Heat transfer coefficients during the condensation of low mass fluxes in smooth horizontal tubes

Until now, no study that systematically investigated the influence of low mass fluxes and temperature differences on condensation heat transfer coefficients has been conducted. Thus, the purpose of this study was to measure condensing heat transfer coefficients at low mass fluxes in a smooth horizontal tube at different temperature differences. Experiments were conducted by condensing R134a in a smooth horizontal tube with an internal diameter of 8.38 mm and a length of 1.5 m. Experiments were conducted at a saturation temperature of 40°C at different qualities, and at low refrigerant mass fluxes that ranged from 50 to 200 kg/m2s. The experiments were also conducted at different wall and saturation temperature differences varying between 3 and 10°C. The flow patterns were recorded with a high speed video camera at the inlet and outlet of the test section through sight glasses. An uncertainty analysis showed that the maximum uncertainty of the heat transfer coefficients and vapour qualities of the data presented in this study were 12% and 5%, respectively. Results showed that the flow patterns during condensation were predominantly stratified and stratified wavy. It was also found that the heat transfer coefficients were dependent on the temperature difference between the temperature of the wall on which condensation occurs and the temperature of the condensing refrigerant. Furthermore, it was found that the heat transfer coefficient decreased with an increase in this temperature difference. When comparing the results of this study at low mass fluxes to the literature, it was found that the absolute mean deviation varied up to 38%. An amendment was suggested in a stratified heat transfer coefficient term from literature. It has been found that, with this amendment, the heat transfer coefficients of low mass fluxes could be estimated with errors of ± 5%.