Faculty Publications
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Item Comparative study of pool boiling heat transfer from various microchannel geometries(Elsevier Ltd, 2018) Walunj, A.; Sathyabhama, A.This paper presents the experimental investigation of pool boiling heat transfer enhancemrent using open microchannels. Rectangular, parabolic and stepped microchannels are fabricated on the 10 mm diameter circular copper test piece and their boiling characteristics are compared with that of the plain surface at saturated condition of distilled water. The effect of channel shape and geometrical parameter on the boiling heat transfer is studied. The channel top width and channel base width are varied from 250 ?m to 500 ?m and 500 ?m to 800 ?m, respectively. The fin tip thickness is varied from 200 ?m to 500 ?m. The channel height is fixed at 500 ?m. The morphology of bubble is observed by using high-speed camera. Compound study of bubble departure diameter and bubble frequency has proved the significant performance of parabolic and stepped microchannel. All the microchannel geometries enhanced the heat transfer rate. The modification of rectangular channel into parabolic and stepped microchannel resulted in the maximum of 88% and 169% enhancement, respectively at 11.7 °C wall superheat. At heat flux of 100 kW/m2, the heat transfer coefficient increased by 21.49–35.37% for different microchannels. The incipient temperature reduced by 13.72–23.18%. A semi-analytical model is developed to estimate the bubble departure diameter of the microchannel which predicts the present experimental data with mean absolute error of 5.58%. © 2017 Elsevier LtdItem Transient CHF enhancement in high pressure pool boiling on rough surface(Elsevier B.V., 2018) Walunj, A.; Sathyabhama, A.Experimental investigation of transient pool boiling heat transfer (PBHT) to saturated water from thick, non-lumped 20 mm diameter copper sample is carried at 1 bar, 5 bar and 10 bar pressure. The time constant (?) of exponential heat supply is varied from 1 to 6. The unidirectional scratches are made on the surface to obtain wide range of surface roughness varying from Ra = 0.106 ?m to Ra = 4.03 ?m. The effect of surface roughness, pressure and time constant on transient critical heat flux (CHF) is extensively studied. Transient CHF enhancement for Ra = 4.03 ?m when ? = 1 is found to be 98.88%, 76.55% and 53.21% at pressures P = 1 bar, P = 5 bar and P = 10 bar, respectively, however it is found to be lower by 9.38%, 21.40% and 9.73%, compared to steady state CHF enhancement for Ra = 4.03 ?m, at respective pressures. The Gorenflo correlation is modified by including the additional parameter ? and it predicts the present transient HTC values with mean absolute error (MAE) of 14.91%. The CHF model is developed by considering the effect of capillary wicking in the narrow unidirectional scratches and the bubble angle. This model predicts the present transient CHF values with MAE of 11.89%. © 2018 Elsevier B.V.Item Bubble dynamics and enhanced heat transfer during high-pressure pool boiling on rough surface(American Institute of Aeronautics and Astronautics Inc. custserv@aiaa.org, 2019) Walunj, A.; Sathyabhama, A.In the present study, the influence of surface roughness (Ra) on critical heat flux (CHF) of water at pressure of 1, 5, and 10 bar is investigated. The desired value of Ra is achieved by making unidirectional scratches on the flat copper surface. Surface roughness Ra varies from 0.106 to 4.03 ?m. The high-speed camera of 1000 fps is used for the boiling visualization study. The effect of surface roughness on bubble departure diameter and bubble frequency at different pressure is reported. Kim's CHF model is modified to include the contact angle as a function of surface roughness and temperature, which predicts the experimental CHF with mean absolute error (MAE) of 10.50% at pressure up to 10 bar. The correlation developed for bubble departure diameter predicts the experimental values with MAE of 17.09%. The relation between bubble departure diameter and bubble frequency is also developed, which predicts the corresponding experimental values with MAE of 25.26%. © 2019 American Institute of Aeronautics and Astronautics Inc. All rights reserved.Item Experimental investigation on transient pool boiling heat transfer from rough surface and heat transfer correlations(International Information and Engineering Technology Association info@iieta.org, 2019) Walunj, A.; Sathyabhama, A.This paper presents the study of transient boiling characteristics on rough copper sample with surface roughness value (Ra) ranging from 0.106 ?m to 4.03 ?m. The effect of roughness and time constant of exponential heat supply on transient critical heat flux (CHF), maximum heat transfer coefficient (HTC) and onset of nucleate boiling (ONB) is extensively studied. The mechanism of heat transfer in unidirectional scratches of rough sample is discussed. High speed visualization is carried to observe the stages of boiling and bubble dynamics. The HTC correlation developed in this study predicts the present experimental values of HTC with a mean absolute error (MAE) of 9.62 %. © 2019 International Information and Engineering Technology Association. All rights reserved.