@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{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_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{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_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{bhaskaran_ro_park_ramakrishnan_2017, title={Analysis of a Novel Technique for Temperature Rise Abatement in Liquid Piston Compressors-External Gas Injection}, volume={9}, ISSN={["1948-5093"]}, DOI={10.1115/1.4035969}, abstractNote={This paper analyses a novel heat transfer enhancement technique that can be used in compressors to limit the temperature rise during compression. This technique is based on the injection of external high-pressure gas into the chamber during the compression process. The impact of different factors on the effectiveness of this technique has been studied using experimental and computational methods. In the first set of trials, the location and angle of injection of the external air was varied. It was observed that the heat transfer coefficient governing the heat transfer rate from the chamber varied greatly with change in location and angle of injection. In the second set of experiments, the source pressure of the injected gas was varied from 100.66 kPa to 551.58 kPa. It was observed that the temperature rise of air in the chamber was reduced with an increase in source pressure. Additionally, the increase in chamber pressure was steeper in the higher source pressure cases. In the third set of experiments, the injection profile of the injected gas was varied. This parameter did not greatly impact the effectiveness of external gas injection. In the last set of experiments, the time of initiation of injection was varied. Earlier injection had a positive impact on reducing the temperature rise in the chamber. However, the pressure in the chamber was seen to increase more rapidly in the runs with early injection. Considering that these factors could have a positive/negative impact on the temperature and pressure in the chamber (work required for compression), it may be required to optimize the injection of external high-pressure gas depending on the application.}, number={2}, journal={JOURNAL OF THERMAL SCIENCE AND ENGINEERING APPLICATIONS}, author={Bhaskaran, Hari Subramaniam and Ro, Paul and Park, Joong-Kyoo and Ramakrishnan, Kishore Ranganath}, year={2017}, month={Jun} }
 @article{ramakrishnan_ro_patil_2016, title={Temperature Abatement Using Hollow Spheres in Liquid Piston Compressor for Ocean Compressed Air Energy Storage System}, DOI={10.1109/oceans.2016.7761341}, abstractNote={This paper deals with a novel technique to curb the temperature raise during compression in a liquid piston compressor used in Ocean Compressed Air Energy Storage (OCAES) system. Hollow spheres made of various materials, viz. Silicon Carbide (SiC), High Density Polyethylene (HDPE), and Polypropylene (PP) were made to float on the top surface of the liquid column. It was observed that the temperature abatement in each of the three cases was very evident. The heat transfer does not depend on the material of the sphere, but the fact that there is a solid surface between water and air itself plays an important role along with the size of the sphere. The heat transfer per unit area from the simulation and the analytical model have been compared and the values are found to be very similar. Also, polytropic index of the compression process was evaluated in case without and with SiC spheres, and it was found to be closer to the isothermal index of 1 when the spheres are used.}, journal={OCEANS 2016 MTS/IEEE MONTEREY}, author={Ramakrishnan, Kishore Ranganath and Ro, Paul. I. and Patil, Vikram. C.}, year={2016} }