Role of gravity in condensation flow of R1234ze(E) inside horizontal mini/macro-channels

The condensation patterns of R1234ze(E) inside horizontal mini/macro-channels were numerically investigated under normal-gravity and zero-gravity conditions. The gravity effects on condensation heat transfer coefficients, liquid film thickness, film distribution, cross-sectional stream-traces, and liquid-phase velocity were analyzed detailedly. The influence of surface tension on condensation flow was also discussed. The gravity effect on condensation heat transfer coefficients was negligible in mini-channels with D = 1 mm, while was important for D = 2 mm and D = 4.57 mm. The gravity effect can either enhance or weaken the condensation heat transfer coefficient, which was dependent on the tube diameter and vapor quality. The enhancement on heat transfer caused by the gravity was more pronounced at lower vapor quality and mass fluxes with a larger diameter tube. The gravity affected the condensation heat performance through changing the vapor–liquid distribution, rather than the film thickness. The gravity has a great influence on the condensation flow field in both circumferential and axial direction. The surface tension played an important role in heat transfer under zero-gravity condition.