A Comparative CFD Assessment Study of Cryogenic Hydrogen and Liquid Natural Gas Dispersion

The introduction of hydrogen to the commercial market as alternative fuel brings up safety concerns. Its storage in liquid or cryo-compressed state to achieve volumetric efficiency involves additional risks and their study is crucial. This work aims to investigate the behaviour of cryogenic hydrogen release and to study factors that affect the vapor dispersion. We focus on the effect of ambient humidity and air's components (nitrogen and oxygen) freezing, in order to identify the conditions under which these factors have considerable influence. The study reveals that the level of influence depends highly on the release conditions and that humidity can reduce conspicuously the longitudinal distance of the Lower Flammability Limit (LFL). Low Froude (Fr) number (<1000) at the release allows the generated by the humidity phase change buoyancy to affect the dispersion, while for higher Fr number - that usually are met in cryo-compressed releases - the momentum forces are the dominant forces and the buoyancy effect is trivial. Simulations with liquid methane release have been also performed and compared to the liquid hydrogen simulations, in order to detect the differences in the behaviour of the two fuels as far as the humidity effect is concerned. It is shown that in methane spills the buoyancy effect in presence of humidity is smaller than in hydrogen spills and it can be considered almost negligible.