A New Technique to Determine Accommodation Coefficients of Cryogenic Propellants

The control of propellant boil-off is essential in long-term space missions. However, a clear understanding of cryogenic propellant phase change and the values of accommodation coefficients are lacking. To that effect, a new method to determine accommodation coefficients using a combination of neutron imaging, thin film evaporation modeling and CFD modeling has been established. Phase change experiments were conducted in the BT-2 Neutron Imaging Facility at the National Institute of Standards and Technology (NIST) by introducing cryogenic vapor (H2 and CH4) at a set pressure into Al6061 and SS316L test cells placed inside a 70mm cryostat. Condensation is achieved by lowering the cryostat temperature below the saturation condition and vice versa for evaporation. Neutron imaging is used to visualize the liquid-vapor interface inside metallic containers due to the difference in attenuation between the cryogen and the metal. Phase change tests are conducted using liquid hydrogen and methane at a range of saturation points between 80 - 230 kPa and corresponding phase change rates were determined. The contact resistances and other transient heat transfer properties of the cryostat setup is determined from the combination of a CFD thermal transport model and a “dry” thermal cycling test. The calibrated CFD model then allows for the determination of the inner wall temperature profile. Results from neutron imaging and the thermal model serve as boundary conditions to a multiscale evaporation model. A macroscale 2D FEA model is used to compute evaporation flux in the bulk meniscus while a thin film evaporation model is used to account for enhanced evaporation near the contact line. Using a combination of neutron imaging, CFD thermal model and a multiscale evaporation model, there is a possibility to extract the accommodation coefficient while accounting for the curvature, disjoining pressure and a variable interface temperature. The accommodation coefficient of H2 decreases from 0.65±0.12 at 88 kPa to 0.22±0.1 at 226 kPa and is independent of container material/geometry. The error is dominated by the uncertainty in the temperature measurements (±0.25K