Modeling and study of microlayer effects on flow boiling in a mini-channel

Abstract

The microlayer evaporation below the vapor bubble contributes much to heat dissipation and bubble growth, but it is hard to explore this microcosmic phenomenon. This study proposes a method for investigating the subcooled flow boiling heat transfer in a rectangular channel by considering the microlayer, conjugated heat transfer, a reasonable nucleation site density, and an accurate liquid-vapor interface. The implementation procedure of the microlayer model in the numerical simulation is fully demonstrated. Based on that, the bubble growth, flow pattern, heat transfer, microlayer depletion, evolutions of evaporative heat flux and dry patch during the subcooled flow boiling under different initial microlayer thicknesses are presented. Results indicate that microlayer evaporation contributes much to bubble volume and heat dissipation. The vapor volume generated by the microlayer evaporation exceeds that flows out of the channel because of the condensation effect at the liquid-vapor interface induced by the subcooled liquid, indicating bubble growth and expansion are completely attributed to microlayer evaporation. Simultaneously, much heat dissipates from the heat source due to microlayer evaporation, which contributes over 70% to both local and whole heat transfer. Because of microlayer depletion, some dry patches may appear at the elongated bubble tail. Therefore, liquid slug, elongated bubble, and dry patch circulate in the studied mini-channel, corresponding to heat transfer modes of liquid convection, microlayer evaporation, and vapor convection, supporting the three-zone heat transfer model. In addition, the initial microlayer thickness is related to the thermal resistance and affects the microlayer evaporation rate, leading to different flow patterns and heat transfer characteristics. The initial microlayer thickness is related to the slipping velocity of bubbles based on the Taylor model, which is recommended for the flow boiling study in a mini-channel.