Investigation of non-equilibrium plasma induced kinetic ignition of counterflow diffusion flames

Kinetic ignition enhancement of H₂ counterflow diffusion flames by a non-equilibrium plasma discharge of air and ultra-lean pre-mixtures was studied experimentally and computationally through the development of a well defined counterflow system. Measurements of the ignition temperatures, major species and sensitivity analyses were conducted to identify the key kinetic enhancement pathways. It was found that the competition between the catalytic effect of NO_x and the inhibiting effects of H₂O and CH₄ controlled the ignition processes in the system. With air as the oxidizer, ignition was enhanced from the plasma produced NO_x. With H₂ ultra-lean pre-mixtures as the oxidizer, H₂O formation significantly increased the ignition temperature. However with plasma activation, the inhibiting effect of H₂O was significantly reduced because of the dominant role of NO_x. With CH₄ ultra-lean pre-mixtures as the oxidizer, the ignition temperatures increased due to the quenching of radicals by H₂O or CH₄, depending upon the strain rate. However, it was also found that the inhibiting effect was significantly decreased with plasma activation. Unlike vitiated air ignition, plasma enhanced ignition for fuel-air mixtures can suppress the inhibiting effects of H₂O and CH₄ because of the overwhelming catalytic NO_x effect at low temperatures and the conversion to less species with less ignition inhibiting effects.