Liquid hydrogen (LH2) has been identified as a carbon-free fuel in the transition to Net Zero. The transport sector has a particular interest in LH2 due to its increased density (71 kg/m3 at –253 °C, 1 barg) compared to pressurised gaseous hydrogen (24 kg/m3 at 20 °C, 350 barg). The Health and Safety Executive’s (HSE) Science Division has been researching and assessing LH2 transport hazards for over a decade with supporting UK Safe Net Zero as one the organisations key objectives. Previous experiments have shown that large LH2 releases can condense the air around them, forming solidified deposits. One ignited release test exhibited evidence of a higher order explosion equivalent to ~2.7 kg of TNT, a far higher yield than other similar LH2 release ignitions. Several potential mechanisms for this larger explosion have been theorised, with one such mechanism being the ‘oxygen enrichment’ of condensed air deposits which would lead to a much more severe explosion when LH2 and an ignition source are introduced. The experiments presented in this paper examine the condensation of air at a LH2 surface in conditions where the relative buoyancy of the hydrogen gas and air can have a significant effect on the mass transfer to the surface i.e. low airflow conditions. The extent of oxygen enrichment of the condensed air was assessed by measuring the composition of venting gas during the build-up of condensate and subsequent evaporation. Experimental results showed that no oxygen enrichment occurred during formation of the condensate as oxygen and nitrogen condensed in the same ratio as air (1:3.7 O2:N2). It was observed that an air condensate can be come oxygen-enriched as nitrogen evaporates preferentially from the condensate.
