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 micro porous 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 show that the 52 m coating thickness porous surface exhibited 65% to 148% higher heat transfer performance than the smooth surface. However, for the 98 m microporous coating surface, the heat transfer coefficients were only from 41% to 90% higher than those on smooth surface at various mass fluxes. This shows that the thicker coating layer thickness did not provide better heat transfer performance. The improvement on the maximum heat fluxes by applying micro porous coatings was only 3% to 10% in comparing to that on smooth surface. Partial dryout was observed at high and moderate mass fluxes on both smooth and porous coating channels. It happened at lower exit vapor qualities in micro porous coating channels than that in smooth channels. The partial dryout exit vapor qualities increased with decreasing mass fluxes. For the lowest mass flux, owing to the low heat flux and low nucleation suppression, no significant partial dryout was investigated. The pressure drops in micro porous coating channels were around 25 to 47% higher than those in smooth channels. There was not significant influence of micro porous coating layer thickness on flow boiling pressures drops.