Boiling heat transfer, pressure drop, and flow pattern in a horizontal square minichannel

This study experimentally investigated the flow boiling heat transfer, pressure drop, and flow pattern in a horizontal square minichannel with a hydraulic diameter of 2.0 mm, and the effects of mass flux, vapor quality, heat flux, and refrigerant properties on the flow boiling characteristics were clarified. The heat transfer coefficient and pressure drop of R32 and R1234yf were measured in a mass flux range of 50-400 kgm(-2) s(-1) at a saturation temperature of 15 degrees C. The flow pattern of the square minichannel outlet was observed and was classified as plug, wavy, churn, and annular flows. The heat transfer coefficients in the square minichannel were larger than those in the circular minichannel with a similar hydraulic diameter at low mass flux conditions. The heat transfer coefficients of R32 indicated higher values compared with those of R1234yf at same mass flux and qualities. An empirical heat transfer model taking into account the forced convection, nucleate boiling, and thin liquid film evaporation was developed for horizontal square and circular minichannels. The frictional pressure drop of R32 was 1.5-2 times higher than that of R1234yf at same mass flux and vapor quality condition, and the effect of channel shape on the frictional pressure drop was small unlike the boiling heat transfer.