The Effect of Selective Energy Deposition on the Homogeneous Ignition of Methane and its Implication for Flame Initiation and Combustion Enhancement

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Abstract
This paper describes a computational study of the ignition of methane mixtures by selective ignition energy deposition. In this work the ignition energy was added as an initial condition to the combustible mixture in a specific manner, either in the form of heat or a combination of heat and selected, highly reactive atoms or molecules. The resulting induction time for the ignition of homogeneous combustion was found to decrease with increasing fraction of ignition energy deposited into dissociation for the same total ignition energy at a given equivalence ratio. In addition to these calculations performed at constant energy, other calculations were performed where equal amounts of the radicals H, O, and OH were added to assess the effect of direct addition of these radicals on the induction time. Oxygen atoms yielded the shortest induction time in the stoichiometric case and hydrogen atoms yielded the shortest induction time in the lean case. Hydroxyl radicals yielded by far the longest induction time in both cases Also obtained were preliminary but significant findings regarding the consequences of ignition energy distribution on the initiation of a planar flame. Two stoichiometric cases were considered where ignition energy was added as heat and as oxygen dissociation in two different proportions at the same total ignition energy. Flame propagation began earlier for the calculation where the larger fraction of ignition energy was deposited in oxygen dissociation. At least for the conditions of the calculations presented here, these results show that some combustion enhancement can be achieved merely by changing the distribution of ignition energy.

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