Capturing interfacial phase change and flow physics during vertical downflow condensation

Two-phase configurations can address the urgent demand for effective heat dissipation solutions in Naval power and energy systems. A better understanding of thermal transport processes in phase-change flows is critical for developing novel two-phase design tools for naval scientists and engineers. This study investigates interfacial phase change and flow dynamics during condensation flow through numerical simulations. An enhanced phase change model, incorporating a mass transfer intensity coefficient dependent on condensation film thickness, is implemented for vertical downflow condensation. A two-dimensional homogeneous two-phase Reynolds-Averaged Navier-Stokes model, coupled with the Shear-Stress Transport k-ω turbulence model, is employed. The developed solver is thoroughly evaluated against varying mass transfer functions and mesh resolutions, demonstrating minimal dependence on condensation surface temperature predictions. Subsequently, four test cases with varying mass flow rates of 108.67 – 413.0 kg/m²s and surface heat fluxes of 3.46 – 8.67 W/cm² are investigated to validate the model against experimental data. The predicted surface temperature profiles along the tube show excellent agreement with measurements, with mean absolute errors below 2.0 % across all cases. Additionally, detailed interfacial phase change and flow characteristics, including temperature and velocity distributions, are analyzed. The results reveal that the liquid film condensation thickness increases and becomes progressively unstable along the tube. Condensation mass transfer predominantly occurs at the liquid-vapor interface within a thin boundary layer. Furthermore, temperature and velocity profiles within the liquid film exhibit high gradients near the condensation surface and the liquid-vapor interface, following similar trends. Lastly, the influence of turbulence modeling on thermal transport is investigated, particularly the damping factor, and is found to significantly affect surface condensation heat transfer and interfacial liquid-vapor dynamics.