Energy Requirements for Spherical Ignitions in Methane-Air Mixtures at Different Equivalence Ratios

Abstract

Computational studies of ignition are reported where the ignition energy is added as an initial condition in a form which serves to raise the gas temperature (heat) or as a combination of dissociation of the fuel and oxygen with heat. The effect of these different energy addition methods on the time history of the ignition kernel and the minimum ignition energy has been investigated in atmospheric-pressure methane-air mixtures. The ignition energy in all of these calculations is added as an initial condition. At low ignition energy densities where the maximum initial temperature in the center of the ignition kernel region is aboul 2000 K. depositing 10% of the ignition energy in dissociation of methane and oxygen leads to a reduced ignition delay and a reduced minimum ignition energy compared to addition of the ignition energy in the form of heat only. These comparisons were made at the same ignition energy and ignition energy density. However, the observed reductions disappear if the ignition energy density is raised such that the initial maximum temperature is about 3000 K. At this higher energy density the high-temperature, near-equilibrium state in the center of the ignition kernel which is a necessary (but not sufficient) condition for flame initiation to occur is reached at a lime which is independent of the method of ignition energy addition. This opens up the possibility that a successful simulation of spark ignition may not need to take explicitly into account the ionization and dissociation created by the spark channel.