing (r/d) from ten to eight increased the overall cooling effectiveness by 2–5%. It was found that reducing the spanwise hole-to-hole spacing further to r/d = 6 does not affect the cooling effectiveness implying the existence of an optimum spanwise hole-to-hole spacing. Also, the minimum liner cooling effectiveness on the liner wall was found to be downstream of the impingement location, which is not observed in the existing literature for experiments done under nonreacting conditions.}, number={7}, journal={Journal of Turbomachinery}, publisher={ASME International}, author={Ahmed, Shoaib and Wahls, Benjamin H. and Ekkad, Srinath V. and Lee, Hanjie and Ho, Yin-Hsiang}, year={2022}, month={May} }
 @article{wahls_ekkad_2022, title={Temperature reconstruction of an axisymmetric enclosed reactive flow using simultaneous background oriented schlieren and infrared thermography}, volume={33}, ISSN={["1361-6501"]}, DOI={10.1088/1361-6501/ac83e2}, abstractNote={Abstract
               The temperature distribution of a premixed methane air flame running at a Reynolds number of 1300 on a circular burner, 12.7 
                     
                     
                        
                           mm
                        
                     
                     
                   diameter, enclosed in a fused silica cylindrical liner has been experimentally reconstructed using a non-invasive approach combining background oriented schlieren (BOS) and infrared (IR) thermography. BOS is used to characterize both the air ambient to the system, using an existing technique called 3D ray tracing, and the reactive flow inside the enclosure, with a novel modified version of 3D ray tracing. IR thermography is used to characterize the thermal/optical characteristics of the quartz glass enclosure itself, since the information is required as BOS is a line of sight imaging technique. Out of necessity, an approximated species independent relationship is used to calculate flow temperature from refractive index. A simulation is used to show this error is in the range of 5.8%–7%. Additionally, it is found that drastically simplifying the approach by removing the IR thermography system entirely and using the near outer wall air temperature from BOS/3D ray tracing to characterize the internal temperature of the quartz liner itself only causes a 1.5%–3.8% degradation in the accuracy of the reconstructed temperature field. The technique as presented is a relatively inexpensive, experimentally simple approach capable of determining the steady state temperature characteristics of optically accessible axisymmetric reactive flows.}, number={11}, journal={MEASUREMENT SCIENCE AND TECHNOLOGY}, author={Wahls, Benjamin H. and Ekkad, Srinath V}, year={2022}, month={Nov} }
 @article{ekkad_singh_2021, title={A Modern Review on Jet Impingement Heat Transfer Methods}, volume={143}, ISSN={0022-1481 1528-8943}, url={http://dx.doi.org/10.1115/1.4049496}, DOI={10.1115/1.4049496}, abstractNote={Abstract
               Jet impingement cooling is considered as one of the most effective heat transfer enhancement techniques. The primary mode of heat transfer enhancement is due to the flow stagnation. The effectiveness of jet impingement as a cooling technique is well documented; however, the application of jet impingement to different problems has been hindered by inability of manufacturing methods to incorporate impingement systems easily into cooling designs. Impingement heat transfer effectiveness can be further improved by improving the jet strength by modifying the jet holes, enhancing surface features, or adding swirl. With an increased usage of this cooling technique and additional modifications in geometry to further enhance the heat transfer capacity to suit different applications, this paper provides a much-needed review of the advancements in the effectiveness of this cooling technique. A comprehensive look at impingement cooling over a variety of modifications and applications with a focus on improved manufacturing techniques impacting novel design and implementation is provided for a variety of heat transfer enhancement applications.}, number={6}, journal={Journal of Heat Transfer}, publisher={ASME International}, author={Ekkad, Srinath V. and Singh, Prashant}, year={2021}, month={Apr} }
 @article{panse_ekkad_2021, title={A numerical parametric study to enhance thermal hydraulic performance of a novel alternating offset oblique microchannel}, volume={79}, ISSN={1040-7782 1521-0634}, url={http://dx.doi.org/10.1080/10407782.2021.1872259}, DOI={10.1080/10407782.2021.1872259}, abstractNote={Abstract A novel alternating offset oblique microchannel (AOOMC) is proposed aimed at enhancing the thermal–hydraulic performance (THP) in microchannels. 3-D, conjugate numerical simulations are carried out over a range of Reynolds number from 200 to 800. The effect of three geometric parameters, relative oblique channel width relative fin offset width and oblique angle on THP is evaluated. THP shows an increasing–decreasing trend with and increases with increase in and for the range tested. The optimal geometry, and showcases the highest THP of 2.41 at Re = 800 which is superior to other enhanced microchannel geometries from the literature.}, number={7}, journal={Numerical Heat Transfer, Part A: Applications}, publisher={Informa UK Limited}, author={Panse, Sanskar S. and Ekkad, Srinath V.}, year={2021}, month={Jan}, pages={489–512} }
 @article{ramakrishnan_ahmed_ekkad_2021, title={Characterization of Transient Wall Heat Load for a Low NOx Lean Premixed Swirl Stabilized Can Combustor Under Reacting Conditions}, volume={14}, ISSN={1948-5085 1948-5093}, url={http://dx.doi.org/10.1115/1.4051375}, DOI={10.1115/1.4051375}, abstractNote={Abstract
               As stringent emissions controls are being placed on gas turbines, modern combustor design optimization is contingent on the accurate characterization of the combustor flame side heat loads. Power generation turbines are increasingly moving toward natural gas, biogas, and syngas, whose composition is highly dependent on the sourcing location. With fuel flexible nozzles, it is important to understand the heat load from various gas mixtures to optimize the cooling design to make sure the liner is not under/over cooled for some mixtures as this has a larger effect on NOx/CO emissions. In addition to knowing the heat load distribution, it is important to understand the peak heat load under start/stop transient conditions which tend to be much higher than steady-state/cruise altitude heat loads. The present work focuses on the experimental measurement of the transient heat load along a can combustor under reacting conditions for a swirl-stabilized premixed methane–air flame. Tests were carried out under various equivalence ratios, Reynolds numbers, and pilot fuel flowrate. An infrared camera was used to measure the inner and outer wall temperatures of the liner to calculate the liner heat load. Particle image velocimetry (PIV) was employed to visualize the flowfield for various reacting conditions studied in this work. Based on the heat transfer study, a detailed report of transient heat load along the length of the liner wall has been presented here. Initial start transient heat load on the liner wall is ∼10–40% more than the steady-state heat load.}, number={2}, journal={Journal of Thermal Science and Engineering Applications}, publisher={ASME International}, author={Ramakrishnan, Kishore Ranganath and Ahmed, Shoaib and Ekkad, Srinath V.}, year={2021}, month={Jun} }
 @article{bhansali_ramakrishnan_ekkad_2022, title={Effect of Pin Fins on Jet Impingement Heat Transfer Over a Rotating Disk}, volume={144}, ISSN={0022-1481 1528-8943}, url={http://dx.doi.org/10.1115/1.4053371}, DOI={10.1115/1.4053371}, abstractNote={Abstract
               Many engineering applications consist of rotating components that experience high-heat load. Applications like the gas turbine engine consist of rotating disks and the study of heat transfer over such rotating surfaces is of particular interest. In the case of gas turbines, the disk also needs to be protected from the ingress of hot turbine gases caused by the low-pressure region created due to the radially outward pumping of fluid close to the rotating surface. The present experimental study investigates the effects of introducing pin-fins on heat transfer over surface of a rotating gas-turbine disk. The experiments were conducted at rotational Reynolds numbers (ReR) of 5487–12,803 based on the disk diameter (D) and jet Reynolds numbers (Re) of 5000–18,000 based on the jet diameter. The effects of nozzle to target spacing (z/d=2−6), eccentricity of impingement (ε = 0−0.67), angle of impingement—both toward and away from the center (θi and θo=0 deg−20 deg), and the pin fin height (Hf=3.05 mm−19.05 mm) were studied. Steady-state temperature measurements were taken using thermocouples embedded in the disk close to the target surface, and area average Nusselt number (Nu) was calculated. The results have been compared with those for a smooth aluminum disk of equal dimensions and without any pin-fins. The average Nu was significantly enhanced by the presence of pin-fins. The enhancement was higher for lower values of Re, and the maximum enhancement was found to be 3.9 times that of a smooth disk for Re=5000. In the impingement dominant regime, the effect of disk rotation was minimal for a smooth disk, but the heat transfer increased with rotational speed in case of pin-fins. There was no impact of eccentricity on Nu for ε = 0 and 0.33. For ε = 0.67, the maximum reduction in enhancement over a smooth surface (21.95%) was observed when compared with coaxial impingement for stationary impingement for Re=18,000 and z/d=4. The effect of inclination angle was insignificant, and no clear trend could be established. Higher heat transfer rates were observed for z/d=6 with the increasing Re, and this effect diminished with the increase in the rotational speed. With the increase in pin-fin height, especially at higher values of Re, there was in increase in the value of Nu. Qualitative visualization of flow field has been performed for smooth and the pin-fin case using the commercial simulation package Ansys Fluent to further understand the flow features that result in the heat transfer enhancement.}, number={4}, journal={Journal of Heat Transfer}, publisher={ASME International}, author={Bhansali, Pratik S. and Ramakrishnan, Kishore Ranganath and Ekkad, Srinath V.}, year={2022}, month={Feb} }
 @article{madhavan_singh_ekkad_2022, title={Effect of Rotation on Heat Transfer in AR = 2:1 and AR = 4:1 Channels Connected by a Series of Crossover Jets}, volume={144}, ISSN={0889-504X 1528-8900}, url={http://dx.doi.org/10.1115/1.4053237}, DOI={10.1115/1.4053237}, abstractNote={Abstract
               Detailed heat transfer measurements using transient liquid crystal thermography were performed on a novel cooling design covering the mid-chord and trailing edge region of a typical gas turbine blade under stationary and rotating conditions. The test section comprised two channels with aspect ratio (AR) of 2:1 (mid-chord) and 4:1 (trailing edge), where the coolant was fed into the AR = 2:1 channel from the root. Rib turbulators with a pitch-to-rib height ratio (p/e) of 10 and rib height-to-channel hydraulic diameter ratio (e/Dh) of 0.075 were placed in the AR = 2:1 channel at an angle of 60 deg relative to the direction of flow. The coolant after entering this section was routed to the AR = 4:1 section through a set of crossover jets. The purpose of the crossover jets was to induce sideways impingement onto the pin fins that were placed in the 4:1 section to enhance heat transfer. The 4:1 section had a realistic trapezoidal shape that mimics the trailing edge of an actual gas turbine blade. The pin fins were arranged in a staggered array with a center-to-center spacing of 2.5 times the pin diameter in both spanwise and streamwise directions. The trailing edge section consisted of both radial and cutback exit holes for flow exit. Experiments were performed for the Reynolds number (Redh(AR=2:1)) of 20,000 at Rotation numbers (Rodh(AR=2:1)) of 0, 0.1, and 0.14. The channel-averaged heat transfer coefficient on trailing side was ∼28% (AR = 2:1) and ∼7.6% (AR = 4:1) higher than the leading side for Rotation number (Ro) of 0.1. It is shown that the combination of crossover jets and pin fins can be an effective method for cooling wedge-shaped trailing edge channels over axial cooling flow designs.}, number={6}, journal={Journal of Turbomachinery}, publisher={ASME International}, author={Madhavan, Srivatsan and Singh, Prashant and Ekkad, Srinath}, year={2022}, month={Feb} }
 @article{rudragoudar_chhatija_panse_ekkad_2021, title={Effect of channel aspect ratio and fin geometry on fluid flow and heat transfer performance of sectional oblique fin microchannels}, ISSN={["1936-3958"]}, DOI={10.1109/ITherm51669.2021.9503250}, abstractNote={Single-pass micro-channel heat sinks (MCHS) with water as the working fluid have been shown to deliver large heat removal rates with marginal pressure drop penalty. MCHS, because of their high surface area to volume ratio, provide large heat transfer coefficients (HTC) not found in other macro-scale channels and have found applications in power electronic devices and aerospace heat exchangers. Micro-channels with sectional oblique fins in the same direction augment heat transfer by reinitializing the boundary layer at the leading edge of each fin, causing the flow to be in a developing state, resulting in improved thermal performance. A detailed numerical study has been carried out on oblique fin microchannel heat sinks (OMC) to investigate the impact of channel aspect ratio (AR) and fin-pitch to oblique channel width ratio (BETA) on heat transfer and fluid flow characteristics. OMC with an aspect ratio ranging from 0.25 to 1, followed by four BETA values between 4 and 8, have been tested. Results reveal that OMC with AR of 0.6 outperforms the other configurations. A trade-off has been established where taller fins, although providing higher heat transfer area, suffer from poor fin efficiency, significantly impacting Nusselt number. OMC with BETA up to 6 has been shown to have similar hydro-thermal performance. The existing interplay between increasing mean channel velocity and decreasing heat transfer area has been identified at large hydraulic diameters.}, journal={PROCEEDINGS OF THE TWENTIETH INTERSOCIETY CONFERENCE ON THERMAL AND THERMOMECHANICAL PHENOMENA IN ELECTRONIC SYSTEMS (ITHERM 2021)}, author={Rudragoudar, Vaibhav Anandkumar and Chhatija, Harish and Panse, Sanskar S. and Ekkad, Srinath}, year={2021}, pages={42–51} }
 @article{sundaram_madhavan_singh_ekkad_2021, title={Enhanced fin-effectiveness of micro-scale concentric-shape roughened target surface subjected to array jet impingement}, volume={173}, ISSN={0017-9310}, url={http://dx.doi.org/10.1016/j.ijheatmasstransfer.2021.121148}, DOI={10.1016/j.ijheatmasstransfer.2021.121148}, abstractNote={Array jet impingement when combined with strategically placed short height (<1.25 mm) roughness elements on the target wall results in enhanced heat dissipation at high fin-efficiency (>95%) without much penalty on pumping power. Advances in metal-powder-based additive manufacturing enables the printing of roughness scales (< 0.5 mm) with acceptable accuracy. This paper presents an experimental and numerical study on the characterization of heat transfer performance of concentric-shaped micro pin-fins subjected to array jet impingement. The experimental study served the dual purpose of establishing manufacturability of such configurations and validating the computational model. Three concentric micro-pin fin configurations were additively manufactured with AlSi10Mg alloy through Direct Metal Laser Sintering (DMLS) with dimensions between 0.25 - 2.25 mm on the target wall subjected to a 5 × 5 array of jets with normalized jet-to-jet spacing of X/Djet =Y/Djet = 3 and normalized jet-to-target spacing Z/Djet of 1. The resultant Nusselt number (Nu) and fin effectiveness (ε) values have been reported for Reynolds number ranging between 3,000 and 12,000. The three target plate configurations produced effectiveness between 1.8 - 2.4 and the configuration with maximum wetted surface area (wall thickness of 250 µm) and void fraction produced highest effectiveness. Further, a validated numerical model was used to perform a parametric study on geometrical parameters of roughness element viz. pin-fin spacing (S), inner diameter (D1), outer diameter (D2) and height (H) at a ReDjet = 6,000. Significant enhancement in ε was observed with H being the dominant parameter affecting the heat transfer. The effect of void fraction (α), which denotes the free flow area between pin-fin elements (Ab –Apin) was also explored. There exists an αopt at which maximum heat transfer was achieved. A correlation for fin effectiveness is proposed as a function of different non-dimensional variables derived from the parametric study and its predictive capabilities were found to be within ±10%.}, journal={International Journal of Heat and Mass Transfer}, publisher={Elsevier BV}, author={Sundaram, Ramaswamy Devakottai and Madhavan, Srivatsan and Singh, Prashant and Ekkad, Srinath V.}, year={2021}, month={Jul}, pages={121148} }
 @article{panse_ekkad_2021, title={Forced convection cooling of additively manufactured single and double layer enhanced microchannels}, volume={168}, ISSN={0017-9310}, url={http://dx.doi.org/10.1016/j.ijheatmasstransfer.2020.120881}, DOI={10.1016/j.ijheatmasstransfer.2020.120881}, abstractNote={Metal based additive manufacturing (AM) is gaining increasing attention in the thermal management community due to the ability to fabricate intricate internal cooling features with acceptable accuracy. The non-traditional designs possible with AM can be leveraged to fabricate enhanced microchannel geometries which allow removal of high heat fluxes from power electronic devices. This study thereby explores the marriage of additive manufacturing and microchannel cooling where microchannels are fabricated using Direct Metal Laser Sintering (DMLS). Microchannels arranged in single layer and double layer configurations are evaluated for their heat transfer and pressure drop performance over Reynolds number, based on channel hydraulic diameter, ranging from 200 - 900 with water as the coolant. The double layer microchannels present a more complex geometry, with two layers of microchannels stacked one on top of the other which promises improved thermal performance based on literature. Enhancement features in the form of periodic oblique and alternating oblique secondary channels, known to augment fluid mixing and disrupt boundary layer growth are incorporated in both configurations aimed at augmenting the overall performance. Seven AM microchannel geometries, three in single and four in double layer configurations are tested. AM parts analyzed for their surface roughness and dimensional accuracy exhibit high surface roughness and deviation from design intent dimensions. Straight microchannel, fabricated using conventional manufacturing where the channel walls exhibit lower roughness levels serves as the baseline case. Comparing the performance of straight microchannels fabricated via conventional and additive methods reveal roughness induced augmentation in heat transfer and pressure drop of about 10-15%. Incorporation of secondary flow channels results in enhanced thermal hydraulic performance in both single and double layer configurations. Microchannels ranked based on thermal-hydraulic performance show double layer microchannel featuring oblique secondary channels offering the best performance with ~60% higher performance than baseline. The findings from this study highlight the potential of AM in developing sophisticated microchannels.}, journal={International Journal of Heat and Mass Transfer}, publisher={Elsevier BV}, author={Panse, Sanskar S. and Ekkad, Srinath V.}, year={2021}, month={Apr}, pages={120881} }
 @article{ekkad_singh_2021, title={Liquid Crystal Thermography in Gas Turbine Heat Transfer: A Review on Measurement Techniques and Recent Investigations}, volume={11}, ISSN={2073-4352}, url={http://dx.doi.org/10.3390/cryst11111332}, DOI={10.3390/cryst11111332}, abstractNote={Liquid Crystal Thermography is a widely used experimental technique in the gas turbine heat transfer community. In turbine heat transfer, determination of the convective heat transfer coefficient (h) and adiabatic film cooling effectiveness (η) is imperative in order to design hot gas path components that can meet the modern-day engine performance and emission goals. LCT provides valuable information on the local surface temperature, which is used in different experimental methods to arrive at the local h and η. The detailed nature of h and η through LCT sets it apart from conventional thermocouple-based measurements and provides valuable insights into cooling designers for concept development and its further iterations. This article presents a comprehensive review of the state-of-the-art experimental methods employing LCT, where a critical analysis is presented for each, as well as some recent investigations (2016–present) where LCT was used. The goal of this article is to familiarize researchers with the evolving nature of LCT given the advancements in instrumentation and computing capabilities, and its relevance in turbine heat transfer problems in current times.}, number={11}, journal={Crystals}, publisher={MDPI AG}, author={Ekkad, Srinath V. and Singh, Prashant}, year={2021}, month={Oct}, pages={1332} }
 @article{ahmed_ramakrishnan_ekkad_2021, title={Overall Cooling Effectiveness of Effusion Cooled Can Combustor Liner Under Reacting and Non-Reacting Conditions}, volume={14}, ISSN={1948-5085 1948-5093}, url={http://dx.doi.org/10.1115/1.4051371}, DOI={10.1115/1.4051371}, abstractNote={Abstract
               Emphasis on lean premixed combustion in modern low NOX combustion chambers limits the air available for cooling the combustion liner. Hence, the development of optimized liner cooling designs is imperative for effective usage of available coolant. An effective way to cool a gas turbine combustor liner is through effusion cooling. Effusion cooling (also known as full-coverage film cooling) involves uniformly spaced holes distributed throughout the liner’s curved surface area. This study presents findings from an experimental study on the characterization of the overall cooling effectiveness of an effusion-cooled liner wall, which was representative of a can combustor under heated flow (non-reacting) and lean-combustion (reacting) conditions. The model can combustor was equipped with an industrial swirler, which subjected the liner walls to engine representative flow and combustion conditions. In this study, two different effusion cooling liners with an inline and staggered arrangement of effusion holes have been studied. Non-dimensionalized streamwise hole-to-hole spacing (z/d) and spanwise hole-to-hole spacing (r/d) of 10 were used for both the effusion liners. These configurations were tested for five different blowing ratios ranging from 0.7 to 4.0 under both reacting and non-reacting conditions. The experiments were carried out at a constant main flow Reynolds number (based on combustor diameter) of 12,500. The non-reacting experiments were carried out by heating the mainstream air, and the reacting experiments were carried out under flame conditions at a total equivalence ratio of 0.65. Infrared thermography (IRT) was used to measure the liner outer surface temperature, and detailed overall effectiveness values were determined under steady-state conditions. It was observed that overall cooling effectiveness trends were different under reacting and non-reacting conditions. The cooling effectiveness for the non-reacting experiments exhibited a decreasing trend, and no consistent location of minimum cooling effectiveness was observed for the range of blowing ratios investigated in this study. For the reacting cases, the cooling effectiveness first follows a decreasing trend, reaches a distinct minimum, and then increases till the end of the combustor. Under non-reacting conditions, the staggered configuration was 9–25% more effective than inline configuration, and under reacting conditions, the staggered configuration was 4–8% more effective than inline configuration. From this study, it is clear that the coolant flame interaction for the reacting experiments impacted the liner cooling effectiveness and led to different overall cooling effectiveness distribution on the liner when compared with the non-reacting experiments.}, number={2}, journal={Journal of Thermal Science and Engineering Applications}, publisher={ASME International}, author={Ahmed, Shoaib and Ramakrishnan, Kishore Ranganath and Ekkad, Srinath V.}, year={2021}, month={Jun} }
 @article{jia_ajayi_ramakrishnan_negi_liu_ekkad_xu_2020, title={A skin layer made of cured polysilazane and yttria stabilized zirconia for enhanced thermal protection of carbon fiber reinforced polymers (CFRPs)}, volume={404}, ISSN={0257-8972}, url={http://dx.doi.org/10.1016/j.surfcoat.2020.126481}, DOI={10.1016/j.surfcoat.2020.126481}, abstractNote={This study reported for the first time using cured preceramic polymer precursor and their composites as thermal protection layer for the carbon fiber reinforced polymer (CFRP). The fabricated composite skin consists of cured polysilazane (PSZ) and yttria stabilized zirconia (YSZ) as the reinforcements and has a low density of 1.67 g/cm3, which can be used for both aerospace and defense applications. The coated composite showed excellent bonding integrity at the interface between the composite skin and the CFRP substrate. Thermal studies revealed the cured PSZ has excellent thermal stability up to its cured temperatures. Using the 3ω method, the room temperature thermal conductivity of PSZ was measured as 0.17 ± 0.02 W/m·K along the through-thickness direction. Thermal conductance of the skin layer is around 940 W/m2·K, which is about 45% reduction in conductance compared to the CFRP. Thermal insulation test shows that the coated composite can be used up to 250 °C. Experiment results demonstrated the skin layer's effectiveness in protecting the CFRP in high-temperature environments.}, journal={Surface and Coatings Technology}, publisher={Elsevier BV}, author={Jia, Yujun and Ajayi, Tosin D. and Ramakrishnan, Kishore Ranganath and Negi, Ankit and Liu, Jun and Ekkad, Srinath and Xu, Chengying}, year={2020}, month={Dec}, pages={126481} }
 @article{sambamurthy_madhavan_singh_ekkad_2020, title={Array Jet Impingement on High Porosity Thin Metal Foams: Effect of Foam Height, Pore-Density, and Spent Air Crossflow Scheme on Flow Distribution and Heat Transfer}, volume={142}, ISSN={0022-1481 1528-8943}, url={http://dx.doi.org/10.1115/1.4047560}, DOI={10.1115/1.4047560}, abstractNote={Abstract
               Experimental investigation was carried out to study heat transfer and fluid flow in high porosity (93%) thin metal foams (MFs) subjected to array jet impingement, under maximum and intermediate crossflow exit schemes. Separate effects of pore-density (pores per inch: PPI) and jet-to-target spacing (z/d) have been studied. To this end, for a fixed pore-density of 40 PPI foams, three different jet-to-target spacings (z/d = 1, 2, 6) were investigated, and for a fixed jet-to-target spacing (z/d) of 6, three different pore-density of 5, 20, and 40 PPI were investigated. The jet diameter-based Reynolds number was varied between 3000 and 12,000. Both flow and heat transfer experiments were carried out to characterize the flow distribution, crossflow mass flux accumulation, and local Nusselt numbers for different jet impingement configurations. The heat transfer results were obtained through steady-state experiments. Local flow measurements show that, as jet-to-target distance decreases, the mass flux distributions were increasingly skewed with higher mass flux distributed toward the exit(s). It was observed that Nusselt number increased with increasing pore density at a fixed jet-to-target spacing and reduced with increase in jet to target spacing at a fixed pore density. Intermediate crossflow had higher heat transfer than maximum crossflow with significantly lower pumping power. For a fixed pumping power, z/d = 2, 40 ppi foam provided an average heat transfer enhancement of 35% over the corresponding baseline configuration for intermediate crossflow scheme and was found to be the optimum configuration.}, number={11}, journal={Journal of Heat Transfer}, publisher={ASME International}, author={Sambamurthy, Vivek Subramaniam and Madhavan, Srivatsan and Singh, Prashant and Ekkad, Srinath V.}, year={2020}, month={Jul} }
 @misc{ekkad_singh_2021, title={Detailed Heat Transfer Measurements for Rotating Turbulent Flows in Gas Turbine Systems}, volume={14}, ISSN={["1996-1073"]}, DOI={10.3390/en14010039}, abstractNote={Detailed understanding of hot gas path flow and heat transfer characteristics in gas turbine systems is imperative in order to design cooling strategies to meet the stringent requirements in terms of coolant usage to maintain critical components below a certain temperature. To this end, extensive research has been carried out over the past four decades on advanced thermal diagnostic methods to accurately measure heat transfer quantities such as Nusselt number and adiabatic film cooling effectiveness. The need to capture local heat transfer characteristics of these complex flow systems drives the development of measurement techniques and the experimental test facilities to support such measurements. This article provides a comprehensive overview of the state-of-the-art thermal diagnostic efforts pertaining to heat transfer measurements in rotating gas turbine blade internal and external cooling and rotor-stator disc cavity, all under rotating environments. The major investigation efforts have been identified for each of the above three categories and representative experimental results have been presented and discussed.}, number={1}, journal={ENERGIES}, author={Ekkad, Srinath V. and Singh, Prashant}, year={2021}, month={Jan} }
 @inproceedings{panse_ekkad_2020, title={Evaluation of Additively Manufactured Single-Pass and Two-Pass Enhanced Microchannel Heat Sinks}, volume={2020-July}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85091773823&partnerID=MN8TOARS}, DOI={10.1109/ITherm45881.2020.9190581}, abstractNote={Additive manufacturing is proving to be a viable manufacturing technique to fabricate complex structures as it opens up the design space by providing improved design freedom and reduced geometric restrictions. Further, it assures to provide benefit to the heat exchanger industry which relies on novel geometric features to enhance performance and efficiency. This paper explores the heat transfer and pressure drop performance of additively manufactured microchannel heat sinks. Two types of microchannel heat sinks were tested, namely, single-pass and two-pass microchannels with heat flux provided on the bottom wall. Additionally, the microchannel geometry was enhanced by incorporating periodic secondary flow passages with an aim to enhance fluid mixing and disrupt boundary layer development. The enhanced microchannel configurations featured the oblique fin and trapezoidal fin heat sinks. Enhanced microchannels showed superior performance both thermally and hydraulically with 100% increase in heat transfer performance with negligible gain in pressure drop over the baseline straight microchannel geometry. Moreover, the performance gain was more evident for the single-pass microchannels than the two-pass configuration, which showed tremendously high pressure drop due to increase in flow length. Results showed 30% reduction in the overall thermal resistance for a constant pressure drop for the enhanced microchannels in the single pass configuration.}, booktitle={InterSociety Conference on Thermal and Thermomechanical Phenomena in Electronic Systems, ITHERM}, author={Panse, S.S. and Ekkad, S.V.}, year={2020}, pages={38–47} }
 @article{singh_sarja_ekkad_2020, title={Experimental and Numerical Study of Chord-Wise Eight-Passage Serpentine Cooling Design for Eliminating the Coriolis Force Adverse Effect on Heat Transfer}, volume={13}, ISSN={1948-5085 1948-5093}, url={http://dx.doi.org/10.1115/1.4047655}, DOI={10.1115/1.4047655}, abstractNote={Abstract
               Gas turbine blades are equipped with internal cooling channels which are connected by 180 deg bends. Due to combined effects of Coriolis force and centrifugal buoyancy force, the heat transfer increases on the trailing side (pressure side) and decreases on the leading side (suction side) for radially outward flow. The trend in heat transfer is opposite for radially inward flow. This configuration leads to nonuniform blade temperature which in unfavorable for blade lifespan. This paper presents a novel eight-passage serpentine design, where passages are arranged along the chord of the blade, to rectify the negative effects of Coriolis force on heat transfer and is an extension four- and six-passage smooth channel studies conducted by the authors earlier. Transient liquid crystal thermography (TLCT) is carried out for detailed measurement of heat transfer coefficients. Heat transfer experiments were performed for Reynolds numbers between 14,264 and 83,616 under stationary conditions. For experiments under rotation, non-dimensional Rotation number is set as 0.05. Heat transfer enhancement levels of nearly twice the Dittus–Boelter correlation (for developed flow in smooth tubes) are obtained under stationary conditions. Under rotation, it is seen that the heat transfer enhancement levels on the leading and trailing sides are similar to each other and also with the stationary condition. Some differences in heat transfer are observed on local level, when rotation cases are compared against the stationary cases. Numerically predicted flow field is presented to support the experimental findings.}, number={1}, journal={Journal of Thermal Science and Engineering Applications}, publisher={ASME International}, author={Singh, Prashant and Sarja, Ajay and Ekkad, Srinath V.}, year={2020}, month={Aug} }
 @article{rajamuthu_panse_ekkad_2021, title={Heat Transfer Enhancement Through Array Jet Impingement on Strategically Placed High Porosity High Pore-Density Thin Copper Foams}, volume={143}, ISSN={1043-7398 1528-9044}, url={http://dx.doi.org/10.1115/1.4049173}, DOI={10.1115/1.4049173}, abstractNote={Abstract
               High porosity, high pore-density (pores per inch: PPI) metal foams are a popular choice in high heat flux cooling applications as they offer large heat transfer area over a given volume, however, accompanied by a concomitant increase in pumping power requirements. This experimental study aims toward developing a novel metal-foam based cooling configuration featuring thin copper foams (3 mm) subjected to orthogonal air jet array impingement. The foam configurations allowed strategic and selective placement of high pore-density (90 PPI) and high porosity (∼96%) copper foam on the heated surface with respect to the jet array in the form of foam stripes aiming to enhance heat transfer and reduce pressure drop penalty. The thermal-hydraulic performance was evaluated over range of Reynolds numbers, jet-to-jet (x/dj, y/dj) and jet-to-target (z/dj) spacings and compared with a baseline smooth surface. The effect of pore density was further analyzed by studying 40 PPI copper foam and compared with corresponding 90 PPI foam arrangement. The thermal-hydraulic performance was found to be governed by combinational interaction of three major factors: heat transfer area, ease of jet penetration, and foam volume usage. Strategic placement of metal foam stripes allowed better utilization of the foam heat transfer area and available foam volume by aiding penetration of coolant fluid through available foam thickness, thus performing better than the case where entire heat transfer area was covered with foam. For a fixed pumping power of 10 W, the optimal metal foam-jet configuration showed ∼50% higher heat transfer with negligible increase in pumping power requirements.}, number={3}, journal={Journal of Electronic Packaging}, publisher={ASME International}, author={Rajamuthu, Varun Prasanna and Panse, Sanskar S. and Ekkad, Srinath}, year={2021}, month={Jan} }
 @article{jia_ajayi_wahls_ramakrishnan_ekkad_xu_2020, title={Multifunctional Ceramic Composite System for Simultaneous Thermal Protection and Electromagnetic Interference Shielding for Carbon Fiber-Reinforced Polymer Composites}, volume={12}, ISSN={1944-8244 1944-8252}, url={http://dx.doi.org/10.1021/acsami.0c17361}, DOI={10.1021/acsami.0c17361}, abstractNote={Achieving a high electrical conductivity while maintaining a good thermal insulation is often contradictory in the material design for the goal of simultaneous thermal protection and electromagnetic interference shielding. The reason is that materials with a high electrical conductivity often pertain a high thermal conductivity. To address this challenge, this study reports a multifunctional ceramic composite system for carbon fiber-reinforced polymer composites. The fabricated multifunctional ceramic composite system has a multilayer structure. The polymer-derived SiCN ceramic reinforced with yttria-stabilized zirconia fibers serves as the thermal protection and impedance-matching layer, while the yttria-stabilized zirconia fiber-reinforced SiCN ceramic with carbon nanotubes provides the electromagnetic interference shielding. The thermal conductance of the multilayered ceramic composite is about 22.5% lower compared to that of the carbon fiber-reinforced polymer composites. The thermal insulation test during the steady-state condition shows that the hybrid composite can be used up to 300 °C while keeping the temperature reaching the surface of carbon fiber-reinforced polymer composites at around 167.8 °C. The flame test was used to characterize the thermal protection capability under transient conditions. The hybrid composite showed temperature differences of 72.9 and 280.7 °C during the low- and high-temperature settings, respectively. The average total shielding efficiency per thickness of the fabricated four-layered ceramic composite system was 21.45 dB/mm, which showed a high reflection-dominant electromagnetic interference shielding. The average total shielding efficiency per thickness of the eight-layered composite system was 16.57 dB/mm, revealing a high absorption-dominant electromagnetic interference shielding. Typical carbon fiber-reinforced polymer composites reveal a reflection-dominant electromagnetic interference shielding. The electrons can freely move in the percolated carbon nanotubes within the inner layers of the composite material, which provide the improved electromagnetic interference shielding ability. The movement of electrons was impeded by the top and bottom layers whose thermal conduction relies on the lattice vibrations, resulting in a satisfactory thermal insulation of the composite materials and impedance matching with the free space. Results of this study showed that materials with a good thermal insulation and electromagnetic interference shielding can be obtained simultaneously by confining the electron movement inside the materials and refraining their movement at the skin surface.}, number={52}, journal={ACS Applied Materials & Interfaces}, publisher={American Chemical Society (ACS)}, author={Jia, Yujun and Ajayi, Tosin D. and Wahls, Benjamin H. and Ramakrishnan, Kishore Ranganath and Ekkad, Srinath and Xu, Chengying}, year={2020}, month={Dec}, pages={58005–58017} }
 @article{sarja_singh_ekkad_2020, title={Parallel rotation for negating Coriolis force effect on heat transfer}, volume={124}, ISSN={0001-9240 2059-6464}, url={http://dx.doi.org/10.1017/aer.2020.1}, DOI={10.1017/aer.2020.1}, abstractNote={ABSTRACTGas turbine blades feature multi-pass internal cooling channels, through which relatively colder air bled from the compressor is routed to cool internal walls. Under rotation, due to the influence of Coriolis force and centrifugal buoyancy, heat transfer at the trailing side enhances and that at the leading side reduces, for a radially outward flow. This non-uniform temperature distribution results in increased thermal stress, which is detrimental to blade life. In this study, a rotation configuration is presented which can negate the Coriolis force effect on heat and fluid flow, thereby maintaining uniform heat transfer on leading and trailing walls. A straight, smooth duct of unit aspect ratio is considered to demonstrate the concept and understand the fluid flow within the channel and its interaction with the walls. The new design is compared against the conventional rotation design. Numerical simulations under steady-state condition were carried out at a Reynolds number of 25000, where the Rotation numbers were varied as 0, 0.1, 0.15, 0.2, 0.25. Realisable version of k-$\varepsilon$ model was used for turbulence modelling. It was observed that new rotation (parallel) configuration’s heat transfer on leading and trailing sides were near similar, and trailing side was marginally higher compared to leading side. An interesting phenomenon of secondary Coriolis effect is reported which accounts for the minor differences in heat transfer augmentation between leading and trailing walls. Due to centrifugal buoyancy, the fluid is pushed towards the radially outward wall, resulting in a counter-rotating vortex pair, which also enhances the heat transfer on leading and trailing walls when compared to stationary case.}, number={1274}, journal={The Aeronautical Journal}, publisher={Cambridge University Press (CUP)}, author={Sarja, A. and Singh, P. and Ekkad, S.V.}, year={2020}, month={Jan}, pages={581–596} }
 @article{awad_attinger_bejan_beskok_celata_colin_dhir_di marco_ekkad_garimella_et al._2020, title={Professor Satish G. Kandlikar on His 70th Birthday}, volume={12}, ISSN={1948-5085 1948-5093}, url={http://dx.doi.org/10.1115/1.4048813}, DOI={10.1115/1.4048813}, number={6}, journal={Journal of Thermal Science and Engineering Applications}, publisher={ASME International}, author={Awad, Mohamed M. and Attinger, Daniel and Bejan, Adrian and Beskok, Ali and Celata, Gian Piero and Colin, Stephane and Dhir, Vijay K. and Di Marco, Paolo and Ekkad, Srinath V. and Garimella, Srinivas and et al.}, editor={Awad, Mohamed M. and Attinger, Daniel and Bejan, Adrian and Beskok, Ali and Celata, Gian Piero and Colin, Stéphane and Dhir, Vijay K. and Di Marco, Paolo and Ekkad, Srinath V. and Garimella, Srinivas and et al.Editors}, year={2020}, month={Nov} }
 @inproceedings{rajamuthu_panse_ekkad_2020, title={Thermal Hydraulic Performance of High Porosity High Pore Density Thin Copper Foams Subject to Array Jet Impingement}, volume={2020-July}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85091744591&partnerID=MN8TOARS}, DOI={10.1109/ITherm45881.2020.9190241}, abstractNote={Metal foams have shown promise in enhancing heat dissipation from heated surfaces and find applications in forced convection cooling environments like electronics cooling. The thermal and hydraulic performance of metal foams have a strong correlation to its pore density (pores per inch: PPI) and porosity. While high pore density is desired to enhance heat dissipation (due to higher effective heat transfer area), high porosity is suitable to maintain low pressure drop in forced convective cooling applications. Towards this end, an experimental study was carried out to evaluate the thermal-hydraulic performance of high pore density (90 PPI), high porosity (95%), thin Copper foams (3 mm thick) strategically placed over a heated surface of base area 20 mm x 20 mm. Heat transfer was facilitated with air as the working fluid impinging through a 3x3 array (x⁄dj = y⁄dj = 4) of circular nozzles of diameter, dj = 1.5 mm. Two metal foam-heated surface configurations were tested, a full foam configuration; where the metal foam covered the entire heated surface area, and a foam stripes configuration, where metal foam stripes were strategically placed over the heated surface, were studied for their heat transfer, pressure drop and thermal hydraulic performance at Reynolds numbers (Rej) between 3000 and 12000. A smooth surface, without metal foam, served as the baseline case. Additionally, the effect of varying jet-to-target plate distance (z) as z⁄dj = 2, 3, 5, 7 was studied. From experiments, it was observed that the stripes configuration had highest heat transfer enhancement of about 1.45 times that of the smooth surface target, at the expense of a marginal increase in pumping power, thereby making it the best configuration in terms of thermal hydraulic performance.}, booktitle={InterSociety Conference on Thermal and Thermomechanical Phenomena in Electronic Systems, ITHERM}, author={Rajamuthu, V.P. and Panse, S.S. and Ekkad, S.V.}, year={2020}, pages={15–22} }
 @inproceedings{panse_singh_ekkad_2019, title={Air-based cooling in high porosity, aluminum foams for compact electronics cooling}, volume={2019-May}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85073908478&partnerID=MN8TOARS}, DOI={10.1109/ITHERM.2019.8757458}, abstractNote={An experimental study was carried out to evaluate heat transfer and pressure drop performance of high porosity (93%) aluminum foams with pore densities 5 pores per inch (PPI), 20 PPI and 40 PPI. Tests were run with air as the working fluid in a rectangular channel of aspect ratio 8:3. The metal foam samples were 50.8 mm ($W$), 20 mm ($H$) and 254 mm ($L$). A smooth channel without metal foam was tested to establish baseline performance curves. Studies were undertaken to investigate the distribution of local Nusselt number along the streamwise direction as a function of metal foam pore density. All foam samples exhibited short thermal developing length of $x/D_{h}\sim 4.2$ after which the heat transfer converged to almost constant value. As expected, heat transfer performance improved with increase in foam pore density, however, accompanied by higher pressure drop. For a given pumping power, the 40 PPI foam sample showed the highest heat transfer performance. Heat transfer with the 40 PPI foam was found to be 27.5 higher than the baseline smooth surface. Metal foams showed excellent promise in high heat flux cooling applications like electronics cooling.}, booktitle={InterSociety Conference on Thermal and Thermomechanical Phenomena in Electronic Systems, ITHERM}, author={Panse, S.S. and Singh, P. and Ekkad, S.V.}, year={2019}, pages={376–383} }
 @inproceedings{ahmed_singh_ekkad_2019, title={Comparison of different combustion liner cooling techniques under non-reacting conditions for a lean premixed fuel nozzle}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85083941517&partnerID=MN8TOARS}, DOI={10.2514/6.2019-0532}, booktitle={AIAA Scitech 2019 Forum}, author={Ahmed, S. and Singh, P. and Ekkad, S.V.}, year={2019} }
 @inproceedings{sarja_madhavan_singh_ekkad_2019, title={Effect of blade profile on four-passage serpentine configuration designed to negate coriolis effect on heat and fluid flow}, volume={5A-2019}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85075419427&partnerID=MN8TOARS}, DOI={10.1115/GT2019-91718}, abstractNote={Abstract
               Gas turbine blades are equipped with serpentine internal cooling channels with 180-degree bends, through which relatively colder air is routed to cool the internal walls. It has been established that under the influence of rotation, pressure and suction side internal wall heat transfer characteristics are very different, which leads to non-uniform metal temperatures, and hence higher levels of thermal stresses. Present study addresses this non-uniformity in heat transfer using parallel rotation to negate Coriolis effect. Further, the blade curvature does not allow rectangular or trapezoidal passages, which are typically studied. In this paper, we have numerically investigated a realistic design for the four-passage channel, where the cooling design can actually be incorporated in a blade. Four-passage configuration also features 90-degree square shaped rib turbulators, and the corresponding baseline case is smooth channel. Numerical simulations have been carried out at Reynolds numbers of 5000, 10000 and 25000 and Rotation numbers were varied between 0 and 0.25. For smooth case, heat transfer enhancement was found to be higher on suction (leading) side compared to pressure (trailing) side under both stationary and rotating conditions. The enhancement levels between stationary and rotation conditions varied marginally in these designs, indicating that buoyancy effects were insignificant. For ribbed case, the effect of 90-degree rib turbulators on local heat transfer was more pronounced on the suction side when compared to smooth case. Under rotating conditions, it was found that the cooling levels were similar to the stationary condition for both pressure and suction side internal walls.}, booktitle={Proceedings of the ASME Turbo Expo}, author={Sarja, A. and Madhavan, S. and Singh, P. and Ekkad, S.V.}, year={2019} }
 @inproceedings{madhavan_sambamurthy_singh_ekkad_2019, title={Effect of pore density on jet impingement onto thin metal foams under intermediate crossflow scheme}, volume={8}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85078824202&partnerID=MN8TOARS}, DOI={10.1115/IMECE2019-10748}, abstractNote={Abstract
               Array jet impingement heat transfer onto thin metal foams of different pore densities has been experimentally investigated in the current study. Aluminum foams with high porosity (93%) and different pore densities of 5, 20 and 40 ppi are subjected to array jet impingement under an intermediate crossflow exit scheme. The jets are arranged such that the streamwise jet-to-jet spacing is x/dj = 8 and spanwise jet-to-jet spacing is y/dj = 4. Jet to target plate spacing was maintained at z/dj = 6 where ‘z’ is the distance between the jet plate and the target surface on which metal foams were installed. A steady state heat transfer technique has been used to obtain local heat transfer coefficients along the streamwise direction. It is observed that heat transfer enhancement levels increase as pore density increases. An enhancement of 50–100% over the baseline case of impingement onto smooth surface is obtained over the flow range tested (3000 < Redj < 12000). At a constant pumping power of 40 W, an enhancement of 26–33% is obtained for the different pore densities tested.}, booktitle={ASME International Mechanical Engineering Congress and Exposition, Proceedings (IMECE)}, author={Madhavan, S. and Sambamurthy, V.S. and Singh, P. and Ekkad, S.}, year={2019} }
 @inproceedings{ramakrishnan_singh_madhavan_ekkad_2019, title={Effect of twist ratio on heat transfer enhancement by swirl impingement}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85084099558&partnerID=MN8TOARS}, DOI={10.1115/HT2019-3530}, abstractNote={Abstract
               Steady state experimental work has been carried out to compare a conventional single jet of diameter 12.7mm with a swirling impinging jet. In this study swirl inserts with three different twist ratios 3, 4.5 and 6 were used to induce the swirling motion to the working fluid. The Reynolds number based on conventional impinging jet’s diameter is varied from 10000 to 16000. It is observed that with increase in twist ratio, the average heat transfer enhancement is reduced. However, with higher twist ratios more uniform distribution of heat transfer enhancement is observed.}, booktitle={ASME 2019 Heat Transfer Summer Conference, HT 2019, collocated with the ASME 2019 13th International Conference on Energy Sustainability}, author={Ramakrishnan, K.R. and Singh, P. and Madhavan, S. and Ekkad, S.V.}, year={2019} }
 @article{kaur_singh_ekkad_2020, title={Enhanced thermal hydraulic performance by V-shaped protrusion for gas turbine blade trailing edge cooling}, volume={149}, ISSN={0017-9310}, url={http://dx.doi.org/10.1016/j.ijheatmasstransfer.2019.119221}, DOI={10.1016/j.ijheatmasstransfer.2019.119221}, abstractNote={Modern high-pressure stage gas turbine blades are equipped with several high-performance cooling concepts, which vary based on the external heat load and the blade-topology. Blade trailing edge cooling is challenging due to the space constraints, where channel aspect ratio (AR: channel width-to-height ratio) is ~ 4:1. With an aim to enhance the thermal hydraulic performance of such narrow channels, a novel V-shaped protrusion configuration is proposed in the present study. The V-shaped protrusion derives its profile from a known concept V-shaped concavity. Numerical investigation of inline configurations of V-shaped concavities and protrusions has been carried out for a 4:1 AR channel for Reynolds number ranging from 10,000 to 60,000. The configurational parameters of both concavity and protrusion were identical. Concavity/protrusion depth-height-to-diameter (δ/D) ratio was 0.3. The streamwise (Sx) and spanwise (Sy) pitch values were 3.2D in both the configurations. The fluid flow and heat transport in the two cooling configurations are discussed in detail. Globally averaged Nusselt number, friction factor and thermal-hydraulic performance have been reported for the investigated Re range. Results show that V-shaped protrusion had the highest thermal-hydraulic performance ~ 2.23, which was about 40.4% higher than the V-shaped concavity configuration for a representative Reynolds number of 10,000.}, journal={International Journal of Heat and Mass Transfer}, publisher={Elsevier BV}, author={Kaur, Inderjot and Singh, Prashant and Ekkad, Srinath V.}, year={2020}, month={Mar}, pages={119221} }
 @inproceedings{madhavan_singh_ramakrishnan_ekkad_2019, title={Experimental investigation of crossflow diverters in jet impingement cooling}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85084099754&partnerID=MN8TOARS}, DOI={10.1115/HT2019-3538}, abstractNote={Abstract
               Jet impingement is a cooling technique commonly employed in combustor liner cooling and high-pressure gas turbine blades. However, jets from upstream impingement holes reduce the effectiveness of downstream jets due to jet deflection in the direction of crossflow. In order to avoid this phenomenon and provide an enhanced cooling on the target surface, we have attempted to come up with a novel design called “crossflow diverters”. Crossflow diverters are U-shaped ribs that are placed between jets in the crossflow direction (under maximum crossflow condition). In this study, the baseline case is jet impingement onto a smooth surface with 10 rows of jet impingement holes, jet-to-jet spacing of X/D = Y/D = 6 and jet-to-target spacing of Z/D = 2. Crossflow diverters with thickness ‘t’ of 1.5875 mm, height ‘h’ of 2D placed in the streamwise direction at a distance of X = 2D from center of the jet have been investigated experimentally. Transient liquid crystal thermography technique has been used to obtain detailed measurement of heat transfer coefficient for four jet diameter based Reynolds numbers of 3500, 5000, 7500, 12000. It has been observed that crossflow diverters protect the downstream jets from upstream jet deflection thereby maximizing their stagnation cooling potential. An average of 15–30% enhancement in Nusselt number is obtained over the flow range tested. However, this comes at the expense of increase in pumping power. Pressure drop for the enhanced geometry is 1–1.5 times the pressure drop for baseline impingement case. At a constant pumping power, crossflow diverters produce 9–15% enhancement in heat transfer coefficient as compared to baseline smooth case.}, booktitle={ASME 2019 Heat Transfer Summer Conference, HT 2019, collocated with the ASME 2019 13th International Conference on Energy Sustainability}, author={Madhavan, S. and Singh, P. and Ramakrishnan, K.R. and Ekkad, S.V.}, year={2019} }
 @inproceedings{ramakrishnan_ahmed_wahls_singh_aleman_granlund_ekkad_liberatore_ho_2019, title={Gas turbine combustor liner wall heat load characterization for different gaseous fuels}, volume={8}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85078826749&partnerID=MN8TOARS}, DOI={10.1115/IMECE2019-11283}, abstractNote={Abstract
               The knowledge of detailed distribution of heat load on swirl stabilized combustor liner wall is imperative in the development of liner-specific cooling arrangements, aimed towards maintaining uniform liner wall temperatures for reduced thermal stress levels. Heat transfer and fluid flow experiments have been conducted on a swirl stabilized lean premixed combustor to understand the behavior of Methane-, Propane-, and Butane-based flames. These fuels were compared at different equivalence ratios for a matching adiabatic flame temperature of Methane at 0.65 equivalence ratio. Above experiments were carried out a fixed Reynolds number (based on the combustor diameter) of 12000, where the pre-heated air temperature was approximately 373K. Combustor liner in this setup was made from 4 mm thick quartz tube. An infrared camera was used to record the inner and outer temperatures of liner wall, and two-dimensional heat conduction model was used to find the wall heat flux at a quasi-steady state condition. Flow field in the combustor was measured through Particle Image Velocimetry. The variation of peak heat flux on the liner wall, position of peak heat flux and heat transfer, and position of impingement of flame on the liner have been presented in this study. For all three gaseous fuels studied, the major swirl stabilized flame features such as corner recirculation zone, central recirculation zone and shear layers have been observed to be similar. Liner wall and exhaust temperature for Butane was highest among the fuel tested in this study which was expected as the heat released from combustion of Butane is higher than that of Methane and Propane.}, booktitle={ASME International Mechanical Engineering Congress and Exposition, Proceedings (IMECE)}, author={Ramakrishnan, K.R. and Ahmed, S. and Wahls, B. and Singh, P. and Aleman, M.A. and Granlund, K. and Ekkad, S. and Liberatore, F. and Ho, Y.-H.}, year={2019} }
 @inproceedings{panse_madhavan_singh_ekkad_2019, title={Impingement heat transfer of various lobe-shaped nozzles}, volume={8}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85078863127&partnerID=MN8TOARS}, DOI={10.1115/IMECE2019-10660}, abstractNote={Abstract
               This paper presents heat transfer characteristics of lobed nozzles, three different lobe configurations viz. three-, four- and six-lobe jets have been tested over a range of Reynolds numbers (based on the effective jet diameter, de) between 8000 and 16000 and normalized jet-to-target spacings (z/de) of 1.6, 3.2 and 4.8. The heat transfer results of lobed configurations were compared to the baseline configuration of circular jets. Steady-state infrared thermography (IRT) experiments were carried out for convective heat transfer coefficient calculations. Experimental results show that the three lobe configuration has a superior heat transfer performance compared to other configurations. Jet-to-target plate standoff distance had drastic effect on the heat transfer performance and contour plots for the lobed nozzles, as heat transfer performance diminished with increase in z/de. For the lobe configurations, with increase in jet-to-target spacing (z/de), the heat transfer coefficient maps tend towards a more circular profile due to the effect of jet diffusion.}, booktitle={ASME International Mechanical Engineering Congress and Exposition, Proceedings (IMECE)}, author={Panse, S.S. and Madhavan, S. and Singh, P. and Ekkad, S.V.}, year={2019} }
 @article{madhavan_singh_ekkad_2019, title={Jet Impingement Heat Transfer Enhancement by Packing High-Porosity Thin Metal Foams Between Jet Exit Plane and Target Surface}, volume={11}, ISSN={1948-5085 1948-5093}, url={http://dx.doi.org/10.1115/1.4043470}, DOI={10.1115/1.4043470}, abstractNote={High-porosity metal foams are known for providing high heat transfer rates, as they provide a significant increase in wetted surface area as well as highly tortuous flow paths resulting in enhanced mixing. Further, jet impingement offers high convective cooling, particularly at the jet footprint areas on the target surface due to flow stagnation. In this study, high-porosity thin metal foams were subjected to array jet impingement, for a special crossflow scheme. High porosity (92.65%), high pore density (40 pores per inch (ppi)), and thin foams (3 mm) have been used. In order to reduce the pumping power requirements imposed by full metal foam design, two striped metal foam configurations were also investigated. For that, the jets were arranged in 3 × 6 array (x/dj = 3.42, y/dj = 2), such that the crossflow is dominantly sideways. Steady-state heat transfer experiments have been conducted for varying jet-to-target plate distance z/dj = 0.75, 2, and 4 for Reynolds numbers ranging from 3000 to 12,000. The baseline case was jet impingement onto a smooth target surface. Enhancement in heat transfer due to impingement onto thin metal foams has been evaluated against the pumping power penalty. For the case of z/dj = 0.75 with the base surface fully covered with metal foam, an average heat transfer enhancement of 2.42 times was observed for a concomitant pressure drop penalty of 1.67 times over the flow range tested.}, number={6}, journal={Journal of Thermal Science and Engineering Applications}, publisher={ASME International}, author={Madhavan, Srivatsan and Singh, Prashant and Ekkad, Srinath}, year={2019}, month={May} }
 @article{madhavan_ramakrishnan_singh_ekkad_2019, title={Jet Impingement Heat Transfer Enhancement by U-Shaped Crossflow Diverters}, volume={12}, ISSN={1948-5085 1948-5093}, url={http://dx.doi.org/10.1115/1.4045514}, DOI={10.1115/1.4045514}, abstractNote={AbstractArray-jet impingement is typically used in gas turbine blade near-wall cooling, where high rates of heat dissipation is required. The accumulated crossflow mass flux results in significant reduction in jet effectiveness in the downstream rows, leading to reduced cooling performance. In this paper, a jet impingement system equipped with U-shaped ribs (hereafter referred as “diverter”) was used for diverting the crossflow away from the jets emanating from the nozzle plate. To this end, a baseline configuration of array-jet impingement onto smooth target surface is considered, where the normalized jet-to-jet spacing (x/dj = y/dj) was 6 and the normalized jet-to-target spacing (z/dj) was 2. Crossflow diverters with thickness t of 1.5875 mm and height h of 2dj (= z) were installed at a distance of 2dj from the respective jet centers. Detailed heat transfer coefficients have been calculated through transient liquid crystal experiments carried out over Reynolds numbers ranging from 3500 to 12,000. It has been observed that crossflow diverters protect the downstream jets from upstream jet deflection, thereby maximizing their stagnation cooling potential. An average of 15–30% enhancement in Nusselt number is obtained over the flow range tested. This benefit in heat transfer came at a cost of increased pumping power to maintain similar flow rate in the system. At a given pumping power, crossflow diverters yielded an enhancement of 9–15% in heat transfer compared with the baseline case.}, number={4}, journal={Journal of Thermal Science and Engineering Applications}, publisher={ASME International}, author={Madhavan, Srivatsan and Ramakrishnan, Kishore Ranganath and Singh, Prashant and Ekkad, Srinath}, year={2019}, month={Dec} }
 @article{singh_ji_ekkad_2019, title={Multi-Pass Serpentine Cooling Designs for Negating Coriolis Force Effect on Heat Transfer: Smooth Channels}, volume={141}, ISSN={0889-504X 1528-8900}, url={http://dx.doi.org/10.1115/1.4042565}, DOI={10.1115/1.4042565}, abstractNote={The combined action of Coriolis and centrifugal buoyancy forces results in nonuniform heat transfer coefficient on pressure and suction side internal walls, hence leading to nonuniform metal temperatures and increased thermal stresses. The present study addresses the problem of nonuniform heat transfer distribution due to rotation effect and proposes novel designs for serpentine cooling passages, which are arranged along the chord of the blade. The two configurations were four-passage and six-passage serpentine smooth channels. Detailed heat transfer coefficients were measured using transient liquid crystal thermography under stationary and rotating conditions. Heat transfer experiments were carried out for Reynolds numbers ranging from 12,294 to 85,000 under stationary conditions. Rotation experiments were carried out for the Rotation numbers of 0.05 and 0.11. Heat transfer enhancement levels of approximately two times the Dittus–Boelter correlation (for developed flow in smooth tubes) were obtained under stationary conditions. Under rotating conditions, we found that the four-passage configuration had slightly lower heat transfer compared with the stationary case, and the six-passage configuration had higher heat transfer on both the leading and trailing sides compared with the stationary case. The leading and trailing side heat transfer characteristics were near-similar to each other for both the configurations, and the rotating heat transfer was near-similar to the stationary condition heat transfer.}, number={7}, journal={Journal of Turbomachinery}, publisher={ASME International}, author={Singh, Prashant and Ji, Yongbin and Ekkad, Srinath V.}, year={2019}, month={Feb} }
 @article{singh_ji_ekkad_2019, title={Multipass Serpentine Cooling Designs for Negating Coriolis Force Effect on Heat Transfer: 45-deg Angled Rib Turbulated Channels}, volume={141}, ISSN={0889-504X 1528-8900}, url={http://dx.doi.org/10.1115/1.4042648}, DOI={10.1115/1.4042648}, abstractNote={Rotation-induced Coriolis and centrifugal buoyancy forces result in significant modification of cooling characteristics of blade pressure and suction side internal walls. The nonuniformity in cooling, coupled with high-speed rotation, results in increased levels of thermal stresses. To address this problem, this study presents two multipassage configurations featuring 45-deg angled turbulators, in four- and six-passage designs. Experiments were carried out under stationary and rotating conditions using transient liquid crystal thermography to measure detailed heat transfer coefficient. It has been shown through experimental data that heat transfer characteristics of the new configurations’ pressure and suction side internal walls were very similar under rotating conditions, at both local and global scales. The heat transfer levels under rotating conditions were also similar to those of the stationary conditions. The contribution of multiple passages connected with 180-deg bends toward overall frictional losses has been evaluated in terms of pumping power and normalized friction factor. The configurations are ranked based on their thermal hydraulic performances over a wide range of Reynolds numbers. The four-passage ribbed configuration had slightly higher heat transfer levels compared with those of the corresponding six-passage ribbed configuration.}, number={7}, journal={Journal of Turbomachinery}, publisher={ASME International}, author={Singh, Prashant and Ji, Yongbin and Ekkad, Srinath V.}, year={2019}, month={Feb} }
 @article{faghri_amano_cheng_chiu_djilali_ekkad_farhanieh_han_jaluria_kabelac_et al._2019, title={Professor Bengt Sundén on his 70th Birthday}, volume={141}, ISSN={0017-9310}, url={http://dx.doi.org/10.1016/j.ijheatmasstransfer.2019.03.148}, DOI={10.1016/j.ijheatmasstransfer.2019.03.148}, journal={International Journal of Heat and Mass Transfer}, publisher={Elsevier BV}, author={Faghri, Mohammad and Amano, Ryo and Cheng, Ping and Chiu, Wilson and Djilali, Ned and Ekkad, Srinath and Farhanieh, Bijan and Han, Je-Chin and Jaluria, Yogesh and Kabelac, Stephan and et al.}, year={2019}, month={Oct}, pages={1315–1317} }
