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

Condensation heat transfer experiments for two refrigerants, R-407C and R-22, both mixed with polyol-ester (POE) and mineral oils were performed in straight and U-bend sections of a microfin tube. Experimental parameters included oil concentration (varied from 0% to 5%), mass flux (varied from 100 to 400 kg/m2 s) and inlet quality (varied from 0.5 to 0.9). The enhancement factors (EFs) for both R-22 and R-407C refrigerants at the first straight section decreased continuously as the oil concentration increased. They decreased rapidly as mass flux decreased and inlet quality increased. The heat transfer coefficients had the maximum at the 90° position of the U-bend. The heat transfer coefficients in the straight section after the U-bend, within a length 48 times that of the inner tube diameter, were larger by a maximum of 33% than the average heat transfer coefficient in the straight section before the U-bend.