Nucleate pool boiling heat transfer of R-1234ze(E) and R-134a on GEWA-B5H and smooth tube with the influence of POE oil

Nucleate pool boiling heat transfer characteristics of pure refrigerants of R-134a, R-1234ze and their corresponding lubricant/refrigerant mixture of R-134a/POEA-68, and R-1234ze/POEA-68 on horizontally arranged enhanced tube (GEWA-B5H) are reported in detail. Tests are conducted at saturation temperatures of 10 degrees C, 0 degrees C, and-6 degrees C having oil concentration from 0.28% to 5%. Tests are conducted with decreasing heat flux range from 90 kW/m(2) to 10 kW/m(2). It appears that the heat transfer coefficient (HTC) of pure R-1234ze on the GEWA-B5H tube is nearly 18%, 10%, and 7% lower than those of R-134a at saturation temperatures of 10 degrees C, 0 degrees C, and-6 degrees C, respectively. For the smooth tube, it was found that the HTC of pure R-1234ze is nearly 27% lower than that of R-134a at all the saturation temperatures. Generally, the presence of a small amount of lubricant oil impairs the HTC on an enhanced surface. But the present experimental investigation shows that the presence of lubricant oil on an enhanced surface may alter the HTC appreciably, and it is highly dependent on the saturation temperature, reduced pressure, and transport properties of refrigerant-oil mixtures. The results indicated that the highest enhancement ratio prevailed at a heat flux of 30 kW/m(2) and the HTC is about 17% and 35% with a 2% oil concentration for R-134a/POEA-68 oil mixture at saturation temperature 0 degrees C, and-6 degrees C respectively. Correspondingly, R-1234ze(E)/POEA-68 oil mixtures showed the highest 26% and 43% enhancement in the HTC at 0 degrees C and-6 degrees C saturation temperature subject to 0.28% oil concentration at a heat flux of 70 kW/m(2). The HTC enhancement ratios of the GEWA-B5H tube were 2.9 - 8.8 for R-1234ze and 2.8 - 6.6 for R-134a compared to the smooth tube.