The effect of heating direction on flow boiling heat transfer of R134a in micro-channels

摘要

This paper presents effects of heating directions on heat transfer performance of R134a flow boiling in micro-channel heat sink.The heat sink has 30 parallel rectangular channels with cross-sectional dimensions of 500μm width 500μm depth and 30mm.length.The experimental operation condition ranges of the heat flux and the mass flux were 13.48 to 82.25 W/cm2 and 373.3 to 1244.4 kg/m2s respectively.The vapor quality ranged from 0.07 to 0.93.The heat transfer coefficients of top heating and bottom heating both were up to 25 kW/m2K.Two dominate transfer mechanisms of nucleate boiling and convection boiling were observed according to boiling curves.The experimental results indicated that the heat transfer coefficient of bottom heating was 13.9％ higher than top heating in low heat flux, while in high heat flux, the heat transfer coefficient of bottom heating was 9.9％.higher than the top heating, because bubbles were harder to divorce the heating wall.Anda modified correlation was provided to predict heat transfer of top heating.