Flow boiling in microchannels has received considerable attention from researchers worldwide in the last decade. A scaling analysis is presented to identify the roles of different forces on the boiling process at microscale. Based on this scaling analysis, the insight gained from the careful visualization and thermal measurements by previous investigators, and the earlier results obtained by the author's group with specifically designed experiments and numerical simulations, a mechanistic description of heat transfer during flow boiling in microchannels is presented. Similarities between heat transfer around a nucleating bubble in pool boiling and in the elongated bubble/slug flow pattern in flow boiling are brought out. The roles of microlayer evaporation and transient conduction/microconvection are discussed. Further, it is pointed out that the convective contribution cannot be ruled out on the basis of experimental data that shows no dependence of heat transfer coefficient on mass flow rate, since the low liquid flow rate during flow boiling in microchannels at low qualities leads to laminar flow, where heat transfer coefficient is essentially independent of the mass flow rate.
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