The effects of release failure conditions on jet impingement ignition of pressurised hydrogen jets

Pressurised hydrogen storage vessel failure scenarios are analysed through numerical simulation to understand the influence of the surrounding geometry. The separation distance from a wall, failure diameter, storage pressure and wall curvature are varied in independent scenarios. Varying separation distance reveals a range within which ignition upon failure is possible due to a coupling of the shock-front compressive heating and the diffusion of the hydrogen plume. At reduced separation distances, there is insufficient time for the shock-wave to expand and provide sufficient shock-heating. When separation distance is increased, the separation between the shock-front and the stoichiometric mixture fraction, ‘lag’, is increased and concurrent heating and flammable conditions are absent. Variation to the release jet exit diameter suggests that smaller failures lack the required momentum to reach ignition temperatures upon failure. At smaller jet exit diameters, the specific enthalpy across the shock-wave is decreased due to the greater radial expansion. Increasing storage pressure does not alter the likelihood of ignition, instead, a minor reduction in the ignition delay after release is observed. Analyses reveal a non-linear relationship between separation distance and ignition, prompting an investigation to characterise the range within which ignition may occur, indicating that for a 5 mm jet exit diameter, separation distances greater than 4 mm, but less than 7 mm undergo ignition. Further analysis is conducted in batch reactors, revealing the relationship between impingement temperature, mixture fraction, residence time and ignition. These results provide valuable insight into the safe storage of pressurised hydrogen. • Pressurised hydrogen jets may ignite upon impingement with an adjacent wall. • Competing effects of shock-heating and expansive cooling influence ignition. • Separation distances near the jet exit diameter are residence time dependent.