 @article{zhang_singh_ekkad_2019, title={Rib Turbulator Heat Transfer Enhancements at Very High Reynolds Numbers}, volume={11}, ISSN={1948-5085 1948-5093}, url={http://dx.doi.org/10.1115/1.4043465}, DOI={10.1115/1.4043465}, abstractNote={High-pressure stage gas turbine blades feature serpentine passages where rib turbulators are installed to enhance heat transfer between the relatively colder air bled off from the compressor and the hot internal walls. Most of the prior studies have been restricted to Reynolds number of 90,000 and several studies have been carried out to determine geometrically optimized parameters for achieving high levels of heat transfer in this range of Reynolds number. However, for land-based power generation gas turbines, the Reynolds numbers are significantly high and vary between 105 and 106. The present study is targeted toward these high Reynolds numbers where traditional rib turbulator shapes and prescribed optimum geometrical parameters have been investigated experimentally. A steady-state liquid crystal thermography technique is employed for measurement of detailed heat transfer coefficient. Five different rib configurations, viz., 45 deg, V-shaped, inverse V-shaped, W-shaped, and M-shaped have been investigated for Reynolds numbers ranging from 150,000 to 400,000. The ribs were installed on two opposite walls of a straight duct with an aspect ratio of unity. For very high Reynolds numbers, the heat transfer enhancement levels for different rib shapes varied between 1.4 and 1.7 and the thermal hydraulic performance was found to be less than unity.}, number={6}, journal={Journal of Thermal Science and Engineering Applications}, publisher={ASME International}, author={Zhang, Mingyang and Singh, Prashant and Ekkad, Srinath V.}, year={2019}, month={May} }
 @article{panse_singh_ekkad_2019, title={Thermal hydraulic performance augmentation by high-porosity thin aluminum foams placed in high aspect ratio ducts}, volume={161}, ISSN={1359-4311}, url={http://dx.doi.org/10.1016/j.applthermaleng.2019.114162}, DOI={10.1016/j.applthermaleng.2019.114162}, abstractNote={High porosity metal foams are known for providing high heat transfer rates, however, the concomitant higher-pressure losses have been a concern towards the realization of enhanced thermal hydraulic performance. Present study aims towards developing metal foam-based cooling configurations subjected to forced convection in a channel by employed very thin and high porosity Aluminum foams, where the goal is to achieve maximum foam volume participation in heat dissipation. To this end, a comprehensive experimental study has been carried out to investigate the effect of foam pore density (pores per inch: PPI) and channel aspect ratio (AR) on the thermal hydraulic performance of thin metal foams. High porosity (93%) thin aluminum foams with pore densities of 5, 20 and 40 PPI were studied, where the convective heat transport was facilitated through air forced through the channel. For the 40 ppi foams, three foam heights of 3.175 mm, 6.35 mm and 19 mm were tested, for 20 ppi foams, two foam heights of 6.35 mm and 19 mm were tested, and for the 5 ppi foams, one height of 19 mm was tested. These three different foam heights yielded in channel ARs of 16:1, 8:1 and 8:3, respectively. Heat transfer gain due to metal foams was evaluated against a geometrically identical smooth channel as well as with the Dittus-Boelter correlation for developed turbulent flow in circular ducts. Experimental data reveals that, for a given AR, an increase in pore-density resulted in both, increase in heat transfer as well as pressure drop. Amongst all three channel configurations, the 40 ppi foams had highest heat transfer, where the gain with respect to smooth channel for Re 5000 was ~21, 12 and 9.5 times, for 8:3, 8:1 and 16:1 channels respectively. Increase in channel AR (thinner foams) demonstrated an increase in heat transfer performance for a given pumping power. The thermal hydraulic trends observed for thin and high pore-density foams proved them to be viable candidates for compact electronics cooling.}, journal={Applied Thermal Engineering}, publisher={Elsevier BV}, author={Panse, Sanskar S. and Singh, Prashant and Ekkad, Srinath V.}, year={2019}, month={Oct}, pages={114162} }
 @article{kaur_singh_ekkad_2020, title={Thermal-Hydraulic Performance Enhancement by the Combination of Rectangular Winglet Pair and V-Shaped Dimples}, volume={12}, ISSN={1948-5085 1948-5093}, url={http://dx.doi.org/10.1115/1.4044169}, DOI={10.1115/1.4044169}, abstractNote={Abstract
               This paper presents numerical study on heat transfer enhancement due to the combination of rectangular winglet pair with V-dimples in an array-type arrangement. Array of rectangular winglet pairs results in heat transfer enhancement, however, at a cost of significant pressure drop, resulting in reduced thermal-hydraulic performance (THP). On the other hand, dimples are associated with lower heat transfer enhancement levels at relatively lower pumping power penalty. To this end, a combination of rectangular winglet pair and V-shaped dimples has been studied in this paper, where the arrangements were intended to achieve enhanced thermal-hydraulic performance. Three different configurations, namely, rectangular winglet pair, rectangular winglet pair with one V-dimple between two consecutive winglets, and rectangular winglet pair with two V-dimples packed in a pitch, are studied here. The variation of heat transfer enhancement, pressure drop gain, and THP with respect to winglet-to-winglet (S) spacing variation for rectangular winglet pair and rectangular winglet pair with one V-dimple configuration is presented at a Reynolds number of 25,000. The THP of the rectangular winglet pair configuration decreases up to S/H equal to 2.5 and then increases (H: channel height). For rectangular winglet pair with one V-dimple, three values of winglet-to-dimple (P) spacings are analyzed. For fixed S/H, the highest P/H configuration provided highest heat transfer enhancement and THP. Among the three configurations studied, rectangular winglet pair with two V-dimples resulted in the highest thermal-hydraulic performance.}, number={2}, journal={Journal of Thermal Science and Engineering Applications}, publisher={ASME International}, author={Kaur, Inderjot and Singh, Prashant and Ekkad, Srinath V.}, year={2020}, month={Apr} }
 @article{ahmed_singh_ekkad_2020, title={Three-Dimensional Transient Heat Conduction Equation Solution for Accurate Determination of Heat Transfer Coefficient}, volume={142}, ISSN={0022-1481 1528-8943}, url={http://dx.doi.org/10.1115/1.4044678}, DOI={10.1115/1.4044678}, abstractNote={Abstract
               Accurate quantification of local heat transfer coefficient (HTC) is imperative for design and development of heat exchangers for high heat flux dissipation applications. Liquid crystal and infrared thermography (IRT) are typically employed to measure detailed surface temperatures, where local HTC values are calculated by employing suitable conduction models, e.g., one-dimensional (1D) semi-infinite conduction model on a material with the low thermal conductivity and low thermal diffusivity. Often times, this assumption of 1D heat diffusion and ignoring its associated lateral conduction effects leads to significant errors in HTC determination. Prior studies have identified this problem and quantified the associated errors in HTC determination for some representative cooling concepts, by accounting for lateral heat diffusion. In this paper, we have presented a procedure for solution of three-dimensional (3D) transient conduction equation using alternating direction implicit (ADI) method and an error minimization routine to find accurate HTCs at relatively lower computational cost. Representative cases of a single jet and an array jet impingement under maximum crossflow condition have been considered here, for IRT and liquid crystal thermography, respectively. Results indicate that the globally averaged HTC obtained using the 3D model was consistently higher than the conventional 1D model by 7–14%, with deviation levels reaching as high as 20% near the stagnation region. Proposed methodology was computationally efficient and is recommended for studies aimed toward local HTC determination.}, number={5}, journal={Journal of Heat Transfer}, publisher={ASME International}, author={Ahmed, Shoaib and Singh, Prashant and Ekkad, Srinath V.}, year={2020}, month={Mar} }
 @inproceedings{ahmed_singh_ekkad_2019, title={Three-dimensional transient heat conduction equation solution for accurate quantification of heat transfer coefficient in transient liquid crystal experiments}, volume={8}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85078858361&partnerID=MN8TOARS}, DOI={10.1115/IMECE2019-10760}, abstractNote={Abstract
               Liquid crystal thermography and infrared thermography techniques are typically employed to measure detailed surface temperatures, where local heat transfer coefficient (HTC) values are calculated by employing suitable conduction models. One such practice, which is very popular and easy to use, is the transient liquid crystal thermography using one-dimensional semi-infinite conduction model. In these experiments, a test surface with low thermal conductivity and low thermal diffusivity (e.g. acrylic) is used where a step-change in coolant air temperature is induced and surface temperature response is recorded. An error minimization routine is then employed to guess heat transfer coefficients of each pixel, where wall temperature evolution is known through an analytical expression. The assumption that heat flow in the solid is essentially in one-dimension, often leads to errors in HTC determination and this error depends on true HTC, wall temperature evolution and HTC gradient. A representative case of array jet impingement under maximum crossflow condition has been considered here. This heat transfer enhancement concept is widely used in gas turbine leading edge and electronics cooling. Jet impingement is a popular cooling technique which results in high convective heat rates and has steep gradients in heat transfer coefficient distribution. In this paper, we have presented a procedure for solution of three-dimensional transient conduction equation using alternating direction implicit method and an error minimization routine to find accurate heat transfer coefficients at relatively lower computational cost. The HTC results obtained using 1D semi-infinite conduction model and 3D conduction model were compared and it was found that the heat transfer coefficient obtained using the 3D model was consistently higher than the conventional 1D model by 3–16%. Significant deviations, as high as 8–20% in local heat transfer at the stagnation points of the jets were observed between h1D and h3D.}, booktitle={ASME International Mechanical Engineering Congress and Exposition, Proceedings (IMECE)}, author={Ahmed, S. and Singh, P. and Ekkad, S.V.}, year={2019} }
 @inproceedings{singh_ekkad_2018, title={An eight-passage serpentine design for negating coriolis force effect on heat transfer}, volume={8B-2018}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85063986857&partnerID=MN8TOARS}, DOI={10.1115/imece2018-86354}, abstractNote={Gas turbine blades are subjected to elevated heat loads due to high temperature gases exiting the combustor section. Complex internal and external cooling techniques are employed in blades to protect them from the hot gases. Blades are equipped with internal cooling passages which are connected to each other by 180-degree bends. The coolant flow is typically from blade root-to-tip and blade tip-to-root. Further, since the blades are subjected to rotation, the fluid dynamics and heat transfer inside these serpentine channels get modified. Under the influence of Coriolis force and centrifugal buoyancy force induced by rotation, the heat transfer for radially outward flow enhances on the trailing side (pressure side) and reduces on the leading side (suction side). A reverse trend in heat transfer is observed for radially inward flow. This heat transfer trend leads to non-uniform blade temperature leading to increase in thermal-stresses. Prolonged operation under critical thermal stresses can lead to significant damage and increase in maintenance and overhaul. This paper presents a novel 8-passgae serpentine design, where passages are arranged along the chord of the blade which has similar heat transfer coefficient distribution on both leading and trailing walls. Detailed heat transfer coefficients were measured using transient liquid crystal thermography under stationary and rotating conditions. Heat transfer experiments were carried out for Reynolds numbers ranging from 14264 to 83616 under stationary conditions. Rotation experiments were carried out at Rotation number of 0.05. Heat transfer enhancement levels of approximately two times the Dittus-Boelter correlation (for developed flow in smooth tubes) were obtained under stationary conditions. Under rotating conditions, we found that the heat transfer levels on the leading and trailing sides were similar to each other and with the stationary condition. Some differences in heat transfer were observed on local level, when rotation cases were compared against the stationary cases.}, booktitle={ASME International Mechanical Engineering Congress and Exposition, Proceedings (IMECE)}, author={Singh, P. and Ekkad, S.}, year={2018} }
 @inproceedings{gadiraju_park_ekkad_limbretore_srinivasan_ho_2018, title={Characterization of heat load on the liner walls during near blowout instabilities}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85044417094&partnerID=MN8TOARS}, DOI={10.2514/6.2018-2128}, number={210059}, booktitle={AIAA Aerospace Sciences Meeting, 2018}, author={Gadiraju, S. and Park, S. and Ekkad, S.V. and Limbretore, F.X. and Srinivasan, R. and Ho, S.}, year={2018} }
 @article{singh_ekkad_2018, title={Detailed Heat Transfer Measurements of Jet Impingement on Dimpled Target Surface Under Rotation}, volume={10}, ISSN={1948-5085 1948-5093}, url={http://dx.doi.org/10.1115/1.4039054}, DOI={10.1115/1.4039054}, abstractNote={The present study investigates the effects of Coriolis force and centrifugal buoyancy force on heat transfer due to jet impingement on dimpled target surface (DT). Detailed heat transfer measurements were carried out using transient liquid crystal (LC) thermography, where the target surface was modeled as one-dimensional (1D) semi-infinite solid. Three different configurations of DT surfaces have been studied. The flow and rotation conditions have been kept the same for all the configurations, where the average Reynolds number (based on jet hole hydraulic diameter: Rej) was 2500 and the rotational speed was 400 rpm (corresponding to Roj of 0.00274). Under nonrotating conditions, DT surface showed positive heat transfer enhancements compared to smooth target surfaces. Under rotating conditions, it was observed that rotation was helpful in enhancing heat transfer on leading and trailing sides for smooth target surface. However, for the DT surfaces, rotation proved to be detrimental to heat transfer enhancement.}, number={3}, journal={Journal of Thermal Science and Engineering Applications}, publisher={ASME International}, author={Singh, Prashant and Ekkad, Srinath V.}, year={2018}, month={Mar} }
 @article{deshpande_ravi_pandit_ma_huxtable_ekkad_2018, title={Effect of Longitudinal Vortex Generator Location on Thermoelectric-Hydraulic Performance of a Single-Stage Integrated Thermoelectric Power Generator}, volume={10}, ISSN={1948-5085 1948-5093}, url={http://dx.doi.org/10.1115/1.4040033}, DOI={10.1115/1.4040033}, abstractNote={Vortex generators have been widely used to enhance heat transfer in various heat exchangers. Out of the two types of vortex generators, transverse vortex generators and longitudinal vortex generators (LVGs), LVGs have been found to show better heat transfer performance. Past studies have shown that the implementation of these LVGs can be used to improve heat transfer in thermoelectric generator systems. Here, a built in module in COMSOL Multiphysics® was used to study the influence of the location of LVGs in the channel on the comprehensive performance of an integrated thermoelectric device (TED). The physical model under consideration consists of a copper interconnector sandwiched between p-type and n-type semiconductors and a flow channel for hot fluid in the center of the interconnector. Four pairs of LVGs are mounted symmetrically on the top and bottom surfaces of the flow channel. Thus, using numerical methods, the thermo-electric-hydraulic performance of the integrated TED with a single module is examined. By fixing the material size D, the fluid inlet temperature Tin, and attack angle β, the effects of the location of LVGs and Reynolds number were investigated on the heat transfer performance, power output, pressure drop, and thermal conversion efficiency. The location of LVGs did not have significant effect on the performance of TEGs in the given model. However, the performance parameters show a considerable change with Reynold's number and best performance is obtained at Reynold number of Re = 500.}, number={5}, journal={Journal of Thermal Science and Engineering Applications}, publisher={ASME International}, author={Deshpande, Samruddhi and Ravi, Bharath Viswanath and Pandit, Jaideep and Ma, Ting and Huxtable, Scott and Ekkad, Srinath}, year={2018}, month={Jun} }
 @article{singh_zhang_ahmed_ramakrishnan_ekkad_2019, title={Effect of micro-roughness shapes on jet impingement heat transfer and fin-effectiveness}, volume={132}, ISSN={0017-9310}, url={http://dx.doi.org/10.1016/j.ijheatmasstransfer.2018.11.135}, DOI={10.1016/j.ijheatmasstransfer.2018.11.135}, abstractNote={With recent advancements in the field of additive manufacturing, the design domain for development of complicated cooling configurations has significantly expanded. The motivation of the present study is to develop high performance impingement cooling designs catered towards applications requiring high rates of heat removal, e.g. gas turbine blade leading edge and double-wall cooling, air-cooled electronic devices, etc. In the present study, jet impingement is combined with strategic roughening of the target surface, to achieve high heat removal rates. Steady state experiments have been carried out to calculate the heat transfer coefficient for jet impingement onto different target surface configurations. The jet-to-jet spacing (x/d = y/d) was varied from 2 to 5, and jet-to-target distance (z/d) was varied from 1 to 5. The target surface configurations featured cylindrical, cubic and concentric shaped roughness elements, fabricated through binder jetting process. The baseline case for the roughened target surface was a smooth target. Heat transfer and pressure drop experiments were carried out at Reynolds numbers ranging from 2500 to 10,000. Further, numerical simulations were carried out to model flow and heat transfer for all configurations at a representative Reynolds number. Through our experiments and numerical results, we have demonstrated that the novel “concentric” roughness shape was the best in terms of fin effectiveness and Nusselt numbers levels, amongst the investigated shapes. The concentric-shape roughened target resulted in fin effectiveness up to 1.6, whereas the cubic- and cylindrical-shape roughened targets yielded in fin effectiveness up to 1.4 and 1.3, respectively. Further, it was experimentally found that the addition of micro-roughness elements does not result in a discernable increment in pressure losses, compared to the impingement on the smooth target surface. Hence, the demonstrated configuration with the highest heat transfer coefficient also resulted in highest thermal hydraulic performance.}, journal={International Journal of Heat and Mass Transfer}, publisher={Elsevier BV}, author={Singh, Prashant and Zhang, Mingyang and Ahmed, Shoaib and Ramakrishnan, Kishore R. and Ekkad, Srinath}, year={2019}, month={Apr}, pages={80–95} }
 @inproceedings{singh_zhang_ahmed_ekkad_2018, title={Effect of nozzle-to-target spacing on fin effectiveness and convective heat transfer coefficient for array jet impingement onto novel micro-roughness structures}, volume={8A-2018}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85063157117&partnerID=MN8TOARS}, DOI={10.1115/IMECE2018-86501}, abstractNote={With recent advancements in the field of additive manufacturing, the design domain for development of complicated cooling configurations has significantly expanded. The motivation of the present study is to develop high-performance impingement cooling designs catered towards application’s requiring high rates of heat removal, e.g. gas turbine blade leading edge and double-wall cooling, air-cooled electronic devices etc. Jet impingement is a popular cooling technique which results in high convective heat rates. In the present study, jet impingement is combined with strategic roughening of the target surface, such that a combined effect of impingement-based and curved-surface area based enhancement in heat transfer coefficient could be achieved. Traditionally, for surface roughening, cylindrical and cubic elements are used. We have demonstrated, through our steady-state experiments, a novel “concentric” shaped roughness element design which has resulted in about 20–60% higher effectiveness compared to smooth target jet impingement, for jet-to-target spacing of one jet diameter. The cubic shaped roughened target yielded about 20% to 40% enhancement in effectiveness, and the cylindrical shaped roughened target yielded 10% to 30% enhancement. Through the plenum pressure measurements, it was found that the addition of the micro-roughness elements does not result in a discernable increment in pressure losses, compared to the standard impingement on the smooth target surface. Hence, the demonstrated configuration with the highest heat transfer coefficient also resulted in the highest thermal hydraulic performance.}, booktitle={ASME International Mechanical Engineering Congress and Exposition, Proceedings (IMECE)}, author={Singh, P. and Zhang, M. and Ahmed, S. and Ekkad, S.V.}, year={2018} }
 @inproceedings{park_gadiraju_pandit_ekkad_liberatore_ho_srinivasan_2018, title={Effects of reacting conditions on flow fields in a swirl stabilized lean premixed can combustor}, volume={4B-2018}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85054151496&partnerID=MN8TOARS}, DOI={10.1115/GT2018-77047}, abstractNote={PIV measurements to understand the flow differences between reacting and non-reacting conditions were conducted in an optically accessible single can combustor. An industrial fuel nozzle was installed at the inlet of the test section to generate the swirl flow for flame stabilization and simulate realistic conditions of a gas turbine combustor. Five different equivalence ratios between 0.50 and 0.75 were tested with propane as fuel. Main air flow was also varied from Reynolds number from 50000 to 110000 with respect to the fuel nozzle diameter. Effect of preheating was tested by changing inlet air temperature from 23 to 200°C. The pressure at the test section was close to atmospheric condition throughout the tests.
               The measurements were performed with a 2-D PIV system. Time-averaged flow velocity, vorticity and turbulent kinetic energy (TKE) were obtained from PIV data and flow structures under different conditions were compared. Swirl jet impingement location on the liner wall was determined as well to understand the impact on the liner wall. Proper orthogonal decomposition (POD) further analyzed the data to compare coherent structures in the reacting and non-reacting flows.}, booktitle={Proceedings of the ASME Turbo Expo}, author={Park, S. and Gadiraju, S. and Pandit, J. and Ekkad, S. and Liberatore, F. and Ho, Y.-H. and Srinivasan, R.}, year={2018} }
 @article{yang_singh_tyagi_pandit_ekkad_ren_2018, title={Experimental Investigation of Rotational Effects on Heat Transfer Enhancement Due to Crossflow-Induced Swirl Using Transient Liquid Crystal Thermography}, volume={10}, ISSN={1948-5085 1948-5093}, url={http://dx.doi.org/10.1115/1.4038538}, DOI={10.1115/1.4038538}, abstractNote={Rotational effects lead to significant nonuniformity in heat transfer (HT) enhancement and this effect is directly proportional to the rotation number (Ro=ΩD/V). Hence, the development of cooling designs, which have less dependence on rotation, is imperative. This paper studied the effect of rotation on crossflow-induced swirl configuration with the goal of demonstrating a new design that has lesser response toward rotational effects. The new design passes coolant from one pass to the second pass through a set of angled holes to induce impingement and swirling flow to generate higher HT coefficients than typical ribbed channels with 180-deg bend between the two passages. Detailed HT coefficients are presented for stationary and rotating conditions using transient liquid crystal (TLC) thermography. The channel Reynolds number based on the channel hydraulic diameter and channel velocity at inlet/outlet ranged from 25,000 to 100,000. The rotation number ranged from 0 to 0.14. Results show that rotation reduced the HT on both sides of the impingement due to the Coriolis force. The maximum local reduction of HT in the present study was about 30%. Rotation significantly enhanced the HT near the closed end because of the centrifugal force and the “pumping” effect, which caused local HT enhancements up to 100%. Compared to U-bend two pass channels, impingement channels had advantages in the upstream channel and the end region, but HT performance was not beneficial on the leading side of the downstream channel.}, number={3}, journal={Journal of Thermal Science and Engineering Applications}, publisher={ASME International}, author={Yang, Li and Singh, Prashant and Tyagi, Kartikeya and Pandit, Jaideep and Ekkad, Srinath V. and Ren, Jing}, year={2018}, month={Jan} }
 @inproceedings{madhavan_singh_ekkad_2018, title={Experimental investigation of heat transfer enhancement through array jet impingement on various configurations of high porosity thin metal foams}, volume={8B-2018}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85063989139&partnerID=MN8TOARS}, DOI={10.1115/imece2018-86432}, abstractNote={High porosity metal foams are known for providing high heat transfer rates, as they provide significant increase in wetted surface area as well as highly tortuous flow paths to coolant flowing over fibers. Further, jet impingement is also known to offer high convective cooling, particularly on the footprints of the jets on the target to be cooled. Jet impingement, however, leads to large special gradients in heat transfer coefficient, leading to increased thermal stresses. In this study, we have tried to use high porosity thin metal foams subjected to array jet impingement, for a special crossflow scheme. One aim of using metal foams is to achieve cooling uniformity also, which is tough to achieve for impingement cooling. High porosity (92.65%) and high pore density (40 pores per inch, 3 mm thick) foams have been used as heat transfer enhancement agents. In order to reduce the pumping power requirements imposed by full metal foam design, we developed two striped metal foam configurations. For that, the jets were arranged in 3 × 6 array (x/d = 3.42, y/d = 2), such that the crossflow is dominantly sideways. This crossflow scheme allowed usage of thin stripes, where in one configuration we studied direct impingement onto stripes of metal foam and in the other, we studied impingement onto metal and crossflow interacted with metal foams. Steady state heat transfer experiments have been conducted for a jet plate configuration with varying jet-to-target plate distance z/d = 0.75, 2 and 4. The baseline case was jet impingement onto a smooth target surface. Jet diameter-based Reynolds number was varied between 3000 to 11000. Enhancement in heat transfer due to impingement onto thin metal foams has been evaluated against the enhancement in pumping power requirements. For a specific case of z/d = 0.75 with the base surface fully covered with metal foam, metal foams have enhanced heat transfer by 2.42 times for a concomitant pressure drop penalty of 1.67 times over the flow range tested.}, booktitle={ASME International Mechanical Engineering Congress and Exposition, Proceedings (IMECE)}, author={Madhavan, S. and Singh, P. and Ekkad, S.V.}, year={2018} }
 @article{park_gomez-ramirez_gadiraju_kedukodi_ekkad_moon_kim_srinivasan_2018, title={Flow Field and Wall Temperature Measurements for Reacting Flow in a Lean Premixed Swirl Stabilized Can Combustor}, volume={140}, ISSN={0742-4795 1528-8919}, url={http://dx.doi.org/10.1115/1.4039462}, DOI={10.1115/1.4039462}, abstractNote={In this study, we provide detailed wall heat flux measurements and flow details for reacting flow conditions in a model combustor. Heat transfer measurements inside a gas turbine combustor provide one of the most serious challenges for gas turbine researchers. Gas turbine combustor improvements require accurate measurement and prediction of reacting flows. Flow and heat transfer measurements inside combustors under reacting flow conditions remain a challenge. The mechanisms of thermal energy transfer must be investigated by studying the flow characteristics and associated heat load. This paper experimentally investigates the effects of combustor operating conditions on the reacting flow in an optical single can combustor. The swirling flow was generated by an industrial lean premixed, axial swirl fuel nozzle. Planar particle image velocimetry (PIV) data were analyzed to understand the characteristics of the flow field. Liner surface temperatures were measured in reacting condition with an infrared camera for a single case. Experiments were conducted at Reynolds numbers ranging between 50,000 and 110,000 (with respect to the nozzle diameter, DN); equivalence ratios between 0.55 and 0.78; and pilot fuel split ratios of 0 to 6%. Characterizing the impingement location on the liner, and the turbulent kinetic energy (TKE) distribution were a fundamental part of the investigation. Self-similar characteristics were observed at different reacting conditions. Swirling exit flow from the nozzle was found to be unaffected by the operating conditions with little effect on the liner. Comparison between reacting and nonreacting flows (NR) yielded very interesting and striking differences.}, number={9}, journal={Journal of Engineering for Gas Turbines and Power}, publisher={ASME International}, author={Park, Suhyeon and Gomez-Ramirez, David and Gadiraju, Siddhartha and Kedukodi, Sandeep and Ekkad, Srinath V. and Moon, Hee-Koo and Kim, Yong and Srinivasan, Ram}, year={2018}, month={May} }
 @inproceedings{park_gadiraju_ekkad_liberatore_srinivasan_2018, title={Flow temperature measurement on a lean premixed swirl stablized combustor under reacting condition}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85044414587&partnerID=MN8TOARS}, DOI={10.2514/6.2018-0393}, number={210059}, booktitle={AIAA Aerospace Sciences Meeting, 2018}, author={Park, S. and Gadiraju, S. and Ekkad, S.V. and Liberatore, F.X. and Srinivasan, R.}, year={2018} }
 @inproceedings{gadiraju_park_singh_pandit_ekkad_liberatore_srinivasan_ho_2018, title={Fuel interchangeability effects on the lean blowout for a lean premixed swirl stabilized fuel nozzle}, volume={4B-2018}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85054146635&partnerID=MN8TOARS}, DOI={10.1115/GT2018-76249}, abstractNote={This work is motivated by an interest in understanding the fuel interchangeability of a fuel nozzle to operate under extreme lean operating conditions. A lean premixed, swirl-stabilized fuel nozzle designed with central pilot hub was used to test various fuel blends for combustion characteristics. Current gas turbine combustion technology primarily focuses on burning natural gas for industrial systems. However, interest in utilizing additional options due to environmental regulations as well as concerns about energy security have motivated interest in using fuel gases that have blends of Methane, Propane, H2, CO, CO2, and N2. For example, fuel blends of 35%/60% to 55%/35% of CH4/CO2 are typically seen in Landfill gases. Syngas fuels are typically composed primarily of H2, CO, and N2. CH4/N2 fuel blend mixtures can be derived from biomass gasification.
               Stringent emission requirements for gas turbines stipulate operating at extreme lean conditions, which can reduce NOx emissions. However, lean operating conditions pose the problem of potential blowout resulting in loss of performance and downtime. Therefore, it is important to understand the Lean Blowout (LBO) limits and involved mechanisms that lead to a blowout. While a significant amount of research has been performed to understand lean blowout limits and mechanisms for natural gas, research on LBO limits and mechanisms for fuel blends has only been concentrated on fuel blends of CH4 and H2 such as syngas. This paper studies the lean blowout limits with fuel blends CH4-C3H8, CH4-CO2, and CH4-N2 and also their effect on the stability limits as the pilot fuel percentage was varied. Experimental results demonstrate that the addition of propane, nitrogen and carbon dioxide has minimal effect on the adiabatic flame temperature when the flame becomes unstable under lean operating conditions. On the other hand, the addition of diluent gas showed a potential blowout at higher adiabatic temperatures.}, booktitle={Proceedings of the ASME Turbo Expo}, author={Gadiraju, S. and Park, S. and Singh, P. and Pandit, J. and Ekkad, S.V. and Liberatore, F. and Srinivasan, R. and Ho, Y.-H.}, year={2018} }
 @inproceedings{panse_singh_ekkad_2018, title={High porosity and high pore density thin copper foams for compact electronics cooling}, volume={8B-2018}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85063987499&partnerID=MN8TOARS}, DOI={10.1115/imece2018-86355}, abstractNote={Porous media like open celled metal foams inherently provide a high heat transfer area per unit volume due to their interconnected cellular structure and are lightweight. High pore density metal foam because of its small overall dimensions and micro feature size shows promise in thermal packaging of compact electronics. An experimental study was carried out to evaluate thermal performance of high porosity (95%) and high pore density (90 PPI) copper foam of size 20 mm × 20 mm × 3 mm in buoyancy induced flow conditions and compared with a baseline smooth surface. The enhanced surface showed about 15% enhancement in average heat transfer coefficient over the baseline case. To optimize the performance further, the foam sample was cut into strips of 20 mm × 5 mm × 3 mm and attached symmetrically on the central 20 mm2 base surface area with inter-spacing of 2.5 mm. This new configuration led to further 15% enhancement in heat transfer even with 25% lesser heat transfer area. This is significant as heat transfer is seen as a strong function of permeability to flow through the structure over heat conduction through it. To test this hypothesis, a third configuration was tested in which the strips were further cut into blocks of 4 mm × 4 mm × 3 mm and attached in a 3 × 3 array on to the base surface. Here, only 36% of the central 20 mm2 base surface area was covered with foam. The heat transfer performance was found to be within ± 10% of the initial metal foam configuration, thereby, supporting the hypothesis. Performance was seen to decrease with increase in inclination from 0° to 30° to 90° with respect to the vertical.}, booktitle={ASME International Mechanical Engineering Congress and Exposition, Proceedings (IMECE)}, author={Panse, S.S. and Singh, P. and Ekkad, S.V.}, year={2018} }
 @inproceedings{singh_ji_ekkad_2018, title={Multi-pass serpentine cooling designs for negating coriolis force effect on heat transfer: 45-degree angled rib turbulated channels}, volume={5A-2018}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85054068519&partnerID=MN8TOARS}, DOI={10.1115/GT2018-76689}, abstractNote={Traditional gas turbine blades are equipped with serpentine passages arranged along the height, wherein the coolant flows radially outward in 1st passage and radially inward in 2nd passage. Prior experimental studies have established that for traditional two-pass rib roughened ducts, under the influence of Coriolis and centrifugal forces, the heat transfer gets enhanced on trailing side for radially outward flow and gets reduced on the leading side for a radially outward flow. A reverse trend in heat transfer is observed for radially inward flow. Rotation induced forces result in non-uniform heat transfer coefficient distribution which results in non-uniform metal temperatures under steady state condition. Present study addresses the problem of non-uniform heat transfer distribution on leading and trailing sides due to rotation effect. Experimental investigation of two configurations has been carried out, where Coriolis effect was negated by aligning the coolant flow vector and rotation vector such that their cross product was effectively a null vector. Novel multi-passage serpentine ducts featuring 45-degree angled rib turbulators with four-passage and six-passage configurations have been studied. Transient liquid crystal thermography experiments were carried out under stationary and rotating conditions. Heat transfer experiments were carried out for Reynolds numbers ranging from 12000 to 80000 under stationary conditions and rotating heat transfer experiments were carried out at two Rotation numbers of 0.05 and 0.11. We found that the heat transfer characteristics of serpentine passages were not influenced by Coriolis force after the 2nd passage. The local heat transfer distribution on leading and trailing sides of serpentine passages were near-similar to each other and comparable with corresponding stationary cases. The contribution of multiple passages connected with 180-degree bends towards overall frictional losses has been evaluated in terms of pumping power and normalized friction factor. The configurations are ranked based on their thermal hydraulic performances over a wide range of Reynolds numbers. The heat transfer enhancement levels of four-passage rib roughened duct was higher than the six-passage configuration and the six-passage configuration had slightly higher thermal hydraulic performance compared to four-passage configuration.}, booktitle={Proceedings of the ASME Turbo Expo}, author={Singh, P. and Ji, Y. and Ekkad, S.V.}, year={2018} }
 @inproceedings{singh_ji_ekkad_2018, title={Multi-pass serpentine cooling designs for negating coriolis force effect on heat transfer: Smooth channels}, volume={5A-2018}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85054059213&partnerID=MN8TOARS}, DOI={10.1115/GT2018-76684}, abstractNote={Gas turbine blades feature serpentine internal cooling passages connected by 180-degree bends, through which coolant bled off from the compressor is routed to cool the internal walls. Under the influence of Coriolis force and centrifugal buoyancy force induced by rotation, the heat transfer for radially outward flow enhances on the trailing side (pressure side) and reduces on the leading side (suction side). A reverse trend in heat transfer is observed for radially inward flow. Rotation induced forces result in non-uniform heat transfer coefficient distribution which results in non-uniform metal temperatures under steady state condition. Present study addresses the problem of non-uniform heat transfer distribution due to rotation effect, by experimental investigation of two configurations where Coriolis effect was negated by aligning the coolant flow vector and rotation vector such that their cross product was effectively a null vector. This paper presents a novel design for serpentine cooling passages which are arranged along the chord of the blade which has similar heat transfer coefficient distribution on both leading and trailing walls. The two configurations were four-passage and six-passage serpentine smooth channels. Detailed heat transfer coefficients were measured using transient liquid crystal thermography under stationary and rotating conditions. Heat transfer experiments were carried out for Reynolds numbers ranging from 12294 to 85000 under stationary conditions. Rotation experiments were carried out at Rotation numbers of 0.05 and 0.11. Heat transfer enhancement levels of approximately two times the Dittus-Boelter correlation (for developed flow in smooth tubes) were obtained under stationary conditions. Under rotating conditions, we found that the four-passage configuration had slightly lower heat transfer compared to stationary case, and the six-passage configuration had higher heat transfer on both leading and trailing sides compared to stationary case. The leading and trailing side heat transfer characteristics were near-similar to each other, for both the configurations and the rotating heat transfer was near-similar to the stationary condition heat transfer.}, booktitle={Proceedings of the ASME Turbo Expo}, author={Singh, P. and Ji, Y. and Ekkad, S.V.}, year={2018} }
 @article{roy_blot_ekkad_ng_lohaus_crawford_abraham_2018, title={Thermal Management of a Transonic Turbine: Leakage Flow and Endwall Contouring Effects}, volume={32}, ISSN={0887-8722 1533-6808}, url={http://dx.doi.org/10.2514/1.T5340}, DOI={10.2514/1.T5340}, abstractNote={Comparison of heat transfer performance of a nonaxisymmetric contoured endwall to a planar baseline endwall in the presence of leakage flow through a stator-rotor rim seal interface is reported in ...}, number={4}, journal={Journal of Thermophysics and Heat Transfer}, publisher={American Institute of Aeronautics and Astronautics (AIAA)}, author={Roy, Arnab and Blot, Dorian M. and Ekkad, Srinath V. and Ng, Wing F. and Lohaus, Andrew S. and Crawford, Michael E. and Abraham, Santosh}, year={2018}, month={Oct}, pages={1031–1044} }
 @inproceedings{ramakrishnan_singh_ekkad_2018, title={Three-tier impingement cooling design for gas turbine blade trailing edge}, volume={8B-2018}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85063989411&partnerID=MN8TOARS}, DOI={10.1115/imece2018-86430}, abstractNote={Gas turbine blades are subjected to elevated heat loads due to highly turbulent hot gases exiting the combustor section. Several internal and external cooling techniques are used to protect the blades from such hostile environment. Trailing edge of a turbine blade is usually cooled with array of staggered cylindrical pins, which connects the pressure and suction side internal walls and hence provide improved structural integrity. However, the heat transfer enhancement levels for array of pin-fins is generally lower than jet impingement and ribbed channels. In this study, we present a three-tier impingement cooling design for blade trailing-edge and part of mid-chord region. In this design, pressure and suction side internal walls are subjected to oblique jet impingement. Three different configurations have been studied where we have systematically varied the jet diameters and number of jets in an array for different tiers. Numerical simulations have been carried out for different flow conditions, which corresponds to Reynolds numbers (based on 1st-passage jet diameter) ranging between 3000 and 46000. First two plenums had high levels of heat transfer due to oblique jet impingement, where the suction side internal wall representative surface, had higher heat transfer compared to the pressure side internal wall. Third tier had the lowest heat transfer due to triangle-like configuration where jets were almost parallel to pressure and suction side surfaces, and hence their effectiveness was lower than the oblique jet impingement in upstream two tiers.}, booktitle={ASME International Mechanical Engineering Congress and Exposition, Proceedings (IMECE)}, author={Ramakrishnan, K.R. and Singh, P. and Ekkad, S.V.}, year={2018} }
 @article{singh_li_ekkad_ren_2017, title={A new cooling design for rib roughened two-pass channel having positive effects of rotation on heat transfer enhancement on both pressure and suction side internal walls of a gas turbine blade}, volume={115}, ISSN={0017-9310}, url={http://dx.doi.org/10.1016/j.ijheatmasstransfer.2017.07.128}, DOI={10.1016/j.ijheatmasstransfer.2017.07.128}, abstractNote={This paper presents a new cooling design for a typical two-pass channel of a high pressure stage turbine blade. Gas turbine blades are subjected to elevated heat loads on the pressure and suction walls. In order to enhance heat transfer from the surfaces to the relatively colder internal air (gas), rib turbulators are installed on the opposite walls of two-pass channel. Combination of Coriolis force and centrifugal buoyancy force result in increase in heat transfer on trailing walls (along the pressure surface, and radially outward coolant flow) and leading walls (along the suction surface, radially inward flow) and vice versa. This leads to non-uniform cooling in both the passes and hence a non-optimum usage of cooling potential. The present study is focused on utilizing the Coriolis force favorably in both the passes by rotating the typical arrangement of two-pass channels by 90°. Detailed heat transfer coefficients were measured by transient liquid crystal thermography under rotating conditions. In order to match the direction of Buoyancy force as it exists in actual engines, colder air was passed during the transient experiment. The heat transfer experiments were carried out at a Reynolds number of 20000 and Rotation numbers of 0 and 0.1. The Nusselt numbers have been reported in two forms, (a) normalized with respect to Dittus-Boelter correlation for developed turbulent flow in circular duct, (b) normalized with corresponding Nusselt number obtained from smooth channel experiments. In order to understand the heat transfer characteristics of both traditional and new design, numerical simulations were also carried out for all configurations and at all experimental conditions to obtain flow and heat transfer predictions. A combined experimental and numerical discussion has been provided to explain the findings of the present study and to support the proposed design. It has been reported that the new design has 11% higher heat transfer enhancement at 8% lower pumping power compared to the traditional two pass rib roughened duct.}, journal={International Journal of Heat and Mass Transfer}, publisher={Elsevier BV}, author={Singh, Prashant and Li, Weihong and Ekkad, Srinath V. and Ren, Jing}, year={2017}, month={Dec}, pages={6–20} }
 @article{ekkad_han_2017, title={A transient liquid crystal image technique for local heat transfer distributions near a sharp 180° turn of a two-pass smooth square channel}, volume={24}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85045919409&partnerID=MN8TOARS}, DOI={10.1615/jflowvisimageproc.v24.i1-4.90}, abstractNote={Local heat transfer distributions are presented near a sharp 180° turn of a two-pass smooth square channel using a transient technique and encapsulated liquid crystal coating. Detailed distributions of the local Nusselt numbers are given for three flow Reynolds numbers of 10,000, 25,000, and 50,000. Results show that the Nusselt numbers are much higher in the region immediately downstream of the turn compared with upstream values for all three Reynolds numbers. The regional averaged results are compared with published heat transfer data.}, number={1-4}, journal={Journal of Flow Visualization and Image Processing}, author={Ekkad, S.V. and Han, J.-C.}, year={2017}, pages={127–140} }
 @inproceedings{gadiraju_park_gomez-ramirez_ekkad_todd lowe_moon_kim_srinivasan_2017, title={Application of proper orthogonal decomposition to high speed imaging for the study of combustion oscillations}, volume={Part F130041-4B}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85030695776&partnerID=MN8TOARS}, DOI={10.1115/GT2017-64602}, abstractNote={The flame structure and characteristics generated by an industrial low emission, lean premixed, fuel swirl nozzle were analyzed for understanding combustion oscillations. The experimental facility is located at the Advanced Propulsion and Power Laboratory (APPL) at Virginia Tech. The experiments were carried out in a model optical can combustor operating at atmospheric pressures. Low-frequency oscillations (<100 Hz) were observed during the reaction as opposed to no reaction, cold flow test cases. The objective of this paper is to understand the frequency and magnitude of oscillations due to combustion using high-speed imaging and associate them with corresponding structure or feature of the flame. Flame images were obtained using a Photron Fastcam SA4 high-speed camera at 500 frames per second. The experiments were conducted at equivalence ratios of 0.65, 0.75; different Reynolds numbers of 50K, 75K; and three pilot fuel to main fuel ratios of 0%, 3%, 6%. In this study, Reynolds number was based on the throat diameter of the fuel nozzle. Since the time averaged flame images are not adequate representation of the flame structures, proper orthogonal decomposition (POD) was applied to the flame images to extract the dominant features. The spatiotemporal dynamics of the images can be decomposed into their constituent modes of maximum spatial variance using POD so that the dominant features of the flame can be observed. The frequency of the dominant flame structures, as captured by the POD modes of the flame acquisitions, were consistent with pressure measurements taken at the exit of the combustor. Thus, the oscillations due to combustion can be visualized using POD. POD was further applied to high-speed images taken during instabilities. Specifically, the instabilities discussed in this paper are those encountered when the equivalence ratio is reduced to the levels approaching lean blowout (LBO). As the equivalence ratio is reduced to near blowout regime, it triggers low-frequency high amplitude instabilities. These low-frequency instabilities are visible as the flapping of the flame. The frequencies of the dominant POD modes are consistent with pressure measurements recorded during these studies.}, booktitle={Proceedings of the ASME Turbo Expo}, author={Gadiraju, S. and Park, S. and Gomez-Ramirez, D. and Ekkad, S.V. and Todd Lowe, K. and Moon, H.-K. and Kim, Y. and Srinivasan, R.}, year={2017} }
 @article{ji_singh_ekkad_zang_2017, title={Effect of crossflow regulation by varying jet diameters in streamwise direction on jet impingement heat transfer under maximum crossflow condition}, volume={72}, ISSN={1040-7782 1521-0634}, url={http://dx.doi.org/10.1080/10407782.2017.1394136}, DOI={10.1080/10407782.2017.1394136}, abstractNote={ABSTRACT Jet impingement heat transfer has been studied numerically for a maximum crossflow condition using a 3 × 9 array of jets. Five-hole configurations have been studied for jet average Reynolds numbers ranging from 10,000 to 20,000. Crossflow has been mitigated by varying the jet diameters in the streamwise direction to reduce the impact of crossflow on downstream jet impingement. The design criteria for all five configurations were to keep the average of the jet diameters equal to the constant jet diameter configuration (baseline). It has been found that the configuration with increasing and then decreasing jet diameters provided higher levels of heat transfer with more uniform cooling when compared to the traditional constant diameter configuration and other configurations.}, number={8}, journal={Numerical Heat Transfer, Part A: Applications}, publisher={Informa UK Limited}, author={Ji, Yongbin and Singh, Prashant and Ekkad, Srinath V. and Zang, Shusheng}, year={2017}, month={Oct}, pages={579–599} }
 @article{singh_ekkad_2017, title={Effects of spent air removal scheme on internal-side heat transfer in an impingement-effusion system at low jet-to-target plate spacing}, volume={108}, ISSN={0017-9310}, url={http://dx.doi.org/10.1016/j.ijheatmasstransfer.2016.12.104}, DOI={10.1016/j.ijheatmasstransfer.2016.12.104}, abstractNote={Present study reports detailed measurements of heat transfer coefficient for jet impingement in an impingement-effusion system at low jet-to-target plate spacing. The heat transfer coefficients were measured experimentally by transient liquid crystal thermography. Heat transfer experiments were carried out at three jet Reynolds numbers – 3500, 6000 and 9000. The jet plate featured 8 × 9 circular jets with normalized streamwise (x/dj) and spanwise (y/dj) spacing of 6. The configurations are divided into two segments based on the characteristics of target surface. The first target surface was smooth without effusion holes, and the second target surface was smooth with effusion holes. The arrangement of effusion holes was staggered with respect to jet plate and the ratio of effusion hole diameter to jet hole diameter was unity. For the smooth target surface without effusion holes, three crossflow schemes were studied – minimum, intermediate and maximum. For the smooth target surface with effusion holes, four different crossflow schemes were studied – zero, minimum, intermediate and maximum. Interesting heat transfer characteristics are reported for different crossflow schemes as it was found that low z/dj (=1) played an important role in the spent air removal from the system. Discharge coefficient of jets is also reported for wide range of plenum pressure ratio. Also reported are the pumping power requirements for each configuration across full range of flow conditions. It has been found that the minimum crossflow scheme (with and without effusion holes) has been the most efficient configuration and the maximum crossflow scheme with target surface without effusion holes has been the least efficient configuration.}, journal={International Journal of Heat and Mass Transfer}, publisher={Elsevier BV}, author={Singh, Prashant and Ekkad, Srinath V.}, year={2017}, month={May}, pages={998–1010} }
 @article{singh_ji_ekkad_2018, title={Experimental and numerical investigation of heat and fluid flow in a square duct featuring criss-cross rib patterns}, volume={128}, ISSN={1359-4311}, url={http://dx.doi.org/10.1016/j.applthermaleng.2017.09.036}, DOI={10.1016/j.applthermaleng.2017.09.036}, abstractNote={This paper presents findings from experimental and numerical study of heat and fluid flow in a straight square duct featuring rib turbulators in a criss-cross pattern formed by 45° angled rib turbulators. Two ribbed configurations with criss-cross pattern – Inline and staggered, have been studied where the baseline case was smooth duct with no heat transfer enhancement feature. Detailed heat transfer coefficients were calculated using transient liquid crystal thermography by employing 1-D semi-infinite conduction model. Heat transfer and pressure drop measurements were carried out for Reynolds number ranging from 30,000 to 60,000. For understanding of heat transfer enhancement mechanism, numerical investigations were carried out using SST k-ω turbulence model. Numerical predictions of near-wall fluid dynamics and turbulent transport has been presented in conjunction with experimentally obtained detailed heat transfer coefficients to demonstrate the heat transfer characteristics of ribbed duct. Nusselt numbers normalized with respect to Dittus-Boelter correlation for developed turbulent flow in circular duct varied between 2.7 and 3.1 for inline and staggered configurations and the thermal hydraulic performance varied between 1.2 and 1.5 for the range of Reynolds number investigated.}, journal={Applied Thermal Engineering}, publisher={Elsevier BV}, author={Singh, Prashant and Ji, Yongbin and Ekkad, Srinath V.}, year={2018}, month={Jan}, pages={415–425} }
 @article{singh_li_ekkad_ren_2017, title={Experimental and numerical investigation of heat transfer inside two-pass rib roughened duct (AR = 1:2) under rotating and stationary conditions}, volume={113}, ISSN={0017-9310}, url={http://dx.doi.org/10.1016/j.ijheatmasstransfer.2017.05.085}, DOI={10.1016/j.ijheatmasstransfer.2017.05.085}, abstractNote={Heat transfer enhancement inside ribbed channels for turbine blades is a critical phenomenon that impacts overall performance and life of the gas turbine. Present study investigates heat and fluid flow in a rectangular duct with heat transfer enhancement features, under rotating and stationary conditions. The heat transfer data obtained experimentally has been explained using numerical prediction of flow features. Detailed heat transfer coefficients have been measured on the walls of two-pass rectangular duct (AR = 1:2) featuring V-shaped rib turbulators, using transient liquid crystal thermography (TLCT). The first pass and second pass featured nine V-shaped ribs each and the bend featured a 90° rib connecting the blade tip underside and the two-pass divider wall. The flow in the first pass was developing in nature. The rib-pitch to rib-height ratio (p/e) was 9.625 and the rib-height to channel hydraulic diameter (e/dh) was 0.125. The baseline case for the rib roughened duct was geometrically identical smooth duct (with no heat transfer enhancement features). Stationary experiments were carried out for Reynolds numbers ranging from 25000 to 75000. The rotation experiments were carried out at 400 RPM (Ro = 0.036) and 700 RPM (Ro = 0.063), at Reynolds number of 25000 (Ro=Ωdh/V,Re=Vdh/ν). Also, numerical simulations were performed for a similar test model under similar flow conditions, using realizable k-∊ turbulence model. Detailed discussion on rib induced secondary flows and rotational effects on heat transfer in smooth and rib roughened duct are presented in this paper using results obtained from detailed heat transfer measurements from experiments and fluid dynamics predictions from numerical simulations.}, journal={International Journal of Heat and Mass Transfer}, publisher={Elsevier BV}, author={Singh, Prashant and Li, Weihong and Ekkad, Srinath V. and Ren, Jing}, year={2017}, month={Oct}, pages={384–398} }
 @inproceedings{boulanger_hutchinson_ng_ekkad_keefe_xu_barker_hsu_2017, title={Experimental based empirical model of the initial onset of sand deposits on hastelloy-X from 1000°C to 1100°C using particle tracking}, volume={2D-2017}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85028971325&partnerID=MN8TOARS}, DOI={10.1115/GT2017-64480}, abstractNote={Deposition experiments are performed on a Hastelloy-X coupon using Arizona Road Test Dust (ARD). A statistical empirical model of the initial onset of ARD deposits is developed from the experimental data. The initial onset of deposits are a quadratic function of local surface temperature and impact velocity components (normal and tangential). A prominent observation is that tangential impact velocity has a significant non-linear, independent effect on deposits relative to normal impact velocity and local surface temperatures. All experiments use 20–40 μm ARD on a bare metal surface over a 1000–1100°C range. Unlike prior mass-based deposit studies, initial deposits for this study are quantified using a Coverage Ratio (CR), which is the area covered by deposits relative to the total area available. The empirical CR model has a strong correlation to coupon surface temperature and impact velocity vectors and is a foundation for future numerical or experimental model comparisons.}, booktitle={Proceedings of the ASME Turbo Expo}, author={Boulanger, A. and Hutchinson, J. and Ng, W.F. and Ekkad, S.V. and Keefe, M.J. and Xu, W. and Barker, B.J. and Hsu, K.}, year={2017} }
 @inproceedings{singh_ekkad_2017, title={Experimental investigation of rotating rib roughened two-pass square duct with two different channel orientations}, volume={5A-2017}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85029022270&partnerID=MN8TOARS}, DOI={10.1115/GT2017-64225}, abstractNote={Effects of rotation on heat transfer on leading and trailing sides of gas turbine blades has been extensively studied in the past. It has been established for typical two-pass channel that radially outward flow (first pass) has higher heat transfer on trailing side and lower heat transfer on leading side and vice versa for radially inward flow (second pass). Rotation induces three forces on the coolant flow — Coriolis, Centrifugal and Buoyancy forces. The direction of Coriolis force depends on the relative angle between the coolant flow and the rotation direction, because of which the direction of Coriolis force is different in individual passes — which in turn results in non-uniform distribution of high heat transfer regions on leading and trailing walls. The present study is focused on utilizing the Coriolis force favorably in both the passes by rotating the typical arrangement of two-pass channels by 90°. Firstly, smooth two pass duct (Model A-smooth) having typical arrangement of coolant flow and rotation direction is studied. The second configuration is the corresponding ribbed channel (Model A-ribbed) featuring V-shaped ribs on both leading and trailing walls. The rib-height-to-channel hydraulic diameter ratio was 0.125, rib pitch-to-rib height ratio was 8, and channel aspect ratio was unity. Model B was obtained by rotating the Model A by 90° and changing the coolant inlet port as well. Model B had three configurations — (a) smooth duct, (b) single sided ribbed duct, and (c) double sided ribbed duct. Detailed heat transfer coefficients were measured by transient liquid crystal thermography under rotating conditions. In order to match the direction of Buoyancy force as it exists in actual engines, colder air was passed during the transient experiment. The heat transfer experiments were carried out at a Reynolds number of 20000 and Rotation numbers of 0, 0.05 and 0.1. The Nusselt numbers have been reported in two forms, (a) normalized with respect to Dittus-Boelter correlation for developed turbulent flow in circular duct, (b) normalized with corresponding Nusselt number obtained from smooth channel experiments. The effects of Coriolis force and centrifugal force on heat transfer has been discussed in detail. A new model has been proposed based on the understanding and findings of the present study, which has positive effects of rotation on both leading and trailing walls.}, booktitle={Proceedings of the ASME Turbo Expo}, author={Singh, P. and Ekkad, S.V.}, year={2017} }
 @article{boulanger_hutchinson_ng_ekkad_keefe_xu_barker_hsu_2017, title={Experimental investigation of the onset of sand deposits on Hastelloy-X between 1,000°C and 1,100°C}, volume={121}, ISSN={0001-9240 2059-6464}, url={http://dx.doi.org/10.1017/AER.2017.48}, DOI={10.1017/AER.2017.48}, abstractNote={ABSTRACTDeposit formation on turbine hardware in propulsion turbine engines can occur in many arid regions globally. Characterising crystalline deposits on metallic substrates can aid in component resilience and health monitor algorithms during particle ingestion. This study has developed two statistical empirical models for prediction from acquired experimental data for the onset of deposits. The prediction models are for crystalline particulate (Arizona Road Test Dust) deposits on a flat rectangular Hastelloy-X test coupon. Particle impingement angles varied between 20° and 80° in experimental flow temperatures of 1,000–1,100°C. Averaged deposits are methodically quantified through normalised particle deposit tallies per area and percent coverage of the surface using microscopic imaging and image processing programs. Deposit accumulation is a quadratic function of both near-surface coupon temperature and coupon angle.}, number={1242}, journal={The Aeronautical Journal}, publisher={Cambridge University Press (CUP)}, author={Boulanger, A. and Hutchinson, J. and Ng, W.F. and Ekkad, S.V. and Keefe, M.J. and Xu, W. and Barker, B. and Hsu, K.}, year={2017}, month={Jun}, pages={1187–1199} }
 @article{singh_ekkad_2017, title={Experimental study of heat transfer augmentation in a two-pass channel featuring V-shaped ribs and cylindrical dimples}, volume={116}, ISSN={1359-4311}, url={http://dx.doi.org/10.1016/j.applthermaleng.2017.01.098}, DOI={10.1016/j.applthermaleng.2017.01.098}, abstractNote={Higher turbine inlet temperature requirements in modern gas turbine airfoils necessitate the development of high performance internal cooling designs for increasing turbine durability and performance. The internal cooling passages of turbine airfoils feature ribs which promote turbulent mixing, increase in near wall shear and hence lead to heat transfer augmentation. One other method of increasing heat transfer enhancement is by adding depressions (commonly called “dimples”) on the walls of internal cooling passages. Earlier studies have investigated different shapes of rib turbulators and dimples, however, the limitations in manufacturability of dimple in actual turbine airfoils has been a concern in the past. Recent advancements in the field of additive manufacturing, e.g. Direct Metal Laser Sintering (DMLS), opens endless possibilities for the development of novel cooling features which can be incorporated in actual engines. The present study investigates the heat transfer and pressure drop characteristics of a two-pass channel (AR = 1) featuring ribs-alone, dimples-alone and combination of ribs and dimples. The ribs are V-shaped with rib-height-to-hydraulic diameter ratio of 0.125 and rib-pitch-to-rib height ratio of 16. Dimples are cylindrical in shape and have a depth-to-print diameter ratio of 0.3. The heat transfer coefficient was measured using transient Liquid Crystal Thermography and detailed Nusselt numbers have been reported and analyzed. Also, static pressure measurements were carried out at several locations to determine the total pressure drop across the two pass channel and the local variation of pressure coefficient. The experiments were carried out for a wide range of Reynolds number (19,500–69,000) to cover the full spectrum typically found in both land-based and air-breathing engines. It has been observed that the combination of ribs and dimples resulted in higher heat transfer augmentation as well as higher thermal hydraulic performance when compared with ribs alone and dimples alone configurations for the range of Reynolds number studied.}, journal={Applied Thermal Engineering}, publisher={Elsevier BV}, author={Singh, Prashant and Ekkad, Srinath}, year={2017}, month={Apr}, pages={205–216} }
 @inproceedings{park_gomez-ramirez_gadiraju_kedukodi_ekkad_moon_kim_srinivasan_2017, title={Flow field and wall temperature measurements for reacting flow in a lean premixed swirl stabilized can combustor}, volume={5C-2017}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85029116922&partnerID=MN8TOARS}, DOI={10.1115/GT2017-64837}, abstractNote={Designing gas turbine combustors requires accurate measurement and prediction of the violent, high-temperature environment in reacting flow. One important factor in combustor design is the heat load on the inner surface of the combustor liner during combustion. To properly analyze the heat load, the mechanisms of thermal energy transfer must be investigated. Of these, the convective heat transfer has not been fully characterized, representing an important challenge in the field of combustor research. The flow field is closely related to the combustion dynamics from the swirling flame in modern burners, and has a direct impact on the convective heat transfer. Most of the flow field measurements reported in the literature have relied on custom research nozzles. However, the development of modern low emission, lean-premixed combustors requires experimental results from realistic industrial fuel nozzles. This paper experimentally investigates the effects of combustor operating conditions on the reacting flow in an optical single can combustor. The swirling flow was generated by an industrial lean pre-mixed, axial swirl fuel nozzle manufactured by Solar Turbines Incorporated.
