Experimental investigation on the heat transfer performance of falling film evaporation with lubrication oil on horizontal tubes having different structures

Abstract

Falling film evaporation which has the advantages of refrigerant charge reduction and heat transfer improvement has been widely used in refrigeration and heat pump systems. Considering the inescapable and complicated influences of lubricating oil, the falling film evaporation heat transfer of R134a with POE-type lubricating oil entrained from 0.5% to 5.1% on horizontal tubes was examined. Experiments were conducted using test tubes with different surface structures at spray density of 0.0065–0.0105 kg/(s·m) and heat fluxes of 12–65 kW/m2. The saturation temperature was set to 6 °C. Experimental results showed that the heat transfer coefficients of the enhanced tubes were higher than that for the smooth tube and the lubricating oil could be conducive to the heat transfer of the tested tubes. The heat transfer coefficient of enhanced tube #1 was increased by more than 30% when the oil content was 5.1%. The mechanism of the function of lubricating oil on the heat transfer is closely related to the surface structure. For tube #1, oil content always supported heat transfer. In the case of tube #2, an optimal oil content at low spray density may exist.