Mechanism study on flow-boiling heat transfer of R447A/lubricating oil in a smooth horizontal tube

Abstract

The mechanism explanation for refrigerant and oil mixture heat transfer has puzzled the researchers for many years. Disagreements on the mechanism of heat transfer in the presence of oil persist. Therefore, in this paper, R447A and R447A/ lubricating oil mixtures were studied in a smooth horizontal cooper tube with 10 mm inner diameter (I.D). The evaporation temperatures were set to 10 and 20°C. The mass flux ranged from 100 to 300 kg/m2•s, and the heat flux ranged from 6 to 24 kW/m2. The oil concentrations ranged from 0.95% to 4.95% by mass. The results showed that the oil could enhance the heat transfer of R447A within a certain oil concentration range. In this range, as the oil concentration was increased, the heat-transfer coefficient of the R447A/oil mixture increased. To make a comprehensive understanding of the mechanism, three auxiliary experiments (phase separation, viscosity and flow pattern transition) were made to support the mechanism explanation. The surface wettability, viscosity, surface tension and phase separation caused by the addition of lubricating oil have different effects on the flow boiling at different vapor quality, and the leading factors are also different at different vapor quality.Based on the calculated mass transfer coefficients, a dimensionless thermal resistance 1/Rt was proposed. Most of the prediction results were less than 20% by using 1/Rt. R447A/oil mixture was also predicted using the new model with modified physical parameters, the prediction result was acceptable.