Boiling heat transfer characteristics in a microchannel array heat sink with low mass flow rate

Flow boiling of R134a in a microchannel array heat sink was experimentally studied for mass fluxes ranging from 10 to 35 kg/m2s. The microchannels are 0.5 mm wide and 0.15 mm deep. The cuboid fins are 1.5 mm wide, 0.15 mm deep, and 3.5 mm long. In a bubbly/slug flow, heat transfer was controlled by convective boiling and evaporation. Heat transfer coefficients significantly increased with heat flux. A high degree of subcooling at the inlet increased the heat transfer rates. In semi-annular and annular flows, the heat transfer coefficients were independent of heat flux. The values moderately increased with mass flux, declined with the increase in vapor quality, and approached a constant value at high vapor qualities. This trend was attributed to partial dry-out of the wall. The annular flow was then dominated by convective evaporation heat transfer. Two correlations were developed to predict the heat transfer coefficients, corresponding to the convective boiling and evaporation region as well as the convective evaporation region.