Condensed Phase Explosions Involving Liquid Hydrogen

Liquid hydrogen may have an important role in the storage and transportation of hydrogen energy. It may also provide the best option for some users of hydrogen energy, notably the aviation sector. In the 1960’s liquid hydrogen spillages in open, uncongested conditions sometimes produced violent condensed phase explosions as well as the familiar gas phase flash and sustained pool fire. Testing showed that burning mixtures of LH2 and solid oxygen/nitrogen readily transitioned to detonation for oxygen concentrations in the solid phase at or above 50%. Such explosive events have been observed in more recent research work on LH2 spillage and the pressure effects could be significant in some accident scenarios. There is a need to understand how solids are produced following spillage and what factors determine the level of oxygen enrichment. This paper describes the physical processes involved in the accumulation of solids during a horizontal discharge at ground level, based on observations made in a recent HSE test that led to a condensed phase explosion. Areas where solids accumulated but remained in intimate contact with LH2 are identified. The paper also includes a thermodynamic and fluid mechanical analysis of the condensation process that includes the calculation of densities of mixtures of LH2 and air in different proportions. When the difference in flow speed between air and underlying LH2 is low, a stable condensation layer can develop above the liquid where the temperature is just under the initial condensation point of air, allowing sustained oxygen enrichment of condensate.