Experimental investigation on inclination effects during condensation heat transfer of R515B inside an inner-grooved tube

To address the gap existing in the open current literature regarding the effect of tilting angle during two-phase heat transfer inside inner-grooved tubes, this work investigates condensation heat transfer within a 7 mm OD inner-grooved tube. The refrigerant used is R515B, which is an azeotropic mixture selected for its favorable characteristics and low global warming potential. Experiments were performed at fixed saturation temperature of 30 °C, varying the mass flux in the range 50–400 kg/m²·s and the mean vapor quality from 0.06 to 0.93. The tube inclination relative to a horizontal plane was adjusted between -60° and +60°, in steps of 30°. Due to its significant effect on gravity-controlled condensation, the difference between the saturation and wall temperatures was kept constant throughout the tests, at selected values of 3 K and 6 K. Moreover, every test is accompanied with the corresponding flow pattern recorded with a high-speed camera to facilitate the understanding of the two-phase heat transfer mechanism. Findings revealed a noteworthy impact of inclination angle on the heat transfer coefficient. Overall, the most efficient thermal performance is achieved with a downward tube inclination of -30°, whereas the worst performance is observed with an upward inclination of +60°. These variations in performance are closely linked to variations in flow regimes and how gravity affects the thickness, distribution, and interfacial turbulence of the liquid film.