Flow boiling heat transfer characteristics of R-22 and R-134a in 0.5, 1.5 and 3.0 mm tubes

An experimental study of R22 and R134a two-phase vaporization in horizontal minichannel was reported. In order to obtain the local heat transfer coefficients, the test was run under a heat flux range of 10 to 40 kW/m2, mass flux range of 200 to 600 kg/m2.s, saturation temperature range of 5 to 10°C, and quality up to 1.0. The test section, which was made of stainless steel tube and heated uniformly by applying an electric current to the tube directly, has inner tube diameters of 0.5, 1.5 and 3.0 mm, and lengths of 0.33 and 2.0 m. The effects on heat transfer coefficient of mass flux, heat flux and inner tube diameter were presented. The experimental heat transfer coefficients were compared with the predictions using existing heat transfer coefficient correlations. A new boiling heat transfer coefficient correlation based on the superposition model, with considering the laminar flow, was developed.