Modes of Detonation Wave Propagation in Water Vapor Concentration Gradients

This numerical study investigates different combustion modes when a Chapman-Jouguet detonation propagates into H 2O-diluted unburnt mixture through a composition gradient layer. A time-accurate and space-adaptive compressible reacting flow solver was used to perform transient detonation simulations of stoichiometric 50%H 2-50%CO/air mixtures with and without H 2O. Concentrations of the water vapor and thicknesses of the gradient layer were varied. From the simulations, three combustion modes were observed: (1) normal detonation propagation, (2) detonation mitigation and re-initiation, (3) detonation suppression. These three modes can be well explained by the theory of shock transmission and reflection in a density-varying medium and the reduction in chemical reactivity due to the weakening of the leading shock. A regime map for limits of each mode was established showing that the mode depends on ζ and η, denoting the normalized ignition delay time including shock reflection effect, and the ratio of the gradient layer thickness to the detonation induction length, respectively. A high value of ζ with a low η indicates the separation of the leading shock and the reaction front; thus, detonation suppression is more probable. These non-dimensional parameters can be further extended to the cases with gradient layers from other gases. In addition, the normalized reactivity gradient, ξ, was used to understand the detonation re-initiation process after the mitigation of the initial detonation.