Numerical study of the enhancement of combustion performance in a scramjet combustor due to injection of electric-discharge-activated oxygen molecules

Abstract

A comprehensive analysis of the efficiency of an approach based on the injection of a thin oxygen stream, subjected to a tailored electric discharge, into a supersonic H 2 -air flow to enhance the combustion performance in the mixing layer and in the scramjet combustor is conducted. It is shown that for such an approach there exist optimal values of reduced electric field E/N and transversal dimension d of the injected oxygen stream, which provide the minimal length of induction zone in the mixing layer. The optimal values of E/N and d depend on air flow parameters and the specific energy put into the oxygen. The injection of a thin oxygen stream (d = 1 mm) subjected to an electric discharge with E/N = 50-100 Td, which produces mostly singlet oxygen O 2 (a 1 g ) and O 2 (b 1 + g ) molecules and atomic oxygen, allows one to arrange stable combustion in a scramjet duct at an extremely low air temperature T air = 900 K and pressure P air = 0.3 bar even at a small specific energy put into the oxygen E s = 0.2 J ncm −3 , and to provide rather high combustion completeness η = 0.73. The advance in the energy released during combustion is much higher (hundred times), in this case, than the energy supplied to the oxygen stream in the electric discharge. This approach also makes it possible to ensure the rather high combustion completeness in the scramjet combustor with reduced length. The main reason for the combustion enhancement of the H 2 -air mixture in the scramjet duct is the intensification of chain-branching reactions due to the injection of a small amount of cold non-equilibrium oxygen plasma comprising highly reactive species, O 2 (a 1 g ) and O 2 (b 1 + g ) molecules and O atoms, into the H 2 -air supersonic flow.

On the role of excited species in hydrogen combustion

“…Since formation of significant amounts of singlet oxygen during ozone decomposition has no experimental evidence, this reaction was excluded from the ozone mechanism [34] and from the recent models of Starik and co-authors [18,32], though it is present in the MSU mechanism [25,30]. In the previous work [34] the rate of collisional quenching of singlet oxygen by O was taken from Cupitt et al [97].…”

“…Note that the rate constants interpreted in the Model HK [5,102] are essentially the lower limit for the rate constant deduced in the Model BK [9,97,101,[103][104][105]. Sharipov and Starik [18,109] recommended the rate constant of reaction (32) nicely crossing experimental points of Cupitt et al [97] and Basevich and Vedeneev [101]. They also concluded that reaction (24c) contributes less than 10% to the total rate of reaction following suggestions that reactive channel (32) is faster than physical quenching [97,101].…”

“…2. Implementation of the rate constant recommended by Starik and Sharipov [73] in the models for hydrogen or syngas [32,18] results in formation rate of singlet oxygen exceeding the rate of formation of the ground state oxygen at typical combustion temperatures that has no experimental evidence. Predictions of Gonzalez et al [124], accepted in the present work, are based on potential energy surface which is in good agreement with that of Xu et al [125].…”

“…Most recent models of Starik et al [13,18,31,32] seem to properly incorporate reaction H 2 + O 2 = H + HO 2 (14) as initiation step in the absence of excited species. However, overall performance of these mechanisms in hydrogen combustion modeling was shown to be unsatisfactory.…”

Physics and chemistry of the influence of excited molecules on combustion enhancement

“…The numerical analysis of the influence of O 2 (a 1 g ) and O 2 (b 1 Σ + g ) molecules on the ignition of H 2 −O 2 (air) and CH 4 −O 2 (air) mixtures in a supersonic flow behind the inclined shock wave was conducted in [26,28,34,35] on the basis of detailed kinetic mechanisms comprising a large set of reactions with O 2 (a 1 g ) and O 2 (b 1 Σ + g ) molecules. However, in past years, novel kinetic data on reactions with O 2 (a 1 g ) and O 2 (b 1 Σ + g ) molecules in the H 2 −O 2 , CO−H 2 −O 2 , CH 4 −O 2 and C 2 H 6 −O 2 mixtures were obtained on the basis of accurate ab initio calculations (herein and hereafter the term 'accurate ab initio calculations' means the quantum-chemical calculations based on solving of the Schrödinger equation for multi-electron systems at a high level of theory), and novel numerical results on the influence of the activation of oxygen molecules by specially arranged electric discharge were reported.…”