Experimental investigation on flow boiling characteristics of zeotropic binary mixtures (R134a/R245fa) in a rectangular micro-channel

In the present study, the flow boiling characteristics of R134a/R245fa zeotropic mixtures with three mass fractions (10/90, 30/70 and 70/30 by wt%) in a single rectangular micro-channel and their pure components were experimentally investigated. The flow boiling in micro-channel was visualized from the top view and heated on three sides with the cross-sectional area of 1 mm × 1 mm and length of 106 mm. For each test refrigerant, the visualization results and heat transfer coefficients were obtained under the parametric conditions comprising the heat flux range of 30-120 kW/m2 and mass flux range of 60-1100 kg/m2 s at the same evaporating temperature of 18.5 °C. The corresponded outlet vapor quality was in the range of 0-1. Results showed that almost all of the test refrigerants experienced five typical flow patterns. Different bubble coexistence phenomena were observed for the pure and mixed refrigerants, respectively. The hysteresis of flow pattern transition for R134a/R245fa zeotropic mixtures was strongly affected by both of the temperature glide and blending ratio. Based on the comparison with pure components, flow boiling heat transfer characteristics of test zeotropic mixtures were analyzed and discussed in detail by considering the mixture effects. Taking account of capillary and Marangoni effect in confined space, as well as other impact factors, the developed prediction method exhibited satisfactory accuracy in predicting the heat transfer coefficient of zeotropic mixtures.