Kinetics of plasma-assisted combustion: effect of non-equilibrium excitation on the ignition and oxidation of combustible mixtures

Abstract

A review of experimental and theoretical investigations of the effect of atomic particles, and electronically and vibrationally excited molecules on the induction delay time and on the shift in the ignition temperature threshold of combustible mixtures is presented. The addition of oxygen and hydrogen atoms to combustible mixtures may cause a significant reduction in the ignition delay time. However, at relatively low initial temperatures, the non-equilibrium effect of the addition of atomic particles in ground electronic states is not pronounced. At the same time, the effect of excited O(¹D) atoms on the oxidation and reforming of combustible mixtures is quite significant due to the high rates of reactions of O(¹D) atoms with hydrogen and hydrocarbon molecules. In fuel–air mixtures, collisions with O(¹D) atoms determine, under certain conditions, the dissociation of hydrocarbon molecules. Singlet oxygen molecules, O₂(a¹Δ _g ), participate both in chain initiation and chain branching reactions, but the effect of O₂(a¹Δ _g ) on the ignition processes is generally less important compared to oxygen atoms. The reactions of vibrationally excited molecules and the processes of VT-relaxation in combustible mixtures are discussed. The production of vibrationally excited N ₂(v) molecules in fuel–air mixtures at relatively low electric field is very important. However, at the moment, the effect of the reactions of N ₂(v) molecules on the oxidation and ignition of combustible mixtures is not completely clear, and requires further investigation. Therefore, with present knowledge, to reduce the ignition delay time and decrease the temperature threshold of combustive mixtures, the use of gas discharge systems with relatively high E/N values is recommended. In this case the reactions of electronically excited molecules, and atomic particles in ground and electronically excited states, are extremely important. The energy stored in electronically excited states of atoms and molecules is spent on the additional dissociation of oxygen and fuel molecules, on fast gas heating, and finally on triggering chain branching reactions.