A Multiscale CFD Model of Evaporating Hydrogen Menisci: Incorporating Subgrid Thin-Film Dynamics and In Situ Accommodation Coefficients

Due to its high energy density, liquid Hydrogen is an essential fuel for both terrestrial energy systems and space propulsion. However, uncontrolled evaporation poses a challenge for cryogenic storage and transport technologies. Accurate modeling of evaporation remains difficult due to the multiscale menisci formed by the wetting liquid phase. Thin liquid films form near the walls of containers, ranging from millimeters to nanometers in thickness. Heat conduction through the solid walls enables high evaporation rates in this region. Discrepancies in the reported values of the accommodation coefficients (necessary inputs to models) further complicate evaporation calculations. In this study, we present a novel multiscale model for CFD simulations of evaporating Hydrogen menisci. Film profiles below 10 μm are computed by a subgrid model using a lubrication-type thin film equation. The microscale model is combined with a macroscale model above 10 μm. Evaporation rates are computed using a kinetic phase change model combined with in situ calculations of the accommodation coefficient using transition state theory. The submodels are implemented in Ansys FluentTM using User-Defined Functions (UDFs), and a method to establish two-way coupling is detailed. The modeling results are in good agreement with cryo-neutron experiments and show improvement over prior models. The model, including UDFs, is made available through a public repository.