An experimental study on flow boiling heat transfer in porous-ribbed micro-channels

Experiments were carried out to investigate flow boiling heat transfer characteristics in a micro-channel heat sink with porous ribs at three rib-heights (i.e. Hrib = 200 mu m, 400 mu m, and 600 mu m), three mass fluxes (i. e. 200 kg/ m2s, 300 kg/m2s, and 400 kg/m2s), and a range of wall heat fluxes (30-1300 kW/m2). The heat transfer coefficients and temperature uniformity in porous-ribbed micro-channels under different conditions were analyzed and compared to the smooth micro-channels (i.e. smooth flat surface and smooth-ribbed surfaces). The flow regimes in both porous and smooth micro-channels were visualized under typical flow boiling conditions. The results showed that micro-channels with higher rib heights experience an earlier transition to the critical heat flux (CHF) region due to the narrower flow path. Compared to the smooth micro-channel, the configuration of porous coating significantly enhances the heat transfer in the micro-channel without ribs or with lower rib height (Hrib = 200 mu m) due to the strong hydrophilicity, good liquid storage, and rewetting ability on porous surfaces. At G = 400 kg/m2s, the porous-flat surface achieves a 67.9 % enhancement of heat transfer coefficient in the micro-channel compared to the smooth-flat micro-channel. However, with a higher rib height (Hrib = 400 mu m and 600 mu m), the thermal resistance is increased which hinders the heat conduction along the rib-height direction. A significant superheat is observed at the bottom of porous surface, thereby deteriorating the heat transfer performance in the porous ribbed micro-channel.