@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{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}, number={6}, journal={Journal of Turbomachinery}, publisher={ASME International}, author={Madhavan, Srivatsan and Singh, Prashant and Ekkad, Srinath}, year={2022}, month={Feb} }
 @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}, number={6}, journal={Journal of Heat Transfer}, publisher={ASME International}, author={Ekkad, Srinath V. and Singh, Prashant}, year={2021}, month={Apr} }
 @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} }
 @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{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{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} }
 @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}, 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{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}, 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}, number={5}, journal={Journal of Heat Transfer}, publisher={ASME International}, author={Ahmed, Shoaib and Singh, Prashant and Ekkad, Srinath V.}, year={2020}, month={Mar} }
 @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} }
 @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={Abstract}, 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{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} }