Scale effects on flow boiling heat transfer in microchannels: A fundamental perspective

Flow boiling in microchannels has received considerable attention from researchers worldwide in the last decade. A scaling analysis is presented to identify the relative effects of different forces on the boiling process at microscale. Based on this scaling analysis, the flow pattern transitions and stability for flow boiling of water and FC-77 are evaluated. From the insight gained through the careful visualization and thermal measurements by previous investigators, 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. Furthermore, it is pointed out that the convective contribution cannot be ruled out on the basis of experimental data which 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. Specific suggestions for future research to enhance the boiling heat transfer in microchannels are also provided.