               Planar particle image velocimetry (PIV) data were acquired and analyzed to understand the characteristics of the flow field. Experiments were conducted at Reynolds numbers ranging between 50000 and 110000 (with respect to the nozzle diameter, DN); equivalence ratios between 0.55 and 0.78; and pilot fuel split ratios of 0 to 6%. Characterizing the impingement location on the liner, and the turbulent kinetic energy (TKE) distribution were a fundamental part of the investigation. Self-similar characteristics were observed at reacting conditions. Jet impingement locations on the liner were at x ≈ 1.16 DN for seven different reacting cases, and it was observed that the impingement location was not significantly affected by the combustion parameters studied. However, non-reacting flow was significantly different in flame structure and impingement locations. Combustor liner wall temperature distributions were measured in reacting condition with an infrared camera for a single case. The temperature profile was explained qualitatively with the flow features measured with PIV. Peak wall temperature close to impingement location on the liner wall reached about 900 K, and peak heat flux was measured as ≈ 23 kW/m2 at x ≈ 2.3 DN.}, booktitle={Proceedings of the ASME Turbo Expo}, author={Park, S. and Gomez-Ramirez, D. and Gadiraju, S. and Kedukodi, S. and Ekkad, S. and Moon, H.-K. and Kim, Y. and Srinivasan, R.}, year={2017} }
 @article{srinivasan_ekkad_tolpadi_2017, title={Heat Transfer Measurements inside Narrow Channels with Ribs and Trenches}, volume={39}, ISSN={0145-7632 1521-0537}, url={http://dx.doi.org/10.1080/01457632.2017.1341198}, DOI={10.1080/01457632.2017.1341198}, abstractNote={ABSTRACT Detailed heat transfer coefficient distributions are obtained for high aspect ratio (width/height = 12.5) duct with rib and trench enhancement features oriented normal to the coolant flow direction. A transient thermochromic liquid crystal technique has been used to experimentally measure heat transfer coefficients from which Nusselt numbers are calculated on the duct surface featuring heat transfer enhancement features. Reynolds number (calculated based on duct hydraulic diameter) ranging from 7100 to 22400 were experimentally investigated. Detailed measurements of heat transfer provided insight into the role of protruding ribs and trenches on the fluid dynamics in the duct. Experimentally obtained Nusselt numbers are normalized by Dittus-Boelter correlation for developed turbulent flow in circular duct. The triangular trenches provide heat transfer enhancement ratios up to 1.9 for low Reynolds numbers. The in-line rib configuration shows similar levels to the trench whereas staggered rib configuration provides heat transfer enhancement ratios up to 2.2 for a low Reynolds number of 7100.}, number={9}, journal={Heat Transfer Engineering}, publisher={Informa UK Limited}, author={Srinivasan, Shreyas and Ekkad, Srinath V. and Tolpadi, Anil}, year={2017}, month={Jul}, pages={750–759} }
 @article{roy_jain_ekkad_ng_lohaus_crawford_abraham_2017, title={Heat Transfer Performance of a Transonic Turbine Blade Passage in the Presence of Leakage Flow Through Upstream Slot and Mateface Gap With Endwall Contouring}, volume={139}, ISSN={0889-504X 1528-8900}, url={http://dx.doi.org/10.1115/1.4037909}, DOI={10.1115/1.4037909}, abstractNote={Comparison of heat transfer performance of a nonaxisymmetric contoured endwall to a planar baseline endwall in the presence of leakage flow through stator–rotor rim seal interface and mateface gap is reported in this paper. Heat transfer experiments were performed on a high turning turbine airfoil passage at Virginia Tech's transonic blow down cascade facility under design conditions for two leakage flow configurations—(1) mateface blowing only, (2) simultaneous coolant injection from the upstream slot and mateface gap. Coolant to mainstream mass flow ratios (MFRs) were 0.35% for mateface blowing only, whereas for combination blowing, a 1.0% MFR was chosen from upstream slot and 0.35% MFR from mateface. A common source of coolant supply to the upstream slot and mateface plenum made sure the coolant temperatures were identical at both upstream slot and mateface gap at the injection location. The contoured endwall geometry was generated to minimize secondary aerodynamic losses. Transient infrared thermography technique was used to measure endwall surface temperature and a linear regression method was developed for simultaneous calculation of heat transfer coefficient (HTC) and adiabatic cooling effectiveness, assuming a one-dimensional (1D) semi-infinite transient conduction. Results indicate reduction in local hot spot regions near suction side as well as area averaged HTC using the contoured endwall compared to baseline endwall for all coolant blowing cases. Contoured geometry also shows better coolant coverage further along the passage. Detailed interpretation of the heat transfer results along with near endwall flow physics has also been discussed.}, number={12}, journal={Journal of Turbomachinery}, publisher={ASME International}, author={Roy, Arnab and Jain, Sakshi and Ekkad, Srinath V. and Ng, Wing and Lohaus, Andrew S. and Crawford, Michael E. and Abraham, Santosh}, year={2017}, month={Oct} }
 @inproceedings{singh_ji_zhang_ekkad_2017, title={Heat transfer enhancement by criss-cross pattern formed by 45° Angled rib turbulators in a straight square duct}, volume={1}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85032891330&partnerID=MN8TOARS}, DOI={10.1115/HT2017-4908}, abstractNote={The need for higher turbine efficiency has been constantly pushing the turbine inlet temperatures to elevated levels. Hot gas path temperatures are much higher than the typical blade material yield temperature. Efficient internal cooling technologies are required for safe operation of gas turbine. Several internal cooling technologies have been developed in order to enhance the heat transfer from relatively hotter walls of turbine blade. For mid-chord region of turbine blade, rib turbulators are typically installed in multi-pass channels. Rib turbulators trip the boundary layer, induce secondary flows which enhance near wall shear as well as enhance turbulent mixing when they interact with surrounding walls. Research has been carried out on several aspects of rib turbulated passages in order to achieve higher thermal hydraulic performance. Generally, rib turbulators are installed on two opposite walls of serpentine passages in order to enhance heat transfer from both pressure and suction sides of blade through coolant flowing through complicated paths. Typical arrangement of rib turbulators were parallel to each other or having some offset from each other. In the present study, an attempt has been made to arrange 45° angled ribs in a way that they form a Criss-Cross pattern. Two ribbed configurations with Criss-Cross pattern - Inline and staggered, have been studied where the baseline case was smooth duct with no rib turbulators. The effective rib-pitch-to-rib-height ratio (p/e) was 8.6 and rib-height-to-channel-hydraulic diameter ratio (e/dh) was 0.1. The channel had a total length of 20 hydraulic diameters and the rib turbulators were installed at a distance of six hydraulic diameters from the inlet of the test section to allow flow development. Detailed heat transfer coefficients were measured using transient liquid crystal thermography employing 1D semi-infinite conduction model. Globally averaged Nusselt numbers are calculated from the detailed measurements and thermal hydraulic performance of configurations have been reported with respect to Reynolds number. The aim of this study was to develop a cooling configuration which has higher thermal-hydraulic performance compared to other traditional rib configurations. It has been found that the heat transfer characteristics of the inline and staggered configurations were similar to each other and ranged between three times D-B correlation to 2.7 times, for Reynolds number ranging from 30000 to 60000. Inline configuration had relatively lower frictional losses, however the thermal hydraulic performances of both the configurations were similar.}, booktitle={ASME 2017 Heat Transfer Summer Conference, HT 2017}, author={Singh, P. and Ji, Y. and Zhang, M. and Ekkad, S.V.}, year={2017} }
 @article{gomez-ramirez_kedukodi_ekkad_moon_kim_srinivasan_2017, title={Investigation of isothermal convective heat transfer in an optical combustor with a low-emissions swirl fuel nozzle}, volume={114}, ISSN={1359-4311}, url={http://dx.doi.org/10.1016/j.applthermaleng.2016.11.154}, DOI={10.1016/j.applthermaleng.2016.11.154}, abstractNote={Modern combustor design optimization is contingent on the accurate characterization of the combustor flame side heat loads. The present work focuses on the experimental measurement of the isothermal (non-reacting) convective heat transfer along a fused silica optical can combustor liner for Reynolds numbers ranging between 11,500 and 138,000. The model combustor was equipped with the SoLoNOx swirl fuel nozzle from Solar Turbines Incorporated, subjecting the liner walls to realistic isothermal flow and turbulence fields. Infrared (IR) imaging through fused silica was demonstrated, and a novel estimation of the three-dimensional conduction heat losses for steady state constant heat flux experiments was developed. A maximum heat transfer augmentation of ∼18 was observed with respect to fully developed turbulent pipe flow correlations. Contrary to other investigations, the augmentation magnitude and distribution are shown to be approximately constant with Reynolds number (particularly away from the impingement location). Particle Image Velocimetry (PIV) was included to support the heat transfer measurements, suggesting that peak heat transfer occurred 0.12 nozzle diameters upstream of the jet reattachment point along the liner. Reynolds-Averaged Navier Stokes (RANS) computations are shown to yield peak heat transfer predictions within 17.4% of the experimental results when using the realizable k-ε turbulence model and enhanced wall treatment. The measurements were further analyzed in the context of results from other heat transfer studies on gas turbine combustors.}, journal={Applied Thermal Engineering}, publisher={Elsevier BV}, author={Gomez-Ramirez, David and Kedukodi, Sandeep and Ekkad, Srinath V. and Moon, Hee-Koo X. and Kim, Yong and Srinivasan, Ram}, year={2017}, month={Mar}, pages={65–76} }
 @article{gomez-ramirez_ekkad_moon_kim_srinivasan_2017, title={Isothermal coherent structures and turbulent flow produced by a gas turbine combustor lean pre-mixed swirl fuel nozzle}, volume={81}, ISSN={0894-1777}, url={http://dx.doi.org/10.1016/j.expthermflusci.2016.10.010}, DOI={10.1016/j.expthermflusci.2016.10.010}, abstractNote={The steady and unsteady isothermal fluid dynamics generated by an industrial low emission, lean premixed, fuel swirl nozzle designed by Solar Turbines Incorporated were investigated in this study. The experiments were carried out in a model optical can combustor operating at atmospheric pressures. Non-time resolved, planar Particle Image Velocimetry (PIV) measurements were taken at Reynolds numbers with respect to the nozzle throat diameter of ∼50 000, ∼100 000, and ∼180 000. The time-averaged velocity fields were approximately self-similar, with the highest mass flow exhibiting a central recirculation zone (CRZ) with a slightly larger diameter. The results were analyzed using a methodology based on Proper Orthogonal Decomposition (POD) to extract the periodic structures in the flow and obtain the underlying stochastic turbulence field. This distinction between stochastic and coherent fluctuations is critical to properly model combustor flows. Coherent flow instabilities such as the precessing vortex core (PVC) and the propagation of axial/radial vortices were observed to significantly contribute to the mixing between the nozzle exit flow and the recirculated mass flow. Over 30% of the total fluctuation (difference between instantaneous and time-averaged velocity fields) kinetic energy was attributed to coherent structures throughout the inner shear layer between the swirling jet exiting the nozzle and the CRZ. Stochastic variability was prevalent close the liner wall and throughout the combustor domain after the swirling jet impinged on the wall, with <20% of the total fluctuation attributed to coherent structures. The normalized coherent and stochastic flow fields were also approximately self-similar with Reynolds number.}, journal={Experimental Thermal and Fluid Science}, publisher={Elsevier BV}, author={Gomez-Ramirez, David and Ekkad, Srinath V. and Moon, Hee-Koo and Kim, Yong and Srinivasan, Ram}, year={2017}, month={Feb}, pages={187–201} }
 @inproceedings{kedukodi_park_gadiraju_ekkad_kim_srinivasan_2017, title={Numerical & experimental investigations for flow fields under non-reacting & reacting conditions through a lean premixed fuel nozzle}, volume={5C-2017}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85029086182&partnerID=MN8TOARS}, DOI={10.1115/GT2017-64911}, abstractNote={Numerical methods coupled with experimental benchmarking approaches, are typically used as effective tools for solving engineering problems due to their significant time saving benefits. In this paper, the swirling flow through an industrial lean-premixed fuel nozzle as used in actual gas turbine combustors is numerically analyzed and compared with experimental observations. The analysis is performed under both non-reacting and reacting conditions for a specific Reynolds number. The reacting experiments were performed using compressed methane as the fuel and air as the oxidizer. A specific inlet Reynolds number flow was studied to understand the combustor flow field with an overall equivalence ratio of 0.65 and 6% pilot fuel.
               Steady state simulations were performed using Fluent solver using Realizable k-ε turbulence model. The reacting flow was simulated using Flamelet Generation Manifold (FGM) model to simulate partially premixed combustion. The non-reacting simulations predicted the combustor flow profiles with certain deviation from Particle Image Velocimetry (PIV) data within the central recirculation region. This deviation may be attributed to the inherent limitations of turbulence model in predicting the central vortex accurately. However, the simulated flow fields were in very good agreement with PIV data under reacting conditions. Additionally, the study was also extended to investigate the sensitivity of inlet swirl on the jet impingement location along the combustor wall. It was found that reaction significantly modifies the jet impingement location for lower inlet swirl angles and showed negligible impact under non-reacting conditions.
               The presented studies in this paper provide a comprehensive summary of modified flow features under non-reacting and reacting conditions and also demonstrates the sensitivity of inlet swirl changes on the location of liner wall impingement. This study is believed to offer a strong base for future studies involving heat transfer characterization along the combustor walls under reacting conditions; and also provide valuable information to the gas turbine combustor design community towards improved liner wall designs using simplistic numerical modeling approaches.}, booktitle={Proceedings of the ASME Turbo Expo}, author={Kedukodi, S. and Park, S. and Gadiraju, S. and Ekkad, S. and Kim, Y. and Srinivasan, R.}, year={2017} }
 @inproceedings{ji_singh_ekkad_zang_2017, title={Numerical investigation of scoop effect on film cooling for cylindrical inclined hole}, volume={8}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85040975494&partnerID=MN8TOARS}, DOI={10.1115/IMECE2017-70184}, abstractNote={Film cooling behavior of a single cylindrical hole inclined at an angle of 35° with respect to a flat surface is numerically predicted in this study. Adiabatic film cooling effectiveness has been presented to evaluate the influence of the scoop placed on the coolant entry side. The effect of blowing ratio (0.65, 1, 1.5 and 2) and the length-to-diameter ratio (1.7 and 4.4) are examined. Three-dimensional Reynolds-averaged Navier-Stokes analysis with SST turbulence model is used for the computations. It has been found that both centerline and laterally averaged adiabatic film cooling effectiveness are enhanced by the scoop and the enhancement increases with the blowing ratio in the investigated range of variables. The scoop was more effective for the higher length-to-diameter ratio cases (L/D = 4.4) because of better velocity distribution at the film hole exit, which makes coolant reattach at a more upstream location after blowing off from the wall.}, booktitle={ASME International Mechanical Engineering Congress and Exposition, Proceedings (IMECE)}, author={Ji, Y. and Singh, P. and Ekkad, S.V. and Zang, S.}, year={2017} }
 @article{barboza_ma_todd lowe_ekkad_ng_2016, title={A Diagnostic Technique for Particle Characterization Using Laser Light Extinction}, volume={138}, ISSN={0742-4795 1528-8919}, url={http://dx.doi.org/10.1115/1.4033468}, DOI={10.1115/1.4033468}, abstractNote={Increased operations of aircraft, both commercial and military in hostile desert environments have increased the risk of micro-sized particle ingestion into engines. The probability of increased sand and dust ingestion results in increased life cycle costs in addition to increased potential for performance loss. Thus, the ability to accurately assess the amount of inlet debris would be useful for engine diagnostics and prognostic evaluation. Previous engine monitoring studies were based on the particle measurements performed a posteriori. Thus, there exists a need for in situ quantification of ingested particles. This paper describes the initial development of a line-of-sight optical technique to characterize the ingested particles at concentrations similar to those experienced by aircraft in brownout conditions using laser extinction with the end goal of producing an onboard aircraft diagnostic sensor. By measuring the amount of light that is transmitted due to the effects of scattering and absorption in the presence of particles over a range of concentrations, a relationship between particle diameters and the laser light extinction was obtained. This relationship was then used to obtain information on diameters and number densities of ingested particles. The particle size range of interest was chosen to be between 1 and 10 μm and the size distribution function was assumed to be lognormal. Tests were performed on polystyrene latex spheres of sizes 1.32 μm, 3.9 μm, and 5.1 μm in water dispersions to measure diameters and concentrations. Measurements were performed over multiple wavelengths to obtain information on the size distribution and number density of particles. Results of tests presented in this paper establish the validity of the laser extinction technique to provide real time information of ingested particles and will serve as an impetus to carry out further research using this technique to characterize particles.}, number={11}, journal={Journal of Engineering for Gas Turbines and Power}, publisher={ASME International}, author={Barboza, Kris and Ma, Lin and Todd Lowe, K. and Ekkad, Srinath and Ng, Wing}, year={2016}, month={May} }
 @inproceedings{kedukodi_gomez-ramirez_ekkad_moon_kim_srinivasan_2016, title={Analysis on impact of turbulence parameters and swirl angle variation on isothermal gas turbine combustor flows}, volume={2}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85002934475&partnerID=MN8TOARS}, DOI={10.1115/HT2016-7134}, abstractNote={The current computational study deals with the isothermal fluid flow and heat transfer analysis of a gas turbine combustor subject to different boundary conditions. A 90 degree sector model was studied computationally in order to identify the impingement and peak heat transfer locations along the combustor liner in addition to heat transfer augmentation. Validation experiments were carried out for the full scale industrial swirler-fuel nozzle using PIV and IR thermography to obtain flow and heat transfer data. Inlet conditions into the swirler were set to a Reynolds number of 50000 and the outlet was set to atmospheric conditions. The swirler vanes provided a radially varying swirl to the flow entering into the combustor. The k-w SST turbulence model was employed to investigate the effects of different inlet turbulence parameters on the accuracy of the simulation, i.e., calculations with experimental inlet turbulent kinetic energy and deduced dissipation rate profiles, and prescribed constant turbulent intensity and length scale. It was observed that the former provided conforming results with the experiments at specific locations and improved convergence, while both cases showed discrepancy in velocity profiles within the central recirculation region of the combustor. The peak heat transfer and impingement location along the liner were in excellent agreement with the experimental data. However the peak magnitude prediction was over-predicted up to 27%. This discrepancy was attributed to the limitations of two-equation turbulence model predictions near the stagnation region. An additional study was performed to investigate the effect of different inlet swirl angles on the impingement location. It was observed that a higher swirl angle shifts the impingement location upstream. Overall, the present study provides a probe into the capability of steady RANS models to predict combustor swirling flows and wall heat transfer; and also aids in using the steady state results as initialization data for the future scale resolved turbulence model based simulations. In spite of the quantitative discrepancies, the liner heat transfer trends are expected to provide valuable insight to the industrial community in the design of combustor liners based on less expensive computational tools.}, booktitle={ASME 2016 Heat Transfer Summer Conference, HT 2016, collocated with the ASME 2016 Fluids Engineering Division Summer Meeting and the ASME 2016 14th International Conference on Nanochannels, Microchannels, and Minichannels}, author={Kedukodi, S. and Gomez-Ramirez, D. and Ekkad, S.V. and Moon, H.-K. and Kim, Y. and Srinivasan, R.}, year={2016} }
 @article{singh_pandit_ekkad_2017, title={Characterization of heat transfer enhancement and frictional losses in a two-pass square duct featuring unique combinations of rib turbulators and cylindrical dimples}, volume={106}, ISSN={0017-9310}, url={http://dx.doi.org/10.1016/j.ijheatmasstransfer.2016.09.037}, DOI={10.1016/j.ijheatmasstransfer.2016.09.037}, abstractNote={Transient heat transfer experiments using liquid crystal thermography have been carried out on a two-pass square channel for testing several unique combinations of ribs and cylindrical dimples. Four different rib shapes, viz. 45° angled, V, W and M have been studied. In compound channels, the rib pitch accommodates cylindrical dimples arranged in the shape of the rib, hence following the direction of rib induced secondary flows. For each rib shape, three different configurations – rib alone, dimple alone and rib–dimpled compound cases were studied. The rib-height-to-channel hydraulic diameter ratio was 0.125 and rib-pitch-to-rib-height ratio was 16. The dimple-depth-to-print diameter ratio was 0.3. The experiments were carried out for a wide range of Reynolds number (19,500–69,000), covering a spectrum typically found in both land-based and air-breathing engines. A total of 52 experiments were carried out to measure detailed heat transfer coefficient on the bottom wall of the two-pass channel. A transient liquid crystal thermography technique was used for heat transfer measurement. Static pressure measurements were carried out to measure the overall pressure drop in the two pass channel. From globally averaged Nusselt number and overall pressure drop, thermal-hydraulic performance of the 13 configurations were determined, compared and analyzed. It has been observed that 45° angled and V compound configurations resulted in higher heat transfer augmentation as well as higher thermal hydraulic performance when compared with their corresponding ribs alone and dimples alone configurations.}, journal={International Journal of Heat and Mass Transfer}, publisher={Elsevier BV}, author={Singh, Prashant and Pandit, Jaideep and Ekkad, Srinath V.}, year={2017}, month={Mar}, pages={629–647} }
 @article{panchal_abraham_roy_ekkad_ng_lohaus_crawford_2016, title={Effect of Endwall Contouring on a Transonic Turbine Blade Passage: Heat Transfer Performance}, volume={139}, ISSN={0889-504X 1528-8900}, url={http://dx.doi.org/10.1115/1.4034411}, DOI={10.1115/1.4034411}, abstractNote={Effect of turbine endwall contouring on its aerodynamic performance has been widely studied, but only a few studies are available in the open literature investigating its effect on heat transfer performance; especially at transonic exit Mach number conditions. In this paper, we report a study of effect of contouring on endwall heat transfer performance of a high-turning high-pressure (HP) turbine blade passage operating under transonic exit conditions. The paper describes comparison of heat transfer performance of two contoured endwall geometries, one aerodynamically optimized (AO) and the other heat transfer optimized (HTO), with a baseline, noncontoured geometry. The endwall geometries were experimentally investigated at Virginia Tech's transient, blow down, transonic linear cascade facility at three exit Mach numbers, Mex= 0.71, 0.88(design) and 0.95, for their heat transfer performance. Endwall surface temperatures were measured using infrared (IR) thermography and local heat transfer coefficient (HTC) values were calculated using measured temperatures. A camera matrix model-based data postprocessing technique was developed to relate the two-dimensional images captured by IR camera to three-dimensional endwall contours. The measurement technique and the methodology for postprocessing of the heat transfer coefficient data have been presented in detail. Discussion and interpretation of experimental results have been augmented using aerodynamic CFD simulations of the geometries. Both the contoured endwalls demonstrated a significant reduction in the overall average heat transfer coefficient values of the order of 10%. The surface Stanton number distributions also indicated a reduction in the level of hot spots for most of the endwall surface. However, at some locations an increase was also observed, especially in the area near the leading edge (LE). The results indicate that the endwall contouring could significantly improve heat transfer performance of turbine passages.}, number={1}, journal={Journal of Turbomachinery}, publisher={ASME International}, author={Panchal, Kapil V. and Abraham, Santosh and Roy, Arnab and Ekkad, Srinath V. and Ng, Wing and Lohaus, Andrew S. and Crawford, Michael E.}, year={2016}, month={Sep} }
 @inproceedings{singh_ekkad_2016, title={Effects of rotation on heat transfer due to jet impingement on cylindrical dimpled target surface}, volume={5B-2016}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84991573843&partnerID=MN8TOARS}, DOI={10.1115/GT2016-56145}, abstractNote={One way to enhance the thermal efficiency of simple gas turbines cycle is by using recuperation to recover some of the exhaust heat. Therefore, this study aims to introduce a new integrated approach to evaluate the techno-economic value of recuperator retrofit on existing industrial gas turbines. The original engines are designed for combined cycles so that the pressure ratios are moderate to secure suitable exhaust temperatures. The developed model is described and implemented for two gas turbines, and the obtained characteristics are evaluated against the actual data. This approach will help the users to select the suitable gas turbine models with favorable recuperator characteristics based on a technical and economic perspective. Besides, the performance results are used to select the optimum thermodynamic and geometrical characteristics of TEMA tubular heat exchanger so that the generated design alternatives are optimized using multi-decision process principle in order to ensure the highest techno-economic value. One of the unique features of the new method is that it depends only on the velocity of recuperator streams to derive the rest of the heat exchanger design and performance characteristics. Moreover, this paper includes a sensitivity study to investigate the effects of power setting, utilization factor and operation availability on the selected recuperator features.}, booktitle={Proceedings of the ASME Turbo Expo}, author={Singh, P. and Ekkad, S.}, year={2016} }
 @article{singh_ravi_ekkad_2016, title={Experimental and numerical study of heat transfer due to developing flow in a two-pass rib roughened square duct}, volume={102}, ISSN={0017-9310}, url={http://dx.doi.org/10.1016/j.ijheatmasstransfer.2016.07.015}, DOI={10.1016/j.ijheatmasstransfer.2016.07.015}, abstractNote={Experimental and numerical study of flow and heat transfer in a two-pass channel featuring different rib geometries has been carried out. The thermal hydraulic performance of four different rib geometries – 45° angled, V, W and M-shaped ribs, have been reported and analyzed. The tests were performed over a Reynolds number range from 19,500 to 69,000. The channel aspect ratio was 1:1 (square), the rib-pitch-to-rib-height ratio (p/e) was 16 and the rib-height-to-channel hydraulic diameter ratio (e/Dh) was 0.125. The detailed Nusselt number distributions on the ribbed wall were obtained using transient liquid crystal thermography. Numerical simulations (using ANSYS Fluent) have been carried out to resolve the complex flow field peculiar to rib turbulator shapes, for detailed understanding of the experimentally measured heat transfer coefficients. For the numerical simulations, realizable version of k-ε model was chosen because of its ability to predict separated flows behind ribs. Also, CFD simulations have been validated with experimentally obtained pressure measurements at several locations in the two pass channel. In addition to flow validation, the numerically obtained heat transfer results are validated and compared with the experiments and discussion has been presented on the role of secondary flows, turbulent kinetic energy etc., on heat transfer augmentation due to the presence of the ribs.}, journal={International Journal of Heat and Mass Transfer}, publisher={Elsevier BV}, author={Singh, Prashant and Ravi, Bharath Viswanath and Ekkad, Srinath V.}, year={2016}, month={Nov}, pages={1245–1256} }
 @inproceedings{singh_ravi_ekkad_2016, title={Experimental investigation of heat transfer augmentation by different jet impingement hole shapes under maximum crossflow}, volume={5B-2016}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84991628479&partnerID=MN8TOARS}, DOI={10.1115/GT2016-57874}, abstractNote={To achieve higher overall efficiency in gas turbine engines, hot gas path components are subjected to high heat transfer loads due to higher turbine inlet temperatures. Jet impingement has been extensively used especially as an internal cooling technique in the leading edge and mid-chord region of first stage vanes, which are subjected to highest heat loads. With the advent of additive manufacturing methods such as Direct Metal Laser Sintering (DMLS), designers are not limited to designing round or race track holes for impingement. The present study is focused on exploring new jet hole shapes, in an arrangement, typical of mid-chord region in a double wall cooling configuration. Transient liquid crystal experiments are carried out to study heat transfer augmentation by jet impingement on smooth target where the spent air is allowed to exit in one direction, thus imposing maximum crossflow condition. The averaged Reynolds number (based on jet hydraulic diameter) is varied from 2500 to 10000. The jet plate has a square array of jets with 7 jets in one row (total number of jets = 49), featuring hole shapes — Racetrack and V, where the baseline case is the round hole. The non-dimensional streamwise (x/dj) and spanwise (y/dj) spacing is 6 and the normalized jet-to-target-plate spacing (z/dj) is 4 and the nozzle aspect ratio (L/dj) is also 4. The criteria for the hole shape design was to keep the effective area of different hole shapes to be the same, which resulted in slightly different hydraulic diameters. The jet-to-target plate spacing (z) has been adjusted accordingly so as to maintain a uniform z/dj of 4, across all three configurations studied. Heat transfer coefficients are measured using a transient Liquid Crystal technique employing a one-dimensional semi-infinite model. Flow experiments are carried out to measure static pressures in the plenum chamber, to calculate the discharge coefficient, for a range of plenum absolute pressure-to-ambient pressure ratios. Detailed normalized Nusselt number contours have been presented, to identify the regions of high heat transfer augmentation locally, so as to help the designers in the organization of jet hole shapes and their patterns in an airfoil depending upon the active heat loads.}, booktitle={Proceedings of the ASME Turbo Expo}, author={Singh, P. and Ravi, B.V. and Ekkad, S.}, year={2016} }
 @article{ramesh_leblanc_narzary_ekkad_anne alvin_2017, title={Film Cooling Performance of Tripod Antivortex Injection Holes Over the Pressure and Suction Surfaces of a Nozzle Guide Vane}, volume={9}, ISSN={1948-5085 1948-5093}, url={http://dx.doi.org/10.1115/1.4035290}, DOI={10.1115/1.4035290}, abstractNote={Film cooling performance of the antivortex (AV) hole has been well documented for a flat plate. The goal of this study is to evaluate the same over an airfoil at three different locations: leading edge suction and pressure surface and midchord suction surface. The airfoil is a scaled up first stage vane from GE E3 engine and is mounted on a low-speed linear cascade wind tunnel. Steady-state infrared (IR) technique was employed to measure the adiabatic film cooling effectiveness. The study has been divided into two parts: the initial part focuses on the performance of the antivortex tripod hole compared to the cylindrical (CY) hole on the leading edge. Effects of blowing ratio (BR) and density ratio (DR) on the performance of cooling holes are studied here. Results show that the tripod hole clearly provides higher film cooling effectiveness than the baseline cylindrical hole case with overall reduced coolant usage on the both pressure and suction sides of the airfoil. The second part of the study focuses on evaluating the performance on the midchord suction surface. While the hole designs studied in the first part were retained as baseline cases, two additional geometries were also tested. These include cylindrical and tripod holes with shaped (SH) exits. Film cooling effectiveness was found at four different blowing ratios. Results show that the tripod holes with and without shaped exits provide much higher film effectiveness than cylindrical and slightly higher effectiveness than shaped exit holes using 50% lesser cooling air while operating at the same blowing ratios. Effectiveness values up to 0.2–0.25 are seen 40-hole diameters downstream for the tripod hole configurations, thus providing cooling in the important trailing edge portion of the airfoil.}, number={2}, journal={Journal of Thermal Science and Engineering Applications}, publisher={ASME International}, author={Ramesh, Sridharan and LeBlanc, Christopher and Narzary, Diganta and Ekkad, Srinath and Anne Alvin, Mary}, year={2017}, month={Jan} }
 @inproceedings{gomez-ramirez_kedukodi_gadiraju_ekkad_moon_kim_srinivasan_2016, title={Gas turbine combustor rig development and initial observations at cold and reacting flow conditions}, volume={5B-2016}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84991629023&partnerID=MN8TOARS}, DOI={10.1115/GT2016-57825}, abstractNote={The present paper describes the first phase of the design and development of a realistic, high-pressure, full-scale research gas turbine combustor at Virginia Tech. The final test rig will be capable of operating at inlet temperatures of 650 K, pressures up to 9.28 Bar (120 psig), maximum air inlet flow rates of 1.27 kg/s (2.8 lbm/s), and allow for variations in the geometry of the combustor model. The first phase consists of a low-pressure (atmospheric) optical combustor for heat transfer and flow-field measurements at isothermal and reacting conditions. The combustor model is equipped with an industrial low emission fuel injector from Solar Turbines Incorporated, used in their land based gas turbine Taurus-60. The primary objective of the developed rig is to provide additional insight into the heat transfer processes that occur within gas turbine combustors, primarily the convective component, which has not been characterized. A future phase of the test rig development will incorporate a pressure vessel that will allow for the operation of the combustor simulator at higher pressures. In the present publication, the design methodology and considerations, as well as the challenges encountered during the design of the first phase of the simulator are briefly discussed. An overview is given on the design of the instrumentation and process piping surrounding the test rig, including ASME codes followed as well as the instrumentation and equipment selected. A detailed description of the test section design is given, highlighting the design for high temperature operation.
               As an example of the capabilities of the rig, representative measurements are presented. Characterization of the isothermal flow field using planar Particle Image Velocimetry (PIV) at a Reynolds number of 50 000 was performed and compared with flame imaging data at the same inlet conditions operating at an equivalence ratio of 0.7. The data suggests that the flame location follows the maximum turbulent kinetic energy as measured in the isothermal field. Representative data from the computational efforts are also presented and compared with the experimental measurements. Future work will expand on both reacting and isothermal PIV and heat transfer measurements, as well as computational validations.}, booktitle={Proceedings of the ASME Turbo Expo}, author={Gomez-Ramirez, D. and Kedukodi, S. and Gadiraju, S. and Ekkad, S.V. and Moon, H.-K. and Kim, Y. and Srinivasan, R.}, year={2016} }
 @article{pandit_ekkad_huxtable_ma_2016, title={Heat  Transfer Optimization for Thermoelectric Power Generation with Automobile Waste Heat Recovery Applications}, volume={19}, ISSN={1049-0787}, url={http://dx.doi.org/10.1615/AnnualRevHeatTransfer.2016015353}, DOI={10.1615/AnnualRevHeatTransfer.2016015353}, abstractNote={This chapter describes the considerations in optimizing the heat exchanger performance of thermoelectric power generators (TEGs) for use in waste heat recovery from automotive exhaust gases. Thermoelectric devices have been in development since the early twentieth century with much of the research focus on improvement in thermoelectric material properties. However, the effective transfer of heat to and from the TEG also has a significant impact on the overall device performance. Strategies and efforts to improve heat transfer for automotive TEG systems are described in this chapter.}, number={1}, journal={Annual Review of Heat Transfer}, publisher={Begell House}, author={Pandit, Jaideep and Ekkad, Srinath V. and Huxtable, Scott T. and Ma, Ting}, year={2016}, pages={241–277} }
 @article{ravi_singh_ekkad_2017, title={Numerical investigation of turbulent flow and heat transfer in two-pass ribbed channels}, volume={112}, ISSN={1290-0729}, url={http://dx.doi.org/10.1016/J.IJTHERMALSCI.2016.09.034}, DOI={10.1016/J.IJTHERMALSCI.2016.09.034}, abstractNote={In this study, the heat transfer and friction characteristics of four different rib geometries- 45° angled, V-shaped, W-shaped and M-shaped ribs in a two-pass stationary channel have been numerically investigated. The aspect ratio (Height to Width) of the cooling channel was 1:1 (square). The rib pitch-to-rib height ratio (p/e) and the rib-height-to-channel hydraulic diameter ratio (e/Dh) were 16 and 0.125 respectively. The Reynolds number was varied from 20,000 to 70,000. For the computations, the Reynolds averaged Navier–Stokes (RANS) equations were solved with the commercial software ANSYS Fluent using the realizable version of k-ε (RKE) model. The heat transfer results were benchmarked with experiments on a test rig with similar geometries and flow conditions. Detailed analysis of the flow characteristics in the two-pass channel was carried out so as to understand the interaction of the rib-induced secondary flows and the bend-induced secondary flows and their contribution to heat transfer enhancement. The heat transfer enhancement provided by V-shaped ribs was 7% higher than 45° ribs, 28% higher than W-shaped ribs and 35% higher than M-shaped ribs. However, the pressure penalty for V-shaped ribs was 19% higher than 45° ribs, 24% higher than W-shaped ribs and 28% higher than M-shaped ribs. On comparing the overall thermal hydraulic performance, V-shaped and 45° ribs were observed to perform significantly better than W-shaped and M-shaped ribs.}, journal={International Journal of Thermal Sciences}, publisher={Elsevier BV}, author={Ravi, Bharath Viswanath and Singh, Prashant and Ekkad, Srinath V.}, year={2017}, month={Feb}, pages={31–43} }
 @article{ma_lu_pandit_ekkad_huxtable_deshpande_wang_2017, title={Numerical study on thermoelectric–hydraulic performance of a thermoelectric power generator with a plate-fin heat exchanger with longitudinal vortex generators}, volume={185}, ISSN={0306-2619}, url={http://dx.doi.org/10.1016/J.APENERGY.2016.01.078}, DOI={10.1016/J.APENERGY.2016.01.078}, abstractNote={In this paper, the effect of longitudinal vortex generators (LVGs) on the performance of a thermoelectric power generator (TEG) with a plate-fin heat exchanger is investigated. A fluid-thermal-electric multi-physics coupled model for the TEG is established on the COMSOL® platform, in which the Seebeck, Peltier, Thomson, and Joule heating effects are taken into account. The equivalent thermal–electrical properties of the thermoelectric (TE) module are used in the numerical simulation. The results indicate that the LVGs produce complex three-dimensional vortices in the cross section downstream from the LVGs, thus enhancing the heat transfer and electric performance compared to a TEG without LVGs. Under baseline operating conditions, the heat input and open circuit voltage of the TEG with LVGs are increased by 41–75% compared to a TEG with smooth channel. The simulations also show that the Reynolds number and hot-side inlet temperature have significant effects on the net power and thermal efficiency of the TEG, but the cold-side temperature has a smaller effect. Additionally, the performance of the TEG under a constant heat transfer coefficient boundary condition is almost the same as the performance under a constant temperature boundary condition. Overall, this work demonstrates that LVGs have great potential to enhance the performance of TEGs for waste heat recovery from vehicle exhaust.}, journal={Applied Energy}, publisher={Elsevier BV}, author={Ma, Ting and Lu, Xing and Pandit, Jaideep and Ekkad, Srinath V. and Huxtable, Scott T. and Deshpande, Samruddhi and Wang, Qiu-wang}, year={2017}, month={Jan}, pages={1343–1354} }
 @inproceedings{boulanger_patel_hutchinson_deshong_xu_ng_ekkad_2016, title={Preliminary experimental investigation of initial onset of sand deposition in the turbine section of gas turbines}, volume={1}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84991396783&partnerID=MN8TOARS}, DOI={10.1115/GT2016-56059}, abstractNote={Particle ingestion into modern gas turbine engines is known to reduce performance and may damage many primary gas path components through erosion or deposition mechanisms. Many studies have been conducted that evaluate the effects of particulate ingestion in primary and secondary gas path components. However, modern gas turbines have gas path temperatures that are above most previous studies. As a result, this study performed particle deposition experiments at the Virginia Tech Aerothermal Rig facility at engine representative temperatures. Arizona Test Dust of 20 to 40 μm was chosen to represent the particle ingested into rotorcraft turbine engines in desert and sandy environments. The experimental setup impinged air and sand particles on a flat Hastelloy X coupon. The gas and sand mixture impacted the coupon at varying angles measured between the gas flow direction and coupon face, hereby referred to as coupon angle. For this study, gas and sand particles maintained a constant flow velocity of about 70 m/s and a temperature of about 1100°C. The coupon angle was varied between 30° to 90° for all experiments. The experimental results indicate sand deposition increased linearly from about 975 °C to 1075 °C for all coupon angles. A multiple linear regression model is used to estimate the amount of deposition that will occur on the test coupon as a function of gas path temperature and coupon angle. The model is adequate in explaining about 67% of the deposition that occurs for the tests. The remaining percentage could be explained with other factors such as particle injection rates and exact surface temperature where the deposits occur.}, booktitle={Proceedings of the ASME Turbo Expo}, author={Boulanger, A. and Patel, H. and Hutchinson, J. and DeShong, W. and Xu, W. and Ng, W. and Ekkad, S.}, year={2016} }
 @article{yang_tyagi_ren_ekkad_jiang_2016, title={Sensitivity of different impingement structures to rotation}, volume={37}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84964686315&partnerID=MN8TOARS}, number={1}, journal={Kung Cheng Je Wu Li Hsueh Pao/Journal of Engineering Thermophysics}, author={Yang, L. and Tyagi, K. and Ren, J. and Ekkad, S. and Jiang, H.-D.}, year={2016}, pages={37–40} }
 @article{ekkad_han_2015, title={A REVIEW OF HOLE GEOMETRY AND COOLANT DENSITY EFFECT ON FILM COOLING}, volume={6}, ISSN={2151-8629}, url={http://dx.doi.org/10.5098/hmt.6.8}, DOI={10.5098/hmt.6.8}, abstractNote={A premiere free-access and peer-reviewed frontier journal site, serving the needs of the thermal-fluids community. See the latest research or submit an article. Quickly share your research with the global thermal-fluids community, for increased citations and impact at no cost.}, number={1}, journal={Frontiers in Heat and Mass Transfer}, publisher={Computers, Materials and Continua (Tech Science Press)}, author={Ekkad, Srinath V and Han, Je-Chin}, year={2015}, month={Jul} }
 @inproceedings{barboza_ma_lowe_ekkad_ng_2015, title={A diagnostic technique for particle characterization using laser light extinction}, volume={6}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84954305414&partnerID=MN8TOARS}, DOI={10.1115/GT2015-43347}, abstractNote={Increased operations of aircraft, both commercial and military in hostile desert environments has increased the risk of micro sized particle ingestion into engines. The probability of increased sand and dust ingestion results in increased life cycle costs in addition to increased potential for performance loss. Thus the ability to accurately assess the amount of inlet debris would be useful for engine diagnostics and prognostic evaluation. Previous engine monitoring studies were based on particle measurements performed a posteriori. Thus there exists a need for in situ quantification of ingested particles. This paper describes the initial development of a line-of-sight optical technique to characterize the ingested particles at concentrations similar to those experienced by aircraft in brownout conditions using laser extinction with the end goal of producing an onboard aircraft diagnostic sensor. By measuring the amount of light that is transmitted due to the effects of scattering and absorption in the presence of particles over a range of concentrations, a relationship between particle diameters and the laser light extinction was obtained. This relationship was then used to obtain information on diameters and number densities of ingested particles. The particle size range of interest was chosen to be between 1–10 μm and the size distribution function was assumed to be lognormal. Tests were performed on polystyrene latex spheres of sizes 1.32 μm, 3.9 μm and 5.1 μm in water dispersions to measure diameters and concentrations. Measurements were performed over multiple wavelengths to obtain information on the size distribution and number density of particles. Results of tests presented in this paper establish the validity of the laser extinction technique to provide real time information of ingested particles and will serve as an impetus to carry out further research using this technique to characterize particles.}, booktitle={Proceedings of the ASME Turbo Expo}, author={Barboza, K. and Ma, L. and Lowe, K.T. and Ekkad, S. and Ng, W.}, year={2015} }
 @article{ramesh_ramirez_ekkad_alvin_2016, title={Analysis of film cooling performance of advanced tripod hole geometries with and without manufacturing features}, volume={94}, ISSN={0017-9310}, url={http://dx.doi.org/10.1016/j.ijheatmasstransfer.2015.11.033}, DOI={10.1016/j.ijheatmasstransfer.2015.11.033}, abstractNote={The present study evaluates the film cooling performance of a set of manufacturable tripod hole designs, with and without shaped exits. The tripod holes with realistic manufacturing features included rounded corners at the hole inlet and outlet, as well as a webbing between the tripod holes. Standard cylindrical and shaped cylindrical (10° fan + laidback) holes were also studied for comparative analysis. Transient heat transfer experiments with a mainstream Red ≈ 3200 were conducted on a flat plate test rig. Different hole geometries were tested at equal mass flow rates, corresponding to a range of blowing ratios equal to 0.5, 1.0, and 2.0 for the cylindrical hole. IR (Infrared) thermography was used to evaluate adiabatic film cooling effectiveness, heat transfer coefficient, and the normalized heat flux on the flat surface. Results showed that the presence of rounded corners or webbing did not lower the performance of the tripod cooling holes. Both tripod hole geometries, with and without manufacturing features, yielded higher film cooling effectiveness compared to the cylindrical holes and slightly higher effectiveness than the shaped holes, while consuming 50% less coolant when operating at the same blowing ratio. The heat transfer coefficient measurements and the overall heat flux ratios further corroborated the thermal advantages of the tripod hole design over traditional cylindrical and shaped holes.}, journal={International Journal of Heat and Mass Transfer}, publisher={Elsevier BV}, author={Ramesh, Sridharan and Ramirez, David Gomez and Ekkad, Srinath V. and Alvin, Mary Anne}, year={2016}, month={Mar}, pages={9–19} }
 @inproceedings{gomez-ramirez_dilip_ravi_deshpande_pandit_ekkad_moon_kim_srinivasan_2015, title={Combustor heat shield impingement cooling and its effect on liner convective heat transfer for a model annular combustor with radial swirlers}, volume={5C}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84954349880&partnerID=MN8TOARS}, DOI={10.1115/GT2015-42868}, abstractNote={Increasing pressure to reduce pollutant emissions such as NOx and CO, while simultaneously increasing the efficiency of gas turbines, has led to the development of modern gas turbine combustors operating at lean equivalence ratios and high compression ratios. These modern combustors use a large portion of the compressor air in the combustion process and hence efficient use of cooling air is critical. Backside impingement cooling is one alternative for advanced cooling in gas turbine combustors. The dome of the combustor is a primary example where backside impingement cooling is extensively used. The dome directly interacts with the flame and hence represents a limiting factor for combustor durability. The present paper studies two aspects of dome cooling: the impingement heat transfer on the dome heat shield of an annular combustor and the effect of the outflow from the spent air on the liner heat transfer. A transient measurement technique using Thermochromic Liquid Crystals (TLCs) was used to characterize the convective heat transfer coefficient on the backside of an industrial heat shield design provided by Solar Turbines, Inc. for Reynolds numbers (with respect to the hole diameter) of ∼ 1500 and ∼ 2500. Reynolds-Averaged Navier Stokes (RANS) calculations using the k-ω SST turbulence model were found to be in good agreement with the experiment. A standard heat transfer correlation for impingement hole arrays overestimated the mean heat transfer coefficient compared to the experiment and computations, however this could be explained by low biases in the results.
               Steady state IR measurements were performed to study the effects that the spent air from the heat shield impingement cooling had on the liner convective heat transfer. Measurements were taken for three Reynolds numbers (with respect to the hydraulic diameter of the combustor annulus) including 50000, 90000, and 130000. A downstream shift in the flow features was observed due to the secondary flow introduced by the outflow, as well as a significant increase in the convective heat transfer close to the dome wall.}, booktitle={Proceedings of the ASME Turbo Expo}, author={Gomez-Ramirez, D. and Dilip, D. and Ravi, B.V. and Deshpande, S. and Pandit, J. and Ekkad, S.V. and Moon, H.-K. and Kim, Y. and Srinivasan, R.}, year={2015} }
 @article{delimont_murdock_ng_ekkad_2015, title={Effect of Temperature on Microparticle Rebound Characteristics at Constant Impact Velocity—Part I}, volume={137}, ISSN={0742-4795 1528-8919}, url={http://dx.doi.org/10.1115/1.4030312}, DOI={10.1115/1.4030312}, abstractNote={Many gas turbine engines operate in harsh environments where the engines ingest solid particles. Ingested particles accelerate the deterioration of engine components and reduce the engine's service life. Understanding particle impacts on materials used in gas turbines at representative engine conditions leads to improved designs for turbomachinery operating in particle-laden environments. Coefficient of restitution (COR) is a measure of particle/wall interaction and is used to study erosion and deposition. In this study, the effect of temperature (independent of velocity) on COR was investigated. Arizona road dust (ARD) of 20–40 μm size was injected into a flow field to measure the effects of temperature and velocity on particle rebound. Target coupon materials used were Stainless Steel 304 (SS304) and Hastelloy X (HX). Tests were performed at three different temperatures: 300 K (ambient), 873 K, and 1073 K. The velocity of the flow field was held constant at 28 m/s. The impingement angle of the bulk sand on the coupon was varied from 30 deg to 80 deg for each temperature tested. The COR was found to decrease substantially from the ambient case to the 873 K and 1073 K cases. The HX material exhibits a larger decrease in COR than the SS304 material. The results are also compared to previously published literatures. The decrease in COR is believed to be due to the changes in the surface of both materials due to oxide layer formation which occurs as the target material is heated.}, number={11}, journal={Journal of Engineering for Gas Turbines and Power}, publisher={ASME International}, author={Delimont, J. M. and Murdock, M. K. and Ng, W. F. and Ekkad, S. V.}, year={2015}, month={Nov} }
 @article{delimont_murdock_ng_ekkad_2015, title={Effect of Temperature on Microparticle Rebound Characteristics at Constant Impact Velocity—Part II}, volume={137}, ISSN={0742-4795 1528-8919}, url={http://dx.doi.org/10.1115/1.4030313}, DOI={10.1115/1.4030313}, abstractNote={When gas turbine engines operate in environments where the intake air has some concentration of particles, the engine will experience degradation. Very few studies of such microparticles approaching their melting temperatures are available in open literature. The coefficient of restitution (COR), a measure of the particles' impact characteristics, was measured in this study of microparticles using a particle tracking technique. Part II of this study presents data taken using the Virginia Tech Aerothermal Rig and Arizona road dust (ARD) of 20–40 μm size range. Data were taken at temperatures up to and including 1323 K, where significant deposition of the sand particles was observed. The velocity at which the particles impact the surface was held at a constant 70 m/s for all of the temperature cases. The target on which the particles impacted was made of a nickel alloy, Hastelloy X. The particle angle of impact was also varied between 30 deg and 80 deg. Deposition of particles was observed as some particles approach their glass transition point and became molten. Other particles, which do not become molten due to different particle composition, rebounded and maintained a relatively high COR. Images were taken using a microscope to examine the particle deposition that occurs at various angles. A rebound ratio was formulated to give a measure of the number of particles which deposited on the surface. The results show an increase in deposition as the temperature approaches the melting temperature of sand.}, number={11}, journal={Journal of Engineering for Gas Turbines and Power}, publisher={ASME International}, author={Delimont, J. M. and Murdock, M. K. and Ng, W. F. and Ekkad, S. V.}, year={2015}, month={Nov} }
 @inproceedings{kedukodi_ekkad_2015, title={Effect of downstream contraction on liner heat transfer in a gas turbine combustor swirl flow}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84964330043&partnerID=MN8TOARS}, DOI={10.1115/GTINDIA2015-1206}, abstractNote={Established numerical approaches for performing detailed flow analysis happens to be an effective tool for industry based applied research. In the present study, computations are performed on multiple gas turbine combustor geometries for turbulent, non-reactive and reactive swirling flow conditions for an industrial swirler. The purpose of this study is to identify the location of peak convective heat transfer along the combustor liner under swirling inlet flow conditions and to investigate the influence of combustor geometry on the flow field. Instead of modeling the actual swirler along with the combustor, an inlet swirl flow profile is applied at the inlet boundary based on previous literature. Initially, the computed results are validated against available experimental data for an inlet Reynolds number flow of 50000 using a 2D axi-symmetric flow domain for non-reacting conditions. A constant heat flux on the liner is applied for the study. Two turbulence models (RNG k-ε and k-ω SST) are utilized for the analysis based on its capability to simulate swirling flows. It is found that both models predict the peak liner heat transfer location similar to experiments. However, k-ε RNG model predicts heat transfer magnitude much closer to the experimental values except displaying an additional peak whereas k-ω model predicts only one peak but tends to over-predict in magnitude. Since the overall characteristic liner heat transfer trend is captured well by the latter one, it is chosen for future computations. A 3D sector (30°) model results also show similar trends as 2D studies. Simulations are then extended to 3 different combustors (Case 1: full cylinder and Case 2 and 3: cylinders with downstream contractions having reduced exit areas) by adopting the same methodology for same inlet flow conditions. Non-reacting simulations predict that the peak heat transfer location is marginally reduced by the downstream contraction of the combustor. However the peak location shifts towards downstream due to the presence of accelerated flow.
