Atmospheric dispersion of hydrogen

The accidental release of gaseous hydrogen or die spillage of liquid hydrogen leads to the evolution and dispersion of a gas cloud whose shape is influenced by the type and rate of release and by the atmospheric conditions as well as by topography. Liquid hydrogen at 20 K which immediately starts to vaporize upon release, behaves like a heavier-than-air gas in the very first phase ( cold sink effect ). This short phase of negative buoyancy with a reduced heat and mass transfer due to the stable stratification effects is replaced by an enhanced positive buoyancy as soon as, due to continuous air entrainment from outside into the gas cloud, the cloud temperature has been raised to 22 K. The dilution 

The first significant experiments to study the dispersion behavior of a hydrogen gas cloud was the NASA series of seven spill tests in 1980 with the release of 5.7 m LH2 per test within a period of 35 - 120 s [26]. Instrumentation allowed for concentration measurements at various positions (see Fig. 8-9). High release rates in connection with release-induced turbulence resulted in intensive cloud formation and rapid mixing with ambient air. The tests demonstrated that the hydrogen cloud was existent for a couple of 100 m downwind, especially if the ground was able to cool sufficiently. 

A whole variety of other experimental series involving many liquids or gases of different densities, bofli on a laboratory scale in wind tunnels and on a large scale in field trials, instantaneously or continuously, have been conducted in the past with the prime goals to observe their behavior and to validate the performance of respective simulation models [120]. 

The need to simulate the atmospheric dispersion of a gas cloud in order to determine extension and lifetime the flammable portions as realistically as possible has led to a lot of approaches, however, with different degrees of success in reaching the ideal profile of a mathematical model. 

In box or slab models, the released gas cloud is assumed to be of cylindrical shape. The processes of advection, i.e., the transport by the mean wind field, of air entrainment, and of gravitational spreading are implemented in empirical correlations derived from experiments. Box models were basically developed to simulate the behavior of a heavier-than-air gas cloud with averaged (in later developments vertical profiles of) temperature and concentration. Acknowledged (extended) box models are, e.g., the US code DEGADIS [57] and the British code HEGADAS [75]. 

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Liquid Hydrogen Pool Spreading and Atmospheric Dispersion of Hydrogen Gas Clouds... 

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