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[en] The process of relaxation of energetic O– ions formed via dissociative attachment of electrons to molecules in the discharge plasmas of water vapor and H2O: O2 mixtures in a strong electric field is studied by the Monte Carlo method. The probability of energetic ions being involved in threshold ion–molecular processes is calculated. It is shown that several percent of energetic O– ions formed via electron attachment to H2O molecules in the course of plasma thermalization transform into OH– ions via charge exchange or are destroyed with the formation of free electrons. The probabilities of charge exchange of O– ions and electron detachment from them increase significantly (up to 90%) when O– ions are formed via electron attachment to O2 molecules in water vapor with an oxygen additive. This effect decreases with increasing oxygen fraction in the mixture but remains appreciable even when the fraction of H2O molecules in the H2O: O2 mixture does not exceed several percent.
[en] Monte Carlo simulation was used to study the translational relaxation of energetic O− ions produced by dissociative electron attachment to O2 molecules in oxygen plasmas in a strong electric field. Initial O− ions have rather high energies and are more reactive than the ions reaching equilibrium with the electric field. Therefore, there is a noticeable probability that the energetic O− ions participate in endothermic reactions prior to energy relaxation of these ions. The probabilities of charge exchange, electron detachment and ion impact vibrational excitation of O2 molecules were calculated versus the reduced electric field. It was shown that up to 6% of energetic O− ions produced in oxygen by dissociative electron attachment to O2 molecules are rapidly transformed to ions due to charge exchange collisions. The probability of electron detachment from energetic O− ions and the probability of vibrational excitation were smaller that the probability of charge exchange. Estimates showed that the increase in the effective rates of the ion–molecule reactions due to high reactivity of energetic O− ions can be important in oxygen plasmas for reduced electric fields of 50–100 Td. (paper)
[en] Ignition of hydrocarbon–oxygen mixtures by means of a nanosecond surface dielectric barrier discharge (NSDBD) was studied experimentally. The propagation velocity of the flame wave and the ignition delay time in mixtures of oxygen with methane, ethane, ethylene, and dimethyl ether were measured using a high-speed camera. The experiments were carried out at room temperature and gas mixture pressures in the range of 0.75–1.25 atm. It is shown that, for all hydrocarbons under study, the flame velocity decreases with reducing pressure and stoichiometric ratio, as well as when the mixture is diluted with molecular nitrogen. Theoretical analysis of the processes in the NSDBD plasma and measurements of the flame velocity in hydrocarbon-containing mixtures without plasma agree qualitatively with the measurement results, except for the increasing dependence of the flame velocity on the pressure, which is decreasing in experiments without a discharge plasma.
[en] The development of a surface barrier discharge in air at atmospheric pressure under the action of a constant voltage of different polarity is simulated numerically. When the polarity of the high-voltage electrode is negative, the discharge develops as an ionization wave that moves along the dielectric surface. When the polarity is positive, the discharge develops as a streamer that first moves above the dielectric surface and then comes into contact with and continues to develop along it. In the case of a high-voltage electrode of positive polarity, the discharge zone above the dielectric surface is approximately five times thicker than that in the case of negative polarity. The characteristic aspects of numerical simulation of the streamer phase of a surface barrier discharge are discussed. The numerical results on the density of the charge stored at the dielectric surface and on the length of the discharge zone agree with the experimental data.
[en] The properties of a surface barrier discharge in atmospheric-pressure air at different polarities of applied voltage were studied experimentally. The influence of the voltage polarity on the spatial structure of the discharge and the electric field in the discharge plasma was determined by means of spectroscopic measurements. It is found that the energy deposited in the discharge does not depend on the voltage polarity and that discharges of positive polarity are more homogenous and the electric fields in them are higher.
[en] 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) x 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 O2+ 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.
[en] Three-body electron attachment to O2 molecules and electron detachment from O2- ions have been theoretically studied in vibrationally excited oxygen and O2-containing mixtures. Assuming that electron attachment and detachment proceed via the formation of vibrationally excited temporary O2- ions, the rates of these processes were determined on the basis of the statistical approach for the vibrational transfer and relaxation in collisions between O2- ions and O2 molecules. The calculated attachment and detachment rate constants turned out to agree well with available measurements in unexcited oxygen. This method was extended to calculate attachment and detachment rates in vibrationally excited oxygen. It was shown that the effect of vibrational excitation on electron detachment is profound, whereas attachment of low-energy electrons to vibrationally excited O2 is inefficient. The calculated vibrational distribution of stable O2- ions turned out to be non-equilibrium in an excited gas and the effective vibrational temperature of the ions was much lower than the vibrational temperature of molecules. An analytical method was suggested to determine this distribution and the effective vibrational temperature. The calculated rate constants were used to simulate the formation and decay of an electron-beam-generated plasma in N2 : O2 mixtures at elevated vibrational temperatures. The calculations showed that vibrational excitation of molecules leads to orders of magnitude increase in the plasma density and in the plasma lifetime, in agreement with available observations.
[en] Electron detachment from O-2 ions has been theoretically studied in oxygen (i) when the gas is vibrationally excited and (ii) when the ions are heated in a strong external electric field. Assuming that electron detachment proceeds via the formation of vibrationally excited temporary O-2 ions, the detachment rate was determined on the basis of the statistical approach for the vibrational transfer and relaxation in collisions between O-2 ions and O2 molecules. The approach used in our previous work to study electron detachment in vibrationally excited oxygen was amended to take into account the conservation of total angular momentum in O-2-O2 collisions. The calculated detachment rates agree well with available measurements in oxygen under equilibrium conditions. The method was used to calculate detachment rates under strongly non-equilibrium conditions in which the vibrational temperature of molecules or ion translational temperature is elevated. The obtained results were compared with available measurements in drift tubes.
[en] Ignition of hydrocarbon-containing gaseous mixtures has been studied experimentally and numerically under the action of a high-voltage nanosecond discharge at elevated temperatures. Ignition delay times were measured behind a reflected shock wave in stoichiometric CnH2n+2 : O2 mixtures (10%) diluted with Ar (90%) for n = 1-5. It was shown that the application of the gas discharge leads to more than an order of magnitude decrease in ignition delay time for all hydrocarbons under consideration. The measured values of ignition delay time agree well with the results of a numerical simulation of the ignition based on the calculation of atom and radical production during the discharge and in its afterglow. The analysis of simulation results showed that a non-equilibrium plasma favours the ignition mainly due to O atoms produced in the active phase of the discharge. (fast track communication)
[en] The electron transport coefficients in weakly ionized nonuniform nonequilibrium plasmas of water vapor and fuel–oxygen mixtures are calculated as functions of the degree of fuel oxidation. The calculations are performed for hydrogen and hydrocarbon fuels. It is shown that the production of water vapor during fuel oxidation strongly affects the electron transport coefficients. Using the calculated coefficients, the parameter ranges are determined in which thermocurrent instability can develop in the gas mixtures under study. The domain of reduced electric fields at which this instability develops extends with increasing the degree of fuel oxidation and becomes the largest under complete fuel oxidation.