Comparative Study of the Flow Boiling Performance of the Hybrid Microchannel-Microgap Heat Sink with Conventional Straight Microchannel and Microgap Heat Sinks

Abstract

In this study, the flow boiling performance of the hybrid microchannel-microgap heat sink (Hybrid 1:1), developed for improved boiling stability, is benchmarked against a conventional straight microchannel heat sink (SMC) and microgap heat sink (MG) having identical footprints of 25mm x 25mm, and microchannel and microgap dimensions of 300μm (Wmc) x 600μm (Hmc), and 25000μm (Wmg) x 600μm (Hmg) respectively. Experiments along with flow visualization are performed using deionized water as the coolant under heat fluxes and mass fluxes in the ranges of 0-128 W/cm2 and 100-399 kg/m2s respectively. SMC experiences the greatest degree of flow instabilities owing to the rapid bi-directional expansion of vapor slugs within the confined flow passages. In comparison, Hybrid 1:1 improves boiling stability particularly under low mass flux conditions on account of the expanding flow configuration that promotes forward expansion of vapor slugs. Similarly, MG also offers a comparable degree of boiling stability as Hybrid 1:1. However, SMC shows the most superior heat transfer performance at all mass fluxes followed by Hybrid 1:1 and MG. Although Hybrid 1:1 stabilizes boiling, the reduced wetted area towards the downstream compromises its heat transfer performance. Whereas the larger wetted area of SMC offsets the degrading effects of boiling instabilities. With the smallest wetted area, MG shows the most inferior heat transfer performance with severe partial dry-out conditions at high heat flux leading to early CHF, particularly under low mass flux conditions. Pressure drops of the heat sinks are in order of the extent of channel confinement, with the highest being for SMC followed by Hybrid 1:1 and MG.