POOL BOILING OF R-134A/POLYOLESTER OIL LUBRICANT MIXTURES ON ENHANCED SURFACES HAVING PORES

Investigations for the pool boiling of refrigerant/oil mixture on the enhanced surfaces are lacking in theWe conducted a study of the pool boiling of R-134a/polyolester oil mixture on the surfaces having a series of pore diameter (0.1 to 0.3 mm) and pitch (0.75 to 3.0 mm). The saturation temperatures were 4.4°C and 26.7°C and the oil concentration varied up to 5%. The results showed that the oil reduced the heat transfer coefficient. This was true for all the pored and smooth surfaces. The heat transfer degradation depended on the heat flux. The samples having the 'small open area' yielded a significant degradation at a low heat flux, whereas the samples having the 'large open area' yielded a noticeable degradation at a high heat flux. Meanwhile, the heat transfer degradation increased as the saturation temperature increased. The reason was attributed to the influence of oil on the surface tension, which was intensified with the increase of the saturation temperature. The flow visualization results showed that, with the increase of oil concentration, the bubble departure diameter decreased. Similarly, the bubble generation frequency and nucleation site density decreased. These changes of the bubble dynamic parameter apparently were responsible for the heat transfer degradation. A correlation was developed to describe the heat transfer degradation as a function of heat flux, pore geometry, saturation temperature, and oil concentration. The correlation predicted 82% of the data within ± 40%.