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 (Formula presented.) O-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 (Formula presented.) -50%CO/air mixtures with and without H (Formula presented.) O. 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 (Formula presented.) and (Formula presented.), 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 (Formula presented.) with a low (Formula presented.) 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, (Formula presented.), was used to understand the detonation re-initiation process after the mitigation of the initial detonation.