Plasma-assisted ignition of methane/air and ethylene/air mixtures: Efficiency at low and high pressures

Abstract

The ignition of methane/air and ethylene/air mixtures by nanosecond pulsed discharges (NSPD) is investigated numerically using a zero-dimensional isochoric adiabatic reactor. A combustion kinetics model is coupled with a non-equilibrium plasma mechanism, which features vibrational and electronic excitation, dissociation, and ionization of neutral particles (O2 and N2) via electron impact. A time to ignition metric τ is defined, and ignition simulations encompassing a wide range of pressures (0.5–30 atm) and pulsing conditions for each fuel are executed. For each fuel, it is found that τ depends primarily on initial pressure and energy deposition rate, and scaling laws are derived. In order to quantify the benefit gained from plasma-assisted ignition (PAI), τ is compared with a thermal ignition time. It is found that for both fuels, PAI leads to a faster ignition at low pressures, while at higher pressures (p0 ≥ 5 atm), methane/air ignition becomes inefficient (meaning a longer ignition time for the same input energy compared to thermal ignition). Ethylene/air PAI shows only a modest deterioration. The drop in performance with pressure is found to be due to the mean electron energy achieved during the pulse, which shows an inverse relationship with pressure, leading to fewer excited species and combustion radicals. The poor performance of methane/air mixture ignition at high pressure is explained by an analysis of the reaction pathways. At high pressures (p0 ∼ 30 atm), H is consumed mostly to form hydroperoxyl (HO2), leading to a bottleneck in the formation of formyl (HCO) from formaldehyde (CH2O). Instead, for ethylene/air ignition, at both low and high pressures there exist several bypass pathways that facilitate the formation of HCO and CO directly from various intermediates, explaining the more robust performance of PAI for ethylene at pressure.