Impact of pressure and equivalence ratio on the plasma-assisted ignition of methane/air and ethylene/air mixtures

摘要

The ignition of CH_4 and C_2H_4 via nanosecond pulsed discharges (NSPD) is studied in a zero-dimensional isochoric and adiabatic reactor. An efficiency metric is defined by considering the time delay between the first pulse and ignition of the reactive mixture. Ignition efficiency is found to have a strong dependence on the pulse energy deposition rate, with a secondary dependence on peak pulse strength. Ambient pressure, temperature, and mixture composition are varied parametrically over ranges that reflect applications of practical interest, namely combustion in scramjets, and gas turbines for energy generation. Pressure is found to have a large effect on the maximum mean electron energy, controlling which excited species are generated, and the amount of energy required to ignite the mixture. Equivalence ratio is shown to have a non-monotonic effect on ignition efficiency, with ignition occurring fastest around moderately fuel-rich conditions. The roles of enhanced radical production and mixture heating in promoting faster ignition are separated by repeating ignition simulations with direct heating of the heavy species, thereby bypassing radical production by electron impact and deexcitation of heavy particles. It is found that C_2H_4 and CH_4 display opposite trends. While radical production promotes faster ignition of ethylene, direct heating of the methane/air mixture is advantageous. A possible explanation rests in the very high activation energy of the first hydrogen abstraction from methane, which benefits from heating more than it does from the presence of radicals. Our results point to the fact that the optimal energy deposition strategy may be fuel-dependent.