Pressure drop of R134a, R32 and R1233zd(E) in diabatic conditions during condensation from superheated vapor

Pressure drop during condensation from superheated vapor is reported in this paper. Three different refrigerants including R134a, R32 and R1233zd(E) are evaluated in two horizontal round copper tubes with 6.1 and 4.0 mm inner diameters. The test conditions cover a range of mass fluxes from 100 kg m−2 s−1 to 400 kg m−2 s−1 and heat fluxes from 5 kW m−2 to 15 kW m−2. The condensing pressures are selected to be those that correspond to 30 °C. The measurements are all conducted with diabatic flows which start in the superheated (SH) region to observe how the onset of condensation affects the pressure drop. The visualizations of condensation are compared with the measured pressure drop, suggesting the pressure drop behaviors in different regions during the condensation process are different due to different flow regimes. The onsets of condensation all happens in the superheated region before specific enthalpy drops to that of bulk quality 1. The pressure drop depends on the mass flux, specific enthalpy and tube diameter. The result also shows that several thermal properties of the refrigerants such as the density ratio and viscosity have an effect on the pressure drop, which can be explained by the generation of waves and the differences in vapor and liquid velocity. A peak in the pressure drop curve is consistently present between the onset and end of condensation around bulk quality 1, which could be a result of the competition between increasing liquid-wave strength and decreasing flow velocity as condensation proceeds after the onset of liquid film. Since none of the current existing correlations are sufficient in accounting for the different mechanisms in different stages of the condensation process, a pressure drop model that is specifically developed for condensation of superheated vapor by taking non-equilibrium into consideration is in need.