Analysis of Evaporative Heat Transfer by Expansion Bubble in a Microchannel for High Heat Flux Cooling

The objectives of this study were to establish a theoretical model of the phase change heat transfer in a microchannel and to investigate the possibility of cooling under higher heat flux than the critical heat flux for nucleate boiling. A bubble expansion model including transient variation of the liquid film thickness was proposed. The heat transfer coefficient and wall superheat were calculated by proposed model. The validity of this model was evaluated by using cited experimental data. It was confirmed that the bubble length derived by this model was in good agreement with that data. In order to evaluate the heat transfer regime in a microchannel, the minimum wall superheat to initiate nucleate boiling was introduced, and the conditions to achieve perfect evaporative heat transfer in a microchannel under the critical heat flux were derived. The diameter of the microchannel where perfect evaporative heat transfer occurs is called the "boiling limit diameter". The boiling limit diameters of various fluids were calculated. The results clarified the influence of the surface tension on the phase change heat transfer in a microchannel. Furthermore, this result also indicated the possibility of high heat flux cooling with perfect evaporative heat transfer in a microchannel.