N. A. Popov

Investigation of the mechanism for rapid heating of nitrogen and air in gas discharges

A rapid heating of nitrogen-oxygen mixtures excited by gas discharges is investigated numerically with allowance for the following main processes: the reactions of predissociation of highly excited electronic states of oxygen molecules (which are populated via electron impact or via the quenching of the excited states of N2 molecules), the reactions of quenching of the excited atoms O(1D) by nitrogen molecules, the VT relaxation reactions, etc. The calculated results adequately describe available experimental data on the dynamics of air heating in gas-discharge plasmas. It is shown that, over a broad range of values of the reduced electric field E/N, gas heating is maintained by a fixed fraction of the discharge power that is expended on the excitation of the electronic degrees of freedom of molecules (for discharges in air, ηE⋍28%). The lower the oxygen content of the mixture, the smaller the quantity ηE. The question of a rapid heating of nitrogen with a small admixture of oxygen is discussed.

Fast gas heating in a nitrogen?oxygen discharge plasma: I. Kinetic mechanism

A model of fast heating of nitrogen?oxygen mixtures excited by a gas discharge in a broad range of reduced electric fields E/N is presented. It is shown that in air at E/N ? 200?Td the main contribution to gas heating occurs due to dissociation reactions by electron impact of O2 molecules and due to processes of quenching of electronically excited N2(B?3?g, C?3?u, ) molecules by oxygen and excited O(1D) atoms by nitrogen. At E/N > 400?Td, dissociation reactions by electron impact of N2 molecules are dominant as well as the processes involving charged particles. The fraction of discharge energy converted to fast gas heating does not exceed 40%. An analysis of the experimental data on fast air heating in discharges at high reduced electric fields E/N is given.It was shown that, in a broad range of reduced electric fields, a fixed fraction of discharge power ?E spent on the excitation of electronic degrees of freedom, ionization and dissociation of molecules is converted to fast heating of nitrogen?oxygen mixtures. In air, the value of ?E is about 30 ? 3%. The value of ?E diminishes with decreasing share of oxygen in a mixture. The significant role of heat release in the pooling reactions of molecules for fast gas heating in pure nitrogen and in nitrogen with small admixtures of oxygen is demonstrated. The simulation results agree with experimental data at E/N < 200?Td within the range of oxygen content ? = 0?20%.

On the possibility of O2(a 1Δg) production by a non-self-sustained discharge for oxygen–iodine laser pumping

O2(a 1Δg) production in a non-self-sustained discharge (ND) in pure oxygen and oxygen mixtures with inert gases (Ar and He) has been studied. A self-consistent model of ND in pure oxygen is developed, allowing us to simulate all the obtained experimental data. Agreement between the experimental and simulated results for pure oxygen over a wide range of reduced electric fields was reached only after taking into account the ion component of the discharge current. It is shown that the correct estimation of the energetic efficiency of O2(a 1Δg) excitation by discharge using the EEDF calculation is possible only with the correct description of the energy deposit into the plasma on the basis of an adequate discharge model. The testing of an O2(a 1Δg) excitation cross-section by direct electron impact, as well as a kinetic scheme of processes involving singlet oxygen, has been carried out by the comparison of experimental and simulated data. The tested model was then used for simulating O2(a 1Δg) production in ND in oxygen mixtures with inert gases. The study of O2(a 1Δg) production in Ar : O2 mixtures with small oxygen content has shown that the ND in these mixtures is spatially non-uniform, which essentially decreases the energetic efficiency of singlet oxygen generation. While simulating the singlet oxygen density dynamics, the process of three-body deactivation of O2(a 1Δg) by O(3P) atoms was for the first time taken into account. The maximal achievable concentration of singlet oxygen in ND can be limited by this quenching. On the basis of the results obtained and the model developed, the influence of hydrogen additives on singlet oxygen kinetics in argon–oxygen–hydrogen mixtures has been analysed. The simulation has shown that fast quenching of O2(a 1Δg) by atomic hydrogen is possible due to significant gas heating in the discharge that can significantly limit the yield of singlet oxygen in hydrogen-containing mixtures.

Fast gas heating in nitrogen?oxygen discharge plasma: II. Energy exchange in the afterglow of a volume nanosecond discharge at moderate pressures

The process of fast gas heating in air in the near afterglow of a pulsed nanosecond spatially uniform discharge has been investigated experimentally and numerically at moderate (3?9?mbar) pressures and high (200?400?Td) reduced electric fields. The temporal behaviour of discharge current, deposited energy, electric field and temperature was measured. The role of processes with participation of excited and charged species was analysed. It was shown that under the considered conditions the main energy release takes place in reactions of nitrogen and oxygen dissociation by electron impact and quenching of electronically excited nitrogen molecules, such as N2( , B?3?g, C?3?u, ) by oxygen and quenching of excited O(1D) atoms by N2. It was shown that about 24% of the discharge energy goes to fast gas heating during the first tens of microseconds after the discharge.

Effect of a pulsed high-current discharge on hydrogen-air mixtures

A self-consistent model describing the influence of a pulsed discharge on H2-air mixtures is developed. The model includes the processes of ionization, dissociation, and excitation of the gas molecules by electron impacts; a set of ion-molecular reactions determining the time evolution of the charged particle densities; the processes involving electronically excited atoms and molecules; and a set of reactions describing the ignition of hydrogen-oxygen mixtures. Results are presented from simulations of the oxidation dynamics of hydrogen molecules in a stoichiometric H2-air mixture and the ignition of such a mixture under the action of a pulsed high-current discharge. The simulation results are compared with available experimental data and calculations performed by other authors.

