Structure and length of chemistry induction zone in hydrogen-air detonations

The induction-zone structure of hydrogen-air detonation is analyzed by employing a reduced chemical kinetic mechanism for hydrogen oxidation chemistry in order to provide preliminary information for linear instability analysis of detonation. The reduced chemistry mechanism, derived by assuming O, OH, and HO₂ steady state, consists of the chain-initiation step, chain-branching step and radical-recombination step. In particular, near the onset condition of detonation instability, the temperature behind the leading shock is higher than the crossover temperature, so that the ignition process throughout the induction zone is found to be dominated by the exothermically neutral chain-branching step rather than the radical recombination step. Just behind the leading shock, the initial radical pool, in which radical concentration is comparable to the ratio of the chain-initiation rate to the chain-branching rate, is produced by the chain-initiation step. Following the initial build-up of a radical pool is the exponential growth of the radical concentration by the chain-branching step until a value of order unity at the end of the induction zone is reached. From the solution for the H-radical profile in the induction zone, the induction time is found to be proportional to the chain-branching time with a large constant multiplier of O(10) arising from the effect of the initial concentration of the radical pool formed by the chain-initiation step. The induction-zone length is calculated as a function of equivalence ratio and overdrive ratio. The results can be utilized in future analysis of linear instability of planar detonation.