Numerical investigation of forced ignition in laminar counterflow non-premixed methane-air flames

Simulations of forced ignition of non-premixed laminar counterflow flames are used to study the effect of strain rate on ignition success. A one dimensional calculation is performed, using detailed methane chemical kinetics and treating the spark as an instantaneous heat release in an inert mixing layer. Ignition success depends on the mixture composition at the spark location, resulting in lean and rich ignitability limits for a given spark that can be different from the fuel's static flammability limits. The difference is attributed to the finite spark width and the diffusion of heat from the spark to the flammable mixture. Ignition is prohibited by excessive strain rates, in some cases at levels well below the extinction value. In the case of successful ignition, the high temperature reached due to the spark energy causes local auto-ignition and subsequently two reaction zones propagate away from the spark to consume the premixed reactants in the mixing layer. In the case of unsuccessful ignition, despite the auto-ignition achieved in the sparked region, the strain rate is sufficiently high for the heat and radicals to diffuse without resulting in a flame.