Facilitated ignition and detonation initiation of hydrogen-oxygen mixtures by ozone doping

This work computationally studies homogeneous and inhomogeneous ozone addition in hydrogen-oxygen ignition and transition to detonation, with emphasis on the ozone-induced two-stage behaviors in the chemical kinetics and wave dynamics. In 0D homogenous ignitions, ozone addition induces an additional first-stage reaction and substantially reduces the ignition delay time (IDT). At intermediate initial temperatures, e.g. T₀ = 600 K, a critical value, α_cr, is identified to delineate different regimes for ozone fraction in the total oxidizer, α. Specifically, the first-stage ignition occurs several orders earlier than IDT for α<α_cr, while the IDTs for two stages are comparable for α>α_cr. Mechanistically, low-to-intermediate temperature HO₂ chemistry introduces a quasi-steady intermediate state in the former regime, whereas ozone reactions directly accelerate high-temperature H₂–O₂ reactions in the latter one. Furthermore, according to the Zel'dovich mechanism, ozone concentration non-uniformity can establish reactivity gradient and hence further initiate double spontaneous waves with multiple dynamics. Coherent coupling between the second-stage heat release and pressure (shock) wave can further evolve into detonation, whereas such synchronized enhancement is not observed in the first-stage ignition due to its large excitation time. Corresponding to the multi-timescales in 0D ignition, when the initial α interval are respectively below, across and above α_cr, the low- and high- temperature reaction fronts always separate, first separate then merge, and always merge, respectively. It is also noted that at higher initial temperatures α_cr decreases, and that the first-stage reaction becomes less concentrated. Consequently, detonation can be initiated with smaller amounts of ozone and is minimally influenced by the first-stage reaction.