Flow pattern transitions and liquid mass transfer resistance of binary zeotropic mixtures during flow boiling

Flow boiling visualization experiments were performed on the binary zeotropic mixtures in a rectangular channel with a cross-sectional area of 8 mm × 8 mm. R134a/R245fa binary mixtures with three mass ratios (20/80, 37/63, and 52/48 wt %) were selected for this study. Encapsulation probe sampling and gas chromatography (GC) techniques were used to measure the liquid mass fraction. The flow patterns, heat transfer, and liquid mass transfer resistances of the zeotropic binary mixtures were studied, with emphasis on the variation of liquid mass transfer resistance during the transition process of the flow patterns. The heat and mass flux were set in the range of 4–21 kW/m2 and 15−150 kg/m2 s, respectively. A wave-stratified to stratified bubble flow pattern transition criterion was proposed based on the experimental conditions, temperature glide and thermophysical properties. The liquid mass transfer resistances were related to the mass fraction of the higher volatile component, temperature glide, and vapor quality. The liquid mass transfer resistance increased during the transition process of the flow patterns dominated by the convective evaporation. The liquid mass transfer resistance decreased at firstly and then increased during the flow pattern dominated by the convective evaporation transitioned to the flow pattern dominated by the nucleate boiling. The stratified bubble flow exhibited better heat transfer performance, while the liquid mass transfer resistance remained unchanged with vapor quality.