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Two-dimensional simulations are conducted to investigate the directinitiation of cylindrical detonation in hydrogen/air mixtures with detailedchemistry. The effects of hotspot condition and mixture composition gradient ondetonation initiation are studied. Different hotspot pressure and compositionare first considered in the uniform mixture. It is found that detonationinitiation fails for low hotspot pressures and supercritical regime dominateswith high hotspot pressures. Detonation is directly initiated from the reactivehotspot, whilst it is ignited somewhere beyond the nonreactive hotspots. Twocell diverging patterns (i.e., abrupt and gradual) are identified and thedetailed mechanisms are analyzed. Moreover, cell coalescence occurs if manyirregular cells are generated initially, which promotes the local cell growing.We also consider nonuniform detonable mixtures. The results show that theinitiated detonation experiences self-sustaining propagation, highly unstablepropagation, and extinction in mixtures with a linearly decreasing equivalenceratio along the radial direction respectively, i.e., 1 to 0.9, 1 to 0.5 and 1to 0. Moreover, the hydrodynamic structure analysis shows that, for theself-sustaining detonations, the hydrodynamic thickness increases at theoverdriven stage, decreases as the cells are generated, and eventually becomealmost constant at the cell diverging stage, within which the sonic plane showsa sawtooth pattern. However, in the detonation extinction cases, thehydrodynamic thickness continuously increases, and no sawtooth sonic plane canbe observed.

Direct detonation initiation in hydrogen/air mixture: effects of compositional gradient and hotspot condition