Annular liquid film thermohydraulic and intermittent dryout in microchannel flow boiling

The annular flow regime in microchannels is commonly described as the steady flow of a vapor core with a surrounding liquid film along the channel length. However, limited studies have suggested temporal disconti-nuity in the liquid film coverage of the microchannel wall. The lack of experimental tools to directly interrogate the liquid flow characteristics and the local surface heat flux has resulted in proliferation of different hypotheses concerning the physics of this process. Surface de-wetting due to high shear stresses at the vapor-liquid interface and liquid film instability and rupture due to perturbation growth at the vapor-liquid interface have been suggested as the underlaying causes of this phenomenon. In this study, a recently developed microsensor array capable of measuring the surface local heat flux and liquid film thickness and velocity is utilized to decipher thermohydraulic characteristics of the annular flow regime and intermittent dryout with unprecedented details. The studies are conducted in a 600 mu m square cross-section microchannel using FC-72 as the test fluid at 70-80% exit vapor quality. The results show that liquid film thinning because of evaporation rather than de-wetting and film rupture is responsible for intermittent surface dryout. Studies at different mass and heat fluxes suggest that for a given surface heat flux, there is a mass flux threshold above which intermittent surface dryout vanishes. Furthermore, the experimental liquid and vapor flow velocities and liquid film thickness are used to calculate the liquid-vapor interfacial shear stress. The results suggest that under the studied conditions, the liquid-vapor interfacial shear stress has little impact on the surface intermittent dryout. This study enhances predictable and reliable design and operation of two-phase heat sinks at high exit vapor qualities.