A decrease in the ignition temperature of molecular systems during nonequilibrium vibrational excitation of reacting mol
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A Decrease in the Ignition Temperature of Molecular Systems during Nonequilibrium Vibrational Excitation of Reacting Molecules A. M. Starik and N. S. Titova Baranov Central Institute of Aviation Motor Building, Moscow, Russia ReceivedDecember30, 1998
Abstract The nonequilibrium vibrational excitation of reacting molecules can result in a substantial (up to 100%) decrease in the temperature threshold of self-ignition in the H 2 + air system. In this case, dissociation reactions are responsible for the formation of active species that initiate chain combustion. An increase in temperature during the induction period caused by the recombination of oxygen and hydrogen atoms and OH radicals formed plays an important role in the intensification of chain reactions and self-ignition of the mixture at low temperatures (-300-400 K). INTRODUCTION The possibility of the activation of ignition of molecular systems by exciting the internal degrees of freedom has long been discussed in the literature [1-3]. The interest in this problem is due to the fact that chemical reactions involving molecules excited even in their lower vibrational or electron states are several orders of magnitude faster than those involving unexcited molecules [4-6]. In this case, many interrelated processes occur in which the vibrational or electron excitation may transfer from one component to another and thus affect both local and integral parameters of combustion. Vibrational excitation of molecules is quite efficient in the electric discharge or in laser-induced combustion [7]. The available data mainly concern the effect of the vibrational excitation of reacting molecules on process kinetics and the dynamic parameters of combustion. However, many practical questions remain unclear. Specifically, the mechanisms for the effect of excitation of molecular vibrations on combustion initiation at low temperatures of H2 + air mixtures and the self-ignition threshold remain unclear. Our work addresses these issues. FORMULATION OF THE PROBLEM AND THEORETICAL ANALYSIS Let us consider the self-ignition of an immobile gas mixture with a constant volume. Let us assume that thermodynamic equilibrium exists between the rotational and translational degrees of freedom at the characteristic times considered in the problem and that the local Boltzmann distribution with a vibrational temperature T~ (~ = 1..... n) is established in each mode ~.
Let us first qualitatively show that the vibrational excitation of initial reactants in the chain mechanism of the initiation of combustion should cause a decrease in the self-ignition temperature. Let us consider the simplest reaction scheme resulting in the generation of the chain carrier, an active radical: A kl ,. r + D (chain initiation), r + A ks ,. C + lr (chain propagation), r + r k3, r2 (chain termination). Here, A is the initial reactant, the decomposition of which results in the generation of the active radical r; r2, C, and D are the reaction products; l = I corresponds to the nonbranched reaction, and l = 2, 3 . . . .
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