Flow-velocity-controlled cryogenic frosting under micro amount of water condition: Frost growth, surface roughness, and morphological evolution

Frost formation under cryogenic and micro amount of water condition is a critical issue in cryogenic wind tunnels, spacecraft thermal-control systems, optical windows, and cryogenic heat exchangers. Unlike conventional frosting in wet air, frost growth in micro amount of water environments is strongly limited by vapor supply and is therefore highly sensitive to gas-flow-induced mass transfer and shear stress. This study was conducted based on a cryogenic visualization experimental system. Under conditions of 120 ± 20 ppmv and 580 ± 20 ppmv, with gas flow velocities ranging from 0 to 1.16 m/s, the effects of gas flow velocity on the thickness, growth rate, surface roughness, and structural evolution of the frost layer were investigated. The results showed that compared to the static airflow condition, the frost layer thickness in the forced convection condition (120 ± 20 ppmv, 0.70 m/s) was 5.3 times greater. As the water vapor content increased, the final thickness of the frost layer at a flow velocity of 0.70 m/s increased from 0.135 mm to 0.218 mm, while at a flow velocity of 1.16 m/s, the final thickness decreased from 0.665 mm to 0.567 mm. The surface roughness also exceeded 29 μm, with a numerical fluctuation of 46.71%. The experimental data demonstrated that in the cryogenic and micro amount of water condition, the shear effect of the airflow was prominent, and it interacted with the mass transfer effect to influence the growth of the frost layer. By changing the flow velocity to regulate the competition balance between shear stress and mass transfer coefficient, the morphology of the frost layer could be controlled. This study clarifies the dual role of gas flow velocity in cryogenic and micro amount of water frosting and provides experimental guidance for frost control in cryogenic engineering systems.