Heat transfer characteristics and flow pattern during two-phase flow boiling of R134a and R407C in a horizontal smooth tube

Experiments to measure flow boiling heat transfer coefficients and pressure drops of R134a and R407C inside a smooth horizontal tube (9.52 mm outside diameter, 1.2 m length) were performed at a refrigerant mass flux range of about 100-400 kg/m2 s varying the inlet temperature of the evaporator within the range of 5-9 °C, with heat fluxes within the range 3-10 kW/m2. The experimental data are described in terms of the heat transfer coefficients and pressure drop gradient as a function of the vapor quality. The experimental results clearly show that the heat transfer coefficients of R134a are always higher than those of R407C (for a minimum of +55% to a maximum of +87%) for same operating conditions. Furthermore, an assessment of predictive methods is provided for heat transfer coefficients and pressure drop gradients; also a comparison of flow regimes visualizations for R134a with a flow pattern map available in the literature has been presented.