A nanosecond surface dielectric barrier discharge at elevated pressures: time-resolved electric field and efficiency of initiation of combustion

We study a nanosecond surface dielectric barrier discharge (SDBD) initiated by negative or positive polarity pulses 10?15?kV in amplitude in a cable, 25?30?ns FWHM, 5?ns rise time, in the regime of a single shot or 3?Hz repetitive frequency. Discharge parameters, namely spatial structure of the discharge and time- and space-resolved electric field are studied in a N2?:?O2?=?4?:?1 mixture for P?=?1?5?atm. The possibility of igniting a combustible mixture with the help of an SDBD is demonstrated using the example of a stoichiometric C2H6?:?O2 mixture at ambient initial temperature and at 1?atm pressure. Flame propagation and ignited volume as a function of time are compared experimentally for two discharge geometries: SDBD and pin-to-pin configurations at the same shape and amplitude of the incident pulse. It is shown that the SDBD can be considered as a multi-point ignition system with maximum energy release near the high-voltage electrode. Numerical modeling of the discharge and subsequent combustion kinetics for the SDBD conditions is performed. The discharge action leads to the production of atoms and radicals as well as to fast gas heating, due to the relaxation of electronic and vibrational degrees of freedom. The calculated ignition delay time is in reasonable agreement with the experimental results.

A nanosecond surface dielectric barrier discharge in air at high pressures and different polarities of applied pulses: transition to filamentary mode

The development of a nanosecond surface dielectric barrier discharge in air at pressures 1–6 bar is studied. At atmospheric pressure, the discharge develops as a set of streamers starting synchronously from the high-voltage electrode and propagating along the dielectric layer. Streamers cover the dielectric surface creating a ‘quasi-uniform’ plasma layer. At high pressures and high voltage amplitudes on the cathode, filamentation of the discharge is observed a few nanoseconds after the discharge starts. Parameters of the observed ‘streamers-to-filaments’ transition are measured; physics of transition is discussed on the basis of theoretical estimates and numerical modeling. Ionization-heating instability on the boundary of the cathode layer is suggested as a mechanism of filamentation.

Effect of singlet oxygen O2(a 1Δg) molecules produced in a gas discharge plasma on the ignition of hydrogen–oxygen mixtures

A kinetic model describing the impact of singlet delta oxygen O2(a 1Δg) produced in a gas discharge plasma on the evolution of hydrogen–oxygen mixture composition is developed. In the framework of this model the possibility of describing all the main experimental data on the dynamics of O2(a 1Δg) quenching in H2 : O2 gas mixtures in the temperature range T0 = 300–1050 K is shown. Most of the collisions between singlet oxygen molecules and atomic hydrogen, O2(a 1Δg) + H, lead to quenching of O2(a 1Δg). The efficiency of this interaction channel is more than 80% and the fraction of the reaction H + O2(a 1Δg) → OH + O(3P) is only 10–20%. The impact of singlet oxygen O2(a 1Δg) admixture on the ignition of H2 : O2 mixtures is investigated. The dominant process determining the degree of O2(a 1Δg) impact is its deactivation by HO2 molecules. As a result, in H2 : O2 mixtures of high pressure, where the number density of the produced HO2 molecules can be sufficiently high, the impact of singlet oxygen on the ignition delay time of these mixtures turns out to be relatively weak. This effect becomes even less noticeable if admixtures of atomic oxygen are present in the mixture. Oxygen atoms produced by the discharge are more efficient in the ignition of hydrogen–oxygen and hydrogen–air mixtures than O2(a 1Δg) molecules. Thus, to reduce the ignition delay time and to decrease the temperature threshold of combustive mixtures, the use of gas discharge systems providing the efficient production of atomic particles is recommended.

Formation and development of a leader channel in air

A self-consistent model is constructed that makes it possible to investigate the formation of a leader channel in air and the evolution of the channel parameters in the developed stage, when the leader is as long as several meters or more. The initial stage of the formation of the channel is characterized by a rapid increase in the electron density and gas temperature, which is a consequence of the onset of thermal-ionizational instability. The radius of a fully developed plasma column at the current I=1 A in air at atmospheric pressure is Rh≅10−2 cm. Then, because of the gas-dynamic and thermal expansion, the plasma radius Rh increases considerably; as a result, the electric field and the reduced field E/N in the corresponding parts of the channel decrease. In the case under consideration, the field in the “oldest” parts of the leader drops to 200 V/cm and even lower and the reduced field becomes as weak as E/N≤10 Td. In this case, the densities of the main species of neutral and charged particles at the center of the channel remain close to their thermodynamically equilibrium values. The results of calculations are compared with the available experimental data.

Study of the formation and propagation of a leader channel in air

Results are presented from calculations of the initial stage of leader channel formation in air. It is shown that the channel forms in two stages: one occurring at an essentially constant gas density on time scales much shorter than the characteristic gas-dynamic time and another in which gas-dynamic rarefaction of the channel becomes important and thermal-ionizational instability develops. The leader propagation velocity is largely determined by the time of channel contraction. The dependence of the leader velocity on the current and initial pressure is calculated. The results of calculations agree with experiment for relatively low leader currents (IL = 0.5–5 A) and for leader currents above tens of amperes. A comparative analysis of the leader propagation velocity in air at different initial pressures is given. In the current range IL = 0.5–30 A, the leader velocity depends weakly on the pressure; however, for currents as high as IL ≥ 50 A, it increases appreciably with pressure.