A continuous-heat-flux phase change model for simulating realistic two-phase unsaturated evaporation processes

Accurately modeling liquid-vapor mass transfer rates is essential for optimizing cryogenic fluid management processes critical to advancing future space missions. This study introduces a continuous-heat-flux phase change model proposed to simulate conditions observed in realistic two-phase unsaturated evaporation phenomena. The mass transfer rate across the two-phase interface is calculated directly based on the local interfacial continuous heat flux on both liquid and vapor phases, effectively accounting for superheated, saturated, and subcooled liquid effects without requiring any empirical tuning parameters. Moreover, phase change occurs exclusively within interfacial cells, ensuring the sharp representation of deformed evaporating interfaces with high accuracy. The proposed model is implemented using user-defined functions in ANSYS Fluent and evaluated against various benchmark evaporation problems, including Stefan and film boiling test cases with nonequilibrium (temperature other than saturation) in a single phase and in both phases. The numerical results, encompassing liquid-vapor interface evolution and temperature distributions, exhibit excellent agreement with published analytical and numerical solutions. Additionally, the model is applied to simulate the complex heat and mass transfer processes in cryogenic tank self-pressurization under two heating configurations: vapor heating and uniform heating. The results demonstrate good agreement with the tank pressure rise trends reported in the literature, validating the applicability of the model to practical evaporation scenarios.