FLOW BOILING HEAT TRANSFER IN MICROCHANNEL HEAT EXCHANGERS WITH MICRO POROUS COATING SURFACE

This study experimentally investigated the heat transfer and pressure drop performance of refrigerant HFC-245fa flow boiling in microchannel heat exchangers with and without microporous coating. The flow boiling heat transfer performance at various mass fluxes, heating rate, exit vapor qualities, and surface coating thickness was compared. The test results showed that the 52 μm coating thickness porous surface exhibited 65–148% higher heat transfer performance than the smooth surface, while the 98 μm microporous coating surface showed a lower improvement of 41–90% at various mass fluxes. Partial dry out was observed at high and moderate mass fluxes on both smooth and porous coating channels. It happened at lower exit vapor qualities in microporous coating channels than that in smooth channels. The partial dry-out exit vapor qualities increased with decreasing mass fluxes. For the lowest mass flux, because of the low heat flux and low nucleation suppression, no significant partial dry out was investigated. The pressure drops in microporous coating channels were around 25–47% higher than those in smooth channels. There was no significant influence of microporous coating layer thickness on flow boiling pressures drops.