Flow pattern and experimental investigation of heat transfer coefficients during the condensation of r134a at low mass fluxes in a smooth horizontal tube.

Abstract

An experimental study of heat transfer and flow pattern visualisation during the condensation of R134a was conducted in a smooth horizontal tube at low mass fluxes. Most previous experimental and analytical studies on in-tube condensation were conducted at high mass fluxes. In these studies, it was found that the heat transfer coefficient was not a function of the temperature difference between the tube wall temperature and condensation temperature. In addition, most heat transfer models developed were for high mass fluxes and failed to predict heat transfer coefficients at low mass fluxes properly. However, the most recent predictive heat transfer models have been based on studying and analysing the flow patterns. In all of these, only very few experimental studies have been coupled with flow pattern identification at different controlled temperature differences and mean vapour qualities at low mass fluxes. Therefore, the purpose of this study was the investigation of R134a condensing at low mass fluxes (20 –100 kg/m2s) and the identification and analysis of the flow patterns observed. The experiments were conducted in a smooth horizontal tube 8.38 mm in internal diameter with a length of 1.5 m at different mean vapour qualities and controlled temperature differences. The average saturation temperature was maintained at 40°C. The flow patterns were recorded simultaneously with a high speed video camera at the inlet and outlet of the test section through transparent sight glasses. The results showed that stratified flow and stratified-wavy were the dominant flow patterns. Stratification would differ with decreasing flow rate of the refrigerant. As the flow rate decreased, the liquid layer at the bottom of the tube increased. The study also revealed the effect of temperature difference between the tube wall and the saturation temperatures with respect to the heat transfer coefficient at low mass flow rates of the refrigerant. The higher the temperature difference, the lower the heat transfer coefficient.