               Reacting flow simulations are performed with Flamelet Generation Manifold (FGM) model for simulating premixed combustion for the same inlet flow conditions as above. It is observed that Case 3 predicts a threefold increase in the exit flow velocity in comparison to non-reacting flow simulations. The liner heat transfer predictions show that both geometries predict similar peak temperatures. However, only one fourth of the initial liner length experiences peak temperature for Case 1 whereas the latter continues to feel the peak till the end. This behavior of Case 3 can be attributed to rapid convection of high temperature products downstream due to the prevailing accelerated flow.}, booktitle={ASME 2015 Gas Turbine India Conference, GTINDIA 2015}, author={Kedukodi, S. and Ekkad, S.}, year={2015} }
 @inproceedings{deshpande_ravi_pandit_ma_huxtable_ekkad_2015, title={Effect of longitudinal vortex generator location on thermoelectric-hydraulic performance of a single stage integrated thermoelectric power generator}, volume={8B-2015}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84974725398&partnerID=MN8TOARS}, DOI={10.1115/IMECE2015-52244}, abstractNote={Vortex generators have been widely used to enhance heat transfer in various heat exchangers. Out of the two types of vortex generators: Transverse vortex generators (TVGs) and longitudinal vortex generators (LVGs), LVGs have been found to show better heat transfer performance. Past studies have shown that the implementation of these LVGs can be used to improve heat transfer in thermoelectric generator systems. Here a built in module in COMSOL Multiphysics® was used to study the influence of the location of LVGs in the channel on the comprehensive performance of an integrated thermoelectric device (ITED). The physical model under consideration consists of a copper interconnector sandwiched between p-type and n-type semiconductors and a flow channel for hot fluid in the center of the interconnector. Four pairs of, LVGs are mounted symmetrically on the top and bottom surfaces of the flow channel. Thus, using numerical methods, the thermo-electric-hydraulic performance of the ITED with a single module is examined. By fixing the material size D, the fluid inlet temperature Tin, and attack angle β; the effects of the location of LVGs and Reynolds number were investigated on the heat transfer performance, power output, pressure drop and thermal conversion efficiency. The location of LVGs did not have significant effect on the performance of TEGs in the given model. However, the performance parameters show a considerable change with Reynold’s number and best performance is obtained at Reynold number of Re = 500.}, booktitle={ASME International Mechanical Engineering Congress and Exposition, Proceedings (IMECE)}, author={Deshpande, S. and Ravi, B.V. and Pandit, J. and Ma, T. and Huxtable, S. and Ekkad, S.}, year={2015} }
 @article{athavale_pandit_ekkad_huxtable_2015, title={Evaluation of Multilouvered-Fin-Based Heat Exchangers for Automobile Exhaust Energy Harvesting Systems}, volume={29}, ISSN={0887-8722 1533-6808}, url={http://dx.doi.org/10.2514/1.T4529}, DOI={10.2514/1.T4529}, abstractNote={Heat exchanger modules with thermoelectric generators are being used to harness energy from automobile exhaust flows. This paper focuses on increasing the hot-side heat transfer for improved performance of the thermoelectric generators using internal louvered fins. The flow and heat transfer behavior inside the exhaust-pipe test section are modeled using computational fluid dynamics. The multilouvered fins basically have multi-flat-plate behavior and enhance the heat transfer by deflecting the air from its original path and aligning it with the plane of the louvers. The heat transfer and pressure drop characteristics are compared with the baseline flow in a channel without louvered fins. Optimization of the louvered fin geometry is performed to determine the configuration that provides highest heat transfer while providing least pressure drop across the pipe length.}, number={4}, journal={Journal of Thermophysics and Heat Transfer}, publisher={American Institute of Aeronautics and Astronautics (AIAA)}, author={Athavale, Jayati and Pandit, Jaideep and Ekkad, Srinath V. and Huxtable, Scott T.}, year={2015}, month={Oct}, pages={785–794} }
 @inproceedings{tyagi_singh_pandit_ramesh_ekkad_tolpadi_2015, title={Experimental study of heat transfer augmentation in high aspectratio channels featuring different dimple configurations}, volume={5A}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84954162550&partnerID=MN8TOARS}, DOI={10.1115/GT2015-43621}, abstractNote={Transient liquid crystal technique was employed for experimental investigation of heat transfer distribution in high aspect ratio rectangular ducts featuring different dimple configurations. Four different dimple configuration shapes, viz. diamond, square, triangular and cylindrical, have been studied. This study also attempts to understand the dimple side wall heat transfer and correlate to the mechanisms of the flow inside the dimple. A brief numerical simulation study was also conducted to understand the vortex formation inside the dimples and how they affect the heat transfer on the dimple walls. Experiments were carried out at three different Reynolds number of 10,000, 16,000 and 21,000. Detailed normalized Nusselt number plots for different configurations has been presented. Pressure drop across each geometry was also recorded and a comparison of thermal–hydraulic performance is given. It was observed that heat transfer enhancement due to dimples was maximum for Re =10,000 case when compared to a flat plate scenario at the same Reynolds number. Thermal–hydraulic performance comparison showed that diamond and triangular dimples performed better due to lower pressure drop penalty.}, booktitle={Proceedings of the ASME Turbo Expo}, author={Tyagi, K. and Singh, P. and Pandit, J. and Ramesh, S. and Ekkad, S.V. and Tolpadi, A.}, year={2015} }
 @inproceedings{ravi_deshpande_ramesh_dhilipkumar_ekkad_2015, title={Film cooling performance of tripod holes on the Endwall upstream of a first stage nozzle guide vane}, volume={8B-2015}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84974738147&partnerID=MN8TOARS}, DOI={10.1115/IMECE2015-53133}, abstractNote={In view of the growing energy demand, there is an increasing need to augment the thermal efficiency of gas turbine engines. The thermal efficiency and power output of gas turbine engines increase with increasing overall pressure ratio which in turn leads to an increase in turbine inlet temperature. The maximum permissible turbine inlet temperature is limited by the material strength of the components of the gas turbine engines. In this regard, it is important to ensure that the endwalls of the first stage nozzle guide vane, which is one of the critical regions, are adequately cooled. The cooling of the endwall is of particular interest because the leading edge region along the endwall of the stator vane experiences high heat transfer rates resulting from formation of horseshoe vortices. In this paper, the performance of upstream purge slot has been compared against discrete film cooling holes. Three different cooling configurations — slot, cylindrical holes and tripod holes have been investigated by comparing the adiabatic film cooling effectiveness. Furthermore, the effect of coolant to mainstream mass flow ratio on the effectiveness of the different cooling schemes has also been studied. The steady-state experiments were conducted in a low speed, linear cascade wind tunnel. Spatially resolved temperature data was captured using infrared thermography technique to compute adiabatic film cooling effectiveness. Amongst the configurations studied, slot ejection offered the best cooling performance at all mass flow ratios. The performance of tripod ejection was comparable to slot ejection at mass flow ratios between 0.5 and 1.5, with the difference in laterally averaged effectiveness being ∼5%. However, at the highest mass flow ratio (MFR=2.5), the difference increased to ∼20%. Low effectiveness values were observed downstream of cylindrical ejection which could be attributed to jet lift-off.}, booktitle={ASME International Mechanical Engineering Congress and Exposition, Proceedings (IMECE)}, author={Ravi, B.V. and Deshpande, S. and Ramesh, S. and Dhilipkumar, P.D. and Ekkad, S.}, year={2015} }
 @inproceedings{yang_tyagi_ekkad_ren_2015, title={Influence of rotation on heat transfer in a two-pass channel with impingement under high reynolds number}, volume={5A}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84954121440&partnerID=MN8TOARS}, DOI={10.1115/GT2015-42871}, abstractNote={Effect of rotation on turbine blade internal cooling is an important factor in gas turbine cooling systems. In order to minimize the impact from the Coriolis force, cooling structures with less rotation-dependent cooling effectiveness are needed. This study presents an impingement design in a two pass channel to reduce impact of rotational forces on non-uniform heat transfer behavior inside these complex channels. A Transient Liquid Crystal(TLC) method was employed to obtain local heat transfer coefficient measurements in a rotating environment. The channel Reynolds number based on the channel diameter ranges from 25,000 to 100,000. The rotation number ranges from 0 to 0.14. A series of computational simulations with the SST model were also utilized to understand the flow field behavior that impacts the heat transfer distributions on the walls. A 1-D correlation of zone averaged Nusselt number distribution was derived from the measurements.
               Results show that rotation reduces the heat transfer on both sides of the impingement, which is due to the Coriolis force and the pressure redistribution. The local change in the present study is about 25%. Rotation significantly enhances the heat transfer near the closed end because of the centrifugal force and the ‘pumping’ effect. Within the parameters of this test, the magnitude of enhancement is 25% to 75%. Compared to U-bended two pass channel, impingement channel has advantages in the upstream channel and the end region, but performance is not beneficial on the leading side of the downstream channel.}, booktitle={Proceedings of the ASME Turbo Expo}, author={Yang, L. and Tyagi, K. and Ekkad, S. and Ren, J.}, year={2015} }
 @inproceedings{kedukodi_ekkad_moon_kim_srinivasan_2015, title={Numerical investigation of effect of geometry changes in a model combustor on swirl dominated flow and heat transfer}, volume={5C}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84954342211&partnerID=MN8TOARS}, DOI={10.1115/GT2015-43035}, abstractNote={Numerical computations are performed on three configurations of a model gas turbine combustor geometry for cold flow conditions. The purpose of this study is to understand the effect of changes to combustor passage section on the location of peak convective heat transfer along the combustor liner. A Reynolds Averaged Navier-Stokes equations based turbulence model is used for all the numerical computations. Simulations are performed on a 3D sector geometry. The first geometry is a straight cylindrical combustor section. The second model has an upstream diverging section before the cylindrical section. Third one has a converging section following the upstream cylindrical section. The inlet air flow has a Reynolds number of 50000 and a swirl number of 0.7. The combustor liner is subjected to a constant heat flux. Finally, liner heat transfer characteristics for the three geometries are compared. It is found that the peak liner heat transfer occurs far downstream of the combustor for full cylinder and downstream convergent cases compared to that in the upstream divergent case. This behavior may be attributed to the resultant pressure distribution due to the combustor passage area changes. Also the magnitude of peak liner heat transfer is reduced for the former two cases since the high turbulent kinetic energy regions within the combustor are oriented axially instead of expanding radially outward. As a consequence, the thermal load on the liner is found to reduce.}, booktitle={Proceedings of the ASME Turbo Expo}, author={Kedukodi, S. and Ekkad, S. and Moon, H.K. and Kim, Y. and Srinivasan, R.}, year={2015} }
 @inproceedings{gomez-ramirez_ekkad_lattimer_moon_kim_srinivasan_2015, title={Separation of radiative and convective wall heat fluxes using thermal infrared measurements applied to flame impingement}, volume={8A-2015}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84982903321&partnerID=MN8TOARS}, DOI={10.1115/IMECE2015-52322}, abstractNote={Flame impingement is critical for the processing and energy industries. The high heat transfer rates obtained with impinging flames are relevant in metal flame cutting, welding, and brazing; in fire research to understand the effects of flames on the structures of buildings; and in the design of high temperature combustion systems. Most of the studies on flame impingement are limited to surfaces perpendicular to the flame, and measurements are often performed using heat flux sensors (such as Schmidt-Boelter heat flux transducers) at discrete locations along the target surface. The use of in-situ probes provides high accuracy but heavily limits the spatial resolution of the measurement. Moreover, flame radiation effects are often neglected, due to the small contribution in non-luminous flames, and the entire heat flux to the target is assumed to be due to convection. Depending on the character of the flame and the impingement surface, local radiative heat transfer can be significant, and the contribution of radiation effects has not been fully quantified.
               This study presents a novel non-intrusive method with high spatial resolution to simultaneously determine the convective and radiative heat fluxes at a wall interacting with a flame or other high temperature environment. Two initial proof of concept experiments were conducted to evaluate the viability of the technique: one consisting of a flame impinging normal to a target and another with a flame parallel to the target surface. Application of the methodology to the former case yielded a stagnation convective heat flux in the order of 106kWm−2 that decreased radially away from the stagnation point. The radiation field for the direct impingement case accounted on average for 4.4% of the overall mean heat flux. The latter experiment exemplified a case with low convective heat fluxes, which was correctly predicted by the measurement. The radiative heat fluxes were consistent between the parallel and perpendicular cases.}, booktitle={ASME International Mechanical Engineering Congress and Exposition, Proceedings (IMECE)}, author={Gomez-Ramirez, D. and Ekkad, S.V. and Lattimer, B.Y. and Moon, H.-K. and Kim, Y. and Srinivasan, R.}, year={2015} }
 @article{ma_pandit_ekkad_huxtable_wang_2015, title={Simulation of thermoelectric-hydraulic performance of a thermoelectric power generator with longitudinal vortex generators}, volume={84}, ISSN={0360-5442}, url={http://dx.doi.org/10.1016/J.ENERGY.2015.03.033}, DOI={10.1016/J.ENERGY.2015.03.033}, abstractNote={This work investigates the feasibility of using LVGs (longitudinal vortex generators) to improve heat transfer in TEG (thermoelectric generator) systems. A coupled fluid-thermal-electric model is established with COMSOL Multiphysics® to study the effects of LVG height, LVG attack angle, and hot-side inlet gas temperature. We find that LVGs can significantly enhance the heat transfer performance, power output, and thermal conversion efficiency due to the generated longitudinal vortices, especially at small LVG attack angles. The performance of the thermoelectric generators with LVGs is best for LVGs that span the full height of the channel at the highest temperature examined (550 K), where the heat input, net power and thermal conversion efficiency are enhanced by 29%–38%, 90%–104% and 31%–36%, respectively, compared to smooth flow channel. As the hot-side inlet gas temperature decreases, the pumping power remains constant and requires a larger portion of the power output since the heat input and power output are significantly reduced. Therefore, it is not beneficial to use tall LVGs at lower hot-side inlet temperatures and higher inlet Reynolds numbers due to the large ratio of pressure drop to power output, but smaller LVGs are still useful under these conditions.}, journal={Energy}, publisher={Elsevier BV}, author={Ma, Ting and Pandit, Jaideep and Ekkad, Srinath V. and Huxtable, Scott T. and Wang, Qiuwang}, year={2015}, month={May}, pages={695–703} }
 @article{ma_pandit_ekkad_huxtable_deshpande_wang_2015, title={Study on Thermoelectric-hydraulic Performance of Longitudinal Vortex Generators in a Large-scale Thermoelectric Power Generator}, volume={75}, ISSN={1876-6102}, url={http://dx.doi.org/10.1016/J.EGYPRO.2015.07.475}, DOI={10.1016/J.EGYPRO.2015.07.475}, abstractNote={In this paper, the effect of longitudinal vortex generators (LVGs) on the performance of a large-scale thermoelectric power generator (TEG) with a plate-fin heat exchanger is investigated. The fluid-thermal-electric multi-physics coupled model for the TEG is established on the COMSOL® platform, in which the Seebeck, Peltier, Thomson, and Joule heating effects are taken into account. The equivalent thermal-electrical properties of the TE module are used in the numerical simulation. The results indicate that the LVGs could produce complex transverse vortices in the cross section downstream from the LVGs, thus enhancing the heat transfer and electric performances of the TEG compared with a TEG without LVGs.}, journal={Energy Procedia}, publisher={Elsevier BV}, author={Ma, Ting and Pandit, Jaideep and Ekkad, Srinath V. and Huxtable, Scott T. and Deshpande, Samruddhi and Wang, Qiuwang}, year={2015}, month={Aug}, pages={639–644} }
 @article{xu_wang_li_ekkad_ma_2015, title={Thermal-Hydraulic Performance of Different Discontinuous Fins Used in a Printed Circuit Heat Exchanger for Supercritical CO2}, volume={68}, ISSN={1040-7782 1521-0634}, url={http://dx.doi.org/10.1080/10407782.2015.1032028}, DOI={10.1080/10407782.2015.1032028}, abstractNote={In this study, four discontinuous fin configurations in parallel and staggered arrangements are investigated to classify their effects on the thermal-hydraulic performance of a printed circuited heat exchanger. Meanwhile, the formation mechanism of the flow resistance of supercritical CO2 is studied in an airfoil fin printed circuited heat exchanger. It shows that the fin configurations have little effect on the overall thermal-hydraulic performance when the mass flow rate of supercritical CO2 is low. The flow resistance dramatically increases during the heating process due to velocity increment caused by increased density, but is not significantly affected by the change in dynamic viscosity.}, number={10}, journal={Numerical Heat Transfer, Part A: Applications}, publisher={Informa UK Limited}, author={Xu, X. Y. and Wang, Q. W. and Li, L. and Ekkad, S. V. and Ma, T.}, year={2015}, month={Jun}, pages={1067–1086} }
 @article{xue_roy_ng_ekkad_2015, title={A Novel Transient Technique to Determine Recovery Temperature, Heat Transfer Coefficient, and Film Cooling Effectiveness Simultaneously in a Transonic Turbine Cascade}, volume={7}, ISSN={1948-5085 1948-5093}, url={http://dx.doi.org/10.1115/1.4029098}, DOI={10.1115/1.4029098}, abstractNote={The study presented in this article provides detailed description about a newly developed experimental technique to determine three key convective heat transfer parameters simultaneously in hot gas path of a modern high pressure turbine–recovery temperature (Tr), heat transfer coefficient (HTC), and adiabatic film cooling effectiveness (Eta). The proposed technique, dual linear regression technique (DLRT), has been developed based on the 1D semi-infinite transient conduction theory, is applicable toward film cooled heat transfer experiments especially under realistic engine environment conditions (high Reynolds number along with high Mach numbers). It addresses the fundamental three temperature problem by a two-test strategy. The current popular technique, curve fitting method (CFM) (Ekkad and Han, 2000, “A Transient Liquid Crystal Thermography Technique for Turbine Heat Transfer Measurements,” Meas. Sci. Technol., 11(7), pp. 957–968), which is widely used in the low speed wind tunnel experiments, is not competent in the transonic transient wind tunnel. The CFM (including schemes for both film cooled and nonfilm cooled experiments) does not provide recovery temperature on the film cooled surface. Instead, it assumes the recovery temperature equal to the mainstream total temperature. Its basic physics model simplifies the initial unsteady flow development within the data reduction period by assuming a step jump in mainstream pressure and temperature, which results in significant under prediction of HTC due to the gradual ramping of the flow Mach/Reynolds number and varying temperature in a transient, cascade wind tunnel facility. The proposed technique is advantageous due to the elimination of these added assumptions and including the effects of compressible flow physics at high speed flow. The detailed discussion on theory and development of the DLRT is followed by validation with analytical calculation and comparisons with the traditional technique by reducing the same set of experimental data. Results indicate that the proposed technique stands out with a higher accuracy and reliability.}, number={1}, journal={Journal of Thermal Science and Engineering Applications}, publisher={ASME International}, author={Xue, Song and Roy, Arnab and Ng, Wing F. and Ekkad, Srinath V.}, year={2015}, month={Mar} }
 @inproceedings{jain_roy_ng_ekkad_lohaus_taremi_2014, title={Aerodynamic performance of a transonic turbine blade passage in presence of upstream slot and mateface gap with endwall contouring}, volume={2C}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84922210927&partnerID=MN8TOARS}, DOI={10.1115/GT2014-26475}, abstractNote={The present article investigates mixed out aerodynamic loss coefficient measurements for a high turning, contoured endwall passage under transonic operating conditions in presence of upstream purge slot and mateface gap. The upstream purge slot represents the gap between stator-rotor interface and the mateface gap simulates the assembly feature between adjacent airfoils in an actual high pressure turbine stage. While the performance of the mateface and upstream slot has been studied for lower Mach number, no studies exist in literature for transonic flow conditions. Experiments were performed at the Virginia Tech’s linear, transonic blow down cascade facility. Measurements were carried out at design conditions (isentropic exit Mach number of 0.88, design incidence) without and with coolant blowing. Upstream leakage flow of 1.0% coolant to mainstream mass flow ratio (MFR) was considered with the presence of mateface gap. There was no coolant blowing through the mateface gap itself. Cascade exit pressure measurements were carried out using a 5-hole probe traverse at a plane 1.0-Cax downstream of the trailing edge. Spanwise measurements were performed to complete the entire 2D loss plane from endwall to midspan, which were used to plot pitchwise averaged losses for different span locations and loss contours for the passage. Results reveal significant reduction in aerodynamic losses using the contoured endwall due to the modification of flow physics compared to a non-contoured planar endwall. The heat transfer experiments, designed to find the heat transfer coefficient and the film cooling effectiveness are described in detail in a separate paper [1].}, booktitle={Proceedings of the ASME Turbo Expo}, author={Jain, S. and Roy, A. and Ng, W. and Ekkad, S. and Lohaus, A.S. and Taremi, F.}, year={2014} }
 @inproceedings{zhang_ma_zeng_ekkad_wang_2014, title={Direct-coupling simulation of thermal-hydraulic and stress analysis in a Cross-Wave primary surface heat exchanger}, volume={8A}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84926368064&partnerID=MN8TOARS}, DOI={10.1115/IMECE2014-37325}, abstractNote={Fluid flow with heat transfer in the Cross Wave (CW) primary surface channels may cause an external stress in the plate due to the non-uniform pressure on the plate surface and non-uniform temperature inside the plate. The plate construction can be deformed under this external stress, which will affect the flow of the fluid by changing the channel dimensions and thus affect the temperature fields. To solve the multi-physical field problem, a direct-coupling simulation method of thermal-hydraulic and stress analysis in a Crow-Wave (CW) primary surface heat exchanger is presented in this paper. The method is based on the commercial code STAR-CD and ABAQUS, and an in house procedure is added to accomplish the direct-coupling simulation in a transient process between thermal-hydraulic and stress modules. Due to the complicated simulation of fluid flow and heat transfer in a two-coupled CW fluid channels in which each channel is separated by the plate (original model) in the CFD (Computer Fluid Dynamics) procedure, an alternative CFD model that is different from the original model is constructed to enable the CFD analysis. The detailed geometry of the original mode is generated in the CAE (Computer Aided Engineering) procedure. A data transition program is developed to control and transform analysis data between the CFD model and the CAE model. The coupling relationship of surface pressure, temperature and the material stress in the CW primary surface heat exchanger is uncovered. The results indicates that the high thermal stress and large displacement about 2.26 × 103 MPa of stress and 8.28 × 10−2 mm of displacement are generated at the beginning of time steps. Therefore, more attention should be paid during the starting up and emergency stop process due to the excessive build of stress during the transient conditions.}, booktitle={ASME International Mechanical Engineering Congress and Exposition, Proceedings (IMECE)}, author={Zhang, J. and Ma, T. and Zeng, M. and Ekkad, S.V. and Wang, Q.W.}, year={2014} }
 @inproceedings{delimont_murdock_ng_ekkad_2014, title={Effect of near melting temperatures on microparticle sand rebound characteristics at constant impact velocity}, volume={1A}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84961342703&partnerID=MN8TOARS}, DOI={10.1115/GT2014-25686}, abstractNote={When gas turbine engines operate in environments where the intake air has some concentration of particles, the engine will experience degradation. Very few studies of microparticles at temperatures approaching the melting temperature of the particles are available in open literature. Coefficient of Restitution (COR), a measure of the particles’ impact characteristics, was measured for microparticles using a particle tracking technique. This study presents data taken using the Virginia Tech Aerothermal Rig and Arizona Road Dust (ARD) of 20–40μm size range. Data was taken at temperatures up to and including 1323 K, where significant deposition of the sand particles was observed. The velocity at which the particles impact the surface was held at a constant 70m/s for all of the temperature cases. The target on which the particles impacted was made of a nickel alloy, Hastelloy X. The particle angle of impact was also varied between 30° and 80°. The COR of the particles decreases slightly as some of the particles approach their glass transition point and start to become molten. Other particles, which do not become molten due to different particle composition, rebound and maintain a relatively high COR. Images were taken using a microscope to examine the particle deposition that occurs at various angles. A rebound ratio is formulated to give a measure of the number of particles which deposit on the surface. The results show an increase in deposition as the temperature approaches the melting temperature of sand.}, booktitle={Proceedings of the ASME Turbo Expo}, author={Delimont, J.M. and Murdock, M.K. and Ng, W.F. and Ekkad, S.V.}, year={2014} }
 @article{pandit_thompson_ekkad_huxtable_2014, title={Effect of pin fin to channel height ratio and pin fin geometry on heat transfer performance for flow in rectangular channels}, volume={77}, ISSN={0017-9310}, url={http://dx.doi.org/10.1016/j.ijheatmasstransfer.2014.05.030}, DOI={10.1016/j.ijheatmasstransfer.2014.05.030}, abstractNote={The efficiency of a thermoelectric generator (TEG) can be defined as the ratio of the power output to the heat input at the hot side of the device. This ratio is governed by the laws of thermodynamics and thus cannot exceed the Carnot efficiency. It follows that the greater the difference between the temperatures of the hot and cold sides of the device, the greater the efficiency and power output from the TEG. This study focuses on effective techniques to enhance heat transfer on the hot side of the TEG in order to increase the total power output from the device. In this study heat transfer enhancement mechanisms are evaluated for the hot side of a TEG system generating power from waste heat in automobile exhaust gases. The use of pin fins was examined, as they are a common and effective way to increase heat transfer in a channel. Heat transfer enhancement measurements are presented with 3-dimensional partial pin fin arrays of circular, triangular, hexagonal, and diamond shapes on the walls of a rectangular channel representing the hot side of the TEG system and the automobile exhaust duct. Channel heights are varied to measure the effect of the pin fin height to channel height ratio while keeping the pin fin height constant. Channel hydraulic diameter and configuration of the fins were chosen based on existing literature. Pin fin performance is studied over a range of Reynolds numbers, calculated based on full channel height. The pin fins with the best initial performance have been further analyzed by varying the channel height in order to change the pin fin to channel height ratio while keeping the hydraulic diameter to pin fin height ratio constant. The experiments use the transient liquid crystal method to measure detailed heat transfer coefficients on the test surface. Results show that the diamond pin fins perform the best in enhancing heat transfer. Lower channel heights that cause pin fins to block 50% of the channel provide significantly higher heat transfer coefficients.}, journal={International Journal of Heat and Mass Transfer}, publisher={Elsevier BV}, author={Pandit, Jaideep and Thompson, Megan and Ekkad, Srinath V. and Huxtable, Scott T.}, year={2014}, month={Oct}, pages={359–368} }
 @inproceedings{delimont_murdock_ng_ekkad_2014, title={Effect of temperature on microparticle rebound characteristics at constant impact velocity}, volume={1A}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84961300111&partnerID=MN8TOARS}, DOI={10.1115/GT2014-25687}, abstractNote={Many gas turbine engines operate in harsh environments where the engines ingest solid particles. Ingested particles accelerate the deterioration of engine components and reduce the engine’s service life. Understanding particle impacts on materials used in gas turbines at representative engine conditions leads to improved designs for turbomachinery operating in particle-laden environments. Coefficient of Restitution (COR) is a measure of particle/wall interaction and is used to study erosion and deposition. In the current study, the effect of temperature (independent of velocity) on COR was investigated. Arizona Road Dust (ARD) of 20–40/μm size was injected into a flow field to measure the effects of temperature and velocity on particle rebound. Target coupon materials used were 304 stainless steel and Hastelloy X. Tests were performed at three different temperatures, 300 K (ambient), 873 K, and 1073 K while the velocity of the flow field was held constant at 28 m/s. The impingement angle of the bulk sand on the coupon was varied from 30 ° to 80 ° for each temperature tested. The COR was found to decrease substantially from the ambient case to the 873 K and 1073 K cases. This decrease is believed to be due to the changes in the surface of both materials due to oxide layer formation which occurs as the target material is heated. The Hastelloy X material exhibits a larger decrease in COR than the stainless steel 304 material. The results are also compared to previously published literature.}, booktitle={Proceedings of the ASME Turbo Expo}, author={Delimont, J.M. and Murdock, M.K. and Ng, W.F. and Ekkad, S.V.}, year={2014} }
 @inproceedings{athavale_pandit_ekkad_huxtable_2014, title={Evaluation of herringbone wavy fin based heat exchanger for heat transfer enhancement in automobile exhaust energy harvesting systems}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85085407349&partnerID=MN8TOARS}, DOI={10.1615/ihtc15.hte.009825}, booktitle={Proceedings of the 15th International Heat Transfer Conference, IHTC 2014}, author={Athavale, J.D. and Pandit, J. and Ekkad, S.V. and Huxtable, S.T.}, year={2014} }
 @inproceedings{athavale_pandit_ekkad_huxtable_2014, title={Evaluation of multi-louvered fin based heat exchangers for use in automobile exhaust energy harvesting systems}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85088760741&partnerID=MN8TOARS}, DOI={10.2514/6.2014-0860}, booktitle={52nd AIAA Aerospace Sciences Meeting - AIAA Science and Technology Forum and Exposition, SciTech 2014}, author={Athavale, J. and Pandit, J. and Ekkad, S.V. and Huxtable, S.T.}, year={2014} }
 @inproceedings{ramesh_ekkad_straub_lawson_alvin_2014, title={Experimental and computational analysis of film cooling hole performance on a high temperature test rig}, volume={8B}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84926431961&partnerID=MN8TOARS}, DOI={10.1115/IMECE2014-38735}, abstractNote={Demand for more power from a gas turbine causes rotor inlet temperature to increase and in order to restrict the metal temperature in hot gas path components within a safe working limit, a better cooling system must be employed. This paper focuses on evaluating the performance of a film cooling hole in engine like conditions. Lab-scale experiments conducted prior to this study have established that tripod holes provide higher effectiveness compared to cylindrical and shaped holes while consuming only half the coolant. In spite of showing potential, it still has to yield superior cooling at engine like conditions. The high temperature test rig facility at NETL can raise the mainstream gas temperature as high as 1175 °C. Coolant temperature is adjusted to study film cooling performance at density ratio 2.8. This study presents results of baseline coupon testing. Metal coupons are made of Haynes230 alloy and are fabricated with cylindrical holes. Surface temperature is recorded using an IR thermographic camera which was calibrated using thermocouples, for different blowing ratios (0.5–2.0). Alongside experiment, a numerical model was also developed in an attempt to provide additional insight on the distribution of surface temperature and overall effectiveness downstream of cooling hole. It was observed that film cooling is effective at M=2.0 and 1.5 and this was associated with high inlet turbulence and swirling velocities disrupting the film cooling performance at lower blowing ratios. CFD predictions seemed to match better at M 1.0 but were found to deviate considerably at other blowing ratios.}, booktitle={ASME International Mechanical Engineering Congress and Exposition, Proceedings (IMECE)}, author={Ramesh, S. and Ekkad, S.V. and Straub, D.L. and Lawson, S.A. and Alvin, M.A.}, year={2014} }
 @inproceedings{ramirez_kumar_ekkad_tafti_kim_moon_srinivasan_2014, title={Flow field and liner heat transfer for a model annular combustor equipped with radial swirlers}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84913533935&partnerID=MN8TOARS}, DOI={10.2514/6.2014-3436}, abstractNote={Swirling flows for combustion stabilization, flame confinement, and proper fuel mixing and recirculation are prevalent in gas turbine combustor applications. Modern gas turbines use swirlers to induce strong rotating vortices and recirculation of the combustion gases to enhance combustion efficiency and stability. This study presents an experimental investigation of the flow field and wall heat transfer characteristics inside a model annular combustor equipped with radial swirlers. 2D Particle Image Velocimetry (2D-PIV) was used to characterize the flow field inside the combustor model. PIV measurements were taken for a single Reynolds number of 70000. To study the recirculation zone, data along the axial direction of the combustor were captured. The data show a slightly asymmetric flow, with the recirculation zone extending up to , where D is the hydraulic diameter of the entire annulus ( m). To study the evolution of the rotating vortex and the flow velocity close to the liner walls, PIV data was also captured at six cross-sections of the annular combustor. The vortex center was observed to be below the center of the swirler, consistent with the asymmetry observed in the axial measurements. Infrared (IR) thermography was used to measure the steady state heat transfer coefficients along the outer and inner liner walls for Reynolds numbers of , , and . The comparison between the heat transfer results for the different Reynolds numbers reveals a relatively constant position for the peak heat transfer at from the swirler exit for both walls. Computational Fluid Dynamics (CFD) calculations were also performed to better understand the characteristics of the flow inside the model combustor. The CFD model reproduces the experimental setup with a mesh of 25 million cells. A ReynoldsAveraged Navier-Stokes (RANS) model was used in the simulation, qualitatively reproducing the overall characteristics observed in the experiment.}, booktitle={50th AIAA/ASME/SAE/ASEE Joint Propulsion Conference 2014}, author={Ramirez, D.G. and Kumar, V. and Ekkad, S.V. and Tafti, D. and Kim, Y. and Moon, H.-K. and Srinivasan, R.}, year={2014} }
 @inproceedings{roy_jain_ekkad_ng_lohaus_crawford_2014, title={Heat transfer performance of a transonic turbine blade passage in presence of leakage flow through upstream slot and mateface gap with endwall contouring}, volume={5B}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84922368826&partnerID=MN8TOARS}, DOI={10.1115/GT2014-26476}, abstractNote={Comparison of heat transfer performance of a non-axisymmetric contoured endwall to a planar baseline endwall in presence of leakage flow through stator-rotor rim seal interface and mateface gap is reported in this paper. Heat transfer experiments were performed on a high turning (∼127°) turbine airfoil passage at Virginia Tech’s transonic blow down cascade facility under design conditions (exit isentropic Mach number 0.88 and 0° incidence) for two leakage flow configurations — 1) mateface blowing only, 2) simultaneous coolant injection from the upstream slot as well as mateface gap. Coolant to mainstream mass flow ratios (MFR) were 0.35% for mateface blowing only, whereas for combination blowing, a 1.0% MFR was chosen from upstream slot and 0.35% MFR from mateface. A common source of coolant supply to the upstream slot and mateface plenum made sure the coolant temperatures were identical at both upstream slot and mateface gap at the injection location. The contoured endwall geometry was generated to minimize secondary aerodynamic losses. Transient IR (Infrared) thermography technique was used to measure endwall surface temperature and a linear regression method was developed for simultaneous calculation of heat transfer coefficient (HTC) and adiabatic cooling effectiveness (ETA), assuming a 1D semi-infinite transient conduction. Results indicate reduction in local hot spot regions near suction side as well as area averaged HTC using the contoured endwall compared to baseline endwall for all coolant blowing cases. Contoured geometry also shows better coolant coverage profiles further along the passage. Detailed interpretation of the heat transfer results along with near endwall flow physics has also been discussed.}, booktitle={Proceedings of the ASME Turbo Expo}, author={Roy, A. and Jain, S. and Ekkad, S.V. and Ng, W.F. and Lohaus, A.S. and Crawford, M.E.}, year={2014} }
 @inbook{ekkad_2014, title={Impingement Cooling for Combustor Liner Backside Cooling}, booktitle={Impingement jet cooling in gas turbines}, publisher={WIT Press}, author={Ekkad, S.V.}, editor={Amano, Ryo and Sunden, BengtEditors}, year={2014} }
 @misc{ekkad_2014, title={Internal Cooling}, url={http://dx.doi.org/10.2514/5.9781624102660.0189.0222}, DOI={10.2514/5.9781624102660.0189.0222}, journal={Turbine Aerodynamics, Heat Transfer, Materials, and Mechanics}, publisher={American Institute of Aeronautics and Astronautics, Inc.}, author={Ekkad, Srinath V.}, year={2014}, month={Jan}, pages={189–222} }
 @inproceedings{alvin_klinger_mcmordie_chyu_siw_miller_reddy_gleeson_anderson_heidloff_et al._2014, title={Netl research efforts on development and integration of advanced material systems and airfoil cooling configurations for future land-based gas turbine engines}, volume={6}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84922817151&partnerID=MN8TOARS}, DOI={10.1115/GT2014-25640}, abstractNote={As future land-based gas turbine engines are being designed to operate with inlet temperatures exceeding 1300°C (2370°F), efforts at NETL have been focused on developing advanced materials systems that are integrated with novel airfoil cooling architectures. Recent achievements in the areas of low cost diffusion bond coat systems applied to single- and poly-crystalline nickel-based superalloys, as well as development of thin nickel-based oxide dispersion strengthened layers are presented in this paper. Integration of these material systems with commercially cast, novel, pin-fin internal cooling airfoil arrays, tripod film cooling hole architectures, trailing edge cooling geometries, and near surface micro-channel concepts is also presented.}, booktitle={Proceedings of the ASME Turbo Expo}, author={Alvin, M.A. and Klinger, J. and McMordie, B. and Chyu, M. and Siw, S. and Miller, N. and Reddy, B.V.K. and Gleeson, B. and Anderson, I. and Heidloff, A. and et al.}, year={2014} }
 @article{singh_tafti_reagle_delimont_ng_ekkad_2014, title={Sand transport in a two pass internal cooling duct with rib turbulators}, volume={46}, ISSN={0142-727X}, url={http://dx.doi.org/10.1016/j.ijheatfluidflow.2014.01.006}, DOI={10.1016/j.ijheatfluidflow.2014.01.006}, abstractNote={Jet engines often operate under dirty conditions where large amounts of particulate matter can be ingested, especially, sand, ash and dirt. Particulate matter in different engine components can lead to degradation in performance. The focus of this study is to investigate the sand transport and deposition in the internal cooling passages of turbine blades. A two pass stationary square duct with rib turbulators subjected to sand ingestion is studied using Large Eddy Simulations (LES). Each pass has ribs on two opposite walls and aligned normal to the main flow direction. The rib pitch to rib height (P/e) is 9.28, the rib height to channel hydraulic diameter (e/Dh) is 0.0625 and calculations have been carried out for a bulk Reynolds number of 25,000. Particle sizes in the range 0.5–25 μm are considered, with the same size distribution as found in Arizona Road Dust (medium). Large Eddy Simulation (LES) with a wall-model is used to model the flow and sand particles are modeled using a discrete Lagrangian framework. Results quantify the distribution of particle impingement density on all surfaces. Highest particle impingement density is found in the first quarter section of the second pass after the 180° turn, where the recorded impingement is more than twice that of any other region. It is also found that the average particle impingement per pitch is 28% higher in the second pass than the first pass. Results show lower particle tendency to impact the region immediately behind the rib in the first pass compared to the second pass where particle impingement is more uniform in the region between two ribs. The rib face facing the flow is by far is the most susceptible to impingement and hence deposition and erosion. The results of this simulation are compared to experiments conducted on an identical two pass geometry with Arizona Road Dust particles. The numerical predictions showed good qualitative agreement with experimental measurements. These results identify the damage prone areas in the internal cooling passages of a turbine blade under the influence of sand ingestion. This information can help modify the geometry of the blade or location of film cooling holes to avoid hole blockage and degradation of heat transfer at the walls.}, journal={International Journal of Heat and Fluid Flow}, publisher={Elsevier BV}, author={Singh, Sukhjinder and Tafti, Danesh and Reagle, Colin and Delimont, Jacob and Ng, Wing and Ekkad, Srinath}, year={2014}, month={Apr}, pages={158–167} }
 @article{phinney_ekkad_2013, title={75th Anniversary Special Issues}, volume={5}, ISSN={1948-5085 1948-5093}, url={http://dx.doi.org/10.1115/1.4023582}, DOI={10.1115/1.4023582}, abstractNote={The celebration of the 75th Anniversary of the establishment of the Heat Transfer Division of the American Society of Mechanical Engineers has been recognized in the dedicated June 2013 issues of the Journal of Heat Transfer and the Journal of Thermal Science and Engineering Applications.In these two issues, historical events of the Heat Transfer Division are reviewed and advances in heat transfer are discussed.These two special issues demonstrate the diversity of activity within the heat transfer community with invited articles from specialists in the field.The articles include analytical, numerical, and experimental developments in conduction, convection (including multiphase heat transfer), and radiation.The Journal of Heat Transfer articles focus on fundaments of heat and mass transfer science; whereas, papers in the Journal of Thermal Science and Engineering Applications present applications of heat and mass transfer.}, number={2}, journal={Journal of Thermal Science and Engineering Applications}, publisher={ASME International}, author={Phinney, L.M. and Ekkad, S.V.}, year={2013}, month={May}, pages={020301} }
 @inproceedings{ekkad_han_2013, title={A review of hole geometry and coolant density effect on film cooling}, volume={3}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84893010995&partnerID=MN8TOARS}, DOI={10.1115/HT2013-17250}, abstractNote={Improved film cooling hole geometries and effect of coolant density on film cooling have been a focus since the 1970s. One of the first studies on modifying hole exit to improve film cooling effectiveness and quantifying coolant density effect was from Prof. Goldstein’s group [1]. This paper traces the development and implementation of hole exit geometries as well as coolant density study from this landmark paper and its impact on future studies of advanced hole geometries under realistic engine-like coolant to mainstream density ratio conditions. This work is not intended to be a comprehensive review of the literature. It is aimed at providing an overview of the influence of ref. [1] over the past 4 decades.}, booktitle={ASME 2013 Heat Transfer Summer Conf. Collocated with the ASME 2013 7th Int. Conf. on Energy Sustainability and the ASME 2013 11th Int. Conf. on Fuel Cell Science, Engineering and Technology, HT 2013}, author={Ekkad, S. and Han, J.-C.}, year={2013} }
 @inproceedings{liu_agarwal_lattimer_ekkad_2013, title={Biomass/coal co-pyrolysis and co-combustion characterization}, volume={3}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84943399074&partnerID=MN8TOARS}, booktitle={8th US National Combustion Meeting 2013}, author={Liu, G. and Agarwal, G. and Lattimer, B. and Ekkad, S.}, year={2013}, pages={2584–2591} }
 @article{carmack_ekkad_kim_moon_srinivasan_2013, title={Comparison of Flow and Heat Transfer Distributions in a Can Combustor for Radial and Axial Swirlers Under Cold Flow Conditions}, volume={5}, ISSN={1948-5085 1948-5093}, url={http://dx.doi.org/10.1115/1.4023890}, DOI={10.1115/1.4023890}, abstractNote={A comparison study between axial and radial swirler performance in a gas turbine can combustor was conducted by investigating the correlation between combustor flow field geometry and convective heat transfer at cold flow conditions for Reynolds numbers of 50,000 and 80,000. Flow velocities were measured using particle image velocimetry (PIV) along the center axial plane and radial cross sections of the flow. It was observed that both swirlers produced a strong rotating flow with a reverse flow core. The axial swirler induced larger recirculation zones at both the backside wall and the central area as the flow exits the swirler, and created a much more uniform rotational velocity distribution. The radial swirler however, produced greater rotational velocity as well as a thicker and higher velocity reverse flow core. Wall heat transfer and temperature measurements were also taken. Peak heat transfer regions directly correspond to the location of the flow as it exits each swirler and impinges on the combustor liner wall.}, number={3}, journal={Journal of Thermal Science and Engineering Applications}, publisher={ASME International}, author={Carmack, Andrew and Ekkad, Srinath and Kim, Yong and Moon, Hee-Koo and Srinivasan, Ram}, year={2013}, month={Jul} }
 @inproceedings{xue_roy_ng_ekkad_2013, title={Comparison of data processing techniques for convective heat transfer measurements in a transient transonic hot wind tunnel}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84896611165&partnerID=MN8TOARS}, DOI={10.1115/GTINDIA2013-3733}, abstractNote={The present study compares two data processing techniques of calculating convective heat transfer coefficient (HTC) for experiments carried out at the Virginia Tech transonic wind tunnel facility. The discussion starts from the physical models and basic assumptions of the two methods denoted as Curve Fitting and Linear Regression, and is followed by application of both methods with the same set of experimental data for a comparative analysis. The linear regression method is found to be superior to the curve fitting method in eliminating the errors caused due to the transient starting up effect of the hot blow down.}, booktitle={ASME 2013 Gas Turbine India Conference, GTINDIA 2013}, author={Xue, S. and Roy, A. and Ng, W.F. and Ekkad, S.V.}, year={2013} }
 @article{lamont_ekkad_anne alvin_2013, title={Effect of Rotation on Detailed Heat Transfer Distribution for Various Rib Geometries in Developing Channel Flow}, volume={136}, ISSN={0022-1481 1528-8943}, url={http://dx.doi.org/10.1115/1.4025211}, DOI={10.1115/1.4025211}, abstractNote={The effects of Coriolis force and centrifugal buoyancy have a significant impact on heat transfer behavior inside rotating internal serpentine coolant channels for turbine blades. Due to the complexity of added rotation inside such channels, detailed knowledge of the heat transfer will greatly enhance the blade designer's ability to predict hot spots so coolant may be distributed more effectively. The effects of high rotation numbers are investigated on the heat transfer distributions for different rib types in near entrance and entrance region of the channels. It is important to determine the actual enhancement derived from turbulating channel entrances where heat transfer is already high due to entrance effects and boundary layer growth. A transient liquid crystal technique is used to measure detailed heat transfer coefficients (htc) for a rotating, short length, radially outward coolant channel with rib turbulators. Different rib types such as 90 deg, W, and M-shaped ribs are used to roughen the walls to enhance heat transfer. The channel Reynolds number is held constant at 12,000 while the rotation number is increased up to 0.5. Results show that in the near entrance region, the high performance W and M-shaped ribs are just as effective as the simple 90 deg ribs in enhancing heat transfer. The entrance effect in the developing region causes significantly high baseline heat transfer coefficients thus reducing the effective of the ribs to further enhance heat transfer. Rotation causes increase in heat transfer on the trailing side, while the leading side remains relatively constant limiting the decrement in leading side heat transfer. For all rotational cases, the W and M-shaped ribs show significant effect of rotation with large differences between leading and trailing side heat transfer.}, number={1}, journal={Journal of Heat Transfer}, publisher={ASME International}, author={Lamont, Justin A. and Ekkad, Srinath V. and Anne Alvin, Mary}, year={2013}, month={Oct} }
 @inproceedings{roy_blot_ekkad_ng_lohaus_crawford_2013, title={Effect of endwall contouring in presence of upstream leakage flow in a transonic turbine blade passage: Heat transfer measurements}, volume={1 PartF}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85071590348&partnerID=MN8TOARS}, DOI={10.2514/6.2013-3635}, abstractNote={Described in this paper is the design of and initial ground test results from a small spacecraft propulsion system. The system consists of a 25 N PMMA and nitrous oxide main hybrid thruster and multiple 1 N ethylene and nitrous oxide bipropellant reaction control thrusters (although only a single reaction control thruster has been tested). The primary objective of the propulsion system development project is to develop an inexpensive system that uses environmentally benign propellants and has performance comparable to the hydrazine and nitrogen tetraoxide bipropellants systems commonly used. Ground test results (using a non flight-weight configuration) show that reliable ignition and efficient and stable combustion have been achieved in the reaction control thruster. Ground testing of the main thruster is underway but is not complete at the point in time.}, booktitle={49th AIAA/ASME/SAE/ASEE Joint Propulsion Conference}, author={Roy, A. and Blot, D. and Ekkad, S. and Ng, W.F. and Lohaus, A.S. and Crawford, M.E.}, year={2013} }
 @inproceedings{leblanc_ramesh_ekkad_alvin_2013, title={Effect of hole exit shaping on film cooling performance for tripod hole injection over a flat surface}, volume={3}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84890154225&partnerID=MN8TOARS}, DOI={10.1115/GT2013-94456}, abstractNote={The effect of hole exit shaping on both heat transfer coefficient and film cooling effectiveness of tripod injection holes is examined experimentally on a flat plate. Previously, it has been clearly proven that tripod hole configurations provide at least 50–60% more cooling effectiveness while using 50% less coolant than standard cylindrical and shaped hole exit geometries. Temperature data is collected using infrared thermography at different operating conditions to determine the benefit of shaping the hole exits for an already proven tripod hole configuration. The test rig consists of a rectangular test section with a main stream flow at 7.9 m/s and coolant flow injected through the bottom surface through the film cooling injection holes. A unique transient IR technique has been used to determine both the adiabatic film effectiveness and heat transfer coefficient from a single test. Two different exit shaping have been considered, one with a 5° flare and layback and one with a 10° flare and layback. Results show that exit shaping improves the performance of these tripod holes compared to the cylindrical hole exits. The 10° flare and layback exit performs slightly better than the 5° flare and layback exit.}, booktitle={Proceedings of the ASME Turbo Expo}, author={LeBlanc, C.N. and Ramesh, S. and Ekkad, S.V. and Alvin, M.A.}, year={2013} }
 @inproceedings{blot_roy_ekkad_ng_lohaus_crawford_2013, title={Effect of upstream purge slot on a transonic turbine blade passage: Part 1 - Aerodynamic performance}, volume={3}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84890181493&partnerID=MN8TOARS}, DOI={10.1115/GT2013-94591}, abstractNote={In this paper, detailed experimental results of total pressure loss and secondary flow field are presented for a high turning (127°) airfoil passage in presence of an upstream purge slot (with and without coolant injection). The experiments were performed at Virginia Tech’s quasi 2D linear turbine cascade operating at transonic conditions. Measurements were made at design exit Mach number 0.88 and design incidence angle. The selected coolant to mainstream mass flow ratio (MFR) was set at 1.0%. In order to match engine representative inlet/exit blade loading, a diverging endwall was utilized where the span increased from the inlet to the exit at a 13 degree angle. A 5-hole probe traverse was used to measure exit total pressure. Pressure loss coefficients were calculated both along pitchwise and spanwise directions at 0.1 axial chord downstream of the blade trailing edge. CFD studies were conducted to compliment the experimental results. The backward facing step present with the upstream slot affects the approaching boundary layer and influences the passage horse-shoe vortex strength. The addition of coolant from the purge slot further increased the aerodynamic losses. However, the backward facing step of the upstream slot seems to be the predominant factor in affecting pressure losses when compared to with or without blowing cases. These results provide further understanding of the passage secondary flow characteristics and aid towards improved design of endwall passages. The heat transfer experiments, designed to find the heat transfer coefficient and the film cooling effectiveness are described in detail in part II of this paper [1].}, booktitle={Proceedings of the ASME Turbo Expo}, author={Blot, D.M. and Roy, A. and Ekkad, S.V. and Ng, W. and Lohaus, A.S. and Crawford, M.E.}, year={2013} }
