Abstract

Recent experiments have shown non-thermal plasma-stimulated ignition in H_2 , CH_4 and C_2 H_4 at temperatures apparently below their respective autoignition thresholds. These experimental data were critically evaluated to determine if energy deposited by the plasma stimulus could explain sub-threshold ignition through the traditional radical-thermal channel, or if some other plasma effect might be responsible. Reported sub-threshold ignition in both H_2 and C_2 H_4 could be explained under conservative assumptions of deposited plasma energy ideally converted into either thermalization in the ignition mixture or dissociation of the ignition mixture into radicals. However, sub-threshold ignition in CH_4 could not be accounted for under this assumption, establishing that the radical-chemical channel may not solely explain some of the sub-threshold ignition observations. Of other possible channels by which plasma may interact with an igniting mixture of hydrogen or hydrocarbons, reclamation of branching radicals from the essentially terminal HO_2 radical was considered in this study. Numerically simulated reactions of active plasma species, in this case vibrationally excited N_2 , with stable and metastable HO_2 showed reductions in ignition threshold temperatures of up to several hundred kelvins at energy inputs comparable to those used in the purportedly sub-threshold ignition experiments. Branching radical reclamation from HO_2 by excited plasma species seems to be a plausible explanation for sub-threshold ignition, but experimentation is needed to substantiate this hypothesis.