Analysis of Hydrogen Diffusion Ignition in Oxygen-Nitrogen Mixtures

Extreme accidental scenarios in nuclear power plants (NPPs) might involve hydrogen formation and pressurized release into the containment building, which eventually may lead to an unintended explosion. Thus, fundamental understanding of the complex physical mechanisms associated with such accidental explosions is necessary for their prevention and mitigation. In the present study, we analyse the mitigating effect of nitrogen dilution on diffusion ignition of hydrogen in air. In particular, we investigate the self-ignition of a diffusion layer behind a shock wave for the initial pressure ratio of 700 between hydrogen and atmospheric air. Numerical simulations are carried out via the open source code Ember for a 1-D unsteady diffusion-reaction problem. Pure H₂ is considered as high-pressure gas and three different gas compositions are considered as low-pressure gas - 21% of O₂ with 79% of N₂, 12% of O₂ with 88% of N₂, and 6% of O₂ with 94% of N₂. For all the cases, it is observed that O₂ is converted into OH and H₂O around the same location within the diffusion layer. However, the ignition delay time is longer for the cases of 12% of O₂ and 6% of O₂ than for pure-air case by less than 0.06 µs and 0.50 µs, respectively.