Effect of viscosity grade (POE) on the smooth-tube pool boiling performance with R-1234ze(E) refrigerant

Abstract

In this study, the nucleate boiling heat transfer performance for R-1234ze(E) subject to lubricant oil is reported. The viscosity grade ranges from low viscosity (68 cSt) to high viscosity (220 cSt) and the tested POE oils include POEA-68, POEA-170, POEC-170, POEA-220, and POEC-220 on a horizontal smooth copper tube. The 30 different refrigerant-oil mixtures were prepared by varying the oil mass concentration ranging from 1 % to 10 % and the corresponding saturation temperatures are 10 °C, 0 °C, and − 6 °C with heat flux ranging from 10 to 90 kW/m2. The results showed that all grades of POE oils are compatible and completely miscible with low GWP refrigerant R-1234ze(E) in the given test range. The present study reveals that the heat transfer characteristics of each refrigerant-oil mixture very much depend on the saturated temperature, heat flux, oil mass concentration, and operating pressure. The highest mass concentration (10 %) consistently deteriorates the heat transfer coefficient (HTC). However, the degradation of HTC with R-1234ze(E) is comparatively less severe than that of R-134a/POE mixture. It is found that a 3 % oil concentration is regarded as the threshold concentration for the highest enhancement in the HTC when compared to the pure refrigerant. The performances of other oil concentrations such as 1 % and 5 % are subject to saturation temperature, heat flux, and evaporator pressure. In terms of the influence of saturation temperature on HTC of the mixture, it was observed that among all the grades of oil, POEC-170 oil shows peculiar behavior and its performance is independent of saturation temperature.