Neutron imaging of evaporation/condensation in cryogenic propellants: an accommodation coefficient study

Cryogenic propellent management is critical to long-term space exploration but our understanding of propellant phase change and subsequent boil-off is limited. This is, in part, because the values of accommodation coefficients (inputs to phase change models) are still lacking and experimental data to compute them is limited. A new method to determine accommodation coefficients is developed. Experiments are conducted in the BT-2 Neutron Imaging Facility at the National Institute of Standards and Technology (NIST) by introducing propellant vapor into cylindrical Al6061 and SS316L cells placed inside a 70mm cryostat. Tests are conducted at a range of saturation points between 80 - 230 kPa using H₂ and CH₄. Phase change is induced through precise control of pressure and/or temperature. Neutron imaging is used to visualize the liquid and evaporation/condensation rates are determined through image processing. Comparing results from a computational model with the experimental data, the accommodation coefficient is explicitly determined. The resulting coefficient values for H₂ (0.19 - 0.41) are generally lower than those for CH₄ (0.68 - 0.86) and exhibit an inverse relationship with saturation vapor pressure. The results agree well with a generalized transition state theory expression.