In this study the hazardous potential of flammable hydrogen-air mixtures with vertical concentration gradients is investigated\nnumerically.The computational model is based on the formulation of a reaction progress variable and accounts for both deflagrative\nflame propagation and autoignition. The model is able to simulate the deflagration-to-detonation transition (DDT) without\nresolving all microscopic details of the flow. It works on relatively coarse grids and shows good agreement with experiments.\nIt is found that a mixture with a vertical concentration gradient can have a much higher tendency to undergo DDT than a\nhomogeneous mixture of the same hydrogen content. In addition, the pressure loads occurring can be much higher. However,\nthe opposite effect can also be observed, with the decisive factor being the geometric boundary conditions.Themodel gives insight\ninto different modes of DDT. Detonations occurring soon after ignition do not necessarily cause the highest pressure loads. In\nmixtures with concentration gradient, the highest loads can occur in regions of very low hydrogen content. These new findings\nshould be considered in future safety studies.
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