Pressure drop of R134a, R32 and R1233zd(E) is measured and reported in diabatic conditions during condensation from superheated vapor inside horizontal smooth round tubes. The test conditions include mass fluxes from 100 to 400 kg/m~2-s, heat fluxes from 5 to 15 kW/m~2 and tube diameters of 4.0 and 6.1 mm at saturation temperatures of 30°C. Compared to a conventional pressure drop model constructed under the assumption of thermal equilibrium, the experimental data clearly shows that the onset and end of condensation is changing instead of being fixed at bulk quality 1 and 0. This discrepancy between the theory and reality results in a deviation between the prediction and data, especially in the condensing superheated (CSH) region. The result shows an increase in pressure drop as mass flux increases. Tube size also affects the pressure drop in that smaller tube yields higher pressure drop. A further comparison between the different refrigerants conditions illustrates the effects of properties such as liquid-vapor density ratio, liquid viscosity, surface tension and latent heat. The results are analyzed along with the visualizations of the flow. While viscosity and velocity gradient determines the magnitude of pressure drop, the waves on the interface and the velocity of the bulk flow are identified as the two competing factors affecting pressure drop when condensation proceeds. The process as described in this paper provides an insight for a mechanistic model that traces the development of the flow to help resolve the issues in conventional "3-zone" pressure drop models.
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