Experimental study of flow boiling heat transfer characteristic by improved surface wettability in multi-microchannel

Abstract

Surface wettability is an important parameter in small scale phenomena because the surface force becomes dominant as the scale size decreases. Especially, in two-phase flow, it changes flow regime that makes the heat transfer, pressure drop, and other characteristics to be different. In this study, we investigated how the surface wettability affects flow boiling heat transfer in multi-microchannel. The experiments were conducted by using distilled water in an aluminum alloy multi-microchannel heat sink. The heat sink was consisted of 10 parallel micro-channels each having a rectangular cross-sectional width and height of 500um and the length of 10mm. Tests were performed for the mass flux of 160 and 480 kg/m2s, heat flux of 320 kW/m2 and saturation temperature of nearly 100℃. The modified and bare aluminum surfaces were also experimented. The modified surface was fabricated by a simple AAO(Anodic Aluminum Oxidation) process with oxalic acids and carbon cathodes. The contact angle of the bare surface was 79° and the modified surface (AAO surface) was less than 10°. In this study, the bare surface showed that the boiling heat transfer coefficient was increased as the mass flux increases since the heat transfer of the single phase forced convection in the liquid film was accelerated. However, the AAO surface showed almost same boiling heat transfer coefficient in spite of the increased mass flux. Thus, the heat transfer coefficient difference between AAO and bare surface was increased as the mass flux increases. This might be related with the very stable liquid film thickness by strong liquid spreading effect of nano, micro surface structure. Moreover, because of this effect, the experiment showed an interesting result that was the delayed partial dry out phenomena.