Comparison of heat transfer coefficient during evaporation of natural refrigerants and R-1234yf in horizontal small tube

An experimental data comparison of evaporation heat transfer coefficient of R-744(CO₂), R-717(NH₃), R-290(C₃H₈) and R-1234yf in horizontal small tubes was conducted under some various flow conditions. The experimental data were obtained over a heat flux range of 5-60 kW m^-2, mass fluxes from 50 to 600 kg m^-2 s^-1, saturation temperatures from 0 to 10 °C, and quality up to 1.0. The test section made of stainless steel tubes with inner diameters of 1.5 mm and 3.0 mm, and a length of 1000 and 2000 mm, respectively, was heated uniformly by applying an electric power to the tubes directly. Effects of mass flux, heat flux, saturation temperature and inner tube diameter on heat transfer coefficient are reported in the present study. Among the present working refrigerants, R-744 has a highest heat transfer coefficient. Laminar flow was observed in the evaporative small tubes and considered in the modification of evaporation heat transfer coefficient correlations.