 @inproceedings{roy_blot_ekkad_ng_lohaus_crawford_2013, title={Effect of upstream purge slot on a transonic turbine blade passage: Part 2 - Heat transfer performance}, volume={3}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84890218479&partnerID=MN8TOARS}, DOI={10.1115/GT2013-94581}, abstractNote={Heat transfer experiments with and without upstream purge cooling were carried out for a high turning (127°) airfoil passage in the presence of an upstream purge slot in a transonic linear cascade. Different coolant to mainstream mass flow ratios (MFR) were investigated at cascade design exit Mach number (0.88) and design incidence angle. The experiments were performed at Virginia Tech’s transient transonic blow down facility. A transient Infrared thermography technique was employed to measure the endwall surface temperature. Heat transfer coefficient (HTC) and film cooling effectiveness (ETA) were calculated from measured temperature assuming a 1-D semi-infinite transient conduction through a solid with convective boundary condition. In this experiment, the blade span increases in the mainstream flow direction in order to match realistic inlet/exit airfoil surface Mach number distribution. Results indicate strong interactions between coolant flow and cross passage secondary flow where significant coolant coverage is observed at higher leakage flow rates through the purge slot. The backward facing step created by the purge slot seems to be the driving factor on influencing endwall HTC compared to with or without blowing cases. Three-dimensional viscous CFD has also been performed for further insight of flow characteristics and to support experimental data. Aerodynamic measurements at cascade exit plane are provided in the companion paper GT2013-94951 [1], “Effect of upstream purge slot on a transonic turbine blade passage: Part – 1 – Aerodynamic performance”.}, booktitle={Proceedings of the ASME Turbo Expo}, author={Roy, A. and Blot, D.M. and Ekkad, S.V. and Ng, W.F. and Lohaus, A.S. and Crawford, M.E.}, year={2013} }
 @inproceedings{gomez-ramirez_srinivasan_ramesh_miranda_ekkad_alvin_2013, title={Experimental and computational evaluation of flow characteristics for advanced film cooling hole geometries}, volume={3}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84892957601&partnerID=MN8TOARS}, DOI={10.1115/HT2013-17463}, abstractNote={Film cooling is crucial in the field of gas turbines to protect the blade surfaces from the hot combustion gases. Several hole geometries have been studied in the past in an effort to optimize the cooling effectiveness of the holes while maintaining the structural integrity of the blade and low manufacturing costs. To understand the cooling effectiveness of the various hole geometries, the flow structures that develop as the coolant jet interacts with the hot mainstream must be understood. The present paper compares the results obtained from 2D Particle Image Velocimetry (PIV) measurements with CFD predictions using standard Reynolds-Averaged Navier Stokes (RANS) models with a commercially available code. The study is conducted for flat plate film cooling via conventional cylindrical holes, shaped holes (10° flare/laidback), and a tripod anti-vortex hole (AV) design. A constant blowing ratio (BR) of 0.5 was used for all the experiments, except for an additional measurement for the AV design at a BR of 1.0. Computational fluid dynamic (CFD) calculations were made with a standard k-epsilon model and compared to PIV results. The results show the counter-rotating vortices developing for cylindrical and shaped holes up to 5D and 3D respectively from the hole exit. AV holes showed no vortex formation, further supporting its higher cooling performance. Moreover, the present results indicate no separation of the coolant jet for AV or shaped holes as expected, while cylindrical holes displayed a small separation with a vertical extent of ∼0.1D. The CFD model was able to capture the main structures of the flow, but further efforts will concentrate in improving the representation of the flow normal to the flat plate surface.}, booktitle={ASME 2013 Heat Transfer Summer Conf. Collocated with the ASME 2013 7th Int. Conf. on Energy Sustainability and the ASME 2013 11th Int. Conf. on Fuel Cell Science, Engineering and Technology, HT 2013}, author={Gomez-Ramirez, D. and Srinivasan, S. and Ramesh, S. and Miranda, M. and Ekkad, S.V. and Alvin, M.A.}, year={2013} }
 @inproceedings{pandit_thompson_ekkad_huxtable_2013, title={Experimental investigation of heat transfer across a thermoelectric generator for waste heat recovery from automobile exhaust}, volume={1}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84892966336&partnerID=MN8TOARS}, DOI={10.1115/HT2013-17438}, abstractNote={The study investigates the temperature gradients achieved across a thermoelectric generator by using the exhaust gases from a vehicle as a heat source and the radiator coolant as the cold sink. Various heat transfer enhancement features are employed in order to achieve as high a temperature gradient as possible. Effect of flow Reynolds numbers and inlet temperatures are examined to create a body of data predicting total power output from the TEG. Data is normalized against results from baseline heat exchanger designs investigated in the past. The experiments are carried out at 1/5th scale of the previously examined geometry. Impingement geometry is employed on the coolant side to enhance heat transfer. The experimental test sections are fabricated using metal 3D printing. Water is used instead of radiator coolant and heated air is used for exhaust gases. The results from the experiments provide valuable data which can be used for system level optimization.}, booktitle={ASME 2013 Heat Transfer Summer Conf. Collocated with the ASME 2013 7th Int. Conf. on Energy Sustainability and the ASME 2013 11th Int. Conf. on Fuel Cell Science, Engineering and Technology, HT 2013}, author={Pandit, J. and Thompson, M. and Ekkad, S.V. and Huxtable, S.}, year={2013} }
 @article{leblanc_narzary_ekkad_2013, title={Film-Cooling Performance of Antivortex Hole on a Flat Plate}, volume={135}, ISSN={0889-504X 1528-8900}, url={http://dx.doi.org/10.1115/1.4023436}, DOI={10.1115/1.4023436}, abstractNote={Improved film cooling performance and coolant flow usage have a significant effect on overall engine performance. In the current study, film cooling performance of an improved antivortex or tripod hole geometry is evaluated on a flat plate surface with steady-state IR (infrared thermography) technique and compared to traditional baseline geometry. The baseline geometry is a simple cylindrical hole design inclined at 30 deg from the surface with pitch-to-diameter ratio of 3.0. The proposed improvement is a tripod design where the two side holes, also of the same diameter, branch out from the root of the main hole at 15 deg angle with a larger pitch-to-diameter ratio of 6.0 between the main holes. The third geometry studied is the same tripod design embedded in a trench to enhance two-dimensional film performance. The mainstream Reynolds number is 3110 based on the coolant hole inlet diameter. Two secondary fluids, air and carbon dioxide, were used to study the effects of coolant-to-mainstream density ratio (DR = 0.95 and 1.45) on film cooling effectiveness. Several blowing ratios in the range 0.5–4.0 were investigated independently at the two density ratios. Results indicate significant improvement in effectiveness with the tripod holes compared to cylindrical holes at all the blowing ratios studied. The trenched design shows improved effectiveness in the trench region and reduced effectiveness in the downstream region. At any given blowing ratio, the tripod hole designs use 50% less coolant and provide at least 30%–40% overall averaged higher cooling effectiveness. The use of relatively dense secondary fluid improves effectiveness immediately downstream of the antivortex holes but leads to poor performance downstream.}, number={6}, journal={Journal of Turbomachinery}, publisher={ASME International}, author={LeBlanc, Christopher and Narzary, Diganta P. and Ekkad, Srinath}, year={2013}, month={Sep} }
 @article{phinney_ekkad_2013, title={Foreword}, volume={135}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84878309195&partnerID=MN8TOARS}, DOI={10.1115/1.4023545}, number={6}, journal={Journal of Heat Transfer}, author={Phinney, L.M. and Ekkad, S.}, year={2013} }
 @article{lamont_ramesh_ekkad_tolpadi_kaminski_salamah_2013, title={Heat Transfer Enhancement in Narrow Diverging Channels}, volume={135}, ISSN={0889-504X 1528-8900}, url={http://dx.doi.org/10.1115/1.4007740}, DOI={10.1115/1.4007740}, abstractNote={Detailed heat transfer coefficient distributions have been obtained for narrow diverging channels with and without enhancement features. The cooling configurations considered include rib turbulators and concavities (or dimples) on the main heat transfer surfaces. All of the measurements are presented at a representative Reynolds number of 28,000. Pressure drop measurements for the overall channel are also presented to evaluate the heat transfer enhancement geometry with respect to the pumping power requirements. The test models were studied for wall heat transfer coefficient measurements using the transient liquid crystal technique. The model wall inner surfaces were sprayed with thermochromic liquid crystals and a transient test was used to obtain the local heat transfer coefficients from the measured color change. An analysis of the results shows that the choice of designs is limited by the available pressure drop, even if the design provides significantly higher heat transfer coefficients. Dimpled surfaces provide appreciably high heat transfer coefficients and a reasonable pressure drop, whereas ribbed ducts provide significantly higher heat transfer coefficients and a higher overall pressure drop.}, number={4}, journal={Journal of Turbomachinery}, publisher={ASME International}, author={Lamont, Justin and Ramesh, Sridharan and Ekkad, Srinath V. and Tolpadi, Anil and Kaminski, Christopher and Salamah, Samir}, year={2013}, month={Jun} }
 @inproceedings{lamont_chatterjee_ekkad_ledezma_kaminski_tolpadi_2013, title={Heat transfer in multiple parallel high aspect ratio ducts with triangular trench enhancement features}, volume={3}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84892995827&partnerID=MN8TOARS}, DOI={10.1115/HT2013-17325}, abstractNote={Detailed heat transfer coefficient distributions and pressure drop have been obtained for high aspect ratio (AR = Width/Height = 12.5) ducts with triangular trench enhancement features oriented normal to the coolant flow direction. Numerical and experimental approaches analyze the performance of triangular trenches for six geometrically identical ducts branching from a common plenum. The numerical approach is based on a Reynolds Averaged Navier Stokes (RANS) turbulence model with an unstructured mesh. A transient liquid crystal (TLC) technique is used to experimentally calculate Nu on the ducts surfaces. Reynolds number (Re = 7080, 14800, and 22400, with respect to the duct hydraulic diameter are explored. As Computational Fluid Dynamics (CFD) and TLC results are both detailed, qualitative and quantitative comparisons are made. Experimental results show the closest and furthest ducts from the entrance of the plenum are considerably affected, as recirculation zones develop which partially choke the inlet the respective ducts. Results from the experiments are compared to CFD predictions from Duct 4. In addition, the experimental data are recalculated with the maximum bias in TLC temperature to indicate an improved matching between CFD and experimental methods to demonstrate that CFD captures the wall heat transfer coefficient trends similar to experimental results. The triangular trenches enhance heat transfer in the ducts two-fold compared to smooth wall Dittus-Boelter Nusselt number correlation for flow in tubes.}, booktitle={ASME 2013 Heat Transfer Summer Conf. Collocated with the ASME 2013 7th Int. Conf. on Energy Sustainability and the ASME 2013 11th Int. Conf. on Fuel Cell Science, Engineering and Technology, HT 2013}, author={Lamont, J. and Chatterjee, K. and Ekkad, S.V. and Ledezma, G. and Kaminski, C. and Tolpadi, A.}, year={2013} }
 @article{reagle_delimont_ng_ekkad_rajendran_2013, title={Measuring the coefficient of restitution of high speed microparticle impacts using a PTV and CFD hybrid technique}, volume={24}, ISSN={0957-0233 1361-6501}, url={http://dx.doi.org/10.1088/0957-0233/24/10/105303}, DOI={10.1088/0957-0233/24/10/105303}, abstractNote={A novel particle tracking velocimetry (PTV)/computational fluid dynamics (CFD) hybrid method for measuring coefficient of restitution (COR) has been developed which is relatively simple, cost-effective, and robust. A laser and camera system is used in the Virginia Tech Aerothermal Rig to measure velocity trajectories of microparticles. The method solves for particle impact velocity at the impact surface using a CFD solution and Lagrangian particle tracking. The methodology presented here attempts to characterize a difficult problem by a combination of established techniques, PTV and CFD, which have not been used in this capacity before. Erosion and deposition are functions of particle/wall interactions and COR is a fundamental property of these interactions. COR depends on impact velocity, angle of impact, temperature, particle composition, and wall material. Two sizes of Arizona road dust and one size of glass beads are impacted on to a 304 stainless steel coupon. The particles are entrained into a free jet of 27 m s−1 at room temperature. Impact angle was varied from 85° to 25° depending on particle. Mean results collected using this new technique compare favorably with trends established in literature. The utilization of this technique to measure COR of microparticle sand will help develop a computational model and serve as a baseline for further measurements at elevated air and wall temperatures.}, number={10}, journal={Measurement Science and Technology}, publisher={IOP Publishing}, author={Reagle, C J and Delimont, J M and Ng, W F and Ekkad, S V and Rajendran, V P}, year={2013}, month={Sep}, pages={105303} }
 @article{parida_ekkad_ngo_2013, title={Multi-Layer Mini-Channel and Ribbed Mini-Channel Based High Performance Cooling Configurations for Automotive Inverters—Part A: Design and Evaluation}, volume={5}, ISSN={1948-5085 1948-5093}, url={http://dx.doi.org/10.1115/1.4023604}, DOI={10.1115/1.4023604}, abstractNote={Necessitated by the dwindling supply of petroleum resources, various new automotive technologies have been actively developed from the perspective of achieving energy security and diversifying energy sources. Hybrid electric vehicles and electric vehicles are a few such examples. Such diversification requires the use of power control units essentially for power control, power conversion, and power conditioning applications such as variable speed motor drives (dc–ac conversion), dc–dc converters and other similar devices. The power control unit of a hybrid electric vehicle or electric vehicle is essentially the brain of the hybrid system as it manages the power flow between the electric motor generator, battery and gas engine. Over the last few years, the performance of this power control unit has been improved and size has been reduced to attain higher efficiency and performance, causing the heat dissipation as well as heat density to increase significantly. Efforts are constantly being made to reduce this size even further. As a consequence, a better high performance cooler/heat exchanger is required to maintain the active devices temperature within optimum range. Cooling schemes based on multiple parallel channels are a few solutions which have been widely used to dissipate transient and steady concentrated heat loads and can be applied to existing cooling system with minor modifications. The aim of the present study has therefore been to study the various cooling options based on mini-channel and rib-turbulated mini-channel cooling for application in a hybrid electric vehicle and other similar consumer products, and perform a parametric and optimization study on the selected designs. Significant improvements in terms of thermal performance, reduced overall pressure drop, and volume reduction have been shown both experimentally and numerically. This paper is the first part in a two part submission and focuses on the design and evaluation of mini-channel and rib-turbulated mini-channel cooling configurations. The second part of this paper discusses the manufacturing and testing of the cooling device.}, number={3}, journal={Journal of Thermal Science and Engineering Applications}, publisher={ASME International}, author={Parida, Pritish R. and Ekkad, Srinath V. and Ngo, Khai}, year={2013}, month={Jun} }
 @inproceedings{numerical modeling of fluid flow and thermal behavior in geothermal heat exchangers_2013, volume={6 B}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84903452518&partnerID=MN8TOARS}, DOI={10.1115/IMECE2013-65098}, abstractNote={Geothermal heat exchangers are highly dependent on the local heat exchange capacity and ground thermal properties. The ability to simulate performance and testing of geothermal heat exchangers is limited. Therefore thermal conductivity tests are usually required for correct sizing of geothermal heat exchange systems. To better understand these tests in relation to energy piles, a 3D finite element model was created using the COMSOL software to simulate the thermal conductivity test of a 12inch (30cm) energy pile. The finite element simulation was created and validated using experimental data to expand the comparisons made between geothermal boreholes and energy piles. In this study, the numerical finite element model has been recreated using the commercial computational fluid dynamics (CFD) software ANSYS which incorporates fluid flow effects. Confirming that the CFD model can accurately model the thermal conductivity test provides an additional tool that will be valuable in modeling geothermal heat exchangers. Results show that parametric variations in terms of fluid flow rate and fluid selection are easier to evaluate using the CFD model compared to the finite element model. Results are also compared with discrete thermal conductivity measurements obtained from real geothermal heat exchangers.}, booktitle={ASME International Mechanical Engineering Congress and Exposition, Proceedings (IMECE)}, year={2013} }
 @article{reagle_delimont_ng_ekkad_2013, title={Study of Microparticle Rebound Characteristics Under High Temperature Conditions}, volume={136}, ISSN={0742-4795 1528-8919}, url={http://dx.doi.org/10.1115/1.4025346}, DOI={10.1115/1.4025346}, abstractNote={Large amounts of tiny microparticles are ingested into gas turbines over their operating life, resulting in unexpected wear and tear. Knowledge of such microparticle behavior at gas turbine operating temperatures is limited in published literature. In this study, Arizona road dust (ARD) is injected into a hot flow field to measure the effects of high temperature and velocity on particle rebound from a polished 304 stainless steel (SS) coupon. The results are compared with baseline (27 m/s) measurements at ambient (300 K) temperature made in the Virginia Tech Aerothermal Rig, as well as previously published literature. Mean coefficient of restitution (COR) was shown to decrease with the increased temperature/velocity conditions in the VT Aerothermal Rig. The effects of increasing temperature and velocity led to a 12% average reduction in COR at 533 K (47 m/s), 15% average reduction in COR at 866 K (77 m/s), and 16% average reduction in COR at 1073 K (102 m/s) compared with ambient results. The decrease in COR appeared to be almost entirely a result of increased velocity that resulted from heating the flow. Trends show that temperature plays a minor role in energy transfer between particle and impact surface below a critical temperature.}, number={1}, journal={Journal of Engineering for Gas Turbines and Power}, publisher={ASME International}, author={Reagle, C. J. and Delimont, J. M. and Ng, W. F. and Ekkad, S. V.}, year={2013}, month={Oct} }
 @inproceedings{reagle_delimont_ng_ekkad_2013, title={Study of microparticle rebound characteristics under high temperature conditions}, volume={2}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84890244778&partnerID=MN8TOARS}, DOI={10.1115/GT2013-95083}, abstractNote={Large amounts of tiny microparticles are ingested into gas turbines over their operating life, resulting in unexpected wear and tear. Knowledge of such microparticle behavior at gas turbine operating temperatures is limited in published literature. In this study, Arizona Road Dust (ARD) is injected into a hot flow field to measure the effects of high temperature and velocity on particle rebound from a polished 304 Stainless Steel (SS) coupon. The results are compared with baseline (27m/s) measurements at ambient (300°K) temperature made in the Virginia Tech Aerothermal Rig, as well as previously published literature. Mean Coefficient of Restitution (COR) was shown to decrease with the increased temperature/velocity conditions in the VT Aerothermal Rig. The effects of increasing temperature and velocity led to a 12% average reduction in COR at 533°K (47m/s), 15% average reduction in COR at 866°K (77m/s), and 16% average reduction in COR at 1073°K (102m/s) compared with ambient results. The decrease in COR appeared to be almost entirely a result of increased velocity that resulted from heating the flow. Trends show that temperature plays a minor role in energy transfer between particle and impact surface below a critical temperature.}, booktitle={Proceedings of the ASME Turbo Expo}, author={Reagle, C.J. and Delimont, J.M. and Ng, W.F. and Ekkad, S.V.}, year={2013} }
 @inproceedings{blanchard_wickersham_yeaton_fleischman_ekkad_ng_vandsburger_ma_lowe_2013, title={Test capabilities in the CCAPS/CSDL augmentor development facility}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85086949414&partnerID=MN8TOARS}, DOI={10.2514/6.2013-32}, abstractNote={The Commonwealth Center for Aerospace and Propulsion Systems (CCAPS) has developed an augmentor development test facility at Virginia Tech’s Combustion System Dynamics Lab (CSDL) that will enable rapid evaluation of augmentor concepts as well as the development of diagnostic techniques for measuring heat release, species concentrations, flame speeds, pressure loss, and other quantities relevant to augmentor performance. This paper provides an overview of the rig’s design and capabilities and the diagnostic techniques that are being developed.}, booktitle={51st AIAA Aerospace Sciences Meeting including the New Horizons Forum and Aerospace Exposition 2013}, author={Blanchard, R. and Wickersham, A. and Yeaton, I. and Fleischman, C. and Ekkad, S. and Ng, W. and Vandsburger, U. and Ma, L. and Lowe, T.}, year={2013} }
 @inproceedings{ramesh_leblanc_ekkad_alvin_2013, title={Thermo-mechanical analysis of various film cooling hole geometries}, volume={3}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84893025970&partnerID=MN8TOARS}, DOI={10.1115/HT2013-17423}, abstractNote={Tripod hole film cooling designs can provide improved cooling with reduced coolant usage. It is essential to determine the impact of strong cooling near hole locations due to increased thermal gradients around the hole region. In this paper, efforts have been made to predict the same using a numerical analysis. Local temperature distribution on the blade surface causes different rates of expansion, resulting in a differential strain, furthering the impact of the numerical study and creating a need to understand the thermo-mechanical behavior of blade design. The thermal stresses that are generated near the film cooling holes are compared for different cooling hole shapes and eventually weigh the thermal advantage of a tripod hole over the cylindrical standard design. Standard k-ε was used to validate the CFD results, ensuring by a fine mesh of 5 million tetrahedrons. Validation includes comparison of the experimental results, with a numerical one where similar blade material and working temperatures are used. Upon validation, CFD was run again at engine temperature conditions with Haynes230 alloy as the blade material. Surface temperature contour enabled structural analysis using a finite element approach. Equivalent stresses (Von-Mises stresses) on the blade emerging from the analysis provided more information about stress dependency on blowing ratio.}, booktitle={ASME 2013 Heat Transfer Summer Conf. Collocated with the ASME 2013 7th Int. Conf. on Energy Sustainability and the ASME 2013 11th Int. Conf. on Fuel Cell Science, Engineering and Technology, HT 2013}, author={Ramesh, S. and Leblanc, C. and Ekkad, S. and Alvin, M.A.}, year={2013} }
 @inproceedings{ramesh_leblanc_ekkad_alvin_2013, title={Tripod hole geometry performance for a vane suction surface near throat location}, volume={3}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84890183573&partnerID=MN8TOARS}, DOI={10.1115/GT2013-94459}, abstractNote={Film cooling performance depends strongly on the hole exit geometry, blowing ratio, and hole location. The goal of this study is to evaluate film cooling geometries that can provide better protection over the suction surface of the airfoil beyond the throat region. This study compares the performance of standard cylindrical; fan-shaped (10° flare/laidback); tripod hole geometry (15° breakout angle); and tripod holes with shaped exits (5° flare on 15° tripod). Film cooling holes are located just upstream of the throat region on the suction side of an airfoil. The airfoil is a scaled up first stage vane from GE E3 engine and is mounted on a low speed linear cascade wind tunnel. A range of blowing ratios from 0.5 to 2.0 was covered for a cylindrical hole, while ensuring all other hole geometries run under similar mass flow rate conditions. Steady state IR (Infra-Red) technique was employed to measure adiabatic film cooling effectiveness. Results show that the tripod holes with and without shaped exits provide much higher film effectiveness than cylindrical and slightly higher effectiveness than shaped exit holes using 50% lesser cooling air while operating at the same blowing ratios. Effectiveness values up to 0.2–0.25 are seen 40-hole diameters downstream for the tripod hole configurations thus providing cooling in the important trailing edge portion of the airfoil.}, booktitle={Proceedings of the ASME Turbo Expo}, author={Ramesh, S. and LeBlanc, C. and Ekkad, S.V. and Alvin, M.A.}, year={2013} }
 @inproceedings{reagle_delimont_ng_ekkad_rajendran_2012, title={A novel optical technique for measuring the coefficient of restitution of microparticle impacts in a forced flowfield}, volume={7}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84881177274&partnerID=MN8TOARS}, DOI={10.1115/GT2012-68252}, abstractNote={Erosion and deposition in gas turbine engines are functions of particle/wall interactions and Coefficient of Restitution (COR) is a fundamental property of these interactions. COR depends on impact velocity, angle of impact, temperature, particle composition, and wall material. The current study attempts to characterize the fundamental behavior of sand at different impact angles. A PIV system is used in the Virginia Tech Aerothermal Rig to measure velocity trajectories of microparticles. A novel method is used that solves for impact velocity in a forced flowfield by numerical methods. Two sizes of Arizona Test Dust and one of Glass beads are impacted into a 304 Stainless Steel coupon. Free jet velocity is 27m/s at room temperature. Impact angle varies from almost 90 to 25 degrees depending on particle. Mean results compare favorably with trends established in literature. This utilization of this technique to measure COR of microparticle sand will help develop a computational model and serve as a baseline for further measurements at elevated, engine representative air and wall temperatures.}, number={PARTS A AND B}, booktitle={Proceedings of the ASME Turbo Expo}, author={Reagle, C.J. and Delimont, J.M. and Ng, W.F. and Ekkad, S.V. and Rajendran, V.P.}, year={2012}, pages={1–9} }
 @inproceedings{carmack_ekkad_kim_moon_srinivasan_2012, title={Comparison of flow and heat transfer distributions in a can combustor for radial and axial swirlers under cold flow conditions}, volume={1}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84892648892&partnerID=MN8TOARS}, DOI={10.1115/HT2012-58550}, abstractNote={A comparison study between axial and radial swirler performance in a gas turbine can combustor was conducted by investigating the correlation between combustor flow field geometry and convective heat transfer at cold flow conditions for Reynolds numbers of 50,000 and 80,000. Flow velocities were measured using Particle Image Velocimetry (PIV) along the center axial plane and radial cross sections of the flow. It was observed that both swirlers produced a strong rotating flow with a reverse flow core. The axial swirler induced larger recirculation zones at both the backside wall and the central area as the flow exits the swirler, and created a much more uniform rotational velocity distribution. The radial swirler however, produced greater rotational velocity as well as a thicker and higher velocity reverse flow core. Wall heat transfer and temperature measurements were also taken. Peak heat transfer regions directly correspond to the location of the flow as it exits each swirler and impinges on the combustor liner wall.Copyright © 2012 by ASME}, booktitle={ASME 2012 Heat Transfer Summer Conf. Collocated with the ASME 2012 Fluids Engineering Div. Summer Meeting and the ASME 2012 10th Int. Conf. on Nanochannels, Microchannels and Minichannels, HT 2012}, author={Carmack, A. and Ekkad, S. and Kim, Y. and Moon, H.-K. and Srinivasan, R.}, year={2012}, pages={873–879} }
 @article{leblanc_ekkad_lambert_rajendran_2012, title={Detailed Heat Transfer Distributions in Engine Similar Cooling Channels for a Turbine Rotor Blade With Different Rib Orientations}, volume={135}, ISSN={0889-504X 1528-8900}, url={http://dx.doi.org/10.1115/1.4006387}, DOI={10.1115/1.4006387}, abstractNote={Detailed Nusselt number distributions are presented for a gas turbine engine similar internal channel geometry used for cooling a modern first stage rotor blade. The cooling design has one leading edge channel and a three-pass channel that covers the rest of the blade. The simulated model, generated from the midspan section of an actual cooling circuit, was studied for wall heat transfer coefficient measurements using the transient liquid crystal technique. The model wall inner surfaces were sprayed with thermochromic liquid crystals, and a transient test was used to obtain the local heat transfer coefficients from the measured color change. Results are presented for a nominal channel inlet leading edge channel Reynolds number of 10,700 and a channel inlet three-pass channel Reynolds number of 25,500. Detailed heat transfer measurements are presented for the simulated leading edge, first pass, second pass and third pass interior walls for different rib configurations. The channels were studied for smooth, 90 deg ribs, and angled ribs geometries in addition to ribs on the divider walls between adjacent passages. Overall pressure drop measurements were also obtained for each passage. Some of these results are compared with the predicted heat transfer from standard correlations used in design practices. Results show very complicated heat transfer behavior in these realistic channels compared to results obtained in simplistic geometry channels from published studies. In some cases, the Nusselt numbers predicted by correlations are 50–60% higher than obtained from the current experiments.}, number={1}, journal={Journal of Turbomachinery}, publisher={ASME International}, author={LeBlanc, Christopher and Ekkad, Srinath V. and Lambert, Tony and Rajendran, Veera}, year={2012}, month={Oct} }
 @article{lamont_ekkad_alvin_2012, title={Detailed Heat Transfer Measurements Inside Rotating Ribbed Channels Using the Transient Liquid Crystal Technique}, volume={4}, ISSN={1948-5085 1948-5093}, url={http://dx.doi.org/10.1115/1.4005604}, DOI={10.1115/1.4005604}, abstractNote={Coolant flow in rotating internal serpentine channels is highly complex due to the effects of the Coriolis force and centrifugal buoyancy. Detailed knowledge of the heat transfer over a surface will greatly enhance the blade designers’ ability to predict hot spots so coolant may be distributed effectively. The present study uses a novel transient liquid crystal technique to measure heat transfer on a rotating two-pass channel surface with chilled inlet air. The present study examines the differences in heat transfer distributions on channel surfaces with smooth walls, 90 deg rib and W-shaped rib turbulated walls. The test section is made up of two passes to model radially inward and outward flows. To account for centrifugal buoyancy, cold air is passed through a room temperature test section. This ensures that buoyancy is acting in a similar direction to real turbine blades. The inlet coolant-to-wall density ratio is fixed at 0.08, Re = 16,000, and Ro = 0.08. The present study shows that the W-shaped ribs enhance heat transfer in all cases (stationary and rotating) approximately 1.75 times more than the 90 deg ribs. The W-shaped rib channel is least affected by rotation, which may be due to the complex nature of the secondary flow generated by the geometry. A higher pressure drop is associated with the W-shaped ribs than the 90 deg ribs, however, the overall thermal-hydraulic performance of the W-shaped ribs still exceeds that set by the 90 deg ribs.}, number={1}, journal={Journal of Thermal Science and Engineering Applications}, publisher={ASME International}, author={Lamont, Justin A. and Ekkad, Srinath V. and Alvin, Mary Anne}, year={2012}, month={Feb} }
 @inproceedings{leblanc_ramesh_ekkad_alvin_2012, title={Effect of breakout angle on tripod injection hole geometries on flat plate film cooling}, volume={7}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84887263973&partnerID=MN8TOARS}, DOI={10.1115/IMECE2012-89346}, abstractNote={In this study, effect of breakout angle of side holes from the main hole in a tripod hole design on film cooling performance is evaluated on a flat plate surface with steady-state IR (infrared thermography) technique. The designs are compared a cylindrical hole design inclined at 30° from the surface with pitch-to-diameter ratio of 3.0 and a shaped hole design, which is identical to the cylindrical hole design with the addition of adding a 10° flare and laydown to the exit on the mainstream surface. The two tripod hole designs are one where the two side holes, also of the same diameter, branch from the root at a 15° angle while maintaining the same 30° inclination as the cylindrical and shaped designs witha pitch-to-diameter ratio between the main holes for this design is 6.0. The other tripod hole design is a modified tripod hole design that increases the branch angle to 30°, which has the added effect of increasing the pitch-to-diameter ratio between the main holes to 7.5. Two secondary fluids — air and carbon-dioxide — were used to study the effects of coolant-to-mainstream density ratio (DR = 0.95 and 1.45) on film cooling effectiveness. Several blowing ratios in the range 0.5–4.0 were investigated independently at the two density ratios. Results show that the tripod hole design provides similar film cooling effectiveness as the shaped hole case with overall reduced coolant usage. Increasing the breakout angle from 15° to 30° reduces overall cooling effectiveness but increases jet-to-jet interactions.}, number={PARTS A, B, C, D}, booktitle={ASME International Mechanical Engineering Congress and Exposition, Proceedings (IMECE)}, author={Leblanc, C. and Ramesh, S. and Ekkad, S. and Alvin, M.A.}, year={2012}, pages={1995–2002} }
 @inproceedings{abraham_panchal_ekkad_ng_lohaus_malandra_2012, title={Effect of endwall contouring on a transonic turbine blade passage: Part 1 - Aerodynamic performance}, volume={8}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84881174877&partnerID=MN8TOARS}, DOI={10.1115/GT2012-68425}, abstractNote={The paper presents a detailed experimental and numerical study on the effect of endwall contouring in a quasi 2D cascade, operating at transonic conditions. Aerodynamic performance of two contoured endwalls are studied and compared with a baseline (planar) endwall. The first contoured endwall was generated with the goal of reducing secondary losses (Aero-Optimized contoured endwall) and the second endwall was generated with the objective of reduced overall heat transfer to the endwall (HT-optimized contoured endwall). Midspan total pressure loss, secondary flow field and static pressure measurements on the airfoil surface were measured. The cascade exit Mach numbers range from 0.71 to 0.95 and the turning angle of the airfoil is ∼127°. The inlet span of the airfoils was reduced with respect to the outlet span with the intention of obtaining a realistic inlet/exit Mach number that is observed in a real engine. 3D viscous compressible CFD analysis was carried out to study the detailed behavior of the complex flow structures that develop as a result of endwall contouring. A 3% reduction in area averaged losses was achieved at 0.1 Cax downstream of the trailing edge and a 17% reduction in mixed out losses was achieved at 1.0 Cax downstream location with the Aero-Optimized contoured endwall.}, number={PARTS A, B, AND C}, booktitle={Proceedings of the ASME Turbo Expo}, author={Abraham, S. and Panchal, K. and Ekkad, S.V. and Ng, W. and Lohaus, A.S. and Malandra, A.}, year={2012}, pages={1089–1098} }
 @inproceedings{panchal_abraham_ekkad_ng_lohaus_crawford_2012, title={Effect of endwall contouring on a transonic turbine blade passage: Part 2 - Heat transfer performance}, volume={4}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84881146253&partnerID=MN8TOARS}, DOI={10.1115/GT2012-68405}, abstractNote={Contouring of turbine endwalls has been widely studied for aerodynamic performance improvement of turbine passages. However, it is equally important to investigate the effect of contouring on endwall heat transfer, because a substantial increase in endwall heat transfer due to contouring will render the design impractical. In this paper, the effect of contouring on endwall heat transfer performance of a high-turning HP-turbine blade passage, operating under transonic exit Mach number conditions, is reported. Three endwall geometries were experimentally investigated at three different passage exit Mach numbers, 0.71, 0.88(design) and 0.95, for their heat transfer performance. One endwall is a non-contoured baseline endwall and the other two are contoured endwall geometries. One of the contoured endwall geometry was generated with the goal of reduction in stagnation pressure losses and the other was generated with the goal of reduced overall heat transfer through the endwall. The experiments were carried out in Virginia Tech’s transient, blow down, transonic linear cascade facility. Endwall surface temperatures were measured using infrared thermography technique. Local heat transfer coefficient values were calculated using the measured temperatures. The heat transfer coefficient values were then related to the endwall geometries using a camera matrix model. The measurement technique and the methodology for the post-processing of the heat transfer coefficient data have been presented in detail. Details of the flow behavior for these endwalls were obtained using CFD simulations and have been used to assist the interpretation of the experimental results. In this study, the heat transfer performance of the contoured endwalls in comparison to the non-contoured baseline case is presented. Both the contoured endwalls demonstrated a significant reduction in the overall average heat transfer coefficient values. The surface heat transfer coefficient distributions also indicated a reduction in the level of hot spots for most of the endwall surface. However, increase in the heat transfer coefficient values was observed especially in the area near the leading edge. The results indicate that, in addition to a probable improvement in aerodynamic performance, endwall contouring may also be used to improve the heat transfer performance of turbine passages. Additionally, aerodynamic behavior of these endwalls is discussed in detail in the companion paper GT2012-68425, “Effect of endwall contouring on a transonic turbine endwall: Part 1 – Aerodynamic performance.”}, number={PARTS A AND B}, booktitle={Proceedings of the ASME Turbo Expo}, author={Panchal, K.V. and Abraham, S. and Ekkad, S.V. and Ng, W. and Lohaus, A.S. and Crawford, M.E.}, year={2012}, pages={151–161} }
 @inproceedings{pandit_dove_ekkad_huxtable_2012, title={Effect of partial 3-dimensional pin fin geometry for heat transfer enhancement in high aspect ratio channels}, volume={7}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84887316518&partnerID=MN8TOARS}, DOI={10.1115/IMECE2012-88251}, abstractNote={The efficiency of a TEG (Thermoelectric Generator) may be thought of as the ratio of the power output to the heat input at the hot junction. This ratio is governed by the laws of thermodynamics and cannot thus exceed the Carnot efficiency. It follows that the greater the difference between the hot and cold side temperatures, the greater the efficiency of the TEG. Heat transfer enhancement measurements with 3-Dimensional partial pin-fin arrays of varying shapes on a flat plate are presented. The fin height is fixed at 15% of channel height. The hydraulic diameter and configuration of the fins are chosen based on existing literature. The study is carried out at various Reynolds numbers based on full channel height. The shapes studied are circular, semi-circular, triangular, hexagonal and diamond shaped. These shapes are compared against a baseline case without fins. The experiment uses the transient liquid crystal (TLC) method to calculate the heat transfer coefficient on the test surface. The data shows that diamond shaped fins provide the highest turbulent mixing downstream of the pins leading to the highest heat transfer coefficients.}, number={PARTS A, B, C, D}, booktitle={ASME International Mechanical Engineering Congress and Exposition, Proceedings (IMECE)}, author={Pandit, J. and Dove, M. and Ekkad, S.V. and Huxtable, S.}, year={2012}, pages={1747–1754} }
 @inproceedings{lamont_ekkad_2012, title={Effect of rotation on jet impingement heat transfer for various jet configurations}, volume={1}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84892652650&partnerID=MN8TOARS}, DOI={10.1115/HT2012-58023}, abstractNote={The present study uses a novel transient liquid crystal technique to measure heat transfer on a rotating, radially outward coolant channel with jet impingement and a crossflow outlet condition. The jet impingement cooling scheme is studied on the leading and trailing sides of a gas turbine internal coolant channel with the jet impingement target surface oriented normal to the direction of rotation. Several aspects of jet impingement are studied under rotating conditions: effect of increasing Rotation number (Ro = 0–0.003), effect of jet inclination angle (90° and 70° from the vertical), and effect of jet-to-target surface distance (H/d = 1, 3, and 5). Heat transfer measurements are obtained on the target surface using the transient liquid crystal technique. All configurations studied have a constant jet-to-jet spacing, P/d = 5. The spacing between the two adjacent rows is P/d = 3. Corresponding flow measurements are taken from stationary conditions. Results show that rotation does not change the heat transfer magnitudes and distributions greatly compared to the stationary results for all H/d and jet orientation cases. As x/d increases, stationary H/d = 5 heat transfer results show a steady decrease, where effectiveness of the jets diminishes. As x/d increases for H/d = 3, the maximum and minimum heat transfer values dampen to a steady constant average value. As x/d increases for H/d = 1, the heat transfer begins very low then steadily increases for higher x/d.© 2012 ASME}, booktitle={ASME 2012 Heat Transfer Summer Conf. Collocated with the ASME 2012 Fluids Engineering Div. Summer Meeting and the ASME 2012 10th Int. Conf. on Nanochannels, Microchannels and Minichannels, HT 2012}, author={Lamont, J.A. and Ekkad, S.V.}, year={2012}, pages={717–726} }
 @article{lamont_ekkad_alvin_2012, title={Effects of Rotation on Heat Transfer for a Single Row Jet Impingement Array With Crossflow}, volume={134}, ISSN={0022-1481 1528-8943}, url={http://dx.doi.org/10.1115/1.4006167}, DOI={10.1115/1.4006167}, abstractNote={The effects of the Coriolis force are investigated in rotating internal serpentine coolant channels in turbine blades. For complex flow in rotating channels, detailed measurements of the heat transfer over the channel surface will greatly enhance the blade designers’ ability to predict hot spots so coolant may be distributed more effectively. The present study uses a novel transient liquid crystal technique to measure heat transfer in a rotating, radially outward channel with impingement jets. A simple case with a single row of constant pitch impinging jets with the crossflow effect is presented to demonstrate the novel liquid crystal technique and document the baseline effects for this type of geometry. The present study examines the differences in heat transfer distributions due to variations in jet Rotation number, Roj, and jet orifice-to-target surface distance (H/dj = 1,2, and 3). Colder air, below room temperature, is passed through a room temperature test section to cause a color change in the liquid crystals. This ensures that buoyancy is acting in a similar direction as in actual turbine blades where walls are hotter than the coolant fluid. Three parameters were controlled in the testing: jet coolant-to-wall temperature ratio, average jet Reynolds number, Rej, and average jet Rotation number, Roj. Results show, such as serpentine channels, the trailing side experiences an increase in heat transfer and the leading side experiences a decrease for all jet channel height-to-jet diameter ratios (H/dj). At a jet channel height-to-jet diameter ratio of 1, the crossflow from upstream spent jets greatly affects impingement heat transfer behavior in the channel. For H/dj = 2 and 3, the effects of the crossflow are not as prevalent as H/dj = 1: however, it still plays a detrimental role. The stationary case shows that heat transfer increases with higher H/dj values, so that H/dj = 3 has the highest results of the three examined. However, during rotation the H/dj = 2 case shows the highest heat transfer values for both the leading and trailing sides. The Coriolis force may have a considerable effect on the developing length of the potential core, affecting the resulting heat transfer on the target surface.}, number={8}, journal={Journal of Heat Transfer}, publisher={ASME International}, author={Lamont, Justin A. and Ekkad, Srinath V. and Alvin, Mary Anne}, year={2012}, month={May} }
 @inproceedings{dove_pandit_ekkad_huxtable_2012, title={Experimental validation of temperature distributions across a heat exchanger for a thermoelectric generator}, volume={7}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84887276008&partnerID=MN8TOARS}, DOI={10.1115/IMECE2012-88282}, abstractNote={Thermoelectric generators (TEGs) are currently a topic of interest in the field of energy harvesting for automobiles. In applying TEGs to the outside of the exhaust tailpipe of a vehicle, the difference in temperature between the hot exhaust gases and the automobile coolant can be used to generate a small amount of electrical power to be used in the vehicle. The amount of power is anticipated to be a few hundred watts based on the temperatures expected and the properties of the materials for the TEG. This study focuses on developing efficient heat exchanger modules in order to maximize the power generation for a given vehicle and TEG. A computational fluid dynamics (CFD) model run by the authors has provided performance predictions for various cases on the cooling side of the heat exchanger. This paper discusses the setup and results of the experimental validation for the CFD model for the proposed TEG heat exchanger module.}, number={PARTS A, B, C, D}, booktitle={ASME International Mechanical Engineering Congress and Exposition, Proceedings (IMECE)}, author={Dove, M. and Pandit, J. and Ekkad, S. and Huxtable, S.}, year={2012}, pages={1755–1760} }
 @book{han_dutta_ekkad_2012, title={Gas Turbine Heat Transfer and Cooling Technology}, ISBN={9780429107115}, url={http://dx.doi.org/10.1201/b13616}, DOI={10.1201/b13616}, abstractNote={Fundamentals Need for Turbine Blade Cooling Turbine-Cooling Technology Turbine Heat Transfer and Cooling Issues Structure of the Book Review Articles and Book Chapters on Turbine Cooling and Heat Transfer New Information from 2000 to 2010 References Turbine Heat Transfer Introduction Turbine-Stage Heat Transfer Cascade Vane Heat-Transfer Experiments Cascade Blade Heat Transfer Airfoil Endwall Heat Transfer Turbine Rotor Blade Tip Heat Transfer Leading-Edge Region Heat Transfer Flat-Surface Heat Transfer New Information from 2000 to 2010 2.10 Closure References Turbine Film Cooling Introduction Film Cooling on Rotating Turbine Blades Film Cooling on Cascade Vane Simulations Film Cooling on Cascade Blade Simulations Film Cooling on Airfoil Endwalls Turbine Blade Tip Film Cooling Leading-Edge Region Film Cooling Flat-Surface Film Cooling Discharge Coefficients of Turbine Cooling Holes 3.10 Film-Cooling Effects on Aerodynamic Losses 3.11 New Information from 2000 to 2010 3.12 Closure References Turbine Internal Cooling Jet Impingement Cooling Rib-Turbulated Cooling Pin-Fin Cooling Compound and New Cooling Techniques New Information from 2000 to 2010 References Turbine Internal Cooling with Rotation Rotational Effects on Cooling Smooth-Wall Coolant Passage Heat Transfer in a Rib-Turbulated Rotating CoolantPassage Effect of Channel Orientation with Respect to the RotationDirection on Both Smooth and Ribbed Channels Effect of Channel Cross Section on Rotating Heat Transfer Different Proposed Correlation to Relate the Heat Transferwith Rotational Effects Heat-Mass-Transfer Analogy and Detail Measurements Rotation Effects on Smooth-Wall Impingement Cooling Rotational Effects on Rib-Turbulated Wall ImpingementCooling New Information from 2000 to 2010 References Experimental Methods Introduction Heat-Transfer Measurement Techniques Mass-Transfer Analogy Techniques Liquid Crystal Thermography Flow and Thermal Field Measurement Techniques New Information from 2000 to 2010 Closure References Numerical Modeling Governing Equations and Turbulence Models Numerical Prediction of Turbine Heat Transfer Numerical Prediction of Turbine Film Cooling Numerical Prediction of Turbine Internal Cooling New Information from 2000 to 2010 References Final Remarks Turbine Heat Transfer and Film Cooling Turbine Internal Cooling with Rotation Turbine Edge Heat Transfer and Cooling New Information from 2000 to 2010 Closure Index}, publisher={CRC Press}, author={Han, Je-Chin and Dutta, Sandip and Ekkad, Srinath}, year={2012}, month={Nov}, pages={1–844} }
 @inproceedings{agarwal_lattimer_ekkad_vandsburger_2012, title={Grid-zone particle hydrodynamics and solid circulation in a multiple jet fluidized bed}, volume={2}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84882360906&partnerID=MN8TOARS}, DOI={10.1115/FEDSM2012-72066}, abstractNote={Particle Image Velocimetry (PIV) and Digital Image Analysis (DIA) were used to investigate the evolution of multiple inlet gas jets located at the distributor base of a two-dimensional fluidized bed setup. Experiments were conducted with varying distributor orifice diameter, orifice pitch, particle density, particle diameter, and fluidization velocity to understand the motion of particles in the grid-zone region of a fluidized bed. Results were used to develop a phenomenological model that quantifies the conditions throughout the entire grid-zone. The results and the model were further analyzed to understand the effect of operating conditions on the solid circulation dynamics of a multiple jet system fluidized bed. It was determined that the solid circulation rate increased linearly with an increase in the fluidization velocity until the jet system transitioned from isolated to an interacting system. The solid circulation increased at a much lower rate in the interacting system of jets. This sudden change in the solid circulation rate has not been reported in the literature possibly due to the lack of multiple jet studies. For multiple jet systems, this phenomenon may indicate the presence of an optimum operating condition with high circulation rate and low air input in the bed.}, booktitle={American Society of Mechanical Engineers, Fluids Engineering Division (Publication) FEDSM}, author={Agarwal, G. and Lattimer, B. and Ekkad, S. and Vandsburger, U.}, year={2012}, pages={55–64} }
 @inproceedings{pandit_dove_ekkad_huxtable_2012, title={Heat exchanger design for waste heat recovery from automobile exhaust using thermoelectric generators}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84873850726&partnerID=MN8TOARS}, DOI={10.2514/6.2012-1011}, booktitle={50th AIAA Aerospace Sciences Meeting Including the New Horizons Forum and Aerospace Exposition}, author={Pandit, J. and Dove, M. and Ekkad, S.V. and Huxtable, S.}, year={2012} }
 @inproceedings{lamont_ramesh_ekkad_tolpadi_kaminski_salamah_2012, title={Heat transfer enhancement in narrow diverging channels}, volume={4}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84881124154&partnerID=MN8TOARS}, DOI={10.1115/GT2012-68486}, abstractNote={Detailed heat transfer coefficient distributions have been obtained for narrow diverging channels with and without enhancement features. The cooling configurations considered include rib turbulators and concavities (or dimples) on the main heat transfer surfaces. All the measurements are presented at a representative Reynolds number of 28,000. Pressure drop measurements for the overall channel are also presented to evaluate the heat transfer enhancement geometry with respect to pumping power requirements. The test models were studied for wall heat transfer coefficient measurements using the transient liquid crystal technique. The model wall inner surfaces were sprayed with thermochromic liquid crystals, and a transient test was used to obtain the local heat transfer coefficients from the measured color change. Analysis of results shows that choice of designs is limited by available pressure drop even if the design provides significantly higher heat transfer coefficients. Dimpled surfaces provide appreciably high heat transfer coefficients and reasonable pressure drop whereas ribbed ducts provide significantly higher heat transfer coefficients and higher overall pressure drop.}, number={PARTS A AND B}, booktitle={Proceedings of the ASME Turbo Expo}, author={Lamont, J. and Ramesh, S. and Ekkad, S.V. and Tolpadi, A. and Kaminski, C. and Salamah, S.}, year={2012}, pages={185–191} }
 @inproceedings{ekkad_parida_ngo_2012, title={High efficiency minichannel and mini-impingement cooling systems for hybrid electric vehicle electronics}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84882309748&partnerID=MN8TOARS}, DOI={10.1115/ICNMM2012-73041}, abstractNote={Over the years, electronic equipment, especially semiconductor based devices, have found their applications in almost all fields of research. The demand for more power and performance from such electronic equipment has constantly been growing resulting in an increased amount of heat dissipation from these devices. While conventional cooling solutions have performed the task of heat removal, no straightforward extension has been possible for significantly high heat fluxes dissipated by smaller and more efficient electronic devices. Thermal management of high-density power control unit for hybrid electric vehicle is one such challenging application. Over the last few years, the performance of this power control unit has been improved and size has been reduced to attain higher efficiency and performance causing the heat dissipation as well as heat density to increase significantly. Efforts are constantly being made to reduce the PCU size even further and also to reduce the manufacturing costs. As a consequence, heat density will go up (∼200–250 W/cm2) and thus, a better high performance cooler/heat exchanger is required which can operate under the existing cooling system design (pressure drop limitation) and at the same time, maintain active devices temperature within optimum range (<120–125°C) for higher reliability.
