Faculty Publications
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Item Bubble dynamics of water-ethanol mixture during subcooled flow boiling in a conventional channel(Elsevier Ltd, 2017) Suhas, B.G.; Sathyabhama, A.In this paper, bubble dynamics in subcooled flow boiling of water-ethanol mixture in horizontal rectangular channels is investigated through visualization. The subcooled flow boiling heat transfer coefficient of water ethanol mixtures are determined for various heat flux, mass flux and ethanol volume fraction. A new empirical correlation is proposed to predict the heat transfer coefficient of pure water based on the parameters like heat flux, bubble departure diameter, waiting period and the growth period. Two types of bubble behaviours are observed after nucleation: (i) Sliding for a distance along the bottom wall of the channel surface before lift-off and (ii) Lift-off from the bottom wall of the channel surface without sliding. Force balance analysis is carried out to determine the reason for bubble lift-off and bubble sliding. The bubble lift-off without sliding is observed at higher ethanol volume fraction, lower heat flux and higher channel inlet temperature. The bubble sliding and lift-off are observed at higher heat flux and lower channel inlet temperature for water and water-ethanol mixture of 25% ethanol volume fraction. However, the effect of mass flux on the bubbles sliding or bubble lift-off is not significant. © 2016 Elsevier LtdItem Augmentation of heat transfer coefficient in pool boiling using compound enhancement techniques(Elsevier Ltd, 2017) Sathyabhama, A.; Dinesh, A.Modern compact electronic chip design demands more efficient and innovative cooling techniques in a limited space. One such method is the immersion cooling by pool boiling heat transfer, which is a highly efficient technique when compared with conventional cooling techniques. The boiling heat transfer coefficient can be enhanced using active and passive techniques. In the present investigation grooves as passive and surface vibration as active techniques were coupled to improve the boiling heat transfer coefficient. The forced vertical vibrations were induced on the copper grooved surface with a mechanical vibrator. The frequency of vibration was varied in the range 0–100 Hz and the amplitude of vibration was varied in the range 0–2.5 mm. The compound technique gave 62% improvement in heat transfer coefficient at 300 kW/m2 heat flux compared to the 29% enhancement due to grooves alone and 10% enhancement due to vibration alone. The experimental results were used to develop a modified Rohsenow correlation which predicts the experimental Nusselt number with an accuracy of ±25%. Boiling visualization was performed and the bubble parameters such as bubble departure diameter, bubble frequency and bubble growth were determined. The bubble departure diameter decreased by almost 36% and the bubble frequency increased by 221% for boiling on vibrated grooved surface. © 2017 Elsevier LtdItem Enhanced boiling heat transfer of water on a liquid-infused surface(Elsevier Ltd, 2023) Prasad Yandapalli, A.V.V.R.; Moreno Resendiz, E.M.; Kuravi, S.; Sathyabhama, S.; Kota, K.The aim of this study was to experimentally demonstrate a counter-intuitive phenomenon that a surface covered with a liquid has the potential for enhancing heat transfer for the boiling of water over it. To this end, a highly-wetting surface with a zero contact angle for multiple liquids, i.e., an Ultra-Omniphilic Surface (UOS) was prepared on aluminum (Al 6061 alloy) using a simple and easy-to-implement bulk micro-manufacturing approach and a non-boiling liquid (NBL) was infused over this surface to occupy its sub-surface micro/nano-cavities. The resulting liquid-infused UOS is called a Binary Surface (BiS) for it has two distinct superficial phases — solid phase as islands and liquid phase as NBL puddles. Saturated nucleate pool boiling experiments were conducted on the BiS and the critical heat flux (CHF) and the boiling heat transfer coefficient (HTC) were measured. The results were compared with the UOS and a plain/polished surface (PS) prepared from the same aluminum alloy sample. In addition, high-speed visualization was employed for capturing the bubble dynamics at different heat fluxes and parameters such as bubble departure diameter (Dd), bubble departure frequency (f), and nucleation site density (NSD) were measured. The results revealed that the nucleate pool boiling performance of water on the BiS surpasses both the PS and the UOS. The HTC on the BiS was 1.33 times and two times larger than the UOS and the PS, respectively. The CHF obtained on the BiS was comparable to that on the UOS and 1.47 times larger than that on the PS even though a considerable portion of the BiS surface area was covered with the NBL and unavailable for boiling. Remarkably, an inspection of the high-speed videos has suggested the presence of the same NBL as the reason for the better boiling heat transfer performance of the BiS. The NBL that was spread over the BiS as puddles was found to (1) prevent the growth of large vapor bubbles and (2) extend the isolated bubble regime by delaying the lateral coalescence of adjacent bubbles. A comparison of the results between UOS and BiS suggests that – as far as boiling enhancement is concerned – mechanisms for tackling vapor bubbles could be superior to those that involve improving surface wettability. © 2023Item Mitigation of secondary flows and pressure side bubble in turbine blade passage using asymmetric endwall contouring: a steady-state analysis(Institute of Physics, 2025) Babu, S.; Jannet, S.; Raja, R.; Lionel, P.; Oommen, L.P.; Surendran, A.In turbine passages, secondary vortices and pressure-side bubbles significantly contribute to aerodynamic losses and reduced blade efficiency issues that are critical in industrial gas turbine performance. Hence, it is very important to mitigate such losses to enhance overall turbine efficiency. Several research attempts have already been made to address this challenge; however, most studies have not focused explicitly on pressure-side bubble mitigation strategies. In the present investigation, an effort has been made to investigate the impact of endwall contouring in minimizing losses caused by secondary vortices, particularly focusing on pressure-side bubble formation. Experimental and numerical investigations are conducted on a low-speed blowing-type turbine cascade wind tunnel. The experimental study involves in-cascade testing, while numerical simulations are performed using ANSYS Reynolds-Averaged Navier–Stokes (RANS) equations with the Shear Stress Transport (SST) turbulence model. Three contouring configurations (EC 1, EC 2, and EC 3) are compared against a non-profiled base case (BC). The results confirm that endwall curvature significantly alters secondary flow behavior and static pressure distribution. While EC 1 and EC 2 generated stagnant zones in the valleys, causing additional losses while the EC 3 profile with optimized hump height and valley depth, redistributed pressure effectively. This effectively suppressed lateral flow migration and pressure-side bubble formation, which in turn enhanced overall turbine performance. In comparison to the base case, the EC 3 design quantitatively reduced total pressure loss by 3.43%, proving its efficacy in improving aerodynamic performance. © 2025 IOP Publishing Ltd. All rights, including for text and data mining, AI training, and similar technologies, are reserved.