Energy requirements for spherical ignitions in atmospheric-pressure methane-air mixtures: A computational study

Abstract

This paper addresses the calculated dependence of the minimum ignition energy and the ignition delay on the form in which the ignition energy is added, either as thermal energy or a combination of thermal energy and dissociation of the fuel or oxygen. The majority of these calculations were performed in atmospheric-pressure, stoichiometric methane-air mixtures. Ignition energy was added as an initial condition in the form of heat and dissociation of methane and oxygen. The distribution of ignition energy between heat and dissociation had only a small effect on the minimum ignition energy, but had a dramatic effect on the ignition delay. As the fraction of ignition energy deposited in dissociation of oxygen and methane increased for these mixtures, the minimum ignition energy decreased by about 6%, then increased. The ignition delay, however, was found to be much shorter when a substantial fraction of the ignition energy was added as dissociation compared to the case where the ignition energy was added in the form of heat only. The minimum ignition energy was found to be most sensitive to the reaction H+O 2 =OH+O. Although the reaction CO+OH=CO 2 +H is the major exothermic contributor in flames, it was not a major, contributor to the exothermic phase of the ignition process. The H+O 2 reaction controls the availability of reactants for the principal exothermic reactions which dominate the heat release during these ignitions.