               The focus of this paper is to discuss the development of various cooling options for high heat flux dissipating devices with severe size constraints. A parametric and optimization study on the selected designs was performed. Finally, the optimized cooler/heat exchanger was tested under actual running conditions. The methodology was to explore various high performance cooling options such as impinging jets, pin fins, and ribbed mini-channels and to arrive at new promising, conceptual designs. These new designs were then compared against similar conventional designs both numerically and experimentally. Additionally, conjugate heat transfer simulations were performed on partial packaging model to compare the various designs. Experiments were also performed to validate the simulation models and characterize the meshing parameters to perform cost and time effective calculations/simulations.}, booktitle={ASME 2012 10th Int. Conf. on Nanochannels, Microchannels, and Minichannels Collocated with the ASME 2012 Heat Transfer Summer Conf. and the ASME 2012 Fluids Engineering Division Sum, ICNMM 2012}, author={Ekkad, S.V. and Parida, P. and Ngo, K.}, year={2012}, pages={669–679} }
 @inproceedings{abraham_panchal_ekkad_ng_lohaus_malandra_2012, title={Measurement of aerodynamic losses for turbine airfoil cascades with varying pitch, operating under transonic conditions}, volume={8}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84881143201&partnerID=MN8TOARS}, DOI={10.1115/GT2012-68424}, abstractNote={The paper presents detailed experimental results of the midspan total pressure losses, secondary flow field, and static pressure measurements on two linear, high-turning turbine cascades at transonic conditions. The airfoils in the two cascades being studied are identical and their aerodynamic loading levels are varied by increasing the pitch of one cascade by 25% with respect to the other. The turbine cascades are referred to as B1-SP and B1-IP. Cascade B1-IP, with its increased pitch, has a Zweifel coefficient increased by 25%. The airfoils have a turning angle of ∼127°. Measurements are made at design and off-design conditions, at exit Mach numbers ranging from 0.71 to 0.95. The exit span of the airfoils are increased relative to the inlet span with the intention of obtaining a ratio of inlet Mach number to exit Mach number that is representative to that encountered in a real engine. This results in one end wall diverging from inlet to exit at a 13 degree angle, which simulates the required leading edge loading as seen in an engine. The objective of this study is to investigate the variation in airfoil loading distribution and the effect it has on aerodynamic performance in terms of pressure losses. Detailed loss measurements, both in the pitchwise as well as spanwise directions, are conducted at 0.1 Cax and 1.0 Cax locations downstream of the trailing edge. Results from 3D viscous numerical simulations have been used to assist the interpretation of experimental results.}, number={PARTS A, B, AND C}, booktitle={Proceedings of the ASME Turbo Expo}, author={Abraham, S. and Panchal, K. and Ekkad, S.V. and Ng, W. and Lohaus, A.S. and Malandra, A.}, year={2012}, pages={1081–1088} }
 @inproceedings{ramesh_narzary_ekkad_2012, title={Optimization of low jet-to-target spacing ratio for double wall impingement cooling applications}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84872834669&partnerID=MN8TOARS}, DOI={10.2514/6.2012-367}, abstractNote={Double wall cooling is a very effective technique for increasing heat transfer in hot gas path components utilizing a narrow channel near the surface of the component. Multiple techniques including the use of impingement jets, turbulators and microchannels have been studied so far. The focus of these researches had been limited to jet-to-wall spacing ratio (H/D) greater than one. Experimental studies were conducted to understand thermal behavior of impinging jets under the conditions of low jet holes-to-target plate spacing (H/D<1). Effects of selected parameters were examined to understand heat transfer behavior and. The parameters studied include jet-to-wall spacing (H/D) and Reynolds number and jet array configuration: staggered and inline. Irrespective of the Reynolds number and jet array configuration, the lowest jet-to-target spacing (H/D = 0.125) resulted in higher heat transfer coefficient when compared with the baseline case (H/D = 1).}, booktitle={50th AIAA Aerospace Sciences Meeting Including the New Horizons Forum and Aerospace Exposition}, author={Ramesh, S. and Narzary, D. and Ekkad, S.V.}, year={2012} }
 @inproceedings{leblanc_narzary_ekkad_alvin_2012, title={Performance of tripod antivortex injection holes on vane film cooling}, volume={1}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84892653952&partnerID=MN8TOARS}, DOI={10.1115/HT2012-58135}, abstractNote={Film cooling performance of a tripod hole anti-vortex geometry is evaluated on cascade vane pressure and suction surfaces with steady-state IR (infrared thermography) technique and compared to a baseline cylindrical hole geometry performance. The base geometry is a simple cylindrical hole design inclined at 30° from the surface with pitch-to-diameter ratio of 3.0. The tripod hole geometry, also called an anti-vortex design, is where two side holes, also of the same diameter, branch out from the root at 15° angle. The pitch-to-diameter ratio between the main holes for this design is 6.0. Two secondary fluids — air and carbon-dioxide — were used to study the effects of coolant-to-mainstream density ratio (DR = 0.95 and 1.45) on film cooling effectiveness. Several blowing ratios in the range 0.5 –4.0 were investigated independently at the two density ratios. Results show that the tripod hole design clearly provides higher film cooling effectiveness the baseline case with overall reduced coolant usage on both pressure and suction side of the airfoil. Additional testing was also conducted to measure the aerodynamic effects of injecting coolant through the cylindrical and tripod hole designs. Results show that the coolant issued from a tripod hole design has a significantly smaller effect on the overall aerodynamic performance of the vane.Copyright © 2012 by ASME}, booktitle={ASME 2012 Heat Transfer Summer Conf. Collocated with the ASME 2012 Fluids Engineering Div. Summer Meeting and the ASME 2012 10th Int. Conf. on Nanochannels, Microchannels and Minichannels, HT 2012}, author={LeBlanc, C. and Narzary, D. and Ekkad, S. and Alvin, M.A.}, year={2012}, pages={745–755} }
 @inproceedings{singh_reagle_delimont_tafti_ng_ekkad_2012, title={Sand transport in a two pass internal cooling duct with rib turbulators}, volume={1}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84892634213&partnerID=MN8TOARS}, DOI={10.1115/HT2012-58100}, abstractNote={A two pass stationary square duct with rib turbulators subjected to sand ingestion is studied using Large Eddy Simulations (LES). Each pass has ribs on two opposite walls and aligned normal to the main flow direction. The rib pitch to rib height (P/e) is 9.28, the rib height to channel hydraulic diameter (e/Dh) is 0.0625 and calculations have been carried out for a bulk Reynolds number of 25,000. Particle sizes in the range 0.5–25 μm are considered, with the same size distribution as found in Arizona Road Dust (medium). Large Eddy Simulation (LES) with wall-model is used to model the flow and sand particles are modeled using a discrete Lagrangian framework. 220,000 particles are injected at the inlet and perfectly elastic collisions with the wall are considered. Results quantify the distribution of particle impingement density on all surfaces. Highest particle impingement density is found in the first quarter section of the second pass after the 180° turn, where the recorded impingement is more than twice that of any other region. It is also found that the average particle impingement per pitch is 28% higher in the second pass than the first pass. Results show lower particle tendency to hit the region immediately behind the rib in the first pass compared to the second pass where particle impingement is more uniform in the region between two ribs. The smooth walls do not show much particle impingement except the wall in second pass where the flow impinges after the turn. The rib face facing the flow is by far is the most susceptible to impingement and hence deposition and erosion. The results of this simulation were also compared with results obtained from experiments conducted on an identical two pass geometry with Arizona Road Dust particles. The particle impingement pattern is recorded by using a sticky tape on all surfaces to capture the particles. The numerical predictions showed good qualitative agreement with experimental measurements. These results help identifying the damage prone areas in the internal cooling passages of a turbine blade under the influence of sand ingestion. This information can help modify the geometry of the blade or location of film cooling holes to avoid hole blockage and degradation of heat transfer at the walls.Copyright © 2012 by ASME}, booktitle={ASME 2012 Heat Transfer Summer Conf. Collocated with the ASME 2012 Fluids Engineering Div. Summer Meeting and the ASME 2012 10th Int. Conf. on Nanochannels, Microchannels and Minichannels, HT 2012}, author={Singh, S. and Reagle, C. and Delimont, J. and Tafti, D. and Ng, W. and Ekkad, S.}, year={2012}, pages={727–735} }
 @inproceedings{xue_ng_ekkad_moon_zhang_2012, title={The performance of fan-shaped hole film cooling on a gas turbine blade at transonic conditon with high freestream turbulence}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84872845939&partnerID=MN8TOARS}, DOI={10.2514/6.2012-368}, abstractNote={An experimental investigation was performed to study the cooling performance and convective heat transfer on a film cooled turbine blade surface. A 2D linear cascade model of the first stage turbine rotor blade of a land-based gas turbine was employed in the study. The film cooling configuration on the blade comprises of 2 rows of fan-shaped holes on the pressure side (PS), and 1 row of fan-shaped hole on the suction side (SS). The tests were performed in the Virginia Tech transonic wind tunnel facility, which simulates engine representative conditions of high turbulence intensity at the inlet and transonic Mach numbers at the exit, with matching Reynolds number to that of the real engine. All the tests were performed at inlet turbulence intensity of 12% with integral length scale of 0.26 normalized by cascade blade pitch. Exit Mach number of 0.67, 0.84, and 1.01 were chosen for the tests. Two combinations of blowing ratios at different rows of cooling holes were tested. (Nominal blowing ratio settings are: suction side injection BR=1.2 and 1.6; pressure side row 1 BR=2.8 and 3.8; pressure side row 2 BR=2.2, and 2.8.) The shock wave/boundary layer interaction effect on Nusselt number and adiabatic effectiveness was observed on the suction side (SS) with a cascade exit Mach number of 1.01. The trends for Nusselt number and adiabatic effectiveness across the shock impinged region suggest the existence of a separation bubble at the location of shock impingement. The detailed discussion of the shock effect on film cooling is presented.}, booktitle={50th AIAA Aerospace Sciences Meeting Including the New Horizons Forum and Aerospace Exposition}, author={Xue, S. and Ng, W. and Ekkad, S. and Moon, H.K. and Zhang, L.}, year={2012} }
 @inproceedings{leblanc_ekkad_lambert_rajendran_2011, title={Detailed heat transfer distributions in engine similar cooling channels for a turbine rotor blade with different rib orientations}, volume={5}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84865500616&partnerID=MN8TOARS}, DOI={10.1115/GT2011-45254}, abstractNote={Detailed Nusselt number distributions are presented for a gas turbine engine similar internal channel geometry used for cooling a modern first stage rotor blade. The cooling design has one leading edge channel and a three-pass channel that covers the rest of the blade. The simulated model, generated from the midspan section of an actual cooling circuit, was studied for wall heat transfer coefficient measurements using the transient liquid crystal technique. The model wall inner surfaces were sprayed with thermochromic liquid crystals, and a transient test was used to obtain the local heat transfer coefficients from the measured color change. Results are presented for a nominal channel inlet leading edge channel Reynolds number of 10700 and a channel inlet three-pass channel Reynolds number of 25500. Detailed heat transfer measurements are presented for the simulated leading edge, first pass, second pass and third pass interior walls for different rib configurations. The channels were studied for smooth, 90° ribs, and angled ribs geometries in addition to ribs on the divider walls between adjacent passages. Overall pressure drop measurements were also obtained for each passage. Some of these results are compared with the predicted heat transfer from standard correlations used in design practices. Results show very complicated heat transfer behavior in these realistic channels compared to results obtained in simplistic geometry channels from published studies. In some cases, the Nusselt numbers predicted by correlations are 50–60% higher than obtained from the current experiments.}, number={PARTS A AND B}, booktitle={Proceedings of the ASME Turbo Expo}, author={LeBlanc, C. and Ekkad, S.V. and Lambert, T. and Rajendran, V.}, year={2011}, pages={1109–1116} }
 @inproceedings{lamont_ekkad_2011, title={Detailed heat transfer measurements inside rotating ribbed channels using the transient liquid crystal technique}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85087998196&partnerID=MN8TOARS}, DOI={10.1115/ajtec2011-44127}, abstractNote={The effects of the Coriolis force and centrifugal buoyancy are well known in rotating internal serpentine coolant channels in turbine blades. As channel flow in rotation is highly complex, detailed knowledge of the heat transfer over a surface will greatly enhance the blade designer’s ability to predict hot spots so coolant may be distributed effectively. The present study uses a novel transient liquid crystal technique to measure heat transfer on a rotating two-pass channel surface with chilled inlet air. The present study examines the differences in heat transfer distributions of three channel types in rotation: smooth wall, 90° ribs, and W-shaped ribs. The two channels in the test section model radially inward and outward flow. To account for centrifugal buoyancy, cold air is passed through a room temperature test section. This ensures that buoyancy is acting in a similar direction to real turbine blades. Three parameters were controlled in the testing: inlet coolant-to-wall density ratio, channel Reynolds number, and Rotation number. Results were compared to previous studies with similar test conditions. The present study shows that the W-shaped ribs enhance heat transfer in all cases (stationary and rotating) approximately 2–3 times better than the 90° ribs. The W-shaped ribbed channel is least affected by rotation due to the complex nature of the flow generated by the geometry.}, booktitle={ASME/JSME 2011 8th Thermal Engineering Joint Conference, AJTEC 2011}, author={Lamont, J.A. and Ekkad, S.V.}, year={2011} }
 @inproceedings{abraham_panchal_ekkad_ng_brown_malandra_2011, title={Effect of airfoil shape and turning angle on turbine airfoil aerodynamic performance at transonic conditions}, volume={6}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84869155002&partnerID=MN8TOARS}, DOI={10.1115/imece2011-62167}, abstractNote={Performance data for high turning gas turbine blades under transonic Mach numbers is significantly lacking in literature. Performance of three gas turbine airfoils with varying turning angles at transonic flow conditions was investigated in this study. Midspan total pressure loss, secondary flow field and static pressure measurements on the airfoil surface in a linear cascade setting were measured. Airfoil curvature and true chord were varied to change the loading vs. chord for each airfoil. Airfoils A, D and E are designed to operate at different velocity triangles. Velocity triangle requirements (inlet/exit Mach number and gas angles) come from 1D and 2D models that include calibrated loss systems. One of the goals of this study was to use the experimental data to confirm/refine loss predictions for the effect of various Mach numbers and gas turning angles. The cascade exit Mach numbers were varied within a range from 0.6 to 1.1. The airfoil turning angle ranges from 120° to 138°. A realistic inlet/exit Mach number ratio, that is representative of that seen in a real engine, was obtained by reducing the inlet span with respect to the exit span of the airfoil, thereby creating a quasi 2D cascade. In order to compare the experimental results and study the detailed flow characteristics, 3D viscous compressible CFD analysis was also carried out.}, number={PARTS A AND B}, booktitle={ASME 2011 International Mechanical Engineering Congress and Exposition, IMECE 2011}, author={Abraham, S. and Panchal, K. and Ekkad, S.V. and Ng, W. and Brown, B.J. and Malandra, A.}, year={2011}, pages={1199–1208} }
 @inproceedings{abraham_panchal_ekkad_ng_brown_malandra_2011, title={Effect of turbine airfoil shape on aerodynamic losses for turbine airfoils operating under transonic conditions}, volume={7}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84865447168&partnerID=MN8TOARS}, DOI={10.1115/GT2011-45188}, abstractNote={Profile and secondary loss correlations have been developed and improved over the years to include the induced incidence and leading edge geometry and to reflect recent trends in turbine design. All of these investigations have resulted in better understanding of the flow field in turbine passages. However, there is still insufficient data on the performance of turbine airfoils with high turning angles operating at varying incidence angles at transonic Mach numbers. The paper presents detailed aerodynamic measurements for three different turbine airfoils with similar turning angles but different aerodynamic shapes. Midspan total pressure loss, secondary flow field, and static pressure measurements on the airfoil surface in the cascades are presented and compared for the three different airfoil sets. The airfoils are designed for the same velocity triangles (inlet/exit gas angles and Mach number). Airfoil curvature and true chord are varied to change the loading vs. chord. The objective is to investigate the type of loading distribution and its effect on aerodynamic performance (pressure loss). Measurements are made at +10, 0 and −10 degree incidence angles for high turning turbine airfoils with ∼127 degree turning. The cascade exit Mach numbers were varied within a range from 0.6 to 1.1. In order to attain a ratio of inlet Mach number to exit Mach number that is representative to that encountered in a real engine, the exit span is increased relative to the inlet span. This results in one end wall diverging from inlet to exit at a 13 degree angle, which simulates the required leading edge loading as seen in an engine. 3D viscous compressible CFD analysis was carried out in order to compare the results with experimentally obtained values and to further investigate the flow characteristics of the airfoils under study.}, number={PARTS A, B, AND C}, booktitle={Proceedings of the ASME Turbo Expo}, author={Abraham, S. and Panchal, K. and Ekkad, S.V. and Ng, W. and Brown, B.J. and Malandra, A.}, year={2011}, pages={511–521} }
 @inproceedings{lamont_ekkad_2011, title={Effects of Rotation on jet impingement channel heat transfer}, volume={5}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84865459680&partnerID=MN8TOARS}, DOI={10.1115/GT2011-45744}, abstractNote={The effects of the Coriolis force and centrifugal buoyancy is investigated in rotating internal serpentine coolant channels in turbine blades. For complex flow in rotating channels, detailed measurements of the heat transfer over the channel surface will greatly enhance the blade designer’s ability to predict hot spots so coolant air may be distributed more effectively. The present study uses a novel transient liquid crystal technique to measure heat transfer in a rotating, radially outward channel with impingement jets. This is the beginning of a comprehensive study on rotational effects on jet impingement. A simple case with a single row of constant pitch impinging jets with crossflow effect is presented to demonstrate the novel liquid crystal technique and document the baseline effects for this type of geoemtry. The present study examines the differences in heat transfer distributions due to variations in jet Rotation number and jet orifice-to-target surface distance. Colder air below room temperature is passed through a room temperature test section to simulate the centrifugal buoyancy effect seen in a real engine environment. This ensures that buoyancy is acting in a similar direction as in actual turbine blades where walls are hotter than the coolant fluid. Three parameters were controlled in the testing: jet coolant-to-wall temperature ratio, average jet Reynolds number, and average jet Rotation number. Results show, like serpentine channels, the trailing side experiences an increase in heat transfer and the leading side experiences a decrease for all jet channel height to jet diameter ratios (H/dj). At a jet channel height to jet diameter ratio of 1, the cross-flow from upstream spent jets greatly affects impingement heat transfer behavior in the channel.}, number={PARTS A AND B}, booktitle={Proceedings of the ASME Turbo Expo}, author={Lamont, J.A. and Ekkad, S.V.}, year={2011}, pages={1269–1278} }
 @article{parida_ekkad_ngo_2011, title={Experimental and numerical investigation of confined oblique impingement configurations for high heat flux applications}, volume={50}, ISSN={1290-0729}, url={http://dx.doi.org/10.1016/j.ijthermalsci.2011.01.010}, DOI={10.1016/j.ijthermalsci.2011.01.010}, abstractNote={Breakthroughs in recent cutting-edge electronic technologies have become increasingly dependent on the ability to safely dissipate large amount of heat from small areas. Improvements in cooling methodologies are therefore required to avoid unacceptable temperature rise and at the same time maintain a high efficiency. Jet impingement is one such cooling scheme which has been widely used to dissipate transient and steady concentrated heat loads. The configuration examined in the present paper aims at wall-integrated inclined impinging jets in a confined environment. Coolant outlet is perpendicular to the plane of the impinging jets and is along the cross-flow direction. The main objective of the present work is to gain insight both experimentally and numerically into designing and analysis of a jet impinging cooling scheme for high heat density applications systems such as micro- and meso-scale electronic systems and trailing edge of a turbine blade. An overall enhancement of 150%–200% in the maximum heat transfer coefficient has been recorded both experimentally and computationally due to impingement and associated swirl. Results are presented to show the effect of the wall induced swirl and the associated enhanced heat transfer mechanism. The presence of fins between the jets further increases the cooling area and adds additional conduction area. The present scheme is therefore expected to provide alternatives for overcoming the existing heat distribution and cooling problems in high heat flux dissipating devices.}, number={6}, journal={International Journal of Thermal Sciences}, publisher={Elsevier BV}, author={Parida, Pritish R. and Ekkad, Srinath V. and Ngo, Khai}, year={2011}, month={Jun}, pages={1037–1050} }
 @article{agarwal_lattimer_ekkad_vandsburger_2012, title={Experimental study on solid circulation in a multiple jet fluidized bed}, volume={58}, ISSN={0001-1541}, url={http://dx.doi.org/10.1002/aic.13703}, DOI={10.1002/aic.13703}, abstractNote={AbstractParticle image velocimetry was used to investigate the evolution of multiple inlet gas jets located at the distributor base of a two‐dimensional fluidized bed setup. Results were used to estimate the solid circulation rate of the fluidized bed as well as particle‐entrainment into the individual jets. The effects of fluidization velocity, orifice diameter, orifice pitch, particle diameter, and particle density were studied. It was determined from this study that the solid circulation rate linearly increased with an increase in the fluidization velocity until the multiple jet system transitioned from isolated to an interacting system. In the interacting system of jets, the solid circulation increased with fluidization velocity but at a much lower rate. For multiple jet systems, this phenomenon may indicate the presence of an optimum operating condition with high circulation rate and low air input in the bed. © 2011 American Institute of Chemical Engineers AIChE J, 58: 3003–3015, 2012}, number={10}, journal={AIChE Journal}, publisher={Wiley}, author={Agarwal, Gaurav and Lattimer, Brian and Ekkad, Srinath and Vandsburger, Uri}, year={2012}, month={Oct}, pages={3003–3015} }
 @inproceedings{roy_ekkad_vandsburger_2011, title={Experimental validation of syngas composition of an entrained flow gasifier model under different operating conditions}, volume={2}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84877599974&partnerID=MN8TOARS}, booktitle={28th Annual International Pittsburgh Coal Conference 2011, PCC 2011}, author={Roy, A. and Ekkad, S.V. and Vandsburger, U.}, year={2011}, pages={870–878} }
 @inproceedings{narzary_leblanc_ekkad_2011, title={Film-cooling performance of anti-vortex hole on a flat plate}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84990174925&partnerID=MN8TOARS}, DOI={10.1115/ajtec2011-44161}, abstractNote={Film cooling performance of two hole geometries is evaluated on a flat plate surface with steady-state IR (infrared thermography) technique. The base geometry is a simple cylindrical hole design inclined at 30° from the surface with pitch-to-diameter ratio of 3.0. The second geometry is an anti-vortex design where the two side holes, also of the same diameter, branch out from the root at 15° angle. The pitch-to-diameter ratio is 6.0 between the main holes. The mainstream Reynolds number is 3110 based on the coolant hole diameter. Two secondary fluids — air and carbon-dioxide — were used to study the effects of coolant-to-mainstream density ratio (DR = 0.95 and 1.45) on film cooling effectiveness. Several blowing ratios in the range 0.5 –4.0 were investigated independently at the two density ratios. Results indicate significant improvement in effectiveness with anti-vortex holes compared to cylindrical holes at all the blowing ratios studied. At any given blowing ratio, the anti-vortex hole design uses 50% less coolant and provides at least 30–40% higher cooling effectiveness. The use of relatively dense secondary fluid improves effectiveness immediately downstream of the anti-vortex holes but leads to poor performance downstream.}, booktitle={ASME/JSME 2011 8th Thermal Engineering Joint Conference, AJTEC 2011}, author={Narzary, D.P. and LeBlanc, C. and Ekkad, S.}, year={2011} }
 @inproceedings{lamont_ekkad_alvin_2011, title={Heat transfer distribution of various rib geometries for developing flow at high rotation numbers}, volume={10}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84869169448&partnerID=MN8TOARS}, DOI={10.1115/imece2011-62610}, abstractNote={The Coriolis force and centrifugal buoyancy have a significant effect on the cooling performance for rotating internal serpentine coolant channels in gas turbine blades. As coolant flow in rotation is highly complex, detailed knowledge of the heat transfer over a surface will greatly enhance the blade designer’s ability to predict hot spots so coolant may be distributed effectively. The present study uses a novel transient liquid crystal technique to measure heat transfer on a rotating, radially outward coolant channel, which is a simplified model of the actual coolant channels. Various rib types such as 90°, W, and M-shaped ribs of varying types are used to roughen the walls. The present study measures the effects of high rotation numbers (Ro) on the performance and heat transfer distribution of different rib types in developing flow. The present study measures how effective the ribs are up to Ro = 0.5. The Reynolds number (Re) is held constant at 12,000. Results show that in the developing region, the W and M-shaped “high-performance” ribs are just as effective as the simple 90° ribs for increasing heat transfer. The entrance effect in the developing region causes significantly high baseline heat transfer enhancement which may explain why ribs are not as effective as they are in the fully developed region. As the rotation number is increased, results show that the heat transfer on the trailing side increases, while the leading side decreases to a limit and remains constant. For all rotational cases, the W and M-shaped ribs show large changes to the heat transfer distributions on the leading and trailing sides.}, number={PARTS A AND B}, booktitle={ASME 2011 International Mechanical Engineering Congress and Exposition, IMECE 2011}, author={Lamont, J.A. and Ekkad, S.V. and Alvin, M.A.}, year={2011}, pages={1039–1047} }
 @article{parida_ekkad_ngo_2012, title={Impingement-based high performance cooling configurations for automotive power converters}, volume={55}, ISSN={0017-9310}, url={http://dx.doi.org/10.1016/j.ijheatmasstransfer.2011.10.024}, DOI={10.1016/j.ijheatmasstransfer.2011.10.024}, abstractNote={Necessitated by the dwindling supply of petroleum resources, various new automotive technologies have been actively developed from the perspective of achieving energy security and diversifying energy sources. Hybrid electric vehicles and electric vehicles are a few such examples. Such diversification requires the use of power control units essentially for power control, power conversion and power conditioning applications such as variable speed motor drives (dc–ac conversion), dc–dc converters and other similar devices. Power control unit of a hybrid electric vehicle or electric vehicle is essentially the brain of the hybrid system as it manages the power flow between the electric motor generator, battery and gas engine. Over the last few years, the performance of this power control unit has been improved and size has been reduced to attain higher efficiency and performance causing the heat dissipation as well as heat density to increase significantly. Efforts are constantly being made to reduce this size even further. As a consequence, a better high performance cooler/heat exchanger is required to maintain the active devices temperature within optimum range. Jet impingement is one such cooling scheme which has been widely used to dissipate transient and steady concentrated heat loads and can be applied to existing cooling system with minor modifications. The aim of the present study has therefore been to study the various cooling options based on impingement for application in hybrid electric vehicle and other similar consumer products and perform parametric and optimization study on the selected designs. Significant improvements in terms of thermal performance and volume reduction have been shown both experimentally and numerically.}, number={4}, journal={International Journal of Heat and Mass Transfer}, publisher={Elsevier BV}, author={Parida, Pritish R. and Ekkad, Srinath V. and Ngo, Khai}, year={2012}, month={Jan}, pages={834–847} }
 @article{agarwal_lattimer_ekkad_vandsburger_2011, title={Influence of multiple gas inlet jets on fluidized bed hydrodynamics using Particle Image Velocimetry and Digital Image Analysis}, volume={214}, ISSN={0032-5910}, url={http://dx.doi.org/10.1016/j.powtec.2011.08.002}, DOI={10.1016/j.powtec.2011.08.002}, abstractNote={A rectangular fluidized bed setup was developed to study the evolution of inlet gas jets located at the distributor. Experiments were conducted with varying distributor types and bed media to understand the motion of particles and jets in the grid-zone region of a fluidized bed. Particle Image Velocimetry and Digital Image Analysis were used to quantify the parameters that characterize these jets. A grid-zone phenomenological model was developed to compare these parameters with those available in the literature. It was determined from this study that jet penetration length behavior is consistently different for fluidization velocities below and above the minimum fluidization. For velocities above minimum fluidization, jet lengths were found to increase more rapidly with increase in orifice velocity than for operating conditions below minimum fluidization.}, number={1}, journal={Powder Technology}, publisher={Elsevier BV}, author={Agarwal, Gaurav and Lattimer, Brian and Ekkad, Srinath and Vandsburger, Uri}, year={2011}, month={Nov}, pages={122–134} }
 @inproceedings{panchal_abraham_ekkad_ng_brown_malandra_2011, title={Investigation of effect of end wall contouring methods on a transonic turbine blade passage}, volume={7}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84865553526&partnerID=MN8TOARS}, DOI={10.1115/GT2011-45192}, abstractNote={End wall contouring has been widely studied during past two decades for secondary loss reduction in turbine passages. Recent non-axisymmetric end wall contouring methods have shown more promise for loss reduction as compared to the axisymmetric end wall contouring methods used in initial studies. The end wall contouring methods have shown definite promise, especially, for the turbine passages at low design exit Mach numbers. A class of methods exists in the literature where the end wall surface is defined by using a combination of two curves. These curves specify surface topology variation in streamwise and pitchwise directions. Another class of methods depends on surface contour optimization, in which the modification of surface contours is achieved by changing the control point locations that define the surface topology. A definitive, passage design parameter based method of contouring is still not available. However, a general guideline for the trend of contour variation, along pitchwise and streamwise direction, can certainly be extrapolated from the existing literature. It is not clear, however, whether such a trend can be fitted to any blade profile to achieve, least of all a nonoptimum but a definite, reduction in losses. Moreover, almost all of the existing studies have focused on end wall contouring of passages with low exit Mach numbers. Some researchers, indeed, have used blades designed for high turning and high exit Mach number. However, such studies were done at Mach number well below the intended design condition. A study of effect of end wall contouring on a high turning blade with high design exit Mach number is not available in open literature. The present study investigates the effect of application of three different types of end wall contouring methods through numerical simulation, on a high turning transonic turbine blade passage. The main contouring method is based on total loss reduction criterion which is described here in detail. The contouring methodology described here avoids the deficiency of current commercial mesh generation software in context of automated meshing and provides a robust end wall optimization methodology. The geometry that gives minimum SKE values is compared with this loss optimized geometry. Additionally, a normalized contoured surface topology was extracted from a previous study that has similar blade design parameters and this surface was fitted to the turbine passage under study in order to investigate the effect of such trend based surface fitting. This contour geometry has also been compared with the other two contour geometries. Aerodynamic response of these geometries has been compared in detail with the baseline case without any end wall contouring. A comparison of shape and location of end wall contours on aerodynamic performance has been provided. The results indicate that end wall contouring for transonic turbine blades may not result in as significant gains at design conditions as those claimed for low speed turbine passages in previous studies.}, number={PARTS A, B, AND C}, booktitle={Proceedings of the ASME Turbo Expo}, author={Panchal, K. and Abraham, S. and Ekkad, S.V. and Ng, W. and Brown, B.J. and Malandra, A.}, year={2011}, pages={523–534} }
 @inproceedings{stoakes_ekkad_2011, title={Optimized impingement configurations for double wall cooling applications}, volume={5}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84865521645&partnerID=MN8TOARS}, DOI={10.1115/GT2011-46143}, abstractNote={Double wall cooling is a very effective technique for increasing heat transfer in hot gas path components utilizing a narrow channel near the surface of the component. Multiple techniques exist to increase the heat transfer within the narrow channel, including the use of impingement jets, turbulators and microchannels. A preliminary study has been performed using computational fluid dynamics (CFD) to determine the heat transfer benefits of double wall cooling technology when compared to a smooth wall square channel and a ribbed wall square channel. Conjugate CFD simulations of flow through an aluminum channel were performed to include the effects of conduction through the solid and convection within the main channel. The design for the preliminary study consists of a square main channel and a narrow impingement channel connected by a series of holes creating impingement jets on the outer surface of the impingement channel. The study examines multiple parameters to increase heat transfer without increasing the pumping power required. The parameters studied include diameter of impingement jets, jet-to-jet spacing, number of impingement jets, and jet-to-wall spacing. Results show that the impingement channel height-to-diameter ratio has a strong impact on heat transfer effectiveness. This study also provides a new optimization methodology for improving cooling designs with specific targets.}, number={PARTS A AND B}, booktitle={Proceedings of the ASME Turbo Expo}, author={Stoakes, P. and Ekkad, S.}, year={2011}, pages={1535–1543} }
 @inproceedings{leblanc_narzary_ekkad_alvin_2011, title={Performance of tripod antivortex injection holes on vane suction side film cooling}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84880695195&partnerID=MN8TOARS}, booktitle={47th AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit 2011}, author={LeBlanc, C. and Narzary, D.P. and Ekkad, S. and Alvin, M.A.}, year={2011} }
 @inproceedings{roy_ekkad_vandsburger_2011, title={Prediction and validation of performance of an entrained flow gasifier model}, volume={4}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84869162837&partnerID=MN8TOARS}, DOI={10.1115/imece2011-63770}, abstractNote={Computational fluid dynamics (CFD) simulation of a single stage, dry-feed entrained flow gasifier is carried out to predict several physical and chemical processes within the gasifier. The model is developed using a commercial software package FLUENT. The CFD model is based on an Eulerian-Lagrangian framework, where the continuous fluid phase is modeled in Eulerian approach and the particle flow trajectory is simulated in Lagrangian frame. The two phases are coupled by appropriate source terms in the conservation equations. The gasification process can be divided into the following sub-processes, which are inert heating, moisture release, coal devolatilization, char gasification and gas phase reactions. Discrete Phase Model (DPM) is used to model the coal particles and coupled with heterogeneous particle surface reactions in Species Transport module. The interaction between reaction chemistry and turbulence is described by Finite-rate/Eddy dissipation model. The simulation provides detailed information of temperature field and species concentration profile inside the gasifier. The temperature distribution clearly indicates the three different reaction zones for devolatilization, gasification and reduction. Steady state model predictions are compared with benchmark experimental data from literature. The trend of the predicted species mole fraction distribution is in good agreement within error bound of the experiment. The model thus provides a validated set of model parameters along with an insight to the underlying flow physics and chemical reactions of gasification process that can be employed to improve design of experiments. This study also develops the basis to achieve further accuracy incorporating complex effects such as detailed reaction kinetic mechanisms, proper devolatilization models, effect of ash-slag transition and particle deposition.}, number={PARTS A AND B}, booktitle={ASME 2011 International Mechanical Engineering Congress and Exposition, IMECE 2011}, author={Roy, A. and Ekkad, S.V. and Vandsburger, U.}, year={2011}, pages={1609–1617} }
 @article{patil_sedalor_tafti_ekkad_kim_dutta_moon_srinivasan_2011, title={Study of Flow and Convective Heat Transfer in a Simulated Scaled Up Low Emission Annular Combustor}, volume={3}, ISSN={1948-5085 1948-5093}, url={http://dx.doi.org/10.1115/1.4004531}, DOI={10.1115/1.4004531}, abstractNote={Modern dry low emissions (DLE) combustors are characterized by highly swirling and expanding flows that makes the convective heat load on the gas side difficult to predict and estimate. A coupled experimental–numerical study of swirling flow inside a DLE annular combustor model is used to determine the distribution of heat transfer on the liner walls. Three different Reynolds numbers are investigated in the range of 210,000–840,000 with a characteristic swirl number of 0.98. The maximum heat transfer coefficient enhancement ratio decreased from 6 to 3.6 as the flow Reynolds number increased from 210,000 to 840,000. This is attributed to a reduction in the normalized turbulent kinetic energy in the impinging shear layer, which is strongly dependent on the swirl number that remains constant at 0.98 for the Reynolds number range investigated. The location of peak heat transfer did not change with the increase in Reynolds number since the flow structures in the combustors did not change with Reynolds number. Results also showed that the heat transfer distributions in the annulus have slightly different characteristics for the concave and convex walls. A modified swirl number accounting for the step expansion ratio is defined to facilitate comparison between the heat transfer characteristics in the annular combustor with previous work in a can combustor. A higher modified swirl number in the annular combustor resulted in higher heat transfer augmentation and a slower decay with Reynolds number.}, number={3}, journal={Journal of Thermal Science and Engineering Applications}, publisher={ASME International}, author={Patil, Sunil and Sedalor, Teddy and Tafti, Danesh and Ekkad, Srinath and Kim, Yong and Dutta, Partha and Moon, Hee-Koo and Srinivasan, Ram}, year={2011}, month={Aug} }
 @inproceedings{reagle_newman_xue_ng_ekkad_moon_zhang_2010, title={A transient infrared technique for measuring surface and endwall heat transfer in a transonic turbine cascade}, volume={4}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-82055194137&partnerID=MN8TOARS}, DOI={10.1115/GT2010-22975}, abstractNote={This paper describes a method for obtaining surface and endwall heat transfer in an uncooled transonic cascade facility using infrared thermography measurements. Midspan heat transfer coefficient results are first presented for an engine representative first stage nozzle guide vane at exit Mach number of 0.77, Reynolds number of 1.05×106 and freestream turbulence intensity of 16%. The results obtained from infrared thermography are compared with previously published results using thin film gauges in the same facility on the same geometry. There is generally good agreement between the two measurement techniques in both trend and overall level of heat transfer coefficient over the vane surface. Stanton number contours are then presented for a blade endwall at exit Mach number of 0.88, Reynolds number of 1.70×106 and freestream turbulence intensity of 8%. Infrared thermography results are qualitatively compared with results from a published work obtained with liquid crystals at similar flow conditions. Results are qualitatively in agreement.}, number={PARTS A AND B}, booktitle={Proceedings of the ASME Turbo Expo}, author={Reagle, C. and Newman, A. and Xue, S. and Ng, W. and Ekkad, S. and Moon, H.K. and Zhang, L.}, year={2010}, pages={405–411} }
 @article{patil_abraham_tafti_ekkad_kim_dutta_moon_srinivasan_2011, title={Experimental and numerical investigation of convective heat transfer in a gas turbine can combustor}, volume={133}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-79251510862&partnerID=MN8TOARS}, DOI={10.1115/1.4001173}, abstractNote={Experiments and numerical computations are performed to investigate the convective heat transfer characteristics of a gas turbine can combustor under cold flow conditions in a Reynolds number range between 50,000 and 500,000 with a characteristic swirl number of 0.7. It is observed that the flow field in the combustor is characterized by an expanding swirling flow, which impinges on the liner wall close to the inlet of the combustor. The impinging shear layer is responsible for the peak location of heat transfer augmentation. It is observed that as Reynolds number increases from 50,000 to 500,000, the peak heat transfer augmentation ratio (compared with fully developed pipe flow) reduces from 10.5 to 2.75. This is attributed to the reduction in normalized turbulent kinetic energy in the impinging shear layer, which is strongly dependent on the swirl number that remains constant at 0.7 with Reynolds number. Additionally, the peak location does not change with Reynolds number since the flow structure in the combustor is also a function of the swirl number. The size of the corner recirculation zone near the combustor liner remains the same for all Reynolds numbers and hence the location of shear layer impingement and peak augmentation does not change.}, number={1}, journal={Journal of Turbomachinery}, author={Patil, S. and Abraham, S. and Tafti, D. and Ekkad, S. and Kim, Y. and Dutta, P. and Moon, H.-K. and Srinivasan, R.}, year={2011} }
 @inproceedings{abraham_panchal_xue_ekkad_ng_brown_malandra_2010, title={Experimental and numerical investigations of a transonic, high turning turbine cascade with a divergent endwall}, volume={1}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-80054982232&partnerID=MN8TOARS}, DOI={10.1115/FEDSM-ICNMM2010-30393}, abstractNote={The paper presents detailed measurements of midspan total pressure loss, secondary flow field, static pressure measurements on airfoil surface at midspan, near hub and near the end walls in a transonic turbine airfoil cascade. Numerous low-speed experimental studies have been carried out to investigate the performance of turbine cascades. Profile and secondary loss correlations have been developed and improved over the years to include the induced incidence and leading edge geometry and to reflect recent trends in turbine design. All of the above investigations have resulted in better understanding of flow field in turbine passages. However, there is still insufficient data on the performance of turbine blades with high turning angles operating at varying incidences angles at transonic Mach numbers. In the present study, measurements were made at +10, 0 and −10 degree incidence angles for a high turning turbine airfoil with 127 degree turning. The exit Mach numbers were varied within a range from 0.6 to 1.1. Additionally, the exit span is increased relative to the inlet span resulting in one end wall diverging from inlet to exit at 13 degree angle. This was done in order to obtain a ratio of inlet Mach number to exit Mach number which is representative to that encountered in real engine and simulates the blade and near end wall loading that is seen in an engine. 3D viscous compressible CFD analysis was carried out in order to compare the results with experimentally obtained values and to further investigate the design and off-design flow characteristics of the airfoil under study. All aerodynamic measurements were compared with CFD analysis and a reasonably good match was observed.}, number={PARTS A, B AND C}, booktitle={American Society of Mechanical Engineers, Fluids Engineering Division (Publication) FEDSM}, author={Abraham, S. and Panchal, K. and Xue, S. and Ekkad, S.V. and Ng, W. and Brown, B.J. and Malandra, A.}, year={2010}, pages={569–576} }
 @inproceedings{parida_ekkad_ngo_2010, title={Innovative liquid cooling configurations for high heat flux applications}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-77955200574&partnerID=MN8TOARS}, DOI={10.1109/ITHERM.2010.5501356}, abstractNote={Breakthroughs in the recent cutting-edge technologies have become increasingly dependent on the ability to safely dissipate large amount of heat from small areas. Improvements in cooling techniques are therefore required to avoid unacceptable temperature rise and at the same time maintain a high efficiency. Jet impingement is one such cooling scheme which has been widely used to dissipate transient and steady concentrated heat loads. But with constantly increasing cooling needs, conventional jet impingement cooling is no longer a viable option. Considerable improvements are therefore required to meets such stringent requirements. A combination of swirl-impingement-fin generating geometry is one such alternative. Even without a fin, an overall enhancement of 150% – 200% in the maximum heat transfer coefficient has been recorded both experimentally and computationally due to impingement and associated swirl. Moreover, the presence of fins further increases the cooling area. The present scheme is therefore expected to overcome the existing heat distribution and cooling problems in high heat flux dissipating devices.}, booktitle={2010 12th IEEE Intersociety Conference on Thermal and Thermomechanical Phenomena in Electronic Systems, ITherm 2010}, author={Parida, P.R. and Ekkad, S.V. and Ngo, K.}, year={2010} }
 @inproceedings{parida_ekkad_ngo_2010, title={Novel PCM and jet impingement based cooling scheme for high density transient heat loads}, volume={7}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84860511271&partnerID=MN8TOARS}, DOI={10.1115/IHTC14-22841}, abstractNote={Breakthroughs in the recent cutting-edge technologies have become increasingly dependent on the ability to safely dissipate large amount of heat from small areas. Improvements in cooling techniques are therefore required to avoid unacceptable temperature rise and at the same time maintain high efficiency. Jet impingement is one such cooling scheme which has been widely used to dissipate transient and steady concentrated heat loads. With constantly increasing transient cooling needs, conventional pin-fin cooling and conventional jet impingement cooling are not meeting the requirements. Considerable improvements are therefore required to meet such stringent requirements without any significant changes in the cooling system. A combined cooling scheme based on jet impingement and phase change materials (PCMs) is presented as one such alternative to existing cooling systems. A high heat storage capability of PCMs in combination with a high heat transfer rates from impingement cooling can help overcome the existing heat distribution and transient cooling problems in high heat flux dissipating devices. Preliminary conjugate CFD simulations show promising results. Additionally, experimental validation of the simulation predictions has also been performed. A reasonably good agreement was found between the predictions and experiments.}, booktitle={2010 14th International Heat Transfer Conference, IHTC 14}, author={Parida, P.R. and Ekkad, S.V. and Ngo, K.}, year={2010}, pages={443–450} }
 @inproceedings{sedalor_patil_ekkad_tafti_kim_moon_srinivasan_2010, title={Study of flow and convective heat transfer in a simulated scaled up low emission annular combustor}, volume={4}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-82055165249&partnerID=MN8TOARS}, DOI={10.1115/GT2010-22986}, abstractNote={Modern Dry Low Emissions (DLE) combustors are characterized by highly swirling and expanding flows that makes the convective heat load on the gas side difficult to predict and estimate. A coupled experimental-numerical study of swirling flow inside a DLE annular combustor model is presented. A simulated scaled up annular combustor shell was designed with a generic fuel nozzle model to create the swirl in the flow. The experiment was simulated with a cold flow and heated combustor walls in a low speed wind tunnel. An infrared camera was used to obtain the temperature distribution along the liner wall. The experiment was conducted at various Reynolds numbers to investigate the effect on the convective heat transfer peak locations. A CFD study performed using FLUENT was used to get a better understanding of high swirl flow field and its effect on the heat transfer on liner walls. Results show that the heat transfer distributions in the annulus have slightly different characteristics for the concave and convex walls. Results also show a much slower drop in heat transfer coefficient enhancement with increasing Reynolds number compared to can combustor liner walls.}, number={PARTS A AND B}, booktitle={Proceedings of the ASME Turbo Expo}, author={Sedalor, T. and Patil, S. and Ekkad, S. and Tafti, D. and Kim, Y. and Moon, H.-K. and Srinivasan, R.}, year={2010}, pages={787–794} }
 @article{parida_mei_jiang_meng_ekkad_2010, title={Experimental Investigation of Cooling Performance of Metal-Based Microchannels}, volume={31}, ISSN={0145-7632 1521-0537}, url={http://dx.doi.org/10.1080/01457630903409654}, DOI={10.1080/01457630903409654}, abstractNote={Metal-based microchannel heat exchangers (MHEs) are of current interest due to the combination of high heat transfer performance and improved mechanical integrity. Efficient methods for fabrication and assembly of functional metal-based MHEs are essential to ensure the economic viability of such devices. Al- and Cu-based high-aspect-ratio microscale structures (HARMS) have been fabricated through molding replication using metallic mold inserts. Such metallic HARMS were assembled through eutectic bonding to form Al- and Cu-based MHEs, on which heat transfer tests were conducted to determine the overall cooling rate and time constants. Electrically heated Cu blocks were placed outside the MHEs and provided a constant flux, and water flowing within the microchannels acted as the coolant. Experimental results show a great influence of the type of metal, flow rate, and the surrounding conditions on the overall cooling performance of the MHEs.}, number={6}, journal={Heat Transfer Engineering}, publisher={Informa UK Limited}, author={Parida, Pritish R. and Mei, Fanghua and Jiang, Jing and Meng, Wen Jin and Ekkad, Srinath V.}, year={2010}, month={May}, pages={485–494} }
 @inproceedings{patil_abraham_tafti_ekkad_kim_dutta_moon_srinivasan_2009, title={Experimental and numerical investigation of convective heat transfer in a gas turbine can combustor}, volume={3}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-77953206941&partnerID=MN8TOARS}, DOI={10.1115/GT2009-59377}, abstractNote={Experiments and numerical computations are performed to investigate the convective heat transfer characteristics of a gas turbine can combustor under cold flow conditions in a Reynolds number range between 50,000 and 500,000 with a characteristic swirl number of 0.7. It is observed that the flow field in the combustor is characterized by an expanding swirling flow which impinges on the liner wall close to the inlet of the combustor. The impinging shear layer is responsible for the peak location of heat transfer augmentation. It is observed that as Reynolds number increases from 50,000 to 500,000, the peak heat transfer augmentation ratio (compared to fully-developed pipe flow) reduces from 10.5 to 2.75. This is attributed to the reduction in normalized turbulent kinetic energy in the impinging shear layer which is strongly dependent on the swirl number that remains constant at 0.7 with Reynolds number. Additionally, the peak location does not change with Reynolds number since the flow structure in the combustor is also a function of the swirl number. The size of the corner recirculation zone near the combustor liner remains the same for all Reynolds numbers and hence the location of shear layer impingement and peak augmentation does not change.}, number={PART B}, booktitle={Proceedings of the ASME Turbo Expo}, author={Patil, S. and Abraham, S. and Tafti, D. and Ekkad, S. and Kim, Y. and Dutta, P. and Moon, H.-K. and Srinivasan, R.}, year={2009}, pages={1363–1371} }
 @article{dhungel_lu_phillips_ekkad_heidmann_2009, title={Film Cooling From a Row of Holes Supplemented With Antivortex Holes}, volume={131}, ISSN={0889-504X 1528-8900}, url={http://dx.doi.org/10.1115/1.2950059}, DOI={10.1115/1.2950059}, abstractNote={The primary focus of this paper is to study the film cooling performance for a row of cylindrical holes each supplemented with two symmetrical antivortex holes, which branch out from the main holes. The antivortex design was originally developed at NASA-Glenn Research Center by James Heidmann, coauthor of this paper. This “antivortex” design is unique in that it requires only easily machinable round holes, unlike shaped film cooling holes and other advanced concepts. The hole design is intended to counteract the detrimental vorticity associated with standard circular cross-section film cooling holes. The geometry and orientation of the antivortex holes greatly affect the cooling performance downstream, which is thoroughly investigated. By performing experiments at a single mainstream Reynolds number of 9683 based on the freestream velocity and film hole diameter at four different coolant-to-mainstream blowing ratios of 0.5, 1, 1.5, and 2 and using the transient IR thermography technique, detailed film cooling effectiveness and heat transfer coefficients are obtained simultaneously from a single test. When the antivortex holes are nearer the primary film cooling holes and are developing from the base of the primary holes, better film cooling is accomplished as compared to other antivortex hole orientations. When the antivortex holes are laid back in the upstream region, film cooling diminishes considerably. Although an enhancement in heat transfer coefficient is seen in cases with high film cooling effectiveness, the overall heat flux ratio as compared to standard cylindrical holes is much lower. Thus cases with antivortex holes placed near the main holes certainly show promising results.}, number={2}, journal={Journal of Turbomachinery}, publisher={ASME International}, author={Dhungel, Alok and Lu, Yiping and Phillips, Wynn and Ekkad, Srinath V. and Heidmann, James}, year={2009}, month={Jan} }
 @inproceedings{abraham_navin_ekkad_2009, title={Film cooling study of novel orthogonal entrance and shaped exit holes}, volume={3}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-77953215706&partnerID=MN8TOARS}, DOI={10.1115/GT2009-60003}, abstractNote={Film cooling effectiveness depends on several geometrical parameters like location on the airfoil, exit shape, orientation and arrangement of the holes. The focus of this investigation is to propose and explore a new film cooling hole geometry. The adiabatic film cooling effectiveness is determined experimentally, downstream of the exit of the film cooling holes on a flat plate using a steady state IR thermography technique. Coolant holes that are perpendicular to the direction of flow detach from the surface and enhance the heat transfer coefficient on the turbine blade without providing any coolant coverage, while angled holes along the mainstream direction result in superior film cooling effectiveness and lower heat transfer to the surface. The objective of this study is to examine the external cooling effects using coolant holes that are a combination of both angled shaped holes as well as perpendicular holes. The inlet of the coolant hole is kept perpendicular to the direction of flow to enhance the internal side heat transfer coefficient and the exit of the coolant hole is expanded and angled along the mainstream flow to prevent the coolant jet from lifting off from the blade external surface. A total of six different cases with variations in exit shape geometry are investigated at different blowing ratios (BR varying from 0.5 to 2.0). Results suggest that the film cooling effectiveness values obtained from these geometries are comparable with those of conventional angled holes. With the added advantage of enhanced heat transfer coefficient on the coolant channel internal side, as proven earlier by Byerley [3], overall superior cooling is accomplished. Furthermore this shaped hole can be made using the same technology being presently used in the industry.}, number={PART B}, booktitle={Proceedings of the ASME Turbo Expo}, author={Abraham, S. and Navin, A.R. and Ekkad, S.V.}, year={2009}, pages={829–838} }
 @article{esposito_ekkad_kim_dutta_2009, title={Novel Jet Impingement Cooling Geometry for Combustor Liner Backside Cooling}, volume={1}, ISSN={1948-5085 1948-5093}, url={http://dx.doi.org/10.1115/1.3202799}, DOI={10.1115/1.3202799}, abstractNote={Impinging jets are commonly used to enhance heat transfer in modern gas turbine engines. Impinging jets used in turbine blade cooling typically operate at lower Reynolds numbers in the range of 10,000–20,000. In combustor liner cooling, the Reynolds numbers of the jets can be as high as 60,000. The present study is aimed at experimentally testing two different styles of jet impingement geometries to be used in backside combustor cooling. The higher jet Reynolds numbers lead to increased overall heat transfer characteristics, but also an increase in crossflow caused by spent air. The crossflow air has the effect of rapidly degrading the downstream jets at high jet Reynolds numbers. In an effort to increase the efficiency of the coolant air, configurations designed to reduce the harmful effects of crossflow are studied. Two main designs, a corrugated wall and extended port, are tested. Local heat transfer coefficients are obtained for each test section through a transient liquid crystal technique. Results show that both geometries reduce the crossflow induced degradation on downstream jets, but different geometries perform better at different Reynolds numbers. The extended port and corrugated wall configurations show similar benefits at the high Reynolds numbers, but at low Reynolds numbers, the extended port design increases the overall level of heat transfer. This is attributed to the developed jet velocity profile at the tube exit. The best possible explanation is that the benefit of the developed jet velocity profile diminishes as jet velocities rise and the air has lesser time to develop prior to exiting.}, number={2}, journal={Journal of Thermal Science and Engineering Applications}, publisher={ASME International}, author={Esposito, E. I. and Ekkad, S. V. and Kim, Yong and Dutta, Partha}, year={2009}, month={Jun}, pages={1–8} }
 @inproceedings{parida_ekkad_2009, title={Numerical prediction of flow and heat transfer rates in metal based microchannels using lattice Boltzmann method}, volume={1}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-70149096230&partnerID=MN8TOARS}, booktitle={2008 Proceedings of the ASME Summer Heat Transfer Conference, HT 2008}, author={Parida, P.R. and Ekkad, S.V.}, year={2009}, pages={233–241} }
 @article{revankar_ekkad_2010, title={Selected Papers From the 19th National & 8th ISHMT-ASME Heat and Mass Transfer Conference}, volume={31}, ISSN={0145-7632 1521-0537}, url={http://dx.doi.org/10.1080/01457630903408268}, DOI={10.1080/01457630903408268}, abstractNote={We are glad to present this special issue of Heat Transfer Engineering with a selection of papers presented at the 19th National & 8th ISHMT-ASME Heat and Mass Transfer Conference, held January 3–5, 2008. The conference was jointly sponsored by the Indian Society of Heat and Mass Transfer (ISHMT) and the American Society of Mechanical Engineers (ASME) and was held at the Jawaharlal Nehru Technological University (JNTU), College of Engineering Kukatpally in Hyderabad, India.}, number={6}, journal={Heat Transfer Engineering}, publisher={Informa UK Limited}, author={Revankar, Shripad T. and Ekkad, Srinath V.}, year={2010}, month={May}, pages={431–432} }
 @article{heidmann_ekkad_2008, title={A Novel Antivortex Turbine Film-Cooling Hole Concept}, volume={130}, ISSN={0889-504X 1528-8900}, url={http://dx.doi.org/10.1115/1.2777194}, DOI={10.1115/1.2777194}, abstractNote={A novel turbine film-cooling hole shape has been conceived and designed at NASA Glenn Research Center. This “antivortex” design is unique in that it requires only easily machinable round holes, unlike shaped film-cooling holes and other advanced concepts. The hole design is intended to counteract the detrimental vorticity associated with standard circular cross-section film-cooling holes. This vorticity typically entrains hot freestream gas and is associated with jet separation from the turbine blade surface. The antivortex film-cooling hole concept has been modeled computationally for a single row of 30 deg angled holes on a flat surface using the 3D Navier–Stokes solver GLENN-HT. A blowing ratio of 1.0 and density ratios of 1.05 and 2.0 are studied. Both film effectiveness and heat transfer coefficient values are computed and compared to standard round hole cases for the same blowing rates. A net heat flux reduction is also determined using both the film effectiveness and heat transfer coefficient values to ascertain the overall effectiveness of the concept. An improvement in film effectiveness of about 0.2 and in net heat flux reduction of about 0.2 is demonstrated for the antivortex concept compared to the standard round hole for both blowing ratios. Detailed flow visualization shows that as expected, the design counteracts the detrimental vorticity of the round hole flow, allowing it to remain attached to the surface.}, number={3}, journal={Journal of Turbomachinery}, publisher={ASME International}, author={Heidmann, James D. and Ekkad, Srinath}, year={2008}, month={May} }
 @article{lu_dhungel_ekkad_bunker_2008, title={Effect of Trench Width and Depth on Film Cooling From Cylindrical Holes Embedded in Trenches}, volume={131}, ISSN={0889-504X 1528-8900}, url={http://dx.doi.org/10.1115/1.2950057}, DOI={10.1115/1.2950057}, abstractNote={The present study is an experimental investigation of film cooling from cylindrical holes embedded in transverse trenches. Different trench depths are considered with two trench widths. Trench holes can occur when blades are coated with thermal barrier coating (TBC) layers. The film-hole performance and behavior will be different for the trench holes compared to standard cylindrical holes that are flush with the surface. The trench width and depth depend on the mask region and the thickness of the TBC layer. Detailed heat transfer coefficient and film effectiveness measurements are obtained simultaneously using a single test transient IR thermography technique. The study is performed at a single mainstream Reynolds number based on freestream velocity and film-hole diameter of 11,000 at four different coolant-to-mainstream blowing ratios of 0.5, 1.0, 1.5, and 2.0. The results show that film effectiveness is greatly enhanced by the trenching due to the improved two-dimensional nature of the film and lateral spreading. The detailed heat transfer coefficient and film effectiveness contours provide a clear understanding of the jet-mainstream interactions for different hole orientations. Computational fluid dynamics simulation using FLUENT was also performed to determine the jet-mainstream interactions to better understand the surface heat transfer coefficient and film effectiveness distributions.}, number={1}, journal={Journal of Turbomachinery}, publisher={ASME International}, author={Lu, Yiping and Dhungel, Alok and Ekkad, Srinath V. and Bunker, Ronald S.}, year={2008}, month={Sep} }
 @article{mei_parida_jiang_meng_ekkad_2008, title={Fabrication, assembly, and testing of Cu- and Al-based microchannel heat exchangers}, volume={17}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-49149131436&partnerID=MN8TOARS}, DOI={10.1109/JMEMS.2008.924276}, abstractNote={Metal-based microchannel heat exchangers (MHEs) are of current interest due to the combination of high heat transfer performance and improved mechanical integrity. Efficient methods for fabrication and assembly of functional metal-based MHEs are essential to ensure the economic viability of such devices. In this paper, the results on fabrication, assembly, and heat transfer testing of Cu- and Al-based MHE prototypes are reported. Efficient fabrication of Cu- and Al-based high-aspect-ratio microscale structures (HARMSs) has been achieved through molding replication using surface-engineered metallic mold inserts. Replicated metallic HARMSs were assembled through eutectic bonding to form entirely Cu- and Al-based MHE prototypes, on which heat transfer tests were conducted to determine the average rate of heat transfer from electrically heated Cu blocks placed outside the MHEs to water flowing within the molding replicated microchannel arrays. Experimentally observed heat transfer rates are higher as compared to those from previous studies on microchannel devices with similar geometries. The potential influence of microchannel surface profile on heat transfer rates is discussed. The present results illustrate the potential of metal-based MHEs in wide-ranging applications.}, number={4}, journal={Journal of Microelectromechanical Systems}, author={Mei, F. and Parida, P.R. and Jiang, J. and Meng, W.J. and Ekkad, S.V.}, year={2008}, pages={869–881} }
 @article{lu_dhungel_ekkad_bunker_2009, title={Film Cooling Measurements for Cratered Cylindrical Inclined Holes}, volume={131}, ISSN={0889-504X 1528-8900}, url={http://dx.doi.org/10.1115/1.2950055}, DOI={10.1115/1.2950055}, abstractNote={Film cooling performance is studied for cylindrical holes embedded in craters. Different crater geometries are considered for a typical crater depth. Cratered holes may occur when blades are coated with thermal barrier coating layers by masking the hole area during thermal barrier coating (TBC) spraying, resulting in a hole surrounded by a TBC layer. The film performance and behavior is expected to be different for the cratered holes compared to standard cylindrical holes. Detailed heat transfer coefficient and film effectiveness measurements are obtained simultaneously using a single test transient IR thermography technique. The study is performed at a single mainstream Reynolds number based on freestream velocity and film-hole diameter of 11,000 at four different coolant-to-mainstream blowing ratios of 0.5, 1.0, 1.5, and 2.0. The results show that film cooling effectiveness is slightly enhanced by cratering of holes, but a substantial increase in heat transfer enhancement negates the benefits of higher film effectiveness. Three different crater geometries are studied and compared to a base line flush cylindrical hole, a trenched hole, and a typical diffuser shaped hole. Computational fluid dynamics simulation using FLUENT was also performed to determine the jet-mainstream interactions associated with the experimental surface measurements.}, number={1}, journal={Journal of Turbomachinery}, publisher={ASME International}, author={Lu, Yiping and Dhungel, Alok and Ekkad, Srinath V. and Bunker, Ronald S.}, year={2009}, month={Jan}, pages={011005} }
