We examine the chemical basis for simplified chemical reaction models by using numerical simulations of adiabatic explosion with detailed chemical kinetic mechanisms under pressure and temperature conditions relevant to detonations. We have studied hydrogen, methane, and ethane to determine the reaction structure and characterize it in terms of three overall features: induction time, energy release pulse width, and reduced effective activation energy. A basic requirement of any realistic reaction model is that these three features should be reproduced over a range of conditions encountered within the detonation front. As part of this study, we have examined the question of the existence of a temperature cutoff, which has been proposed as the basis for formulating previous three-step models. We show that a definite cutoff temperature does not exist for any of the fuels we examine but there is a shift in the principle reaction pathway for the hydrogen-oxygen system in the vicinity of the extended second explosion limit. This shift in mechanism is associated with a peak in the reduced effective activation energy. A five-step reaction model is proposed to represent this shift in pathways, and with the appropriate choice of parameters, we show that the key features of the hydrogen-oxygen mechanism can be reproduced.
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