Condensation heat transfer in smooth inclined tubes for R134a at different saturation temperatures

This paper presents the effects of saturation temperature and inclination angle on convective heat transfer during condensation of R134a in an inclined smooth tube of inner diameter of 8.38 mm. Experiments were conducted for inclination angles ranging from −90° (vertical downwards) to +90° (vertical upwards) for mass fluxes between 100 kg/m2 s and 400 kg/m2 s and mean vapour qualities between 0.1 and 0.9 for saturation temperatures ranging between 30 °C and 50 °C in a test matrix of 637 test data points. The results showed that saturation temperature and inclination angle strongly influenced the heat transfer coefficient. With respect to saturation temperature, an increase in saturation temperature generally led to a decrease in heat transfer coefficient irrespective of the inclination angle. The effect of inclination angle was found to be more pronounced at mass fluxes of 100 kg/m2 s and 200 kg/m2 s for the range of mean vapour qualities considered. Within the region of influence of inclination, there was an optimum angle, which was between −15° and −30° (downward flow). The inclination effect corresponded to the predominance of the effect of gravity on the flow distribution. An increase in saturation temperature increased the effect of inclination on two-phase heat transfer.