 @article{lu_esposito_ekkad_2008, title={Predictions of Flow and Heat Transfer in Low Emission Combustors}, volume={29}, ISSN={0145-7632 1521-0537}, url={http://dx.doi.org/10.1080/01457630701825515}, DOI={10.1080/01457630701825515}, abstractNote={Flow and heat transfer predictions in modern low emission combustors are critical to maintaining the liner wall at reasonable temperatures. This study is the first to focus on a critical issue for combustor design. The objective of this paper is to understand the effect of different swirl angle for a dry low emission (DLE) combustor on flow and heat transfer distributions. This paper provides the effect of fuel nozzle swirl angle on velocity distributions, temperature, and surface heat transfer coefficients. A simple test model is investigated with flow through fuel nozzles without reactive flow. The fuel nozzle angle is varied to obtain different swirl conditions inside the combustor. The effect of flow Reynolds number and swirl number are investigated using FLUENT. Different RANS-based turbulence models are tested to determine the ability of these models to predict the swirling flow. For comparison, different turbulence models such as standard k − ε, realizable k − ε, and shear stress transport (SST) k−ω turbulence model were studied for non-reactive flow conditions. The results show that, for a high degree swirl flow, the SST k−ω model can provide more reasonable predictions for recirculation and high velocity gradients. With increasing swirl angle, the average surface heat transfer coefficient increases while the average static temperature will decrease. Preliminary analysis shows that the k−ω model is the best model for predicting swirling flows. Also critical is the effect of the swirling flows on the liner wall heat transfer. The strength and magnitude of the swirl determines the local heat transfer maxima location. This location needs to be cooled more effectively by various cooling schemes.}, number={4}, journal={Heat Transfer Engineering}, publisher={Informa UK Limited}, author={Lu, Yiping and Esposito, Eric and Ekkad, Srinath V.}, year={2008}, month={Apr}, pages={375–384} }
 @inproceedings{lu_ekkad_bunker_2008, title={Trench film cooling - Effect of trench downstream edge and hole spacing}, volume={4}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-69949112020&partnerID=MN8TOARS}, DOI={10.1115/GT2008-50606}, abstractNote={The present study is a continuation of an experimental investigation of film cooling from cylindrical holes embedded in transverse trenches. In this study, focus is on varying the downstream edge of the trench by angling it along the flow. Different edge angles are studied for the same trench depth. Also, the effect of hole spacing is considered for one of the standard trenches from previous studies to understand the effect of trenching on overall coolant usage. Detailed heat transfer coefficient and film effectiveness measurements are obtained simultaneously using a single test transient IR thermography technique. The study is performed at a single mainstream Reynolds number based on free-stream velocity and film hole diameter of 11000 at four different coolant-to-mainstream blowing ratios of 0.5, 1.0, 1.5 and 2.0. The results show that film effectiveness is greatly enhanced by the trenching due to improved two dimensional nature of the film and lateral spreading. The detailed heat transfer coefficient and film effectiveness contours provide a clear understanding of the jet-mainstream interactions for different hole orientations. The effect of edge angling is minimal on the overall cooling effectiveness but may have an impact on jet-mainstream interaction aerodynamic losses.}, number={PART A}, booktitle={Proceedings of the ASME Turbo Expo}, author={Lu, Y. and Ekkad, S.V. and Bunker, R.S.}, year={2008}, pages={563–569} }
 @inproceedings{heidmann_ekkad_2007, title={A novel anti-vortex turbine film cooling hole concept}, volume={4 PART A}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-34548707084&partnerID=MN8TOARS}, DOI={10.1115/GT2007-27528}, abstractNote={A novel turbine film cooling hole shape has been conceived and designed at NASA Glenn Research Center. This “anti-vortex” design is unique in that it requires only easily machinable round holes, unlike shaped film cooling holes and other advanced concepts. The hole design is intended to counteract the detrimental vorticity associated with standard circular cross-section film cooling holes. This vorticity typically entrains hot freestream gas and is associated with jet separation from the turbine blade surface. The anti-vortex film cooling hole concept has been modeled computationally for a single row of 30 degree angled holes on a flat surface using the 3D Navier-Stokes solver Glenn-HT. A blowing ratio of 1.0 and density ratios of 1.05 and 2.0 are studied. Both film effectiveness and heat transfer coefficient values are computed and compared to standard round hole cases for the same blowing rates. A net heat flux reduction is also determined using both the film effectiveness and heat transfer coefficient values to ascertain the overall effectiveness of the concept. An improvement in film effectiveness of about 0.2 and in net heat flux reduction of about 0.2 is demonstrated for the anti-vortex concept compared to the standard round hole for both blowing ratios. Detailed flow visualization shows that as expected, the design counteracts the detrimental vorticity of the round hole flow, allowing it to remain attached to the surface.}, booktitle={Proceedings of the ASME Turbo Expo}, author={Heidmann, J.D. and Ekkad, S.}, year={2007}, pages={487–496} }
 @inproceedings{esposito_ekkad_kim_dutta_2007, title={Comparing extended port and corrugated wall jet impingement geometry for combustor liner backside cooling}, volume={4 PART B}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-34548764929&partnerID=MN8TOARS}, DOI={10.1115/GT2007-27390}, abstractNote={Impinging jets are commonly used to enhance heat transfer in modern gas turbine engines. Impinging jets used in turbine blade cooling typically operate at lower Reynolds numbers in the range of 10,000 to 20,000. In combustor liner cooling, the Reynolds numbers of the jets can be as high as 60,000. The present study is aimed at experimentally testing two different styles of jet impingement geometries to be used in backside combustor cooling. The higher jet Reynolds numbers lead to increased overall heat transfer characteristics, but also an increase in crossflow caused by spent air. The crossflow air has the effect of rapidly degrading the downstream jets at high jet Reynolds numbers. In an effort to increase the efficiency of the coolant air, configurations designed to reduce the harmful effects of crossflow are studied. Two main designs, a corrugated wall and extended ports, are tested. Local heat transfer coefficients are obtained for each test section through a transient liquid crystal technique. Results show that both geometries reduce the crossflow induced degradation on downstream jets, but the individual geometries perform better at different Reynolds numbers. The extended port and corrugated wall configurations show similar benefits at the high Reynolds numbers, but at low Reynolds numbers, the extended port design increases the overall level of heat transfer. This is attributed to the developed jet velocity profile at the tube exit. The benefit of the developed jet velocity profile diminishes as jet velocities rise and the air has less time to develop prior to exiting.}, booktitle={Proceedings of the ASME Turbo Expo}, author={Esposito, E.I. and Ekkad, S.V. and Kim, Y. and Dutta, P.}, year={2007}, pages={1347–1354} }
 @inproceedings{lu_dhungel_ekkad_bunker_2007, title={Effect of trench width and depth on film cooling from cylindrical holes embedded in trenches}, volume={4 PART A}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-34548710737&partnerID=MN8TOARS}, DOI={10.1115/GT2007-27388}, abstractNote={The present study is an experimental investigation of film cooling from cylindrical holes embedded in transverse trenches. Different trench depths are considered with two trench widths. Trench holes can occur when blades are coated with thermal barrier coating (TBC) layers. The film hole performance and behavior will be different for the trench holes compared to standard cylindrical holes that are flush with the surface. The trench width and depth depends on the mask region and the thickness of the TBC layer. Detailed heat transfer coefficient and film effectiveness measurements are obtained simultaneously using a single test transient IR thermography technique. The study is performed at a single mainstream Reynolds number based on free-stream velocity and film hole diameter of 11000 at four different coolant-to-mainstream blowing ratios of 0.5, 1.0, 1.5 and 2.0. The results show that film effectiveness is greatly enhanced by the trenching due to improved two dimensional nature of the film and lateral spreading. The detailed heat transfer coefficient and film effectiveness contours provide a clear understanding of the jet-mainstream interactions for different hole orientations. CFD simulation using Fluent was also performed to determine the jet mainstream interactions to better understand the surface heat transfer coefficient and film effectiveness distributions.}, booktitle={Proceedings of the ASME Turbo Expo}, author={Lu, Y. and Dhungel, A. and Ekkad, S.V. and Bunker, R.S.}, year={2007}, pages={339–349} }
 @inproceedings{dhungel_lu_phillips_ekkad_heidmann_2007, title={Film cooling from a row of holes supplemented with anti vortex holes}, volume={4 PART A}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-34548772366&partnerID=MN8TOARS}, DOI={10.1115/GT2007-27419}, abstractNote={The primary focus of this paper is to study the film cooling performance for a row of cylindrical holes each supplemented with two symmetrical anti vortex holes which branch out from the main holes. The anti-vortex design was originally developed at NASA-Glenn Research Center by Dr. James Heidmann, co-author of this paper. This “anti-vortex” design is unique in that it requires only easily machinable round holes, unlike shaped film cooling holes and other advanced concepts. The hole design is intended to counteract the detrimental vorticity associated with standard circular cross-section film cooling holes. The geometry and orientation of the anti vortex holes greatly affect the cooling performance downstream, which is thoroughly investigated. By performing experiments at a single mainstream Reynolds number of 9683 based on the free stream velocity and film hole diameter at four different coolant-to-mainstream blowing ratio of 0.5, 1, 1.5, 2 and using the transient IR thermography technique, detailed film cooling effectiveness and heat transfer coefficients are obtained simultaneously from a single test. When the anti vortex holes are nearer to the primary film cooling holes and are developing from the base of the primary holes, better film cooling is accomplished as compared to other anti vortex hole orientations. When the anti vortex holes are laid back in the upstream region, film cooling diminishes considerably. Although an enhancement in heat transfer coefficient is seen in cases with high film cooling effectiveness, the overall heat flux ratio as compared to standard cylindrical holes is much lower. Thus cases with anti vortex holes placed near the main holes certainly show promising results.}, booktitle={Proceedings of the ASME Turbo Expo}, author={Dhungel, A. and Lu, Y. and Phillips, W. and Ekkad, S.V. and Heidmann, J.}, year={2007}, pages={375–384} }
 @inproceedings{lu_dhungel_ekkad_bunker_2007, title={Film cooling measurements for cratered cylindrical inclined holes}, volume={4 PART A}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-34548798091&partnerID=MN8TOARS}, DOI={10.1115/GT2007-27386}, abstractNote={Film cooling performance is studied for cylindrical holes embedded in craters. Different crater geometries are considered for a typical crater depth. Cratered holes may occur when blades are coated with thermal barrier coating layers by masking the hole area during TBC spraying resulting in hole surrounded by TBC layer. The film performance and behavior is expected to be different for the cratered holes compared to standard cylindrical holes. Detailed heat transfer coefficient and film effectiveness measurements are obtained simultaneously using a single test transient IR thermography technique. The study is performed at a single mainstream Reynolds number based on free-stream velocity and film hole diameter of 11000 at four different coolant-to-mainstream blowing ratios of 0.5, 1.0, 1.5 and 2.0. The results show that film cooling effectiveness is slightly enhanced by cratering of holes but a substantial increase in heat transfer enhancement negates the benefits of higher film effectiveness. Three different crater geometries are studied and compared to a baseline flush cylindrical hole, a trenched hole, and a typical diffuser shaped hole. CFD simulation using Fluent was also performed to determine the jet-mainstream interactions associated with the experimental surface measurements.}, booktitle={Proceedings of the ASME Turbo Expo}, author={Lu, Y. and Dhungel, A. and Ekkad, S.V. and Bunker, R.S.}, year={2007}, pages={329–338} }
 @article{lu_allison_ekkad_2007, title={Turbine blade showerhead film cooling: Influence of hole angle and shaping}, volume={28}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-34548608683&partnerID=MN8TOARS}, DOI={10.1016/j.ijheatfluidflow.2007.01.002}, abstractNote={Detailed film cooling measurements are presented on a turbine blade leading edge model with three rows of showerhead holes. Experiments are run at a mainstream Reynolds number of 19,500 based on cylindrical leading edge diameter. One row of holes is located on the stagnation line and the other two rows are located at 615° on either side of the stagnation line. The three rows have compound angle holes angled 90° in the flow direction, 30° along the spanwise direction, and the two holes on either side of the stagnation row have and additional angle of 0°, 30°, and 45° in the transverse direction. The effect of hole shaping of the 30° and 45° holes is also considered. Detailed heat transfer coefficient and film effectiveness measurements are obtained using a transient infrared thermography technique. The results are compared to determine the advantages of shaping the compound angle for rows of holes off stagnation row. Results show that, the additional compound angle in the transverse direction for the two rows adjacent to the stagnation row provide significantly higher film effectiveness than the typical leading edge holes with only two angles. Results also show that, the shaping of showerhead holes provides higher film effectiveness than just adding an additional compound angle in the transverse direction and significantly higher effectiveness than the baseline typical leading edge geometry. Heat transfer coefficients are higher as the spanwise angle for this study is larger than typical leading edge geometries with an angle of 30° compared to 20° for other studies.}, number={5}, journal={International Journal of Heat and Fluid Flow}, author={Lu, Y. and Allison, D. and Ekkad, S.V.}, year={2007}, pages={922–931} }
 @inproceedings{lu_ekkad_2007, title={Understanding the effect of trenching on film cooling}, volume={3}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-43449133295&partnerID=MN8TOARS}, DOI={10.1115/HT2007-32598}, abstractNote={Recently, there has been a strong focus on film cooling holes embedded in trenches. In this study, film cooling predictions are used to understand the mechanisms of the jets that exit these trenched holes. The present work employs RSM (Reynolds stress transport models) for simulation of turbulent flows in film cooling and the simulation was run using FLUENT computer code. Comparisons are made with experimental data for the film effectiveness distributions. Results show that the film cooling jet exiting the trenched hole shows more two-dimensional flow than the typical cylindrical holes. The jet appears to remain closer to the surface providing more coverage to the surface.}, booktitle={2007 Proceedings of the ASME/JSME Thermal Engineering Summer Heat Transfer Conference - HT 2007}, author={Lu, Y. and Ekkad, S.V.}, year={2007}, pages={591–596} }
 @inbook{ekkad_2006, title={Calculation of Convective Heat Transfer Coefficient Using a Semi-infinite Solid Assumption}, booktitle={Heat-transfer calculations}, publisher={McGraw-Hill}, author={Ekkad, S.V.}, editor={Kutz, MyerEditor}, year={2006} }
 @inproceedings{esposito_ekkad_dutta_kim_greenwood_2006, title={Corrugated wall jet impingement geometry for combustor liner backside cooling}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84920630604&partnerID=MN8TOARS}, DOI={10.1115/IMECE2006-13300}, abstractNote={The present study investigates alternative jet impingement geometries aimed at the reduction of detrimental crossflow effects for use in combustor liner backside cooling. Through the use of a corrugated wall design, the spent air of upstream jets is routed past downstream jets with minimal interference. Three configurations of the design are studied. First, the jet spacing is held constant, and the design of the corrugations is changed for sparse arrays. The second part of the study studied the effects of the corrugated wall on dense arrays. The average jet Reynolds number, Red, is varied and tested for 20000, 40000, and 60000. Local Nusselt number distributions were evaluated using a transient liquid crystal technique. The results show that the corrugated wall design can significantly reduce the negative effects of crossflow especially at higher jet Reynolds numbers. Further, the design of the corrugations has a substantial impact on the performance of the geometry. The corrugated wall geometries with smaller bypass channels outperformed the geometries tested with larger channels. The use of corrugated jet impingement configurations would allow larger jet impingement arrays without sacrificing heat transfer effectiveness.}, booktitle={American Society of Mechanical Engineers, Heat Transfer Division, (Publication) HTD}, author={Esposito, E.I. and Ekkad, S.V. and Dutta, P. and Kim, Y. and Greenwood, S.}, year={2006} }
 @article{ekkad_ou_rivir_2006, title={Effect of jet pulsation and duty cycle on film cooling from a single jet on a leading edge model}, volume={128}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-33845689956&partnerID=MN8TOARS}, DOI={10.1115/1.2185122}, abstractNote={The effect of jet pulsation and duty cycle on film effectiveness and heat transfer was investigated on a film hole located on the circular leading edge of a blunt body. A transient infrared technique was used to measure both heat transfer coefficients and film effectiveness from a single test. Detailed Frossling number and film effectiveness distributions were obtained for all flow conditions. Jet pulsing frequencies of 5 Hz, 10 Hz, and 20 Hz have been studied. The effect of duty cycle created by the valve opening and closing times was also set at different levels of 10%, 25%, 50%, and 75% of designated 100% fully open condition for different blowing ratios from 0.25 to 2.0. The combination of pulse frequency and duty cycle was investigated for different blowing ratios on a single leading edge hole located at 22 deg from geometric leading edge. Results indicate that higher effectiveness and lower heat transfer coefficients are obtained at the reduced blowing ratios, which result from reduced duty cycles. The effect of varying the pulsing frequency from 5 Hz to 20 Hz is not discernable beyond the level of experimental uncertainty. Effective blowing ratio due to lowering of the duty cycle at a given blowing ratio seems to play a more important role in combination with pulsing, which provides improved cooling effectiveness at lower heat transfer coefficients.}, number={3}, journal={Journal of Turbomachinery}, author={Ekkad, S.V. and Ou, S. and Rivir, R.B.}, year={2006}, pages={564–571} }
 @article{lu_nasir_faucheaux_ekkad_2006, title={Film cooling measurements for novel hole configurations}, volume={128}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84969860204&partnerID=MN8TOARS}, DOI={10.1115/1.2221300}, abstractNote={Experimental Procedure: •Blower is set appropriately for required mainstream velocity •Heater is turned on and allowed to heat the air to a desired mainstream temperature •The coolant air is provided from separate compressed air supply and is metered for flow measurement •The mainstream and coolant are triggered at the same instant when the IR system starts taking images and saving to hard drive at set intervals •Images are saved then processed to calculate the heat transfer coefficients and film effectiveness using the theory}, number={8}, journal={Journal of Heat Transfer}, author={Lu, Y. and Nasir, H. and Faucheaux, D. and Ekkad, S.V.}, year={2006}, pages={737} }
 @inproceedings{lu_ekkad_2006, title={Film cooling predictions for cratered cylindrical inclined holes}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84920632974&partnerID=MN8TOARS}, DOI={10.1115/IMECE2006-14406}, abstractNote={Film cooling is studied for cylindrical holes embedded in craters. Different crater geometries are considered for a typical crater depth. Cratered holes occur when blades are coated with thermal barrier coating layers by masking the hole area during TBC spraying. The film performance and behavior will be different for the cratered holes compared to standard cylindrical holes. FLUENT is used to simulate film cooling for the different crater geoemtries and compared to baseline uncratered cylindrical holes. Film effectiveness is reduced by the cratering compared to earlier studies which had two-dimensional trenches instead of craters. The study is performed at a single mainstream Reynolds number based on free-stream velocity and film hole diameter of 11000 at one different coolant-to-mainstream blowing ratio of 1.0.}, booktitle={American Society of Mechanical Engineers, Heat Transfer Division, (Publication) HTD}, author={Lu, Y. and Ekkad, S.V.}, year={2006} }
 @inproceedings{lu_allison_ekkad_2006, title={Influence of hole angle and shaping on leading edge showerhead film cooling}, volume={3 PART A}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-33750852255&partnerID=MN8TOARS}, DOI={10.1115/GT2006-90370}, abstractNote={Detailed film cooling measurements are presented on a turbine blade leading edge model with three rows of showerhead holes. Experiments are run at a mainstream Reynolds number of 19,500 based on cylindrical leading edge diameter. One row of holes is located on the stagnation line and the other two rows are located at ±15° on either side of the stagnation line. The three rows have compound angle holes angled 90° in the flow direction, 30° along the spanwise direction, and the two holes on either side of the stagnation row have and additional angle of 0°, 30°, and 45° in the transverse direction. The effect of hole shaping of the 30° and 45° holes is also considered. Detailed heat transfer coefficient and film effectiveness measurements are obtained using a transient infrared thermography technique. The results are compared to determine the advantages of shaping the compound angle for rows of holes off stagnation row. Results show that, the additional compound angle in the transverse direction for the two rows adjacent to the stagnation row provide significantly higher film effectiveness than the typical leading edge holes with only two angles. Results also show that, the shaping of showerhead holes provides higher film effectiveness than just adding an additional compound angle in the transverse direction and significantly higher effectiveness than the baseline typical leading edge geometry. Heat transfer coefficients are higher as the spanwise angle for this study is larger than typical leading edge geometries with an angle of 30° compared to 20° for other studies.}, booktitle={Proceedings of the ASME Turbo Expo}, author={Lu, Y. and Allison, D. and Ekkad, S.V.}, year={2006}, pages={375–382} }
 @article{esposito_ekkad_2006, title={Jet impingement heat transfer visualization using a steady state liquid crystal method}, volume={128}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-52949104145&partnerID=MN8TOARS}, DOI={10.1115/1.2221301}, abstractNote={Experimental Procedure: •Blower is set appropriately for required jet Reynolds number •Heater is turned on and allowed to reach steady state •A picture is taken of the liquid crystal coated test plate and heater amperage and voltage measured •Heater power is incrementally increased and additional pictures taken to capture temperature and heater flux data at every point in the array •Pictures are converted to Hue and liquid crystal calibration curve used to determine temperature at corresponding point}, number={8}, journal={Journal of Heat Transfer}, author={Esposito, E. and Ekkad, S.V.}, year={2006}, pages={738} }
 @inproceedings{lu_ekkad_2006, title={Predictions of film cooling from cylindrical holes embedded in trenches}, volume={2}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-33845532553&partnerID=MN8TOARS}, booktitle={Collection of Technical Papers - 9th AIAA/ASME Joint Thermophysics and Heat Transfer Conference Proceedings}, author={Lu, Y. and Ekkad, S.V.}, year={2006}, pages={1361–1369} }
 @inproceedings{ibrahim_kochuparambil_ekkad_simon_2005, title={CFD for jet impingement heat transfer with single jets and arrays}, volume={3 PART A}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-27744594135&partnerID=MN8TOARS}, DOI={10.1115/GT2005-68341}, abstractNote={CFD experiments were conducted for heat transfer with jet impingement over solid surfaces. The parameters include: 1) Jet Reynolds number from 3,000 to 23,000, 2) Jet-to-target-plate spacing (z/d), from 2 to 14 (single jet), d is jet diameter, 3) Target plate shape: 3a) flat, 3b) concave, 3c) convex, (single jet), 4) One row of seven jets impinging on a flat surface, the channel has one end closed (at 24d away from the most upstream jet axis), 5) Three rows of seven jets each in-line arrangement impinging on a flat surface, the channel has one end closed (at 24d away from the most upstream jet axis). Four CFD models (utilizing FLUENT commercial code) have been considered: 1) laminar flow (no turbulent transport), and turbulent flow with turbulence modeling by 2) the standard k–ε model, 3) the k–ω model, and 4) the v2–f model. The predictions of Nu number for each case were compared with experimental data available from the literature. It is shown that the v2–f model gives the best overall performance, though the k–ω model gives good predictions for most of the flow, with the exception of near the stagnation zone for some cases. The models are in much better agreement (with the data) as z/d grows and at larger radial locations from the jet axis, as expected. For multiple jets in one row (z/d = 2), again the v2–f showed the best overall agreement with the experimental data. The k–ω model is not as good while k–ε clearly overpredicts the Nusselt numbers. For multiple jets in three inline rows (z/d = 5), all the three models were in overall agreement with the experimental data. However, k–ε and k–ω exhibit an important phenomenon, reported by the experiments: a decrease of the stagnation Nu from the upstream jet to the downstream ones. The v2–f model did not reproduce this feature.}, booktitle={Proceedings of the ASME Turbo Expo}, author={Ibrahim, M.B. and Kochuparambil, B.J. and Ekkad, S.V. and Simon, T.W.}, year={2005}, pages={359–373} }
 @article{nasir_ekkad_bunker_2007, title={Effect of tip and pressure side coolant injection on heat transfer distributions for a plane and recessed tip}, volume={129}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-34248403339&partnerID=MN8TOARS}, DOI={10.1115/1.2366540}, abstractNote={The present study investigates the effects of coolant injection on adiabatic film effectiveness and heat transfer coefficients from a plane and recessed tip of a high pressure turbine first stage rotor blade. Three cases where coolant is injected from (a) five orthogonal holes located along the camber line, (b) seven angled holes located near the blade tip along the pressure side, and (c) combination cases when coolant is injected from both tip and pressure side holes were studied. The pressure ratio (inlet total pressure to exit static pressure for the cascade) across the blade row was 1.2, and the experiments were run in a blow-down test rig with a four-blade linear cascade. The Reynolds number based on cascade exit velocity and axial chord length was 8.61×105 and the inlet and exit Mach numbers were 0.16 and 0.55, respectively. A transient infrared technique was used to measure adiabatic film effectiveness and heat transfer coefficient simultaneously for three blowing ratios of 1.0, 2.0, and 3.0. For all the cases, gap-to-blade span ratio of 1% was used. The depth-to-blade span ratio of 0.0416 was used for the recessed tip. Pressure measurements on the shroud were also taken to characterize the leakage flow and understand the heat transfer distributions. For tip injection, when blowing ratio increases from 1.0 to 2.0, film effectiveness increases for both plane and recessed tip and heat transfer coefficient decreases for both plane and recessed tip. At blowing ratio 3.0, lift-off is observed for both cases. In case of pressure side coolant injection and for plane tip, lift-off is observed at blowing ratio 2.0 and reattachments of jets are observed at blowing ratio 3.0. But, almost no effectiveness is observed for squealer tip at all blowing ratios with pressure side injection with reduced heat transfer coefficient along the pressure side. For combination case, very high effectiveness is observed at blowing ratio 3.0 for both plane and recessed blade tip. It appears that for this high blowing ratio, coolant jets from the tip hit the shroud first and then reattach back onto the blade tip with very high heat transfer coefficients for both plane and recessed blade tip.}, number={1}, journal={Journal of Turbomachinery}, author={Nasir, H. and Ekkad, S.V. and Bunker, R.S.}, year={2007}, pages={151–163} }
 @inproceedings{nasir_ekkad_bunker_2005, title={Effect of tip and pressure side coolant injection on heat transfer distributions for a plane and recessed tip}, volume={3 PART A}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-27744449608&partnerID=MN8TOARS}, DOI={10.1115/GT2005-68595}, abstractNote={The present study investigates the effects of coolant injection on adiabatic film effectiveness and heat transfer coefficients from a plane and recessed tip of a HPT first stage rotor blade. Three cases where coolant is injected from (a) five orthogonal holes located along the camber line, (b) seven angled holes located near the blade tip along the pressure side and (c) combination cases when coolant is injected from both tip and pressure side holes were studied. The pressure ratio (inlet total pressure to exit static pressure for the cascade) across the blade row was 1.2, and the experiments were run in a blow-down test rig with a four-blade linear cascade. The Reynolds number based on cascade exit velocity and axial chord length was 8.61×105 and the inlet and exit Mach number were 0.16 and 0.55, respectively. A transient infrared (IR) technique was used to measure adiabatic film effectiveness and heat transfer coefficient simultaneously for three blowing ratios of 1.0, 2.0, and 3.0. For all the cases, gap-to-blade span ratio of 1% was used. The depth-to-blade span ratio of 0.0416 was used for the recessed tip. Pressure measurements on the shroud were also taken to characterize the leakage flow and understand the heat transfer distributions. For tip injection, when blowing ratio increases from 1.0 to 2.0, film effectiveness increases for both plane and recessed tip. At blowing ratio 3.0, lift off is observed for both cases. In case of pressure side coolant injection and for plane tip, lift off is observed at blowing ratio 2.0 and reattachments of jets are observed at blowing ratio 3.0. But, almost no effectiveness is observed for squealer tip at all blowing ratios with pressure side injection. For combination case, very high effectiveness is observed at blowing ratio 3.0 for both plane and recessed blade tip. It appears that for this high blowing ratio, coolant jets from the tip hit the shroud first and then reattach back on to the blade tip. For tip injection, as blowing ratio increases heat transfer coefficient decreases for both plane and recessed tip. In case of pressure side coolant injection and for plane tip, film injection reduced heat transfer coefficient along the pressure side. Minimal effect is observed for recessed tip at all blowing ratios. For combination case, very high heat transfer coefficient is observed at blowing ratio 3.0 for both plane and recessed blade tip. It appears that for this high blowing ratio, coolant jets from the tip hit the shroud first and then reattach back on to the blade tip.}, booktitle={Proceedings of the ASME Turbo Expo}, author={Nasir, H. and Ekkad, S.V. and Bunker, R.S.}, year={2005}, pages={573–584} }
 @inproceedings{lu_nasir_ekkad_2005, title={Film cooling from a row of holes embedded in transverse slots}, volume={3 PART A}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-27744550896&partnerID=MN8TOARS}, DOI={10.1115/GT2005-68598}, abstractNote={Film cooling performance for a row of cylindrical holes can be enhanced by embedding the row in transverse slots. The geometry of the transverse slot greatly affects the cooling performance downstream of injection. The effect of the slot exit area and edge shape is investigated. Detailed heat transfer coefficient and film effectiveness measurements are obtained simultaneously using a single test transient IR thermography technique. The study is performed at a single mainstream Reynolds number based on free-stream velocity and film hole diameter of 7150 at three different coolant-to-mainstream blowing ratios of 0.5, 1.0, and 1.5. The results show that the film cooling holes provide higher film effectiveness when embedded in a slot. However, in some geometries when the slot begins at the upstream edge of the hole, the film effectiveness diminishes. The heat transfer coefficient enhancement due to the embedding is not significantly higher compared to the typical unembedded cylindrical hole. The overall heat flux ratio comparing film cooling with embedded holes to unembedded holes shows that the full slot and downstream slot spacing after the hole exit produce the highest heat flux reduction. The holes-in-slot geometry is certainly very promising.}, booktitle={Proceedings of the ASME Turbo Expo}, author={Lu, Y. and Nasir, H. and Ekkad, S.V.}, year={2005}, pages={585–592} }
 @inproceedings{lu_fauchcaux_ekkad_2005, title={Film cooling measurements for novel hole configurations}, volume={3}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-29644435324&partnerID=MN8TOARS}, DOI={10.1115/HT2005-72396}, abstractNote={Film cooling performance for a row of cylindrical holes can be. The effect of the slot exit area and shape is investigated. Detailed heat transfer coefficient and film effectiveness measurements are obtained simultaneously using a single test transient IR thermography technique. The study is performed at a single mainstream Reynolds number based on free-stream velocity and film hole diameter of 7150 at three different coolant-to-mainstream blowing ratios of 0.5, 1.0, and 1.5. Two designs with a crescent shaped exit and a slot exit are considered. The results show that the crescent shaped exits provide significantly higher film cooling effectiveness than the cylindrical hole exit at all blowing ratios. The converging slot exit provides similar effectiveness as the crescent for higher blowing ratios. However, the crescent shape also enhances heat transfer coefficients significantly. Overall effectiveness for both crescent and converging slot exits are clearly superior to the standard cylindrical hole.}, booktitle={Proceedings of the ASME Summer Heat Transfer Conference}, author={Lu, Y. and Fauchcaux, D. and Ekkad, S.V.}, year={2005}, pages={59–66} }
 @article{gao_ekkad_bunker_2005, title={Impingement heat transfer, part I: Linearly stretched arrays of holes}, volume={19}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-12744263181&partnerID=MN8TOARS}, DOI={10.2514/1.8551}, abstractNote={This is Part I of a two-part paper to study new configurations for impingement heat transfer. In Part I, linearly stretched arrays of holes are investigated, and in Part II the effect of pressure gradient on jet-impingement heat transfer is studied. Impingement holes in real engines are primarily directed to hot spot locations, thus producing nonsquare arrays. In this study, the spacing between the holes increases in both the streamwise and spanwise direction simulating the stretching of the hole arrays downstream. Two different arrays are investigated with the first array having uniform diameter holes through the array placed in a stretched format and the second array having increasing diameter holes. Three jet Reynolds numbers between 2 x 10 3 and 10 3 are studied for three different jet height-to-diameter ratios between 1 and 5. The measured heat-transfer coefficients for these arrays are then predicted using existing impingement heat-transfer correlations based on regular evenly spaced arrays}, number={1}, journal={Journal of Thermophysics and Heat Transfer}, author={Gao, L. and Ekkad, S.V. and Bunker, R.S.}, year={2005}, pages={57–65} }
 @article{hebert_ekkad_gao_bunker_2005, title={Impingement heat transfer, part II: Effect of streamwise pressure gradient}, volume={19}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-12744263177&partnerID=MN8TOARS}, DOI={10.2514/1.8588}, abstractNote={This is Part II of a two-part paper on jet-impingement heat transfer. The effect of streamwise pressure gradient on jet-impingement heat transfer is investigated. In realistic configurations, impingement arrays do not always impinge in channels that have parallel walls. Converging channels create positive pressure gradient, and diverging channels create adverse pressure gradient. In this study, the effect of nonparallel walls on jet-impingement heat transfer is investigated. Firstly for impingement arrays with jet-to-jet streamwise and spanwise spacing of fourhole diameters and eight-hole diameters and then for the linearly stretched arrays discussed in Part I. Two jet Reynolds numbers are studied for all cases for Re =6 × × 10 3 , and 10 3 . Also, the jet height-to-diameter ratio is increased from 1 to 5 to generate the adverse pressure gradient and decreased from 5 to 1 to generate the positive pressure gradient. Results show that the effect of streamwise pressure gradient alters the flow distribution causing significant variations in heat-transfer distributions. Accelerating flow causes streamwise jet stretching, whereas decelerating flow causes spanwise jet stretching. The results for converging and diverging channels are compared with results for parallel plate channels with different spacing to compare the effect of the streamwise pressure gradient.}, number={1}, journal={Journal of Thermophysics and Heat Transfer}, author={Hebert, R. and Ekkad, S.V. and Gao, L. and Bunker, R.S.}, year={2005}, pages={66–71} }
 @inproceedings{mahmud hasan_ekkad_mensah_2005, title={Simulations of dimensional effects in solid oxide fuel cells}, volume={4}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-29644447790&partnerID=MN8TOARS}, DOI={10.1115/HT2005-72394}, abstractNote={Two dimensional simulation models for a solid oxide fuel cell (SOFC) including gas channels have been studied using the commercial software FEMLAB which has a multi-physics approach. The models include full coupling between the mass balances at the anode and cathode, the balance of the ionic current carried by the oxide ion, and a balance of electronic current with an assumption of isothermal conditions. Three cross-section areas of 1cm × 1cm, 3cm × 3cm, and 5 cm × 5cm were chosen to study the dimensional effect. An additional 1cm × 1cm channel with rectangular protrusions was also studied to investigate geometry effects. Simulation results show expected result of increases in fuel cell properties like convective flux, electric potential with increasing cell flow area. The smaller fuel cell surface is altered by adding rectangular protrusions to increase surface area. The presence of rectangular protrusions also show increased current density production compared to a wall without protrusions. It is expected that the proposed simulations can be used to significantly help the design and operation of a SOFC in practical applications.}, booktitle={Proceedings of the ASME Summer Heat Transfer Conference}, author={Mahmud Hasan, A.B. and Ekkad, S.V. and Mensah, P.}, year={2005}, pages={11–16} }
 @inproceedings{mahmud hasan_guo_ekkad_2005, title={The effects of feeding configurations to water flooding and general performance of a proton exchange membrane fuel cell}, volume={45}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-33645068600&partnerID=MN8TOARS}, DOI={10.1115/IMECE2005-81268}, abstractNote={The performance of a Proton Exchange Membrane Fuel Cell (PEMFC) using different feeding configurations has been studied. Three bipolar plates, namely serpentine, straight channel and interdigitated designs, were arranged in different combinations for the PEMFC anode and cathode sides. Nine combinations in total were tested under different flow rates, working temperatures and loadings. The cell voltage versus current density and the cell power density versus current density curves were obtained. After operating the PEMFC under high current densities, the cell was split and the water flooding in the feeding channels was visually inspected. Experimental results showed that for different feeding configurations, interdigitated bipolar plate in anode side and serpentine bipolar plate in cathode side had the best performance in terms of cell voltage-current density curve, power density output rate, percentage of flooded area in the feeding channels, the pattern of flooding and the fuel utilization rate.}, booktitle={American Society of Mechanical Engineers, Advanced Energy Systems Division (Publication) AES}, author={Mahmud Hasan, A.B. and Guo, S.M. and Ekkad, S.V.}, year={2005}, pages={429–435} }
 @article{ekkad_ou_rivir_2004, title={A transient infrared thermography method for simultaneous film cooling effectiveness and heat transfer coefficient measurements from a single test}, volume={126}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-14744294190&partnerID=MN8TOARS}, DOI={10.1115/1.1791283}, abstractNote={In film cooling situations, there is a need to determine both local adiabatic wall temperature and heat transfer coefficient to fully assess the local heat flux into the surface. Typical film cooling situations are termed three temperature problems where the complex interaction between the jets and mainstream dictates the surface temperature. The coolant temperature is much cooler than the mainstream resulting in a mixed temperature in the film region downstream of injection. An infrared thermography technique using a transient surface temperature acquisition is described which determines both the heat transfer coefficient and film effectiveness (nondimensional adiabatic wall temperature) from a single test. Hot mainstream and cooler air injected through discrete holes are imposed suddenly on an ambient temperature surface and the wall temperature response is captured using infrared thermography. The wall temperature and the known mainstream and coolant temperatures are used to determine the two unknowns (the heat transfer coefficient and film effectiveness) at every point on the test surface. The advantage of this technique over existing techniques is the ability to obtain the information using a single transient test. Transient liquid crystal techniques have been one of the standard techniques for determining h and η for turbine film cooling for several years. Liquid crystal techniques do not account for nonuniform initial model temperatures while the transient IR technique measures the entire initial model distribution. The transient liquid crystal technique is very sensitive to the angle of illumination and view while the IR technique is not. The IR technique is more robust in being able to take measurements over a wider temperature range which improves the accuracy of h and η. The IR requires less intensive calibration than liquid crystal techniques. Results are presented for film cooling downstream of a single hole on a turbine blade leading edge model.}, number={4}, journal={Journal of Turbomachinery}, author={Ekkad, S.V. and Ou, S. and Rivir, R.B.}, year={2004}, pages={597–603} }
 @inproceedings{ekkad_ou_rivir_2004, title={A transient infrared thermography method for simultaneous film cooling effectiveness and heat transfer coefficient measurements from a single test}, volume={3}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-10244229755&partnerID=MN8TOARS}, DOI={10.1115/gt2004-54236}, abstractNote={In film cooling situations, there is a need to determine both local adiabatic wall temperature and heat transfer coefficient to fully assess the local heat flux into the surface. Typical film cooling situations are termed three temperature problems where the complex interaction between the jets and mainstream dictates the surface temperature. The coolant temperature is much cooler than the mainstream resulting in a mixed temperature in the film region downstream of injection. An infrared thermography technique using a transient surface temperature acquisition is described which determines both the heat transfer coefficient and film effectiveness (non-dimensional adiabatic wall temperature) from a single test. Hot mainstream and cooler air injected through discrete holes are imposed suddenly on an ambient temperature surface and the wall temperature response is captured using infrared thermography. The wall temperature and the known mainstream and coolant temperatures are used to determine the two unknowns (heat transfer coefficient and film effectiveness) at every point on the test surface. The advantage of this technique over existing techniques is the ability to obtain the information using a single transient test. Transient liquid crystal techniques have been one of the standard techniques for determining h and η for turbine film cooling for several years. Liquid crystal techniques do not account for non uniform initial model temperatures while the transient IR technique measures the entire initial model distribution. The transient liquid crystal technique is very sensitive to the angle of illumination and view while the IR technique is not. The IR technique is more robust in being able to take measurements over a wider temperature range which improves the accuracy of h and η. The IR requires less intensive calibration than liquid crystal techniques. Results are presented for film cooling downstream of a single hole on a turbine blade leading edge model.}, booktitle={Proceedings of the ASME Turbo Expo 2004}, author={Ekkad, S.V. and Ou, S. and Rivir, R.B.}, year={2004}, pages={999–1005} }
 @inproceedings{hebert_ekkad_khanna_2004, title={Combination of impingement and trip strips for combustor liner backside cooling}, volume={3}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-21544472888&partnerID=MN8TOARS}, DOI={10.1115/ht-fed2004-56536}, abstractNote={Effective cooling of modern low NOx combustor liners is achieved through combinations of impingement and other heat transfer enhancement methods. In the present study, a combination of impingement and trip strips is studied to determine the optimum location of trip strips with respect to impingement jet arrays. Heat transfer with pure impingement has degradation downstream due to increased cross-flow effects. To counter the cross-flow induced heat transfer degradation, a combination technique wherein impingement is combined with ribs placed in between impingement rows or downstream of the impingement array is studied. Three configurations with increased rib placements and reduced impingement holes are studied and compared with pure impingement cases for the same jet Reynolds number. Three jet Reynolds numbers are studied for Rej = 10000, 20000, and 30000. Detailed heat transfer distributions are obtained using the transient liquid crystal technique. Results show that the presence of ribs increases jet impingement heat transfer on the surface with lower mass flows. The effectiveness of the combination ribs and impingement can provide higher heat transfer with reduced cooling air requirements.}, booktitle={Proceedings of the ASME Heat Transfer/Fluids Engineering Summer Conference 2004, HT/FED 2004}, author={Hebert, R. and Ekkad, S.V. and Khanna, V.}, year={2004}, pages={179–185} }
 @article{saxena_nasir_ekkad_2004, title={Effect of blade tip geometry on tip flow and heat transfer for a blade in a low-speed cascade}, volume={126}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-1942425037&partnerID=MN8TOARS}, DOI={10.1115/1.1643385}, abstractNote={A comprehensive investigation of the effect of various tip sealing geometries is presented on the blade tip leakage flow and associated heat transfer of a scaled up HPT turbine blade in a low-speed wind tunnel facility. The linear cascade is made of four blades with the two corner blades acting as guides. The tip section of a HPT first stage rotor blade is used to fabricate the two-dimensional blade. The wind tunnel accommodates an 116 deg turn for the blade cascade. The mainstream Reynolds number based on the axial chord length at cascade exit is 4.83×105. The upstream wake effect is simulated with a spoked wheel wake generator placed upstream of the cascade. A turbulence grid placed even farther upstream generates the required freestream turbulence of 4.8%. The center blade has a tip clearance gap of 1.5625% with respect to the blade span. Static pressure measurements are obtained on the blade surface and the shroud. The effect of crosswise trip strips to reduce leakage flow and associated heat transfer is investigated with strips placed along the leakage flow direction, against the leakage flow and along the chord. Cylindrical pin fins and pitch variation of strips over the tip surface are also investigated. Detailed heat transfer measurements are obtained using a steady-state HSI-based liquid crystal technique. The effect of periodic unsteady wake effect is also investigated by varying the wake Strouhal number from 0. to 0.2, and to 0.4. Results show that the trip strips placed against the leakage flow produce the lowest heat transfer on the tips compared to all the other cases with a reduction between 10–15% compared to the plain tip. Results also show that the pitch of the strips has a small effect on the overall reduction. Cylindrical pins fins and strips along the leakage flow direction do not decrease the heat transfer coefficients and in some cases enhance the heat transfer coefficients by as much as 20%.}, number={1}, journal={Journal of Turbomachinery}, author={Saxena, V. and Nasir, H. and Ekkad, S.V.}, year={2004}, pages={130–138} }
 @inproceedings{ekkad_ou_rivir_2004, title={Effect of jet pulsation and duty cycle on film cooling from a single jet on a leading edge model}, volume={375}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-20344398461&partnerID=MN8TOARS}, DOI={10.1115/IMECE2004-60466}, abstractNote={The effect of jet pulsation and duty cycle on film effectiveness and heat transfer was investigated on a film hole located on the circular leading edge of a blunt body. A transient infrared technique was used to measure both heat transfer coefficients and film effectiveness from a single test. Detailed Frossling number and film effectiveness distributions were obtained for all flow conditions. Jet pulsing frequencies of 5 Hz, 10 Hz, and 20 Hz have been studied. The effect of duty cycle created by the valve opening and closing times was also set at different levels of 10%, 25%, 50%, and 75% of designated 100% fully open condition for different blowing ratios from 0.25 to 2.0. The combination of pulse frequency and duty cycle was investigated for different blowing ratios on a single leading edge hole located at 22-deg from geometric leading edge. Results indicate that higher effectiveness and lower heat transfer coefficients are obtained at the reduced blowing ratios which result from reduced duty cycles. The effect of varying the pulsing frequency from 5 Hertz to 20 Hertz is not discernable beyond the level of experimental uncertainty. Effective blowing ratio due to lowering of the duty cycle at a given blowing ratio seems to plays a more important role in combination with pulsing which provides improved cooling effectiveness at lower heat transfer coefficients.}, number={1}, booktitle={American Society of Mechanical Engineers, Heat Transfer Division, (Publication) HTD}, author={Ekkad, S.V. and Ou, S. and Rivir, R.B.}, year={2004}, pages={575–582} }
 @article{saxena_ekkad_2004, title={Effect of squealer geometry on tip flow and heat transfer for a turbine blade in a low speed cascade}, volume={126}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-7244247150&partnerID=MN8TOARS}, DOI={10.1115/1.1777580}, abstractNote={Abstract
               A detailed investigation on the effect of squealer geometries on the blade tip leakage flow and associated heat transfer is presented for a scaled up high pressure turbine blade in a low-speed wind tunnel facility. The linear cascade is made of four blades with the two corner blades acting as guides. The tip profile of a first stage rotor blade is used to fabricate the two-dimensional blade. The wind tunnel accommodates an 116° turn for the blade cascade. The mainstream Reynolds number based on the axial chord length based on cascade exit velocity is 4.83×105. An upstream wake effect is simulated with a spoked wheel wake generator placed upstream of the cascade. A turbulence grid placed even farther upstream generates a free-stream turbulence of 4.8%. The center blade has a tip clearance gap of 1.56% with respect to the blade span. Static pressure measurements are obtained on the blade surface and the shroud. Results show that the presence of the squealer alters the tip gap flow field significantly and produces lower overall heat transfer coefficients. The effects of different squealer arrangements are basically to study the effect of squealer rim placement on tip leakage flow and associated heat transfer. Detailed heat transfer measurements are obtained using a steady state liquid crystal technique. The effect of periodic unsteady wake effect is also investigated by varying the wake Strouhal number from 0–0.4. Results show that suction side squealers may be favorable in terms of overall reduction in heat transfer coefficients over the tip surface. However, the presence of a full squealer is most beneficial in terms of reducing overall heat load on the tip surface. There is reasonable effect of wake induced periodicity on tip heat transfer.}, number={4}, journal={Journal of Heat Transfer}, author={Saxena, V. and Ekkad, S.V.}, year={2004}, pages={546–553} }
 @article{nasir_ekkad_kontrovitz_bunker_prakash_2004, title={Effect of tip gap and squealer geometry on detailed heat transfer measurements over a high pressure turbine rotor blade tip}, volume={126}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-7644220874&partnerID=MN8TOARS}, DOI={10.1115/1.1731416}, abstractNote={The present study explores the effects of gap height and tip geometry on heat transfer distribution over the tip surface of a HPT first-stage rotor blade. The pressure ratio (inlet total pressure to exit static pressure for the cascade) used was 1.2, and the experiments were run in a blow-down test rig with a four-blade linear cascade. A transient liquid crystal technique was used to obtain the tip heat transfer distributions. Pressure measurements were made on the blade surface and on the shroud for different tip geometries and tip gaps to characterize the leakage flow and understand the heat transfer distributions. Two different tip gap-to-blade span ratios of 1% and 2.6% are investigated for a plane tip, and a deep squealer with depth-to-blade span ratio of 0.0416. For a shallow squealer with depth-to-blade span ratio of 0.0104, only 1% gap-to-span ratio is considered. The presence of the squealer alters the tip gap flow field significantly and produces lower overall heat transfer coefficients. The effects of different partial squealer arrangements are also investigated for the shallow squealer depth. These simulate partial burning off of the squealer in real turbine blades. Results show that some partial burning of squealers may be beneficial in terms of overall reduction in heat transfer coefficients over the tip surface.}, number={2}, journal={Journal of Turbomachinery}, author={Nasir, H. and Ekkad, S.V. and Kontrovitz, D.M. and Bunker, R.S. and Prakash, C.}, year={2004}, pages={221–228} }
 @inproceedings{nasir_ekkad_bunker_prakash_2004, title={Effects of tip gap film injection from plain and squealer blade tips}, volume={3}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-10244221127&partnerID=MN8TOARS}, DOI={10.1115/gt2004-53455}, abstractNote={The present study investigates the effect of orthogonal tip gap film injection from a plain and squealer tip of a HPT first stage rotor blade. The pressure ratio (inlet total pressure to exit static pressure) for the cascade used was 1.2, and the experiments were run in a blow-down test rig with a four-blade stationary linear cascade. The Reynolds number based on cascade exit velocity and axial chord length was 8.61×105 and the inlet and exit Mach numbers were 0.16 and 0.55, respectively. Five holes are located along the camber line of the blade tip. A transient infrared technique was used to measure the local heat transfer coefficient and film effectiveness from a single transient test. All measurements were made for three blowing ratios of 1.0, 1.5, and 2.0. For all the cases, a small tip gap-to-blade span ratio of 0.5% was used. The squealer depth-to-blade span ratio of 4.16% was used for the squealer. Results show that the film injection has some effect for plain tips but has no effect or negligible effect for squealer tips.}, booktitle={Proceedings of the ASME Turbo Expo 2004}, author={Nasir, H. and Ekkad, S.V. and Bunker, R.S. and Prakash, C.}, year={2004}, pages={419–427} }
 @inproceedings{hebert_ekkad_khanna_abreu_moon_2004, title={Heat transfer study of a novel low-crossflow design for jet impingement}, volume={375}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-20344408308&partnerID=MN8TOARS}, DOI={10.1115/IMECE2004-60468}, abstractNote={Impingement heat transfer is significantly affected by initial cross-flow or by the presence of cross-flow from upstream spent jets. In this study, a zero cross-flow design is presented. The zero-crossflow design creates spacing between hole arrays to allow for spent flow to be directed away from impinging jets. Three configurations with different impingement holes placements are studied and compared with pure impingement with spent crossflow cases for the same jet Reynolds number. Three jet Reynolds numbers are studied for Rej = 10000, 20000, and 30000. Detailed heat transfer distributions are obtained using the transient liquid crystal technique. The zero-cross flow design clearly shows minimal degradation of impingement heat transfer due to crossflow compared to conventional design with lower mass flow rate requirement and lesser number of overall impingement holes due to the reduced cross-flow effect on the impingement region.}, number={1}, booktitle={American Society of Mechanical Engineers, Heat Transfer Division, (Publication) HTD}, author={Hebert, R. and Ekkad, S.V. and Khanna, V. and Abreu, M. and Moon, H.-K.}, year={2004}, pages={583–588} }
 @article{ekkad_nasir_2003, title={Dimple Enhanced Heat Transfer in High Aspect Ratio Channels}, volume={10}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-1542708321&partnerID=MN8TOARS}, DOI={10.1615/JEnhHeatTransf.v10.i4.40}, abstractNote={
 Detailed heat transfer measurements are presented for a rectangular channel with dimples on one wall. Dimpled surfaces provide high heat transfer enhancement comparable to ribbed surfaces with reduced overall pressure drop. The heat transfer coefficients were measured using a transient liquid crystal technique. The effect of channel flow Reynolds number was investigated for a wide range from 10000 to 65000. The channel is a 25.4 mm × 101.6 mm (1” × 4”) rectangular cross-section with the dimples on one of the 101.6 mm wall. Heat transfer enhancement around three times that of a smooth channel were achieved for all flow conditions. The overall pressure drop through the dimpled section of the passage was also measured. The resulting thermal performance of the dimples surfaces is significantly higher compared to channels with protruding ribs.}, number={4}, journal={Journal of Enhanced Heat Transfer}, author={Ekkad, S.V. and Nasir, H.}, year={2003}, pages={395–405} }
 @inproceedings{saxena_nasir_ekkad_2003, title={Effect of blade tip geometry on tip flow and heat transfer for a blade in a low speed cascade}, volume={5 A}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0346335565&partnerID=MN8TOARS}, DOI={10.1115/GT2003-38176}, abstractNote={A comprehensive investigation of the effect of various tip sealing geometries is presented on the blade tip leakage flow and associated heat transfer of a scaled up HPT turbine blade in a low-speed wind tunnel facility. The linear cascade is made of four blades with the two corner blades acting as guides. The tip section of a HPT first stage rotor blade is used to fabricate the 2-D blade. The wind tunnel accommodates an 116° turn for the blade cascade. The mainstream Reynolds number based on the axial chord length at cascade exit is 4.83 × 105. The upstream wake effect is simulated with a spoked wheel wake generator placed upstream of the cascade. A turbulence grid placed even farther upstream generates the required free-stream turbulence of 4.8%. The center blade has a tip clearance gap of 1.5625% with respect to the blade span. Static pressure measurements are obtained on the blade surface and the shroud. The effect of crosswise trip strips to reduce leakage flow and associated heat transfer is investigated with strips placed along the leakage flow direction, against the leakage flow and along the chord. Cylindrical pin fins and pitch variation of strips over the tip surface are also investigated. Detailed heat transfer measurements are obtained using a steady state HSI-based liquid crystal technique. The effect of periodic unsteady wake effect is also investigated by varying the wake Strouhal number from 0. to 0.2, and to 0.4. Results show that the trip strips placed against the leakage flow produce the lowest heat transfer on the tips compared to all the other cases with a reduction between 10–15% compared to the plain tip. Results also show that the pitch of the strips has a small effect on the overall reduction. Cylindrical pins fins and strips along the leakage flow direction do not decrease the heat transfer coefficients and in some cases enhance the heat transfer coefficients by as much as 20%.}, booktitle={American Society of Mechanical Engineers, International Gas Turbine Institute, Turbo Expo (Publication) IGTI}, author={Saxena, V. and Nasir, H. and Ekkad, S.V.}, year={2003}, pages={81–90} }
 @inproceedings{nasir_ekkad_kontrovitz_bunker_prakash_2003, title={Effect of tip gap and squealer geometry on measured heat transfer over a HPT rotor blade tip}, volume={374}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-1842534972&partnerID=MN8TOARS}, DOI={10.1115/IMECE2003-41294}, abstractNote={The present study explores the effects of gap height and tip geometry on heat transfer distribution over the tip surface of a HPT first stage rotor blade. The pressure ratio (inlet total pressure to exit static pressure for the cascade) used was 1.2, and the experiments were run in a blow-down test rig with a four-blade linear cascade. A transient liquid crystal technique was used to obtain the tip heat transfer distributions. Pressure measurements were made on the blade surface and on the shroud for different tip geometries and tip gaps to characterize the leakage flow and understand the heat transfer distributions. Two different tip gap-to-blade span ratio of 1% and 2.6% are investigated for a plane tip and a deep squealer with depth-to-blade span ratio of 0.0416. For a shallow squealer with depth-to-blade span ratio of 0.0104, only 1% gap-to-span ratio is considered. The presence of the squealer alters the tip gap flow field significantly and produces lower overall heat transfer coefficients. The effects of different partial squealer arrangements are also investigated for the shallow squealer depth. These simulate partial burning off of the squealer in real turbine blades. Results show that in some cases, partial burning of squealers along the pressure surface may be beneficial in terms of overall reduction in heat transfer coefficients over the tip surface compared to the plain tip.}, number={2}, booktitle={American Society of Mechanical Engineers, Heat Transfer Division, (Publication) HTD}, author={Nasir, H. and Ekkad, S.V. and Kontrovitz, D.M. and Bunker, R.S. and Prakash, C.}, year={2003}, pages={11–21} }
 @article{nasir_ekkad_acharya_2003, title={Flat surface film cooling from cylindrical holes with discrete tabs}, volume={17}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0042767685&partnerID=MN8TOARS}, DOI={10.2514/2.6786}, abstractNote={The effect of discrete delta-shaped tabs on film cooling performance from a row of cylindrical angled holes is investigated. The holes are inclined at 35 deg along the streamwise direction. Four tab locations are investigated: 1) tabs placed along the upstream edge of the hole covering 33% of the hole, 2) tabs placed along the upstream edge covering 11% of the hole, 3) tabs placed along the downstream edge of the hole, and 4) tabs placed along the lateral edges of the hole. They are compared to the baseline case without tabs. Measurements were carried out in a low-speed wind tunnel using the transient liquid crystal technique. The mainstream velocity and freestream turbulence intensity in the low-speed wind tunnel are 8.5 m/s and 6%, respectively, and the Reynolds number based on hole diameter is 6375. Three blowing ratios of 0.56, 1.13, and 1.7 are tested}, number={3}, journal={Journal of Thermophysics and Heat Transfer}, author={Nasir, H. and Ekkad, S.V. and Acharya, S.}, year={2003}, pages={304–312} }
 @inproceedings{gao_ekkad_bunker_2003, title={Impingement heat transfer under linearly stretched arrays of holes}, volume={5 A}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0347596456&partnerID=MN8TOARS}, DOI={10.1115/GT2003-38178}, abstractNote={Impingement heat transfer for linearly stretched arrays of holes is investigated. In real engine configurations, impingement arrays are not always square with evenly spaced holes both in streamwise and spanwise direction. They are primarily directed to hot spot locations thus producing nonsquare arrays. In this study, the spacing between the holes increases in both the streamwise and spanwise direction simulating the stretching of the hole arrays downstream. Two different arrays are investigated with the first array having uniform diameter holes through the array placed in a stretched format. The second array has holes placed in the same locations with increasing diameter along the streamwise direction. The measured heat transfer coefficients for these arrays are then predicted using existing impingement heat transfer correlations based on regular evenly spaced arrays. Results show that the published correlations over-predict the effect of cross-flow. Also, the correlation was extrapolated for this study due to lack of information for extremely strong cross-flow effects. All measurements were obtained using the transient liquid crystal technique.}, booktitle={American Society of Mechanical Engineers, International Gas Turbine Institute, Turbo Expo (Publication) IGTI}, author={Gao, L. and Ekkad, S.V. and Bunker, R.S.}, year={2003}, pages={91–100} }
 @article{nasir_acharya_ekkad_2003, title={Improved film cooling from cylindrical angled holes with triangular tabs: Effect of tab orientations}, volume={24}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0042284896&partnerID=MN8TOARS}, DOI={10.1016/S0142-727X(03)00082-1}, abstractNote={The effect of discrete delta (or triangular)-shaped tabs with different orientations on the film cooling performance from a row of cylindrical holes is investigated. The holes are inclined at 35° along the streamwise direction and the tabs are located along the upstream edge of the holes. Three tab orientations are investigated: (1) tabs placed parallel to the film cooled surface covering a part of the hole; (2) tabs oriented downward at −45°; and (3) tabs oriented upwards at 45°. Measurements were carried out in a low-speed wind tunnel using the transient liquid crystal technique. The mainstream velocity and free-stream turbulence intensity in the low-speed wind tunnel are 9 m/s and 7% respectively and the mainstream Reynolds number based on hole diameter is around 7100. Three blowing ratios of 0.56, 1.13, and 1.7 are tested. Results show that the tabs oriented downwards provide the highest effectiveness at a blowing ratio of 0.56 while the tabs oriented horizontally provides the highest film effectiveness at blowing ratios of 1.13 and 1.7. The higher effectiveness with the tabs is due to the generation of secondary eddies counter-rotating with respect to the kidney pair; these eddies reduce jet penetration and thus increase film-cooling effectiveness. The horizontally oriented tabs produce higher discharge coefficients (lower pressure drop) over the entire range of blowing ratios.}, number={5}, journal={International Journal of Heat and Fluid Flow}, author={Nasir, H. and Acharya, S. and Ekkad, S.}, year={2003}, pages={657–668} }
 @inproceedings{ekkad_gao_hebert_2002, title={Effect of jet-to-jet spacing in impingement arrays on heat transfer}, volume={372}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0346884482&partnerID=MN8TOARS}, DOI={10.1115/IMECE2002-32108}, abstractNote={Detailed heat transfer measurements are presented for jet impingement through arrays of jet holes. The effect of jet-to-wall spacing, hole-to-hole spacing are studied for inline arrays of holes. The axial and spanwise spacing (S/D) of holes are varied to produce square and rectangular arrays of holes. The results are presented at a jet average Reynolds numbers of 5000, 10000, and 15000. The jet-to-wall spacing is varied from 1 to 5. The arrays of 25 holes are placed to create four different configurations. The first configuration has an axial jet-to-jet spacing (SX/D) of 4 and a jet-to-jet spanwise spacing (SY/D) of 4, the second configuration has a SX/D of 8 and SY/D of 4, and the last configuration has a SX/D and SY/D of 8. Detailed heat transfer measurements are obtained using the transient liquid crystal technique. Results show that increase in jet-to-wall spacing reduces cross-flow effect. Results also show that the increase spacing between jets increases lateral spreading.}, number={4}, booktitle={American Society of Mechanical Engineers, Heat Transfer Division, (Publication) HTD}, author={Ekkad, S.V. and Gao, L. and Hebert, R.T.}, year={2002}, pages={237–244} }
