E. M. Anokhin

Nanosecond discharge ignition in acetylene-containing mixtures

We study experimentally and numerically the kinetics of ignition in lean and stoichiometric C2H2?:?O2?:?Ar mixtures after a high-voltage nanosecond discharge. The ignition delay time is measured behind a reflected shock wave with and without the discharge using detection of CH radiation. Generation of the discharge plasma is shown to lead to a decrease in ignition delay time. Discharge processes followed by chain chemical reactions with energy release are simulated during ignition in the C2H2?:?O2?:?Ar mixtures. The generation of atoms, radicals and excited and charged particles in the discharge phase is numerically simulated. The calculations are based on the measured time-resolved discharge current and electric field. The calculated densities of the active particles produced in the discharge on a nanosecond time scale are employed as input data to simulate plasma-assisted ignition on a microsecond scale. The calculated ignition delay times are compared with the experimental data. It is shown that the effect of the discharge plasma on ignition of the acetylene-containing mixtures is associated with active species production in the discharge phase rather than with gas heating during the discharge and in its afterglow. A sensitivity analysis is made to determine limiting reactions in acetylene autoignition and ignition after the discharge under the conditions studied.

Plasma decay in air and O2 after a high-voltage nanosecond discharge

This paper presents the results of experimental and theoretical studies of an afterglow in room temperature air and O2 excited by a high-voltage nanosecond discharge for pressures between 1 and 10?Torr. We measured time-resolved electron density by a microwave interferometer for initial electron densities in the range (2?3)???1012?cm?3. Discharge uniformity was investigated by optical methods. The balance equations for charged particles and electron temperature were numerically solved to describe the temporal evolution of the densities of electrons and ions in the discharge afterglow. It was shown that the loss of electrons is governed by dissociative and three-body electron recombination with ions under the conditions considered. Good agreement between the calculated and measured electron density histories could be obtained only when the rate of three-body recombination was increased by an order of magnitude and when the dependence of the recombination rate on electron temperature was changed. This could testify that the well-understood mechanism of three-body electron recombination with atomic ions could be noticeably modified in the case of molecular ions.

Ignition of hydrocarbon : air mixtures by a nanosecond surface dielectric barrier discharge

Ignition of stoichiometric hydrocarbon : air mixtures by a nanosecond surface dielectric barrier discharge has been experimentally studied at room temperature and atmospheric and subatmospheric pressures. Observations were made for different voltage polarities and shapes of the high-voltage electrode. The ignition delay time and the velocity of the combustion wave were measured in a C2H2 : air mixture versus applied voltage by processing discharge gap images. It was concluded that the mixtures are ignited easier by the discharge for a negative voltage polarity and when it develops from a gear-like electrode. A 2D simulation of the discharge was performed to calculate the temporal and spatial distributions of generated active species and gas temperature during the discharge and in its afterglow for both electrode polarities. It was shown that the voltage threshold for ignition by a negative-polarity discharge is lower than that for a positive-polarity discharge, in qualitative agreement with observations. This is due to the formation of a region with efficient active species production and fast gas heating in the immediate vicinity of the high-voltage electrode when a voltage of negative polarity is applied to it.

Plasma decay in the afterglow of a high-voltage nanosecond discharge in air

The decay of air plasma produced by a high-voltage nanosecond discharge at room temperature and gas pressures in the range of 1–10 Torr was studied experimentally and theoretically. The time dependence of the electron density was measured with a microwave interferometer. The initial electron density was about 1012 cm−3. The discharge homogeneity was monitored using optical methods. The dynamics of the charged particle densities in the discharge afterglow was simulated by numerically solving the balance equations for electron and ions and the equation for the electron temperature. It was shown that, under these experimental conditions, plasma electrons are mainly lost due to dissociative and three-body recombination with ions. Agreement between the measured and calculated electron densities was achieved only when the rate constant of the three-body electron-ion recombination was increased by one order of magnitude and the temperature dependence of this rate constant was modified. This indicates that the mechanism for three-body recombination of molecular ions differs from that of the well-studied mechanism of atomic ion recombination.

Plasma decay in high-voltage nanosecond discharges in oxygen-containing mixtures

Plasma decay in high-voltage nanosecond discharges in CO2: O2 and Ar: O2 mixtures at room gas temperature and a pressure of 10 Torr is studied experimentally and theoretically. The time dependence of the electron density during plasma decay is measured using microwave interferometry. The time evolution of the charged particle density, ion composition, and electron temperature is simulated numerically. It is shown that, under the given conditions, the discharge plasma is dominated for the most time by O2+ ions and plasma decay is determined by dissociative and three-body electron−ion recombination. As in the previous studies performed for air and oxygen plasmas, agreement between measurements and calculations is achieved only under the assumption that the rate of three-body recombination of molecular ions is much greater than that for atomic ions. The values of the rate constant of three-body recombination of electrons with О2+ ions in a wide range of electron temperatures (500–5500 K), as well as for thermal (300 K) electrons, are obtained by processing the experimental results.

Dynamics of radiation in a CO:N2 mixture behind strong shock waves

Experiments are performed in determining the spectral composition and dynamics of radiation in a CO:N2 mixture behind the front of incident shock wave at velocities up to 6.5 km/s and initial pressures in the mixture of 1–9 torr. Absolute values of radiation intensity are obtained. It is determined that the main source of radiation under the conditions being investigated is provided by the violet system of CN. Also considered are the fourth positive system of CO and the Swan system of C2 molecule. Comparison is made of the radiation intensity and time dynamics of the molecular systems identified above.

Plasma Decay in the Afterglow of High-Voltage Nanosecond Discharges in Unsaturated and Oxygenated Hydrocarbons

Results of experimental and theoretical study of plasma decay in the afterglow of high-voltage nanosecond discharges in gaseous ethylene and dimethyl ether at room temperature and pressures from 2 to 20 Torr are presented. Using a microwave interferometer, the time behavior of the electron density in the range from 2 × 1010 to 3 × 1012 cm–3 during plasma decay is investigated. By processing the experimental data, the effective coefficients of electron–ion recombination as functions of the gas pressure are obtained. It is found that these coefficients substantially exceed the recombination coefficients of simple hydrocarbon ions. This distinction, as well as the increase in the effective recombination coefficient with pressure, is explained by the formation of cluster ions in three-body collisions, which recombine with electrons more efficiently than simple molecular ions. The coefficients of three-body conversion of simple molecular ions into cluster ions in the plasmas of ethylene and dimethyl ether, as well as the coefficients of recombination of electrons with cluster ions in these gases, are determined by analyzing the experimental data.

Development of high-voltage nanosecond discharge in combustible mixtures

The discharge development inmethane- and hydrogen-oxygenmixtures under repeated high-voltage nanosecond pulses is studied. It is shown that the fraction of the energy deposited in the methane-oxygen-mixture discharge and the maximum discharge current pass through a minimum with increasing number of pulses, and the plasma decay rate, on the contrary, reaches a maximum. The observed features are explained by partial fuel oxidation with the result that intermediate components are accumulated in the combustiblemixture, which result in rapid electron loss and decay plasma enhancement.