Kinetics Paths to Radical-induced Ignition of Methane/air Mixtures

Non-equilibrium plasmas may be used to dissociate reactants into radicals. In turn, these radicals react releasing energy and broadening the radical pool and thereby ignite or aid the ignition of a fuel=air mixture. Using methane as an archetypical fuel, this work studies the kinetic pathways that control homogeneous ignition in the presence of radicals, which were previously formed by external, non-chemical means. This type of ignition generally follows a well-defined fourstage chain sequence starting with reactions of the original radicals, followed by the reactions of their products, up to the point of ignition. The early chemical kinetic processes were found to be relatively insensitive to pressure, initial temperature, and=or stoichiometry of the mixture. Radical-induced ignition has two major advantages over thermal ignition. First, a major part of the ignition energy comes from exothermic reactions between the radicals and the oxidizer. As a result, radical-induced ignition can potentially require less energy than, for example, sparkplugs to which all the thermal energy must be supplied in the form of electricity. Second, radical ignition can reduce the ignition delay time by many orders of magnitude to values, under extreme conditions, of the order of few hundred nanoseconds, which are nearly 5 orders of magnitude lower than the tens of milliseconds ignition delays that are typical of thermal ignition.