Mechanisms of trace water vapor frosting on a cryogenic surface in nitrogen gas flow

Abstract

Water vapor contamination control is currently one of the key technical issues faced in cryogenic wind tunnels. However, the behaviors and the underlying mechanisms of trace water vapor frosting under cryogenic conditions still remain unknown due to the extreme experimental conditions and difficulties in direct measurement. A computational fluid dynamics (CFD) model was developed for describing the frosting process of trace water vapor on a cryogenic surface in nitrogen gas flow. The calculated results were validated against published experimental data and showed good agreements. The macroscopic and local growth behaviors of the frost layer were further studied based on the validated model. The variation in the nitrogen gas density under cryogenic conditions and its impact on the frost layer properties were analyzed. The change in the nitrogen gas density displayed a major difference between the frosting under refrigeration temperature conditions and cryogenic conditions, and taking the nitrogen gas density as a constant like previous models would cause great deviations in the calculated frost layer properties. The influence of water vapor content on the frost properties was also investigated. The frost layer thickness showed less sensitivity to changes in the water vapor content compared to the frost weight deposited on per unit area of the cold surface and the frost layer density. The frost layer under extremely low water vapor content might still cause small levels of distributed roughness on the surfaces, thereby affecting the aerodynamic characteristics of the model in the cryogenic wind tunnel.