Experimental investigation on solidification characteristics of air-like nitrogen-oxygen mixtures in liquid hydrogen

Solid air accumulation poses a significant safety risk for liquid hydrogen systems, particularly when oxygen-enriched layers form on the solid surfaces. This study experimentally investigated the solidification of the air-like nitrogen–oxygen gas mixtures with different nitrogen–oxygen ratios in liquid hydrogen, documenting morphological characteristics through visual observation and compositional distributions via gas chromatography. The results showed that for the solidification of the gas mixture with a relative oxygen content of 21.96 %, the size of the formed solid particles varied within the range of 0.7 mm to 1.2 mm. Moreover, most of the solid particles were dispersed in the form of powder in liquid hydrogen, and a small portion of solid particles were accumulated to form sheet-shaped or block-shaped aggregates. Compositional analysis revealed significant oxygen enrichment at the solid surface, reaching the maximum relative proportions of 33.64 %. Similarly, gas mixture with 30.42 % relative oxygen content yielded smaller solid particles with a size from 0.5 mm to 1 mm during the solidification, and the morphology of the formed solid particles was basically identical to that formed under air-like condition. The solid air’s outer layer consistently exhibited oxygen enrichment with higher oxygen content in the injected gas mixture directly increasing oxygen concentration in the outer surface, which was demonstrated by the maximum relative oxygen proportion attaining 34.33 %. This work significantly expands the existing experimental database of the solidification of air in liquid hydrogen, and also provides valuable experimental reference for the safety design of cryogenic systems.