Coupled inflow vapour velocity and surface wettability effect on condensation heat transfer characteristics: a volume-of-fluid method study

This study employs volume-of-fluid-based computational fluid dynamics modelling to investigate the coupled effects of surface wettability and inflow vapour velocity on R134a ( $p/p_{cri}=0.25$ ) condensation heat transfer in horizontal tubes. The results demonstrate that both the condensation heat transfer coefficient (HTC) and Nusselt number consistently increase with rising vapour velocity, indicating enhanced convective heat transfer at higher flow rates. Within this overall trend, the influence of surface wettability varies significantly across different velocity regimes. At moderate inlet velocities (10 m s⁻¹), surface wettability demonstrates maximum impact, with the HTC enhancement exceeding 19.1% between peak and minimum values, optimising at contact angles of 120 $^\circ$ –140 $^\circ$ . As velocity increases to 20 m s⁻¹, while surface wettability effects persist with $\gt$ 11.7 % enhancement, convective heat transfer becomes increasingly dominant, showing $\gt$ 38.8 % improvement in the maximum HTC compared with the 10 m s⁻¹ case. At higher velocities (40 m s⁻¹), the influence of surface wettability diminishes substantially, with the HTC variation reducing to $\gt$ 1.04 %. At extreme velocities (80 m s⁻¹), surface tension effects become negligible compared with vapour shear forces, resulting in minimal (0.53 %) variation across different contact angles. The equivalent Reynolds number peaks at 20 m s⁻¹, indicating optimal conditions for condensate formation and flow characteristics. These findings provide crucial insights for condensation system design, suggesting that while increasing velocity generally enhances heat transfer performance, surface wettability modifications are most effective at moderate velocities, while high-velocity applications should prioritise flow dynamics and system geometry optimisation.