A Numerical Study of Coefficient-free Kinetic Evaporation Modeling in Liquid Hydrogen

Numerical modeling of liquid-vapor phase change has long presented a challenge due to the use of tuning coefficients. The commonly used Hertz-Knudsen-Schrage equation requires accommodation coefficients as necessary inputs but reported values span three orders of magnitude even for common fluids such as water. Data for cryogenic fluids are severely limited. Computational modeling of evaporation in liquid Hydrogen is critical to the development of long-term cryo-storage technologies for deep-space applications. Due to lack of data, the accommodation coefficient is commonly reduced to a non-physical tuning parameter to achieve numerical stability. To alleviate this, we use a Transition State Theory based analytical description of the accommodation coefficients to develop a new coefficient-free computational approach to model evaporation. This approach is used inside a CFD setup in Ansys Fluent to model quasi-steady-state evaporation, where a sharp interface is assumed. Mesh cells adjacent to the sharp interface are identified as the active region where User-Defined Functions are used to apply phase change-related mass, heat, and momentum sources. The source terms are computed using local thermophysical quantities and applied along the entire interface, resulting in non-uniform evaporation. The model is validated by comparison to evaporation rates from recently published cryo-neutron experiment datasets. The importance of drift velocity in the evaporation model is also studied.