Effects of Vibrational Excitation on Nanosecond Discharge Enhanced Methane-Air Ignition

A zero-dimensional model of nonequilibrium plasma-assisted methane-air ignition at atmospheric pressure is presented to study the effects of vibrationally excited species on ignition enhancement. It is found that the vibrationally excited species CH4(v), O-2(v), and especially N-2(v) can be generated efficiently when the reduced electric field strength is below 100 Td in a stoichiometric methane-air mixture. The results show that vibrationally excited species could not effectively enhance ignition via kinetic pathways due to their fast relaxation, but instead via thermal pathways through gas heating. Despite this, the thermal enhancement is much less effective than the kinetic effects due to plasma, indicating that the production of vibrationally excited species consumes the energy deposited in the plasma, and thus limits the production of more active particles, such as O, O(D-1), and N-2(B). A sensitivity analysis is conducted to further understand the detailed chemistry of the ignition enhancement involving vibrational excitation. The results show the reactions e + N-2 -> e + N-2(v) and e + CH4 -> e + CH4(v) have significant inhibitive effects on ignition enhancement in discharge conditions with a fixed plasma energy. The reactions involving the production of O and O( 1 D) show promotive effects in enhancing ignition because they accelerate the radical production through chain branching reactions. Finally, the calculated ignition delay times at different temperatures show that the ignition enhancement by nanosecond discharge and the promotive effect of N are more significant at low temperatures.