Modelling of an Atmospheric–Pressure Air Glow Discharge Operating in High–Gas Temperature Regimes: The Role of the Associative Ionization Reactions Involving Excited Atoms

A model of a stationary glow-type discharge in atmospheric-pressure air operated in high-gas-temperature regimes (1000 K < Tg < 6000 K), with a focus on the role of associative ionization reactions involving N(2D,2P)-excited atoms, is developed. Thermal dissociation of vibrationally excited nitrogen molecules, as well as electronic excitation from all the vibrational levels of the nitrogen molecules, is also accounted for. The calculations show that the near-threshold associative ionization reaction, N(2D) + O(3P) → NO+ + e, is the major ionization mechanism in air at 2500 K < Tg < 4500 K while the ionization of NO molecules by electron impact is the dominant mechanism at lower gas temperatures and the high-threshold associative ionization reaction involving ground-state atoms dominates at higher temperatures. The exoergic associative ionization reaction, N(2P) + O(3P) → NO+ + e, also speeds up the ionization at the highest temperature values. The vibrational excitation of the gas significantly accelerates the production of N2(A3∑u+) molecules, which in turn increases the densities of excited N(2D,2P) atoms. Because the electron energy required for the excitation of the N2(A3∑u+) state from N2(X1∑g+, v) molecules (e.g., 6.2 eV for v = 0) is considerably lower than the ionization energy (9.27 eV) of the NO molecules, the reduced electric field begins to noticeably fall at Tg > 2500 K. The calculated plasma parameters agree with the available experimental data.

Numerical Investigation on the Effects of Chemical Reactions on the Discharge Characteristics and Energy Balance of a Nanosecond Repetitive Pulsed Dielectric Barrier Discharge

Numerical investigation on a nanosecond repetitively pulsed dielectric barrier discharge (NS-DBD) in air is a temporal and spatial multi-scale problem involving a large number of species and chemical reactions. To know the effects of the species and chemical reactions on the discharge characteristics and energy balance, a high voltage repetitive plane to plane NS-DBD is numerically studied. Four groups of species and the corresponding chemical reactions are adopted in the investigation. The most complex one has 31 species and 99 chemical reactions that contains all reaction types, in particular, the vibrational-translational relaxation reactions, whereas the simplest one has only 4 species and 4 reactions, which represents the main kinetic processes. The others are in between. The discharge energy reaches to a periodic phase equality state after the second pulse in the repetitive pulses, and the present analysis is focused on the 7th pulse. All the N 2 / O 2 mixture reaction models predict almost the same discharge energies, which are qualitatively similar with that in the simplified 4-species model. The prediction of the discharge energy is determined by the electronic excitation and the energy gain by ions, but the vibrational excitation, negative ions, associative ionization, dissociation of nitrogen and oxygen molecules have very weak effects. The gas heating is determined by the exothermic reactions and the ions. The main processes in the fast and slow gas heating are the energy release of ions and the exothermic reactions, respectively. The negative ions, vibrational excitation, and associative ionization have very weak effects on the gas heating during the high voltage pulse, but they have considerable effects at a larger time scale. The magnitudes of the fast gas heating efficiency ( η G H ) are in the range of 41%∼47% in the N 2 / O 2 mixture reduced kinetic models, but η G H is higher in the 4-reaction model.

Glow Discharge in a High-Velocity Air Flow: The Role of the Associative Ionization Reactions Involving Excited Atoms

A kinetic scheme for non-equilibrium regimes of atmospheric pressure air discharges is developed. A distinctive feature of this model is that it includes associative ionization with the participation of N(2D, 2P) atoms. The thermal dissociation of vibrationally excited nitrogen molecules and the electronic excitation from all the vibrational levels of the nitrogen molecules are also accounted for. The model is used to simulate the parameters of a glow discharge ignited in a fast longitudinal flow of preheated (T0 = 1800–2900 K) air. The results adequately describe the dependence of the electric field in the glow discharge on the initial gas temperature. For T0 = 1800 K, a substantial acceleration in the ionization kinetics of the discharge is found at current densities larger than 3 A/cm2, mainly due to the N(2P) + O(3P) → NO+ + e process; being the N(2P) atoms produced via quenching of N2(A3∑u+) molecules by N(4S) atoms. Correspondingly, the reduced electric field noticeably falls because the electron energy (6.2 eV) required for the excitation of the N2(A3∑u+) state is considerably lower than the ionization energy (9.27 eV) of the NO molecules. For higher values of T0, the associative ionization N(2D) + O(3P) → NO+ + e process (with a low–activation barrier of 0.38 eV) becomes also important in the production of charged particles. The N(2D) atoms being mainly produced via quenching of N2(A3∑u+) molecules by O(3P) atoms.