Comprehensive analysis of combustion enhancement mechanisms in a supersonic flow of CH4–O2 mixture with electric-discharge-activated oxygen molecules

Mechanisms of ignition and combustion enhancement in a supersonic CH4–O2 reactive flow behind an inclined shock wave front are analyzed numerically when chemically active species are produced by electric discharges with different values of reduced electric field. The analysis is carried out based on thermally nonequilibrium kinetic models both for the discharge and for the shock-induced combustion, taking into account the disruption of thermodynamic equilibrium between vibrational and translational degrees of freedom of the reacting molecules and treating the vibrational-electronic-chemistry coupling. It is demonstrated that the presence of electronically excited O2 molecules O2(a 1Δg) and O2(b 1 Σg+) as well as O atoms in oxygen plasma allows one to intensify the chain mechanism of CH4–O2 combustion even if the energy put to O2 molecules in the discharge is sufficiently low. The excitation of O2 molecules to the singlet states O2(a 1Δg) and O2(b 1 Σg+) is more preferable in shortening the induction zone length than the production of O atoms in the course of dissociation of O2 molecules due to electron impact. However, both excitation of O2 molecules to the singlet delta and singlet sigma states and production of O atoms in an electric discharge are much more effective in combustion enhancement than merely heating the mixture. (paper)