EXPERIMENTAL INVESTIGATION OF EVAPORATIVE HEAT TRANSFER CHARACTERISTICS IN A SMALL-DIAMETER TUBE USING R-134a

Abstract

Evaporative heat transfer characteristics of R-134a in a small diameter tube have been investigated. Stainless steel tube with an inner diameter of 2.2 mm was used as a test section. Test section was uniformly heated by electric current which was applied to the tube wall. The saturation temperature of refrigerant is calculated from the measured saturation pressure by using an equation of state for refrigerant. Inner wall temperature was calculated from measured outer wall temperature, accounting for heat generation in the tube and heat conduction through the tube wall. Mass quality of refrigerant was calculated by considering the heat input to the pre-heater and the test section. Heat fluxes were set at 19, 36, 46, and 64 kW/m², and mass fluxes were set at 380, 470, and 570 kg/m²s for each heat flux condition. From this study, flow patterns in a small-diameter tube were predicted by using flow pattern map, where the flow was presumed to have slug flow pattern for low quality region and to have annular flow pattern for high quality region. Heat transfer coefficients in a small diameter tube have been provided with respect to quality for several mass flux and heat flux conditions. Heat transfer in a small diameter tube is affected by heat flux as well as mass flux for a wide range of mass quality. Finally, the results in this study are compared with Gungor and Winterton correlation, which gives the absolute average deviation of 27%.