Boiling heat transfer of refrigerant R-600a/R-290-oil mixtures in the serpentine small-diameter U-tubes

Flow boiling experiments for refrigerant R-600a, and R-290 mixed with the lubricating oil (EMKARATE) in the serpentine small-diameter (2.46 mm) U-tubes are reported. The tests were conducted at the nominal inlet pressure of 186.2 kPa, vapor qualities (0–0.76), mass flux of 100–320 (kg/m²s) and inlet oil concentrations from 0 to 5 mass% oil. It was noted that a significant degradation of heat transfer coefficients presented at high qualities and high oil concentrations. The present study investigated that whether the average heat transfer coefficient increased as mass fluxes and the numbers of the U-bend increased. In addition, the ratios (the enhanced heat transfer factor, EF) for both R-600a and R-290 refrigerants increased as the oil concentration increased up to 1% then decreased with each increase in oil concentration. Moreover, pressure dropped during evaporation increased with the addition of a lubricant, mass fluxes and the numbers of the U-turn. The values of the pressure drop penalty factor PF were generally larger than l and increased rapidly as the oil concentration and the numbers of the U-turn increased.