Relaxation of electronic excitation in nitrogen/oxygen and fuel/air mixtures: fast gas heating in plasma-assisted ignition and flame stabilization

Fast gas heating (FGH) is an abrupt increase in gas temperature in non-equilibrium low-temperature plasma due to relaxation of electronically excited states of atoms and molecules. In the active flow control, fast gas heating is responsible for thermal frequency perturbations in the range of unstable frequencies of flow instabilities. In plasma-assisted combustion, abrupt temperature increase due to FGH, together with generation of radicals in plasma, induces acceleration of combustion chemistry providing shortening of the induction delay time and intensification of combustion. Over the last decade, significant progress has been made towards the understanding of kinetics of the fast gas heating. New observations of fast gas heating in air and nitrogen/oxygen mixtures have been reported. The result of experiments, reporting heating to thousands of kelvins during tens of nanoseconds at atmospheric pressure in non-combustible mixtures, have provided new opportunities in the development of kinetic models. Electron-impact dissociation, quenching of electronically excited states of atoms and molecules, ion-molecular reactions, recombination of charged particles are reviewed analysing their role in the fast gas heating. The fraction of energy spent on fast gas heating η_R has been suggested as a universal parameter to generalize the results of empirical research on energy relaxation. This paper considers the dependence of η_R on reduced electric field, specific delivered energy, oxygen fraction in the mixture and other parameters. The analysis is grouped over three different ranges of the reduced electric field: E/N ≤ 150 Td, E/N = 150–400 Td and E/N > 400 Td. Non-numerous experimental and theoretical studies of the fast gas heating in hydrogen- and hydrocarbon-containing mixtures are discussed and compared to the results in non-flammable mixtures. This article is to provide a comprehensive overview of the progress of kinetics of fast gas heating and to indicate the lack of experimental data and consequently, the gap in the knowledge of energy relaxation in discharges in combustible mixtures.