Nikolay Aleksandrov

Plasma decay in N2, CO2 and H2O excited by high-voltage nanosecond discharge

Plasma decay after a high-voltage nanosecond discharge was studied experimentally and numerically in room temperature N2, CO2 and H2O for pressures between 1 and 10 Torr. The time-resolved electron density was measured by a microwave interferometer for initial electron densities in the range 8 × 1011–3 × 1012 cm−3 and the effective electron–ion recombination coefficient was determined. It was shown that this coefficient varies in time and depends on pressure. A numerical simulation was carried out to describe the temporal evolution of the densities of charged particles under the conditions considered. A good agreement was obtained between the calculated and the measured electron density histories. It was shown that the loss of electrons is governed by dissociative recombination with complex ions, their density being dependent on pressure. In N2 at low pressures, a hindered electron thermalization in collisions with molecules led to a delay in the plasma decay. This effect was observed both experimentally and theoretically.

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.

Plasma decay in air and N2 : O2 : CO2 mixtures at elevated gas temperatures

Plasma decay after a high-voltage nanosecond discharge has been studied experimentally and numerically behind incident and reflected shock waves in high temperature (600?2400?K) air and N2?:?O2?:?CO2 mixtures for pressures between 0.05 and 1.2?atm. Time-resolved electron density history was measured by a microwave interferometer for initial electron densities in the range (1?3) ? 1012?cm?3 and the effective electron?ion recombination coefficient was determined. A numerical simulation was carried out to describe the temporal evolution of the densities of charged and neutral particles under the conditions considered. It was shown that the loss of electrons in this case is determined by dissociative recombination with ions, whereas the effect of complex ions is negligible. Electron attachment to O2 to form negative ions is not important because of fast electron detachment in collisions with O atoms produced in the discharge. In the absence of O atoms the electron density could decay as if the loss of charged particles were governed by electron?ion recombination with the effective rate coefficient being much higher than the dissociative recombination coefficient.

Kinetics in Gas Mixtures for Problem of Plasma Assisted Ignition

Experiments and numerical modelling of ignition of hydrocarbon–containing mixtures under the action of pulsed nanosecond discharge have been performed. The experiments were carried out using a shock tube technique in a set of stoichiometric mixtures CnH2n+2 :O 2 (10%) diluted by Ar (90%) for hydrocarbons from CH4 to C5H12. The temperature behind the reflected shock wave (T5) varied from 950 to 2000 K, and the pressure (P5 )w as 0.2 to 1.0 atm. For each set of experimental parameters, the ignition by the discharge was compared with autoignition. Numerical simulation was divided into two steps. First, we calculated production of active species of a stage of the discharge and its nearest afterglow. At this stage, experimentally obtained dependencies of electric field and energy input vs time were used to calculate gas excitation by the discharge. In early afterglow immediately after discharge electronically excited species, like Ar, were quenched giving an additional production of atoms and radicals. Second, the obtained density of atoms and radicals was taken as an initial parameter for calculations of chemical kinetics in a microsecond time scale. As a result, the shift of the ignition delay time for all investigated mixtures has been obtained and compared with the experimental data.

Kinetic mechanism of plasma recombination in methane, ethane and propane after high-voltage nanosecond discharge

The results of the experimental and numerical study of high-voltage nanosecond discharge afterglow in pure methane, ethane and propane are presented for room temperature and pressures from 2 to 20 Torr. Time-resolved electron density during the plasma decay was measured with a microwave interferometer for initial electron densities in the range between 5 × 1010 and 3 × 1012 cm−3 and the effective recombination coefficients were obtained. Measured effective recombination coefficients increased with gas pressure and were much higher than the recombination coefficients for simple molecular hydrocarbon ions. The properties of plasma in the discharge afterglow were numerically simulated by solving the balance equations for charged particles and electron temperature. Calculations showed that electrons had time to thermalize prior to the recombination. The measured data were interpreted under the assumption that cluster hydrocarbon ions are formed during the plasma decay that is controlled by the dissociative electron recombination with these ions at electron room temperature. Based on the analysis of the experimental data, the rates of three-body formation of cluster ions and recombination coefficients for these ions were estimated.

Comparative Shock-Tube Study of Autoignition and Plasma-Assisted Ignition of C2-Hydrocarbons

The kinetics of ignition in C2H2:O2:Ar, C2H4:O2:Ar, C2H6:O2:Ar and C2H5OH:O2:Ar mixtures was analyzed in experiments using a shock tube with a discharge cell. Ignition delay time was measured behind a reflected shock wave after a high-voltage nanosecond discharge and in its absence. Numerical simulation was used to show the main mechanisms that lead to peculiarities of ignition properties of C2-hydrocarbons.

Analysis of energetic efficiency and kinetics of intermediates in the problem of plasma assisted ignition

Analysis of kinetics in the afterglow of pulsed nanosecond high–voltage discharge for conditions of plasma assisted ignition at elevated temperatures for hydrocarbons:oxygen mixtures diluted with argon has been performed. Computer modelling based on results of the experimental data on ignition delay times, demonstrates a special behavior of the kinetic curves of intermediate components in the process of plasma assisted ignition. Calculated kinetic curves for electrons, O, OH radicals, and main reagents and products have been analyzed. Kinetic mechanism has been suggested to describe plasma assisted ignition of methane:air mixtures diluted with argon. Efficiency of O-atoms production was analysed for different gas mixtures.

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.

Comparative study of nonequilibrium plasma generation and plasma-assisted ignition for different C2 hydrocarbons

Using a shock tube with a discharge cell, ignition delay time was measured in a lean (φ = 0.5) C2H6:O2:Ar mixture and in lean (φ = 0.5) and stoichiometric C2H4:O2:Ar mixtures with a high-voltage nanosecond discharge and without it. The measured results were compared to the measurements made previously with the same setup for C2H6-, C2H5OH- and C2H2-containing mixtures. It was shown that the effect of plasma on ignition is almost the same for C2H6, C2H4 and C2H5OH-containing mixtures. The efficiency of nonequilibrium excitation is smaller for mixtures with C2H2 because of efficient production of atoms and radicals in thermally-equilibrium chemical reactions at elevated temperature. The effect of non-equilibrium plasma on C2-hydrocarbon ignition is dominated by the electron-impact production of O and H atoms and hydrocarbon radicals in the discharge and in its afterglow. The dominant mechanisms of the production of these active species are electron-impact dissociation of fuel and oxygen, and electron-impact excitation of Ar followed by dissociative quenching of excited Ar atoms in collisions with O2 and hydrocarbons. In addition, although the hydrocarbon fraction in the mixtures under consideration is small, direct electron-impact dissociation of hydrocarbons plays an important role in the stoichiometric C2H2- and C2H4-containing mixtures due to high dissociation cross sections for these hydrocarbon molecules. In the C2H5OH-containing mixtures, the rate of electron-impact dissociation is several times lower than the same process of other hydrocarbon molecules. However, this does not lead to a lower efficiency for plasma assisted ignition in ethanol-containing mixtures because in these mixtures the rate of electron-impact ionization is high due to the low ionization energy of ethanol and the high values of the corresponding ionization cross section. As a result, the specific deposited energy in the discharge is higher in C2H5OH-containing mixtures. The deposited energy increase compensates for the smaller electron-impact dissociation rate coefficient for C2H5OH molecules. The overall efficiency of plasma-assisted ignition for mixtures with C2H5OH is close to that of C2H4.