Experimental study on condensation flow patterns inside inclined U-bend tubes

Effect of inclination angle/flow direction on the patterns of a two-phase flow with low mass flux in a U-bend tube is not well documented. In this study, flow patterns and transitions for condensation of refrigerant R-134a inside U-bend tubes and their contiguous straight tubes are visually observed and analyzed. In order to study the condensation flow patterns, an experimental setup is developed. The experiments have been performed for downward and upward condensation flows with different inclination angles. Low refrigerant mass fluxes were selected to vary in the range of 52–225 kg/m² s. The observations showed that, the flow regime is influenced by interfacial shear stress, surface tension, centrifugal force and gravitational force. Analysis of the collected data showed that the tube inclination angle strongly influences the flow pattern. Moreover, the variety of flow regimes for the horizontal cases, vertical downward and vertical upward U-bend tube, is more than other inclinations. Considering the present range of the experimental parameters, inclinations of central region of the U-bend tube as well as the inclination angle significantly impact the two-phase flow patterns. The observed flow patterns are correlated via Weber number for different inclination angles, flow directions, and refrigerant mass fluxes.