Condensation heat-transfer and pressure drop characteristics of refrigerant R-290/R-600a–oil mixtures in serpentine small-diameter U-tubes

Flow condensing experiments for refrigerant R-290, and R-600a mixed with the lubricating oil (EMKARATE RL 32H) in serpentine small-diameter (2.46 mm) U-tubes are reported. The tests were run at the saturation temperature of 40 °C, vapor qualities of 0.41–0.82, mass flux of 300–600 (kg/m2s) and inlet oil concentrations from 0 to 5 mass% oil. It was found that the condensation heat-transfer coefficients increased as mass flux values, vapor quality and the number of tube bends increased, but it decreased as the oil concentration increased. In addition, the two-phase pressure drops increased with increases in mass flux values, the number of tube bends and the oil concentration.