Prediction of subcooled flow boiling local heat transfer coefficient in microchannel heatsinks

This study aims to evaluate previous models that describe the average heat transfer coefficient (HTC) of subcooled flow boiling in microchannels and modify them to predict the local HTC. To enrich and extend the databases of these models, three-dimensional computational fluid dynamics simulations of HFE-7100, ethanol, and water in a wide range of heat fluxes were performed. The effect of the inlet degree of subcooling, or the difference between saturation and bulk fluid temperatures, on thermal performance is first investigated. Following that, the effect of the channel height-to-width ratio is investigated by examining four microchannels of different sections. Although decreasing the inlet subcooling tends to increase the overall HTC, it results in a lower maximum allowable heat flux due to the increase in bubble coverage of the bottom wall. A greater height-to-width ratio may improve the HTC, but there is an optimum ratio beyond which thermal performance degrades. Finally, the predictability of five models describing the average HTC of the subcooled flow boiling region is investigated, and adjusted correlations for accurate local HTC prediction are presented. A new correlation predicting the length of the onset of nucleate boiling is also demonstrated.