Evaporation heat transfer for R-22 and R-407C refrigerant–oil mixture in a microfin tube with a U-bend

Evaporation heat transfer experiments for two refrigerants, R-407C and R-22, mixed with polyol ester and mineral oils were performed in straight and U-bend sections of a microfin tube. Experimental parameters include an oil concentration varied from 0 to 5%, an inlet quality varied from 0.1 to 0.5, two mass fluxes of 219 and 400 kg m−2s−1 and two heat fluxes of 10 and 20 kW m−2. Pressure drop in the test section increased by approximately 20% as the oil concentration increased from 0 to 5%. Enhancement factors decreased as oil concentration increased under inlet quality of 0.5, mass flux of 219 kg m−2 s−1, and heat flux of 10 kW m−2, whereas they increased under inlet quality of 0.1, mass flux of 400 kg m−2 s−1, and heat flux of 20 kW m−2. The local heat transfer coefficient at the outside curvature of an U-bend was larger than that at the inside curvature of a U-bend, and the maximum value occurred at the 90° position of the U-bend. The heat transfer coefficient was larger in a region of 30 tube diameter length at the second straight section than that at the first straight section.