Evaporation Heat Transfer Characteristics of CO2R-22 and R-134a in a Horizontal Smooth Tube

Evaporation heat transfer coefficients of carbon dioxide(), R-22, and R-134a in a horizontal smooth tube were measured and analyzed as a function of heat flux, mass flux, and evaporating temperature. The experiments were carried out by varying heat flux from 10 to 20 , mass flux from 170 to 340 , and saturation temperatures of 5 and . It was found that the heat transfer coefficient of decreased with a rise of quality due to an earlier liquid-film dryout as compared to R-22 and R-134a. Averaged heat transfer coefficients of were 22-63% higher than those of R-22 and R-134a at all test conditions. The effects of mass flux and heat flux on averaged heat transfer coefficients were much greater in than in R-22 and R-134a. When comparing test results with the correlations in the literature, the existing models yielded large deviations at medium and high qualities. Therefore, a generalized correlation for evaporation heat transfer needs to be developed by including the effects of dryout phenomenon.