Experimental investigation on boiling heat transfer of R22 outside three doubly-enhanced tubes

This paper presents information about boiling heat transfer of R22 outside three doubly-enhanced tubes(E30, E32 and E33) at the saturation temperature of 6°C. A modified Wilson plot technique was used to determine the refrigerant heat transfer coefficient. The results indicated that the overall heat transfer coefficient of tube E30 is 1.145 times of that of tube E32, and is 1.02 times of that of the tube E33 at the flow velocity of 4.4m s^-1. Boiling heat transfer coefficient of the enhanced tube E30 was better than E32 and E33, specially enhanced tube E30 was in the range of 11-14.6kW.m^-2K^-1, E32 was 9.8-13.2kW m^-2 K^-1 and E33 was 11.2-14.4 kW.m^-2.K^-1. Typically, boiling heat transfer performance of tube E30 is 1.17 times of that of tube E32, and is 1.02 times of that of tube E33 under the same heat flux. The average enhancement factor of tube E30, E32 and E33 are 2.57, 2.37 and 2.52, respectively. The in-tube convective heat transfer coefficients of the enhanced tubes were 2.325-2.737 times of that of the smooth one. At the Reynolds number of 3.7×10⁴, the percentage of water and refrigerant side of the enhanced tubes are almost the same.