Quantifying the Potential Consequences of a Detonation in a Hydrogen Jet Release

The unconfined release of high-pressure hydrogen can create a large flammable jet with the potential to generate significant damage. To properly understand the separation distances necessary to protect the immediate surroundings, it is important to accurately assess the potential consequences. In these events, the possibility for a detonation cannot be excluded, and would generally result in the worst case scenario, from the standpoint of damaging overpressure. The strong concentration gradients created by a jet release, however, raises the question of what portion of the flammable cloud should be considered. Often all of the fuel within the limits of fast-flame acceleration, or even all of the fuel within the flammability range, is considered, which typically comprises the majority of the flammable cloud. In this work, prior detonation studies are reviewed to illustrate the inherently unstable nature of detonations, with a focus on the critical dimensions and concentration gradients that can support a propagating detonation wave. These criteria are then applied to the flammable cloud concentration distributions generated by an unconfined jet release of hydrogen. By evaluating these limits, it is found that the portion of the flammable cloud that is likely to participate is significantly reduced. These results are compared with existing experimental data on the ignition of unconfined hydrogen releases, and the peak pressures that were measured are consistent with a detonation of a mass of fuel that is equivalent to the model prediction for the mass of fuel within the detonable limits. This work demonstrates how the critical conditions for detonation propagation can be used to estimate the portion of a hydrogen release that could participates in a detonation and how these criteria can be readily incorporated into existing dispersion modelling approaches.