 @inproceedings{pramanick_ekkad_2002, title={Effect of rotation on flow and temperature distributions in a two-pass channel connected by two rows of holes}, volume={3 A}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0036998264&partnerID=MN8TOARS}, DOI={10.1115/GT2002-30218}, abstractNote={This paper presents a computational study of the effect of rotation on the velocity and thermal field for a two-pass channel connected by two rows of holes on the divider wall. Detailed velocity and temperature distributions are presented inside a rotating two-pass coolant channel connected by two rows of holes on the divider walls. The enhanced cooling in this passage design is achieved by a combination of impingement and crossflow-induced swirl. The cross flow is generated from one coolant passage to the adjoining coolant passage through a series of straight or angled holes placed along the dividing wall. The holes deliver the flow from one passage to another typically achieved in a conventional design by an 180° U-bend. In this study, the configuration has two rows of 12 straight holes placed axially. The two hole rows are placed at 1.27-cm distance from the sidewalls and are angled orthogonally. There are 12 holes in each rows of 1.27-cm diameter each. The holes direct flow perpendicular to the axial direction. Commercial software, FLUENT, is used for predicting the flow using the standard k-ε turbulence model. The results are presented at a channel flow Reynolds number of 25000. The effect of rotation number from 0, 0.1, and 0.2 is studied along with inlet coolant-to-wall density ratio of 0.05, 0.1, and 0.15. Results show that the impingement and swirl flow are affected by rotation induced Coriolis and centrifugal forces. There appears to be little effect of centrifugal buoyancy for this geometry as velocity profiles are unaffected by the wall temperature changes.}, booktitle={American Society of Mechanical Engineers, International Gas Turbine Institute, Turbo Expo (Publication) IGTI}, author={Pramanick, A. and Ekkad, S.V.}, year={2002}, pages={543–550} }
 @inproceedings{yang_acharya_ekkad_prakash_bunker_2002, title={Flow and heat transfer predictions for a flat-tip turbine blade}, volume={3 A}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0037003472&partnerID=MN8TOARS}, DOI={10.1115/GT2002-30190}, abstractNote={Numerical calculations are performed to simulate the tip leakage flow and heat transfer on the GE-E3 High-Pressure-Turbine (HPT) rotor blade. The calculations are performed for a single blade with periodic conditions imposed along the two boundaries in the circumferential-pitch direction. Cases considered are a flat blade tip at three different tip gap clearances of 1%, 1.5% and 2.5% of the blade span. The numerical results are obtained for two different pressure ratios (ratio of inlet total pressure to exit static pressure) of 1.2 and 1.32 and an inlet turbulence level of 6.1%. To explore the effect of turbulence models on the heat transfer results, three different models of increasing complexity and computational effort (standard high Re k-ε model, RNG k-ε and Reynolds Stress Model) are investigated. The predicted tip heat transfer results are compared with the experimental data of Azad [1], and show satisfactory agreement with the data. Hear transfer predictions for all three turbulence models are comparable, and no significant improvements are obtained with the Reynolds-stress model.}, booktitle={American Society of Mechanical Engineers, International Gas Turbine Institute, Turbo Expo (Publication) IGTI}, author={Yang, H. and Acharya, S. and Ekkad, S.V. and Prakash, C. and Bunker, R.}, year={2002}, pages={271–283} }
 @article{ekkad_kontrovitz_nasir_pamula_acharya_2002, title={Heat transfer in two-pass turbulated channels connected by holes}, volume={16}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0036638366&partnerID=MN8TOARS}, DOI={10.2514/2.6694}, abstractNote={This is part of a continuing study of a new internal channel cooling design for modern gas-turbine blades. In previous studies, the enhanced cooling in thesecond pass of a serpentine channel was achieved by a combination of impingement and crosse ow-induced swirl. A holed or slotted divider wall replaced the 180-deg U turn connecting the two legs of the serpentine channel. Flow from one coolant pass to the adjoining coolant pass was achieved through a series of straight and angled holes and a two-dimensional slot placed along the dividing wall. The focus is to enhance the heat transfer in the e rst pass of the two-pass channel using traditional rib turbulators. The effect of ribs in the e rst pass on the overall second pass heat transfer enhancement is compared to channels with no rib turbulators. Heat transfer distributions are compared for three channel e ow Reynolds numbers ranging between 1:0 £ 10 4 and 5:0 £ 10 4 . Three different rib cone gurations, 90-deg ribs, 60-deg angled forward facing toward the divider wall, and 60-deg angled backward facing away from divider wall, are studied for all Reynolds numbers and dividerwallgeometries. Thepresenceof ribsin thee rst passdoesnot only decreasethe enhanced heattransfer in the second pass, but also provides higher heat transfer enhancement in the e rst pass, resulting in an increase in overall heat transfer enhancement for the entire two-pass channel.}, number={3}, journal={Journal of Thermophysics and Heat Transfer}, author={Ekkad, S.V. and Kontrovitz, D. and Nasir, H. and Pamula, G. and Acharya, S.}, year={2002}, pages={404–414} }
 @article{ekkad_kontrovitz_2002, title={Jet impingement heat transfer on dimpled target surfaces}, volume={23}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0036471417&partnerID=MN8TOARS}, DOI={10.1016/S0142-727X(01)00139-4}, abstractNote={Detailed heat transfer distributions are presented over a jet impingement target surface with dimples. Jet impingement by itself is an extremely effective heat transfer enhancement technique. This study investigates the effect of jet impingement on a target surface with a dimple pattern. The effect of dimple location, underneath the jets or between the jets, is investigated. The effect of dimple depth is also investigated. The average jet Reynolds number is varied from 4800 to 14 800. The heat transfer measurements are obtained using the transient liquid crystal technique. Results for dimpled target surfaces are normalized with data for plane target surfaces to determine whether the presence of dimples enhances heat transfer. Results show that the presence of dimples on the target surface, in-line or staggered with respect to jet location, produce lower heat transfer coefficients than the non-dimpled target surface. The bursting phenomena associated with flow over dimples produces disturbances of the impingement jet structures resulting in lower levels of heat transfer coefficients on the target surface.}, number={1}, journal={International Journal of Heat and Fluid Flow}, author={Ekkad, S.V. and Kontrovitz, D.}, year={2002}, pages={22–28} }
 @inproceedings{acharya_yang_ekkad_prakash_bunker_2002, title={Numerical simulation of film cooling on the tip of a gas turbine blade}, volume={3 B}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0036992043&partnerID=MN8TOARS}, DOI={10.1115/GT2002-30553}, abstractNote={Numerical simulations of flow and heat transfer are presented for a GE-E3 turbine blade with a film-cooled tip. Results are presented for both a flat tip and a squealer tip. Straight-through coolant holes are considered, and the calculation domain includes the flow development in the coolant delivery tubes. Results are presented with three different tip gaps representing 1%, 1.5% and 2.5% of blade span, a blowing ratio (ratio of coolant-jet-exit velocity to average passage flow velocity) of 1, and an inlet turbulence intensity of 6.1%. On a flat tip, film coolant injection is shown to lower the local pressure ratio and alters the nature of the leakage vortex. High film cooling effectiveness and low heat transfer coefficients are obtained along the coolant trajectory; these values increase slightly with increasing tip clearances. For a squealer tip, the flow inside the squealer cavity exhibits streamwise directed flow, which alters the trajectory of the coolant jets and reduces their effectiveness.}, booktitle={American Society of Mechanical Engineers, International Gas Turbine Institute, Turbo Expo (Publication) IGTI}, author={Acharya, S. and Yang, H. and Ekkad, S.V. and Prakash, C. and Bunker, R.}, year={2002}, pages={1051–1062} }
 @inproceedings{yang_acharya_ekkad_prakash_bunker_2002, title={Numerical simulation of flow and heat transfer past a turbine blade with a squealer-tip}, volume={3 A}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0036998289&partnerID=MN8TOARS}, DOI={10.1115/GT2002-30193}, abstractNote={Numerical calculations are performed to simulate the tip leakage flow and heat transfer on the squealer (recessed) tip of GE-E3 turbine rotor blade. A squealer tip with a 3.77% recess of the blade span is considered in this study, and the results are compared with the predictions for a flat-tip blade. The calculations have been performed for an isothermal blade with an overall pressure ratio of 1.32, an inlet turbulence intensity of 6.1%, and for three different tip gap clearances of 1%, 1.5% and 2.5% of the blade span. These conditions correspond to the experiments reported by Azad et al. [1]. The calculations have been performed for three different turbulence models (the standard high Re k-ε model, the RNG k-ε and the Reynolds Stress Model) in order to assess the capability of the models in correctly predicting the blade heat transfer. The predictions show good agreement with the experimental data, with the Reynolds stress model calculations clearly providing the best results. Substantial reductions in the tip heat transfer and leakage flow is obtained with the squealer tip configuration. With the squealer tip, the heat transfer coefficients on the shroud and on the suction surface of the blade are also considerably reduced.}, booktitle={American Society of Mechanical Engineers, International Gas Turbine Institute, Turbo Expo (Publication) IGTI}, author={Yang, H. and Acharya, S. and Ekkad, S.V. and Prakash, C. and Bunker, R.}, year={2002}, pages={295–307} }
 @inproceedings{ekkad_nasir_2001, title={Dimple enhanced heat transfer in high aspect ratio channels}, volume={369}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0346254091&partnerID=MN8TOARS}, number={5}, booktitle={American Society of Mechanical Engineers, Heat Transfer Division, (Publication) HTD}, author={Ekkad, S.V. and Nasir, H.}, year={2001}, pages={385–391} }
 @article{nasir_ekkad_acharya_2001, title={Effect of compound angle injection on flat surface film cooling with large streamwise injection angle}, volume={25}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0035421930&partnerID=MN8TOARS}, DOI={10.1016/S0894-1777(01)00052-8}, abstractNote={Film cooling measurements are presented over a flat surface through a single row of discrete holes angled 55° along the streamwise direction. The holes are angled 0° and 60° in the lateral direction to study the effect of compound angle injection. Detailed heat transfer coefficient enhancement and film effectiveness distributions are presented for the entire region from downstream of the holes to distances far downstream at about X/d=20. Tests were conducted in a low speed wind tunnel. The mainstream flow Reynolds number based on hole diameter is around 9500 and the free-stream turbulence intensity is set at 11%. Results are presented for three blowing ratios of 0.5, 1.0, and 1.5 and a coolant-to-mainstream density ratio of 1.0. A transient liquid crystal technique will be used to measure both the local heat transfer coefficient and film effectiveness results simultaneously. The technique uses two similar tests to resolve the heat transfer coefficient and film effectiveness. The detailed heat transfer coefficient and film effectiveness contours provide a clear understanding of the jet–mainstream interactions for different hole orientations with large streamwise angle injection. Results show that adding a compound angle to a hole with large streamwise angle produces significant variations in the detailed film effectiveness distributions and enhances local heat transfer coefficients.}, number={1-2}, journal={Experimental Thermal and Fluid Science}, author={Nasir, H. and Ekkad, S.V. and Acharya, S.}, year={2001}, pages={23–29} }
 @inproceedings{ekkad_kontrovitz_nasir_pamula_acharya_2001, title={Effect of rib turbulators in the first pass on heat transfer distributions in a two-pass channel connected by two rows of holes}, volume={3}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-82055208723&partnerID=MN8TOARS}, DOI={10.1115/2001-GT-0184}, abstractNote={This paper is a continued study of new internal channel cooling designs for modern gas turbine blades. In previous studies, the enhanced cooling in the second pass of a serpentine channel was achieved by a combination of impingement and crossflow-induced swirl. A holed or slotted divider wall replaced the 180° U-turn connecting the two legs of the serpentine channel. Flow from one coolant passage to the adjoining coolant passage was achieved through a series of straight and angled holes and a two-dimensional slot placed along the dividing wall. In this study, the focus is to enhance the heat transfer in the first pass of the two-pass channel using traditional rib turbulators. The effect of ribs in the first pass on the overall second pass heat transfer enhancement is compared to channels with no rib turbulators. Heat transfer distributions are compared for channels with and without ribs for three-channel flow Reynolds numbers ranging between 1.0×104 − 5.0×104. Results show that the presence of the ribs in the first pass reduces the heat transfer coefficients slightly in the second pass compared to the no-ribs channels. However, the first pass heat transfer is significantly enhanced over the case without ribs. In effect, the overall heat transfer enhancement for the combined two passes is significantly enhanced. Three different rib configurations, 90° ribs, 60° angled forward facing towards divider wall, and 60° angled backward facing away from divider wall, are studied for all Reynolds numbers and divider wall geometries. The presence of ribs in the first pass does not only decrease the enhanced heat transfer in the second pass but also provides higher heat transfer enhancement in the first pass resulting in an increase in overall heat transfer enhancement for the entire two-pass channel.}, booktitle={Proceedings of the ASME Turbo Expo}, author={Ekkad, S.V. and Kontrovitz, D. and Nasir, H. and Pamula, G. and Acharya, S.}, year={2001} }
 @inproceedings{nasir_acharya_ekkad_2001, title={Film cooling from a single row of cylindrical angled holes with triangular tabs having different orientations}, volume={3}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84905734720&partnerID=MN8TOARS}, DOI={10.1115/2001-GT-0124}, abstractNote={The effect of discrete delta (or triangular)-shaped tabs with different orientations on the film cooling performance from a row of cylindrical holes is investigated. The holes are inclined at 35° along the streamwise direction and the tabs are located along the upstream edge of the holes. Three tab orientations are investigated: (1) tabs placed parallel to the film cooled surface covering a part of the hole; (2) tabs oriented downward at –45 degrees; and (3) tabs oriented upwards at 45 degrees. Measurements were carried out in a low-speed wind tunnel using the transient liquid crystal technique. The mainstream velocity and free-stream turbulence intensity in the low speed wind tunnel are 9 m/s and 7% respectively and the mainstream Reynolds number based on hole diameter is around 7,100. Three blowing ratios of 0.56, 1.13, and 1.7 are tested. Results show that the tabs oriented horizontally and those oriented downward provides the highest film effectiveness at blowing ratios of 0.56 and 1.13. At blowing ratio of 1.7, the horizontal tabs have the highest effectiveness. The higher effectiveness (200-300%) and higher heat transfer coefficient (25-30%) with the tabs are caused due to the generation of secondary eddies counter-rotating with respect to the kidney pair; these eddies reduce jet penetration and thus increase film cooling effectiveness.}, booktitle={Proceedings of the ASME Turbo Expo}, author={Nasir, H. and Acharya, S. and Ekkad, S.}, year={2001} }
 @article{han_ekkad_2001, title={Recent development in turbine blade film cooling}, volume={7}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0011716251&partnerID=MN8TOARS}, DOI={10.1155/S1023621X01000033}, abstractNote={Gas turbines are extensively used for aircraft propulsion, land-based power generation, and industrial applications. Thermal efficiency and power output of gas turbines increase with increasing turbine rotor inlet temperature (RIT). The current RIT level in advanced gas turbines is far above the .melting point of the blade material. Therefore, along with high temperature material development, a sophisticated cooling scheme must be developed for continuous safe operation of gas turbines with high performance. Gas turbine blades are cooled internally and externally. This paper focuses on external blade cooling or so-called film cooling. In film cooling, relatively cool air is injected from the inside of the blade to the outside surface which forms a protective layer between the blade surface and hot gas streams. Performance of film cooling primarily depends on the coolant to mainstream pressure ratio, temperature ratio, and film hole location and geometry under representative engine flow conditions. In the past number of years there has been considerable progress in turbine film cooling research and this paper is limited to review a few selected publications to reflect recent development in turbine blade film cooling.}, number={1}, journal={International Journal of Rotating Machinery}, author={Han, J.-C. and Ekkad, S.}, year={2001}, pages={21–40} }
 @article{ekkad_han_2000, title={A transient liquid crystal thermography technique for gas turbine heat transfer measurements}, volume={11}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0034235042&partnerID=MN8TOARS}, DOI={10.1088/0957-0233/11/7/312}, abstractNote={This paper presents in detail the transient liquid crystal technique for convective heat transfer measurements. A historical perspective on the active development of liquid crystal techniques for convective heat transfer measurement is also presented. The experimental technique involves using a thermochromic liquid crystal coating on the test surface. The colour change time of the coating at every pixel location on the heat transfer surface during a transient test is measured using an image processing system. The heat transfer coefficients are calculated from the measured time responses of these thermochromic coatings. This technique has been used for turbine blade internal coolant passage heat transfer measurements as well as turbine blade film cooling heat transfer measurements. Results can be obtained on complex geometry surfaces if visually accessible. Some heat transfer results for experiments with jet impingement, internal cooling channels with ribs, flow over simulated TBC spallation, flat plate film cooling, cylindrical leading edge and turbine blade film cooling are presented for demonstration.}, number={7}, journal={Measurement Science and Technology}, author={Ekkad, S.V. and Han, J.-C.}, year={2000}, pages={957–968} }
 @article{du_ekkad_han_lee_2001, title={Detailed film cooling measurements over a gas turbine blade using a transient liquid crystal image technique}, volume={7}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-52549104471&partnerID=MN8TOARS}, DOI={10.1155/S1023621X01000367}, abstractNote={Detailed heat transfer coefficient and film effectiveness distributions over a gas turbine blade with film cooling are obtained using a transient liquid crystal image technique. The test blade has three rows of film holes on the leading edge and two rows each on the pressure and suction surfaces. A transient liquid crystal technique maps the entire blade midspan region, and helps provide detailed measurements, particularly near the film hole. Tests were performed on a five-blade linear cascade in a low-speed wind tunnel. The mainstream Reynolds number based on cascade exit velocity is5.3×105. Two different coolants (air andCo2) were used to simulate coolant density effect. Coolant blowing ratio was varied between 0.8 and 1.2 for air injection and 0.4–1.2 forCo2injection. Results show that film injection promotes earlier laminar-turbulent boundary layer transition on the suction surface and also enhances local heat transfer coefficients (up to 80%) downstream of injection. An increase in coolant blowing ratio produces higher heat transfer coefficients for both coolants. This effect is stronger immediately downstream of injection holes. Film effectiveness is highest at a blowing ratio of 0.8 for air injection and at a blowing ratio of 1.2 forCo2injection. Such detailed results will help provide insight into the film cooling phenomena on a gas turbine blade.}, number={6}, journal={International Journal of Rotating Machinery}, author={Du, H. and Ekkad, S.V. and Han, J.-C. and Lee, C.P.}, year={2001}, pages={415–424} }
 @article{ekkad_pamula_shantiniketanam_2000, title={Detailed heat transfer measurements inside straight and tapered two-pass channels with rib turbulators}, volume={22}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0034280281&partnerID=MN8TOARS}, DOI={10.1016/S0894-1777(00)00022-4}, abstractNote={Most of the studies on gas turbine blade internal channels have focused on constant cross-sectional areas from entrance to turn. Gas turbine blades are typically tapered from hub to tip to reduce thermal loading. These channels exist inside high-performance turbine blades for providing effective cooling to the blade external surface, which is exposed to high-temperature gas flow. Heat transfer measurements are presented for both the straight and tapered square channels including the turn region with and without rib turbulators. The straight channels will have a uniform square cross-section area of 5.08×5.08cm2. For the tapered channels, the square cross-sectional area reduces from entrance into the first pass (5.08×5.08cm2) to the 180° turn (2.54×2.54cm2) and then expands from turn to exit in the second pass (5.08×5.08cm2). The heat transfer results for tapered channels are compared with results for straight channels. Results show that heat transfer in tapered smooth channels is enhanced significantly due to flow acceleration in the first pass, a combination of taper and turn and flow deceleration in the second pass. Overall, the tapered channels significantly produce higher heat transfer enhancements compared to the Dittus–Boelter correlation for fully developed flow especially in the after-turn region. Based on the results from this study, the heat transfer inside tapered channels in the after-turn region cannot be predicted by calculating local Reynolds numbers and using straight channel heat transfer correlations. However, the first pass Nusselt number enhancement distributions are similar for both straight and tapered channels when normalized using the local Nusselt number based on local Reynolds number. The difference in the after-turn region between the straight and tapered channels is reduced with the addition of rib turbulators.}, number={3-4}, journal={Experimental Thermal and Fluid Science}, author={Ekkad, S.V. and Pamula, G. and Shantiniketanam, M.}, year={2000}, pages={155–163} }
 @article{ekkad_han_2000, title={Film cooling measurements on cylindrical models with simulated thermal barrier coating spallation}, volume={14}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0033742951&partnerID=MN8TOARS}, DOI={10.2514/2.6531}, abstractNote={Detailed heat transfer coefficient and film effectiveness distributions are presented on a cylindrical leading-edge model with simulated thermal barrier coating spallation using a transient liquid crystal technique. Tests were performed in a low-speed wind tunnel on a cylindrical model in a crossflow with two rows of injection holes. Mainstream Reynolds number based on the cylinder diameter was 1.009 x 10 5 . The two rows of injection holes were ±15 deg from stagnation. The film holes were spaced four hole diameters apart and were angled 30 and 90 deg to the surface in the spanwise and streamwise directions, respectively. The simulated spallation cavities were rectangular in shape and had rounded edges. The simulated spallation was placed at two locations, 20-40 deg (S3) and 35-55 deg (S4), respectively. The cylinder surface was coated with thermochromic liquid crystals, and a transient test was run to obtain the heat transfer coefficients and film effectiveness. The effect of coolant blowing ratio was studied for blowing ratios of 0.4 and 0.8. Results show that the Nusselt numbers increase and film effectiveness values decrease with an increasing blowing ratio}, number={2}, journal={Journal of thermophysics and heat transfer}, author={Ekkad, S.V. and Han, J.C.}, year={2000}, pages={194–200} }
 @inproceedings{ekkad_nasir_acharya_2000, title={Film cooling on a flat surface with a single row of cylindrical angled holes: Effect of discrete tabs}, volume={366}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0347705413&partnerID=MN8TOARS}, booktitle={American Society of Mechanical Engineers, Heat Transfer Division, (Publication) HTD}, author={Ekkad, S.V. and Nasir, H. and Acharya, S.}, year={2000}, pages={3–12} }
 @article{ekkad_huang_han_2000, title={Impingement heat transfer measurements under an array of inclined jets}, volume={14}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0033734436&partnerID=MN8TOARS}, DOI={10.2514/2.6524}, abstractNote={References 1Marotta, E. E., and Fletcher, L. S., “Thermal Contact Conductance of Selected Polymeric Materials,” Journal of Thermophysics and Heat Transfer, Vol. 10, No. 2, 1996, pp. 334–342. 2Mikic, B. B., “Thermal Contact Conductance; Theoretical Considerations,” International Journal of Heat and Mass Transfer, Vol. 17, 1974, pp. 205–214. 3Cooper, M., Mikic, B. B., and Yovanovich, M. M., “Thermal Contact Conductance,” International Journal of Heat and Mass Transfer, Vol. 12, 1969, pp. 279–300. 4Parihar, S. K., and Wright, N. T., “Thermal Contact Resistance at Elastomer toMetal Interfaces,” InternationalCommunicationsinHeat andMass Transfer, Vol. 24, No. 8, 1997, pp. 1083–1092. 5Makushkin, A. P., “Study of Stress-Strain of Polymer Layer During Spherical Indenter Penetration,” Trenie I Iznos, Vol. 5, No. 5, 1984, pp. 823–832. 6Greenwood, J. A., and Williamson, J. B. P., “Contact of Nominally Flat Surfaces,” Proceeding of the Royal Society of London, Series A: Mathematical and Physical Sciences, Vol. A295, 1966, pp. 300–319.}, number={2}, journal={Journal of thermophysics and heat transfer}, author={Ekkad, S. and Huang, Y. and Han, J.-C.}, year={2000}, pages={286–288} }
 @inproceedings{pamula_ekkad_acharya_2000, title={Influence of cross-flow induced swirl and impingement on heat transfer in a two-pass channel connected by two rows of holes}, volume={3}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84955163751&partnerID=MN8TOARS}, DOI={10.1115/2000-GT-0235}, abstractNote={Detailed heat transfer distributions are presented inside a two-pass coolant square channel connected by two rows of holes on the divider walls. The enhanced cooling is achieved by a combination of impingement and crossflow-induced swirl. Three configurations are examined where the cross flow is generated from one coolant passage to the adjoining coolant passage through a series of straight and angled holes and a two-dimensional slot placed along the dividing wall. The holes/slots deliver the flow from one passage to another typically achieved in a conventional design by a 180° U-bend. Heat transfer distributions will be presented on the sidewalls of the passages. A transient liquid crystal technique is applied to measure the detailed heat transfer coefficient distributions inside the passages. Results for the three hole supply cases are compared with the results from the traditional 180° turn passage for three channel flow Reynolds numbers ranging between 10000 and 50000. Results show that the new feed system, from first pass to second pass using crossflow injection holes, produce significantly higher Nusselt numbers on the second pass walls. The heat transfer enhancement in the second pass of these channels are as high as 2–3 times greater than that obtained in the second pass for a channel with a 180° turn. Results are also compared with channels that have only one row of discharge holes.}, booktitle={Proceedings of the ASME Turbo Expo}, author={Pamula, G. and Ekkad, S.V. and Acharya, S.}, year={2000} }
 @article{ekkad_pamula_acharya_2000, title={Influence of crossflow-induced swirl and impingement on heat transfer in an internal coolant passage of a turbine airfoil}, volume={122}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0034251863&partnerID=MN8TOARS}, DOI={10.1115/1.1289020}, abstractNote={Detailed heat transfer distributions are presented inside a two-pass coolant channel with crossflow-induced swirl and impingement. The impingement and passage crossflow are generated from one coolant passage to the adjoining coolant passage through a series of straight or angled holes along the dividing wall. The holes provide for the flow turning from one passage to another typically achieved in a conventional design by a 180-deg U-bend. The holes direct the flow laterally from one passage to another and generate different secondary flow patterns in the second pass. These secondary flows produce impingement and swirl and lead to higher heat transfer enhancement. Three different lateral hole configurations are tested for three Reynolds numbers (Re=10,000, 25,000, 50,000). The configurations were varied by angle of delivery and location on the divider wall. A transient liquid crystal technique is used to measure the detailed heat transfer coefficient distributions inside the passages. Results with the new crossflow feed system are compared with the results from the traditional 180-deg turn passage. Results show that the crossflow feed configurations produce significantly higher Nusselt numbers on the second pass walls without affecting the first pass heat transfer levels. The heat transfer enhancement is as high as seven to eight times greater than obtained in the second pass for a channel with a 180-deg turn. The increased measured pressure drop (rise in friction factor) caused by flow through the crossflow holes are compensated by the significant heat transfer enhancement obtained by the new configuration. [S0022-1481(00)03103-0]}, number={3}, journal={Journal of Heat Transfer}, author={Ekkad, S.V. and Pamula, G. and Acharya, S.}, year={2000}, pages={587–597} }
 @article{pamula_ekkad_acharya_2001, title={Influence of crossflow-induced swirl and impingement on heattransfer in a two-pass channel connected by two rows of holes}, volume={123}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0035299108&partnerID=MN8TOARS}, DOI={10.1115/1.1343467}, abstractNote={Detailed heat transfer distributions are presented inside a two-pass coolant square channel connected by two rows of holes on the divider walls. The enhanced cooling is achieved by a combination of impingement and crossflow-induced swirl. Three configurations are examined where the crossflow is generated from one coolant passage to the adjoining coolant passage through a series of straight and angled holes and a two-dimensional slot placed along the dividing wall. The holes/slots deliver the flow from one passage to another. This is typically achieved in a conventional design by a 180 deg U-bend. Heat transfer distributions will be presented on the sidewalls of the passages. A transient liquid crystal technique is applied to measure the detailed heat transfer coefficient distributions inside the passages. Results for the three-hole supply cases are compared with the results from the traditional 180 deg turn passage for three channel flow Reynolds numbers ranging between 10,000 and 50,000. Results show that the new feed system, from first pass to second pass using crossflow injection holes, produces significantly higher Nusselt numbers on the second pass walls. The heat transfer enhancements in the second pass of these channels are as much as two to three times greater than that obtained in the second pass for a channel with a 180 deg turn. Results are also compared with channels that have only one row of discharge holes.}, number={2}, journal={Journal of Turbomachinery}, author={Pamula, G. and Ekkad, S.V. and Acharya, S.}, year={2001}, pages={281–287} }
 @article{du_ekkad_han_1999, title={Effect of unsteady wake with trailing edge coolant ejection on film cooling performance for a gas turbine blade}, volume={121}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0032704069&partnerID=MN8TOARS}, DOI={10.1115/1.2841337}, abstractNote={The effect of unsteady wakes with trailing edge coolant ejection on surface heat transfer coefficients and film cooling effectiveness is presented for a downstream film-cooled turbine blade. The detailed heat transfer coefficient and film effectiveness distributions on the blade surface are obtained using a transient liquid crystal technique. Unsteady wakes are produced by a spoked wheel-type wake generator upstream of the five-blade linear cascade. The coolant jet ejection is simulated by ejecting coolant through holes on the hollow spokes of the wake generator. For a blade without film holes, unsteady wake increases both pressure side and suction side heat transfer levels due to early boundary layer transition. Adding trailing edge ejection to the unsteady wake further enhances the blade surface heat transfer coefficients particularly near the leading edge region. For a film-cooled blade, unsteady wake effects slightly enhance surface heat transfer coefficients but significantly reduces film effectiveness. Addition of trailing edge ejection to the unsteady wake has a small effect on surface heat transfer coefficients compared to other significant parameters such as film injection, unsteady wakes, and grid generated turbulence, in that order. Trailing edge ejection effect on film effectiveness distribution is stronger than on the heat transfer coefficients.}, number={3}, journal={Journal of Turbomachinery}, author={Du, H. and Ekkad, S.V. and Han, J.-C.}, year={1999}, pages={448–455} }
 @article{ekkad_han_1999, title={Heat transfer distributions on a cylinder with simulated thermal barrier coating spallation}, volume={13}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0032739395&partnerID=MN8TOARS}, DOI={10.2514/2.6403}, abstractNote={Detailed heat transfer distributions are presented over a turbine blade leading-edge model with simulated thermal barrier coating spoliation. The blade leading-edge region is simulated by a cylinder in a crossflow with a tailboard. The heat transfer measurements are presented only on one side of the front half of the cylinder. The simulated spoliation cavities are rectangular in shape and have rounded corners. The effect of a spallation cavity is studied at four different locations (0-20, 10-30, 20-40, and 35-55 deg). Two different cavity depths are studied at each location to understand the effect of spallation depth on local heat transfer distributions. The effect of free stream turbulence on detailed heat transfer is also presented for each case. Detailed heat transfer measurements are obtained using a transient liquid crystal technique. Detailed heat transfer distributions present the local high-heat transfer and low-heat transfer regions inside and outside the spallation. Results show that spallations can enhance heat transfer up to two times compared with that for a smooth surface. Results also show that the spallation location and depth have a strong effect on local heat transfer distributions on the leading edge. An increase in freestream turbulence further increases the heat transfer coefficients caused by the spallation. Tt Tu Tw = Too = t = Nomenclature}, number={1}, journal={Journal of Thermophysics and Heat Transfer}, author={Ekkad, S.V. and Han, J.-C.}, year={1999}, pages={76–81} }
 @article{ekkad_huang_han_1999, title={Impingement heat transfer on a target plate with film cooling holes}, volume={13}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0033355454&partnerID=MN8TOARS}, DOI={10.2514/2.6471}, abstractNote={Detailedheat-transferdistributionsarepresentedforanarray ofjetsimpinging onatargetplatewithastaggered array of e lm cooling holes. The e ow impinges on the target plate through a row of impingement holes and exits the channel from the sides and through the e lm holes. The top plate has 12 rows of impingement holes, and the target platehas11 rows ofe lm holes. Theimpingementholes and the e lm holes have thesame diameterandarestaggered such that the air from the impingement hole does not exit directly through the e lm hole. The setup is typical of an impingement/transpiration cooled gas turbine airfoil. Additional to the e ow exiting through the e lm holes, there is an exit for crosse ow after impingement. The exit opening of the impingement channel is changed to provide three different spent air exit directions. The detailed heat-transfer coefe cient distributions were measured using a transient liquid crystal technique. Results are presented for a range of jet Reynolds numbers between 0.4 ££ 10 3 and 2.0 ££ 10 4 with different exit e ow orientations. Heat-transfer results for the target plate with e lm holes are compared with those without e lm holes under the same e ow conditions. Film extraction reduces crosse ow effects on jet impingement heat transfer. However, overall averaged heat-transfer rates on the target surface appear less affected by presence of e lm hole for cases where the crosse ow is generated in only one direction.}, number={4}, journal={Journal of thermophysics and heat transfer}, author={Ekkad, S.V. and Huang, Y. and Han, J.-C.}, year={1999}, pages={522–528} }
 @inproceedings{ekkad_pamula_acharya_1999, title={Influence of cross-flow induced swirl and impingement on heat transfer in an internal coolant passage of a turbine airfoil}, volume={364-1}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0033313153&partnerID=MN8TOARS}, booktitle={American Society of Mechanical Engineers, Heat Transfer Division, (Publication) HTD}, author={Ekkad, S.V. and Pamula, G. and Acharya, S.}, year={1999}, pages={227–233} }
 @article{ekkad_han_du_1998, title={Detailed film cooling measurements on a cylindrical leading edge model: Effect of free-stream turbulence and coolant density}, volume={120}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0032194608&partnerID=MN8TOARS}, DOI={10.1115/1.2841792}, abstractNote={Detailed heat transfer coefficient and film effectiveness distributions are presented on a cylindrical leading edge model using a transient liquid crystal technique. Tests were done in a low-speed wind tunnel on a cylindrical model in a crossflow with two rows of injection holes. Mainstream Reynolds number based on the cylinder diameter was 100,900. The two rows of injection holes were located at ±15 deg from stagnation. The film holes were spaced four hole diameters apart and were angled 30 and 90 deg to the surface in the spanwise and streamwise directions, respectively. Heat transfer coefficient and film effectiveness distributions are presented on only one side of the front half of the cylinder. The cylinder surface is coated with a thin layer of thermochromic liquid crystals and a transient test is run to obtain the heat transfer coefficients and film effectiveness. Air and CO2 were used as coolant to simulate coolant-to-mainstream density ratio effect. The effect of coolant blowing ratio was studied for blowing ratios of 0.4, 0.8, and 1.2. Results show that Nusselt numbers downstream of injection increase with an increase in blowing ratio for both coolants. Air provides highest effectiveness at blowing ratio of 0.4 and CO2 provides highest effectiveness at a blowing ratio of 0.8. Higher density coolant (CO2) provides lower Nusselt numbers at all blowing ratios compared to lower density coolant (air). An increase in free-stream turbulence has very small effect on Nusselt numbers for both coolants. However, an increase in free-stream turbulence reduces film effectiveness significantly at low blowing ratios for both coolants.}, number={4}, journal={Journal of Turbomachinery}, author={Ekkad, S.V. and Han, J.C. and Du, H.}, year={1998}, pages={799–807} }
 @inproceedings{ekkad_krishna_han_1998, title={Detailed film cooling measurements on a cylindrical model with simulated TBC spallation}, volume={357}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-12844285293&partnerID=MN8TOARS}, number={4}, booktitle={American Society of Mechanical Engineers, Heat Transfer Division, (Publication) HTD}, author={Ekkad, S.V. and Krishna, A. and Han, J.C.}, year={1998}, pages={69–76} }
 @article{ekkad_huang_han_1998, title={Detailed heat transfer distributions in two-pass square channels with rib turbulators and bleed holes}, volume={41}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0032412918&partnerID=MN8TOARS}, DOI={10.1016/S0017-9310(98)00099-4}, abstractNote={Detailed heat transfer coefficient distributions are presented for a two-pass square channel with a 180° turn. One wall of the channel has periodically placed rib turbulators and bleed holes. Four different configurations of 90° parallel, 60° parallel, 60° V ribs, and 60° inverted V ribs are studied in conjunction with the effect of bleed holes on the same wall. The surface is coated with a thin layer of thermochromic liquid crystals and a transient test is run to obtain the detailed heat transfer distributions. Detailed distributions show distinctive peaks in heat transfer levels around bleed holes and on rib turbulator tips. The 60° parallel, 60° V, and 60° inverted V ribbed channels produce similar levels of heat transfer enhancement in the first pass. However, the 60° inverted V ribbed channel produces higher enhancement in the second pass. Regional-averaged heat transfer results indicate that a surface with bleed holes provides similar heat transfer enhancement as that for a surface without bleed holes although 20–25% of the inlet mass flow exits through the bleed holes.}, number={23}, journal={International Journal of Heat and Mass Transfer}, author={Ekkad, S.V. and Huang, Y. and Han, J.-C.}, year={1998}, pages={3781–3791} }
 @article{huang_ekkad_han_1998, title={Detailed heat transfer distributions under an array of orthogonal impinging jets}, volume={12}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0031672665&partnerID=MN8TOARS}, DOI={10.2514/2.6304}, abstractNote={Detailed heat transfer distributions are presented for an array of in-line jets impinging orthogonally on a target plate with different crossflow orientations. A transient liquid crystal technique was used to measure the detailed heat transfer coefficients on the target surface. Different crossflow directions are created by changing the test section open ends. For each exit orientation, measurements are made at four flow Reynolds numbers between 4.8 x 10 3 and 1.83 X 10 4 . Results show that the now exit crossflow direction significantly affects the flow and heat transfer coefficient distributions on the target plate. Local heat transfer coefficient increases with an Increase in average jet Reynolds number over the entire impingement target surface. Highest heat transfer coefficients are obtained for a crossflow orientation where flow exits in both directions. Nusselt number results are correlated for the various exit flow orientations}, number={1}, journal={Journal of Thermophysics and Heat Transfer}, author={Huang, Y. and Ekkad, S.V. and Han, J.-C.}, year={1998}, pages={73–79} }
 @article{du_han_ekkad_1998, title={Effect of unsteady wake on detailed heat transfer coefficient and film effectiveness distributions for a gas turbine blade}, volume={120}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0032194349&partnerID=MN8TOARS}, DOI={10.1115/1.2841793}, abstractNote={Unsteady wake effects on detailed heat transfer coefficient and film cooling effectiveness distributions from a gas turbine blade with film cooling are obtained using a transient liquid crystal technique. Tests were performed on a five-blade linear cascade at a axial chord Reynolds number of 5.3 × 105 at cascade exit. Upstream unsteady wakes are simulated using a spoke-wheel type wake generator. The test blade has three rows of film holes on the leading edge and two rows each on the pressure and suction surfaces. Air and CO2 were used as coolants to simulate different coolant-to-mainstream density ratio effect. Coolant blowing ratio for air injection is varied from 0.8 to 1.2 and is varied from 0.4 to 1.2 for CO2. Results show that Nusselt numbers for a film-cooled blade are much higher compared to a blade without film injection. Particularly, film injection causes earlier boundary layer transition on the suction surface. Unsteady wakes slightly enhance Nusselt numbers but significantly reduce film cooling effectiveness on a film-cooled blade compared with a film-cooled blade without wakes. Nusselt numbers increase slightly but film cooling effectiveness increase significantly with an increase in blowing ratio for CO2 injection. Higher density coolant (CO2) provides higher effectiveness at higher blowing ratios (M = 1.2) whereas lower density coolant (Air) provides higher effectiveness at lower blowing ratios (M = 0.8).}, number={4}, journal={Journal of Turbomachinery}, author={Du, H. and Han, J.C. and Ekkad, S.V.}, year={1998}, pages={808–817} }
 @inproceedings{du_ekkad_han_1998, title={Effect of unsteady wake with trailing edge coolant ejection on film cooling performance for a gas turbine blade}, volume={4}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84973177461&partnerID=MN8TOARS}, DOI={10.1115/98-GT-259}, abstractNote={The effect of unsteady wakes with trailing edge coolant ejection on surface heat transfer coefficients and film cooling effectiveness is presented for a downstream film-cooled turbine blade. The detailed heat transfer coefficient and film effectiveness distributions on the blade surface are obtained using a transient liquid crystal technique. Unsteady wakes are produced by a spoked wheel-type wake generator upstream of the five-blade linear cascade. The coolant jet ejection is simulated by ejecting coolant through holes on the hollow spokes of the wake generator. For a blade without film holes, unsteady wake increases both pressure side and suction side heat transfer levels due to early boundary layer transition. Adding trailing edge ejection to the unsteady wake further enhances the blade surface heat transfer coefficients particularly near the leading edge region. For a film-cooled blade, unsteady wake effects slightly enhance surface heat transfer coefficients but significantly reduces film effectiveness. Addition of trailing edge ejection to the unsteady wake has a small effect on surface heat transfer coefficients compared to other significant parameters such as film injection, unsteady wakes, and grid generated turbulence, in that order. Trailing edge ejection effect on film effectiveness distribution is stronger than on the heat transfer coefficients.}, number={GT}, booktitle={Proceedings of the ASME Turbo Expo}, author={Du, H. and Ekkad, S.V. and Han, J.-C.}, year={1998} }
 @article{ekkad_mehendale_han_lee_1997, title={Combined effect of grid turbulence and unsteady wake on film effectiveness and heat transfer coefficient of a gas turbine blade with air and CO<inf>2</inf> film injection}, volume={119}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0031192855&partnerID=MN8TOARS}, DOI={10.1115/1.2841163}, abstractNote={Experiments were performed to study the combined effect of grid turbulence and unsteady wake on film effectiveness and heat transfer coefficient of a turbine blade model. Tests were done on a five-blade linear cascade at the chord Reynolds number of 3.0 × 105 at cascade inlet. Several combinations of turbulence grids, their locations, and unsteady wake strengths were used to generate various upstream turbulence conditions. The test blade had three rows of film holes in the leading edge region and two rows each on the pressure and suction surfaces. Air and CO2 were used as injectants. Results show that Nusselt numbers for a blade with film injection are much higher than that without film holes. An increase in mainstream turbulence level causes an increase in Nusselt numbers and a decrease in film effectiveness over most of the blade surface, for both density injectants, and at all blowing ratios. A free-stream turbulence superimposed on an unsteady wake significantly affects Nusselt numbers and film effectiveness compared with only an unsteady wake condition.}, number={3}, journal={Journal of Turbomachinery}, author={Ekkad, S.V. and Mehendale, A.B. and Han, J.C. and Lee, C.P.}, year={1997}, pages={594–600} }
 @inproceedings{ekkad_han_du_1997, title={Detailed film cooling measurements on a cylindrical leading edge model: Effect of free-stream turbulence and coolant density}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0031375134&partnerID=MN8TOARS}, booktitle={American Society of Mechanical Engineers (Paper)}, author={Ekkad, Srinath V. and Han, Je-Chin and Du, Hui}, year={1997} }
 @inproceedings{du_han_ekkad_1997, title={Detailed film cooling measurements over a gas turbine blade using a transient liquid crystal image technique}, volume={350}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-4043123444&partnerID=MN8TOARS}, booktitle={American Society of Mechanical Engineers, Heat Transfer Division, (Publication) HTD}, author={Du, H. and Han, J.-C. and Ekkad, S.V.}, year={1997}, pages={245–254} }
 @article{ekkad_han_1997, title={Detailed heat transfer distributions in two-pass square channels with rib turbulators}, volume={40}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0031195217&partnerID=MN8TOARS}, DOI={10.1016/S0017-9310(96)00318-3}, abstractNote={Detailed Nusselt number distributions are presented for a two-pass square channel with one ribbed wall. This wall of the channel is sprayed with thermochromic liquid crystals, and a transient test is run to obtain the local heat transfer coefficients. Results are presented for Reynolds numbers ranging from 6000 to 60 000. The rib height-to-hydraulic diameter ratio is 0.125; the rib pitch-to-height ratio is 10; and the rib configurations are 90° parallel, 60° parallel, 60° V, and 60° broken V. Detailed measurements are presented in the first pass, before the 180° turn, in the turn region, after the turn, and further downstream in the second pass. The detailed distributions provide a clear understanding of the secondary flows induced by the 180° turn and the rib turbulators.}, number={11}, journal={International Journal of Heat and Mass Transfer}, author={Ekkad, S.V. and Han, J.-C.}, year={1997}, pages={2525–2537} }
 @inproceedings{ekkad_ran_1997, title={Detailed heat transfer distributions on a cylindrical model with simulated TBC spallation}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84983098241&partnerID=MN8TOARS}, booktitle={35th Aerospace Sciences Meeting and Exhibit}, author={Ekkad, S.V. and Ran, J.-C.}, year={1997} }
 @article{mehendale_jiang_ekkad_han_1998, title={Effect of film injection location on local heat transfer coefficient on a gas turbine blade}, volume={4}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-54749120172&partnerID=MN8TOARS}, DOI={10.1155/S1023621X98000141}, abstractNote={Experiments were performed to study the effect of film hole location on local heat transfer coefficient distribution of a turbine blade model with air or CO2 film injection to simulate coolant density effect. Tests were performed on a five blade linear cascade at the chord Reynolds number of3×105at cascade inlet. The test blade had three rows of film holes in the leading edge region and two rows each on the pressure and suction surfaces. Film hole locations were varied by leaving the desired ones open and plugging the rest. A combination of turbulence grid and unsteady wake was used to generate upstream high turbulence condition. Results indicate that film injection by itself causes a substantial increase in Nusselt numbers over a blade model without film holes. An increase in mainstream turbulence intensity causes an increase in Nusselt numbers over most of the blade surface, for both coolants, and at all blowing ratios. Film injection promotes an earlier boundary layer transition on the suction surface and the onset of transition depends on the film injection location; but at high turbulence levels, transition location is almost independent of film injection location.}, number={3}, journal={International Journal of Rotating Machinery}, author={Mehendale, A.B. and Jiang, H.W. and Ekkad, S.V. and Han, J.-C.}, year={1998}, pages={163–174} }
 @inproceedings{du_han_ekkad_1997, title={Effect of unsteady wake on detailed heat transfer coefficient and film effectiveness distributions for a gas turbine blade}, volume={3}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84973643393&partnerID=MN8TOARS}, DOI={10.1115/97-GT-166}, abstractNote={Unsteady wake effects on detailed heat transfer coefficient and film cooling effectiveness distributions from a gas turbine blade with film cooling are obtained using a transient liquid crystal technique. Tests were performed on a five-blade linear cascade at a axial chord Reynolds number of 5.3 × 105 at cascade exit. Upstream unsteady wakes are simulated using a spoke-wheel type wake generator. The test blade has three rows of film holes on the leading edge and two rows each on the pressure and suction surfaces. Air and CO2 were used as coolants to simulate different coolant-to-mainstream density ratio effect. Coolant blowing ratio for air injection is varied from 0.8 to 1.2 and is varied from 0.4 to 1.2 for CO2. Results show that Nusselt numbers for a film-cooled blade are much higher compared to a blade without film injection. Particularly, film injection causes earlier boundary layer transition on the suction surface. Unsteady wakes slightly enhance Nusselt numbers but significantly reduce film cooling effectiveness on a film-cooled blade compared with a film-cooled blade without wakes. Nusselt numbers increase slightly but film cooling effectiveness increases significantly with an increase in blowing ratio for CO2 injection. Higher density coolant (CO2) provides higher effectiveness at higher blowing ratios (M = 1.2) whereas lower density coolant (Air) provides higher effectiveness at lower blowing ratios (M = 0.8).}, booktitle={Proceedings of the ASME Turbo Expo}, author={Du, H. and Han, J.-C. and Ekkad, S.V.}, year={1997} }
 @article{du_ekkad_han_1997, title={Effect of unsteady wake with trailing edge coolant ejection on detailed heat transfer coefficient distributions for a gas turbine blade}, volume={119}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0000435010&partnerID=MN8TOARS}, DOI={10.1115/1.2824216}, abstractNote={Detailed heat transfer coefficient distributions on a turbine blade under the combined effects of trailing edge jets and unsteady wakes at various free-stream conditions are presented using a transient liquid crystal image method. The exit Reynolds number based on the blade axial chord is varied from 5.3 × 105 to 7.6 × 105 for a five blade linear cascade in a low speed wind tunnel. Unsteady wakes are produced using a spoked wheel-type wake generator upstream of the linear cascade. Upstream trailing edge jets are simulated by air ejection from holes located on the hollow spokes of the wake generator. The mass flux ratio of the jets to free-stream is varied from 0.0 to 1.0. Results show that the surface heat transfer coefficient increases with an increase in Reynolds number and also increases with the addition of unsteady wakes. Adding grid generated turbulence to the unsteady wake further enhances the blade surface heat transfer coefficients. The trailing edge jets compensate the defect in the velocity profile caused by the unsteady passing wakes and give an increase in free-stream velocity and produce a more uniformly disturbed turbulence intensity profile. The net effect is to increase both the front parts of blade suction and pressure surface heat transfer. However, the jet effect diminishes in and after the transition regions on suction surface, or far away from the leading edge on pressure surface.}, number={2}, journal={Journal of Heat Transfer}, author={Du, H. and Ekkad, S. and Han, J.-C.}, year={1997}, pages={242–248} }
 @article{ekkad_zapata_han_1997, title={Film effectiveness over a flat surface with air and CO<inf>2</inf>injection through compound angle holes using a transient liquid crystal image method}, volume={119}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0031191260&partnerID=MN8TOARS}, DOI={10.1115/1.2841162}, abstractNote={This paper presents detailed film effectiveness distributions over a flat surface with one row of injection holes inclined streamwise at 35 deg for three blowing ratios (M = 0.5, 1.0, 2.0). Three compound angles of 0, 45, and 90 deg with air (D.R. = 0.98) and CO2 (D. R. = 1.46) as coolants are tested at an elevated free-stream turbulence condition (Tu ≈ 8.5 percent). A transient liquid crystal technique is used to measure local heat transfer coefficients and film effectiveness. Detailed film effectiveness results are presented near and around film injection holes. Compound angle injection provides higher film effectiveness than simple angle injection for both coolants. Higher density injectant produces higher effectiveness for simple injection. However, lower density coolant produces higher effectiveness for a large compound angle of 90 deg. The detailed film effectiveness obtained using the transient liquid crystal technique, particularly in the near-hole region, provided a better understanding of the film cooling process in gas turbine components.}, number={3}, journal={Journal of Turbomachinery}, author={Ekkad, S.V. and Zapata, D. and Han, J.C.}, year={1997}, pages={587–593} }
 @article{ekkad_zapata_han_1997, title={Heat transfer coefficients over a flat surface with air and CO<inf>2</inf> injection through compound angle holes using a transient liquid crystal image method}, volume={119}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0031190345&partnerID=MN8TOARS}, DOI={10.1115/1.2841161}, abstractNote={This paper presents the detailed heat transfer coefficients over a flat surface with one row of injection holes inclined streamwise at 35 deg for three blowing ratios (M = 0.5–2.0). Three compound angles of 0, 45, and 90 deg with air (D.R. = 0.98) and CO2 (D.R. = 1.46) as coolants were tested at an elevated free-stream turbulence condition (Tu ≈ 8.5 percent). The experimental technique involves a liquid crystal coating on the test surface. Two related transient tests obtained detailed heat transfer coefficients and film effectiveness distributions. Heat transfer coefficients increase with increasing blowing ratio for a constant density ratio, but decrease with increasing density ratio for a constant blowing ratio. Heat transfer coefficients increase for both coolants over the test surface as the compound angle increases from 0 to 90 deg. The detailed heat transfer coefficients obtained using the transient liquid crystal technique, particularly in the near-hole region, will provide a better understanding of the film cooling process in gas turbine components.}, number={3}, journal={Journal of Turbomachinery}, author={Ekkad, S.V. and Zapata, D. and Han, J.C.}, year={1997}, pages={580–586} }
 @inproceedings{du_ekkad_han_1997, title={Unsteady high turbulence effect on turbine blade film cooling heat transfer performance using a transient liquid crystal technique}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0031345037&partnerID=MN8TOARS}, number={10195}, booktitle={NASA Conference Publication}, author={Du, H. and Ekkad, S.V. and Han, J.C.}, year={1997}, pages={239–259} }
 @inproceedings{huang_ekkad_han_1996, title={Detailed heat transfer coefficient distributions under an array of impinging jets with coolant extraction}, volume={333}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0030386706&partnerID=MN8TOARS}, booktitle={American Society of Mechanical Engineers, Heat Transfer Division, (Publication) HTD}, author={Huang, Y. and Ekkad, S.V. and Han, J.-C.}, year={1996}, pages={143–150} }
 @inproceedings{ekkad_huang_han_1996, title={Detailed heat transfer distributions in two-pass smooth and turbulated square channels with bleed holes}, volume={330}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0030398909&partnerID=MN8TOARS}, booktitle={American Society of Mechanical Engineers, Heat Transfer Division, (Publication) HTD}, author={Ekkad, S.V. and Huang, Y. and Han, J.-C.}, year={1996}, pages={133–140} }
 @inproceedings{du_ekkad_han_1996, title={Effect of unsteady wake with trailing edge coolant ejection on detailed heat transfer coefficient distributions for a gas turbine blade}, volume={327}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0030413744&partnerID=MN8TOARS}, booktitle={American Society of Mechanical Engineers, Heat Transfer Division, (Publication) HTD}, author={Du, H. and Ekkad, S. and Han, J.-C.}, year={1996}, pages={81–88} }
 @article{ekkad_han_1996, title={Heat transfer inside and downstream of cavities using transient liquid crystal method}, volume={10}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0030195475&partnerID=MN8TOARS}, DOI={10.2514/3.818}, abstractNote={Local heat transfer coefficient distributions are investigated inside and downstream of various cavities on a flat surface. The effect of cavity size, depth, and shape are studied using a transient liquid crystal image method. Liquid crystals are sprayed on the test surface and a hot mainstream is imposed, suddenly causing a color change. The time of color change is obtained using an image processing system. An increase in cavity size for the same depth increases heat transfer coefficients on the test surface. An increase hi cavity depth increases downstream heat transfer coefficients. Five cavity shapes are studied to compare local heat transfer behavior for the effect of various shapes. Nomenclature cp = specific heat of test surface material D = hydraulic diameter of cavity d = cavity depth h = local convection heat transfer coefficient with cavity h0 = local convection heat transfer coefficient without cavity k = thermal conductivity of test surface material Rex = Reynolds number based on axial distance, Ree = momentum thickness Reynolds number, T = temperature Tu = freestream turbulence intensity t = time of liquid crystal color change U = mainstream velocity x = axial distance along the mainstream z = spanwise distance a = thermal diffusivity of test surface material A = temperature step 8 = boundary-layer thickness 8* = displacement thickness 6 = momentum thickness of the boundary layer IJL = fluid dynamic viscosity p = fluid density r = time step}, number={3}, journal={Journal of Thermophysics and Heat Transfer}, author={Ekkad, S.V. and Han, J.-C.}, year={1996}, pages={511–516} }
 @article{ekkad_du_han_1996, title={Local heat transfer coefficient and film effectiveness distributions on a cylindrical leading edge model using a transient liquid crystal image method}, volume={3}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-27844443743&partnerID=MN8TOARS}, DOI={10.1615/JFlowVisImageProc.v3.i2-3.20}, abstractNote={A transient liquid crystal technique is presented for measuring detailed heat transfer coefficients and film effectiveness on a cylindrical test model with film cooling. The technique uses a thin liquid crystal coating on the test surface and two similar transients tests. The cylinder, coated with a thin layer of liquid crystals, is heated to a uniform surface temperature and suddenly exposed to a cooler mainstream. The time history of color change at each pixel location is analyzed to obtain the local heat transfer coefficient and film effectiveness. Tests were run at a mainstream Reynolds number based on cylinder diameter of 100,900. The effect of blowing ratio on heat transfer coefficient and film effectiveness was studied for five blowing ratios ranging between 0.2 and 1.2. Two roles of holes at ±15° from stagnation and hole spacing of four-hole diameters apart and angled at 30° and 90° to the surface in the spanwise and streamwise directions were used for coolant ejection. Air was used as coolant. Detailed distributions obtained using the present technique provide a better understanding of the film cooling phenomena on the cylinder surface. The technique provides high resolution and more accurate results compared with classic heat transfer measurement techniques. Some of the results from the present study are compared with results obtained using classic heat transfer measurement methods.}, number={2-3}, journal={Journal of Flow Visualization and Image Processing}, author={Ekkad, S.V. and Du, H. and Han, J.-C.}, year={1996}, pages={129–140} }
 @inbook{han_ekkad_huang_1996, place={New York}, title={Measurements on Augmented Heat Transfer Surfaces Using a Transient Liquid Crystal Technique}, booktitle={Process, Enhanced, and Multiphase Heat Transfer: A Festschrift for A.E. Bergles}, publisher={Begell House, Inc}, author={Han, J.C. and Ekkad, S.V. and Huang, Y.}, editor={Manglik, R.M. and Kraus, A.D.Editors}, year={1996}, pages={339–349} }
 @article{du_ekkad_han_1996, title={Surface heat transfer visualization on a model gas turbine blade using a transient liquid crystal image technique}, volume={3}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-27844522045&partnerID=MN8TOARS}, DOI={10.1615/jflowvisimageproc.v3.i2-3.30}, abstractNote={A transient liquid crystal technique has been developed to visualize the convective heat transfer coefficient distributions on a model gas turbine blade. A five-blade linear cascade is installed into a low-speed wind tunnel to simulate the gas turbine blade cascade. A color image processing system is used to measure the color change of the liquid crystal layer coated on the middle test blade at the center of the cascade. Detailed heat transfer coefficient distributions on a turbine blade are presented for the different flow Reynolds numbers. The cascade exit flow Reynolds number of the flow passing the cascade based on the blade chord is varied from 7.1 × 105 to 1.02 × 106. Results are compared with those obtained with the thin-foil thermocouple method under the same conditions. It is found that the transient liquid crystal image technique gives more detailed information than the classic thin-foil thermocouple method. Some findings with this technique, such as separation bubble effect on heat transfer coefficient on the pressure surface of the blade, flow transition location, and high heat transfer coefficients near the trailing edges on both the suction and the pressure surfaces of the blade, are an improvement over the classic method. The detailed information obtained using this technique may significantly influence the cooling design of the gas turbine blade.}, number={2-3}, journal={Journal of Flow Visualization and Image Processing}, author={Du, H. and Ekkad, S. and Han, J.-C.}, year={1996}, pages={141–152} }
 @inproceedings{ekkad_zapata_han_1995, title={Film effectiveness over a flat surface with air and CO<inf>2</inf> injection through compound angle holes using a transient liquid crystal image method}, volume={4}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84976878628&partnerID=MN8TOARS}, DOI={10.1115/95-GT-011}, abstractNote={This paper presents detailed film effectiveness distributions over a flat surface with one row of injection holes inclined streamwise at 35° for three blowing ratios (M=0.5, 1.0, 2.0). Three compound angles of 0°, 45°, and 90° with air (D.R.=0.98) and CO2 (D.R.=1.46) as coolants are tested at an elevated free-stream turbulence condition (Tu≈8.5%). A transient liquid crystal technique is used to measure local heat transfer coefficients and film effectiveness. Detailed film effectiveness results are presented near and around film injection holes. Compound angle injection provides higher film effectiveness than simple angle injection for both coolants. Higher density injectant produces higher effectiveness for simple injection. However, lower density coolant produces higher effectiveness for a large compound angle of 90°. The detailed film effectiveness obtained using the transient liquid crystal technique, particularly in the near hole region, provided a better understanding of the film cooling process in gas turbine components.}, booktitle={Proceedings of the ASME Turbo Expo}, author={Ekkad, S.V. and Zapata, D. and Han, J.-C.}, year={1995} }
 @inproceedings{ekkad_zapata_han_1995, title={Heat transfer coefficients over a flat surface with air and CO<inf>2</inf> injection through compound angle holes using a transient liquid crystal image method}, volume={4}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84976874954&partnerID=MN8TOARS}, DOI={10.1115/95-GT-010}, abstractNote={This paper presents the detailed heat transfer coefficients over a flat surface with one row of injection holes inclined streamwise at 35° for three blowing ratios (M=0.5–2.0). Three compound angles of 0°, 45°, and 90° with air (D.R.=0.98) and CO2 (D.R.=1.46) as coolants were tested at an elevated free-stream turbulence condition (Tu≈8.5%). The experimental technique involves a liquid crystal coating on the test surface. Two related transient tests obtained detailed heat transfer coefficients and film effectiveness distributions. Heat transfer coefficients increase with increasing blowing ratio for a constant density ratio but decrease with increasing density ratio for a constant blowing ratio. Heat transfer coefficients increase for both coolants over the test surface as the compound angle increases from 0° to 90°. The detailed heat transfer coefficients obtained using the transient liquid crystal technique, particularly in the near hole region, will provide a better understanding of the film cooling process in gas turbine components.}, booktitle={Proceedings of the ASME Turbo Expo}, author={Ekkad, S.V. and Zapata, D. and Han, J.-C.}, year={1995} }
 @article{ekkad_han_1995, title={LOCAL HEAT TRANSFER DISTRIBUTIONS NEAR A SHARP 180&deg; TURN OF A TWO-PASS SMOOTH SQUARE CHANNEL USING A TRANSIENT LIQUID CRYSTAL IMAGE TECHNIQUE}, volume={2}, ISSN={1065-3090}, url={http://dx.doi.org/10.1615/jflowvisimageproc.v2.i3.80}, DOI={10.1615/jflowvisimageproc.v2.i3.80}, abstractNote={Local heat transfer distributions are presented near a sharp 180° turn of a two-pass smooth square channel using a transient technique and encapsulated liquid crystal coating. Detailed distributions of the local Nusselt numbers are given for three flow Reynolds numbers of 10,000, 25,000, and 50,000. Results show that the Nusselt numbers are much higher in the region immediately downstream of the turn compared with upstream values for all three Reynolds numbers. The regional averaged results are compared with published heat transfer data.}, number={3}, journal={Journal of Flow Visualization and Image Processing}, publisher={Begell House}, author={Ekkad, Srinath and Han, Je-Chin}, year={1995}, pages={285–297} }
 @inproceedings{ekkad_du_han_1995, title={Local heat transfer coefficient and film effectiveness distributions on a cylindrical leading edge model using a transient liquid crystal image method}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0029491932&partnerID=MN8TOARS}, booktitle={American Society of Mechanical Engineers (Paper)}, author={Ekkad, Srinath V. and Du, Hui and Han, Je-Chin}, year={1995} }
 @article{mehendale_ekkad_han_1994, title={Mainstream turbulence effect on film effectiveness and heat transfer coefficient of a gas turbine blade with air and CO<inf>2</inf> film injection}, volume={37}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0028534050&partnerID=MN8TOARS}, DOI={10.1016/0017-9310(94)90387-5}, abstractNote={Effect of grid-generated mainstream turbulence on the film effectiveness and heat transfer coefficient of a turbine blade model was investigated at a chord Reynolds number of 3 × 105. The blade model had three rows of film holes in the leading edge region and two rows each on the pressure and suction surfaces. CO2 and air, with density ratios (DRs) of about 1.5 and 1.0, respectively, were used as film injectants. Results indicate that an increase in mainstream turbulence causes an increase in Nusselt numbers and a decrease in film effectiveness, over most of the blade surface, for both density ratio injectants at all blowing ratios.}, number={17}, journal={International Journal of Heat and Mass Transfer}, author={Mehendale, A.B. and Ekkad, S.V. and Han, J.-C.}, year={1994}, pages={2707–2714} }