Experimental study of flow boiling in a hybrid microchannel-microgap heat sink

Abstract

A stable flow boiling operation is key to enhancing the two-phase cooling performance of microchannel heat sinks. To this end, a novel heat sink is developed which integrates a 300 μm × 600 μm straight microchannel array in the upstream region with a 25 mm × 600 μm microgap channel in the downstream region of a 25 mm × 25 mm copper heat sink. Flow boiling experiments are conducted using de-ionized water at 5 different mass fluxes in the range of 100–399 kg/m2 s, supplied at a fixed inlet temperature of 85.5 °C. The downstream heat transfer coefficient in the microgap section shows an M-shaped profile with increasing heat flux and vapor quality. Stable boiling conditions are prevalent across a large span of operating heat flux. Instabilities are experienced only for a short range of low heat flux following ONB. The stabilizing effect is attributed to the larger flow cross-section area offered by the downstream microgap section that allows expanding vapor bubbles to evacuate with lesser hindrance. Flow visualization reveals that a stable annular flow regime is established at moderate to high heat flux during which the stable boiling operation is observed. Thin film evaporation taking place during annular flow conditions results in an increasing trend in the downstream heat transfer coefficient from moderate to high heat flux. Pressure drop associated with the hybrid heat sink is found to be modest and reaches a maximum of 6 kPa at the highest mass flux of 399 kg/m2 s. A brief comparison of this heat sink is made with its straight microchannel and microgap heat sink counterparts. The hybrid heat sink shows a heat transfer performance that is superior to the microgap heat sink while poorer than the straight microchannel heat sink although it offers a better boiling stability than the straight microchannel heat sink. It however lowers the pressure drop compared to the straight microchannel heat sink.