Flow boiling of water in a circular staggered micro-pin fin heat sink

Flow boiling of water across a bank of circular staggered micro-pin fins, 250 μm long and 100 μm diameter with pitch-to-diameter ratio of 1.5, was experimentally studied for mass fluxes ranging from 346 kg/m2 s to 794 kg/m2 s and surface heat fluxes ranging from 20 W/cm2 to 350 W/cm2. Local temperature measurements were used to elucidate the two-phase heat transfer coefficients. The flow was visualized and classified as vapor slug and annular flow patterns. A flow map was constructed and compared with maps for adiabatic micro-scale systems and good agreement was observed. A superposition model for the two-phase heat transfer coefficient in convective boiling regime was developed. Frictional multiplier, ((ϕl)2(ϕl)2), from micro-scale studies was used to define the enhancement factor (F) in the heat transfer coefficient model and reasonable agreement was obtained between experiments and predictions.