N. M. Tarasova

Determination of the gas temperature in high-pressure microwave discharges in hydrogen

Results of experimental determinations of the gas temperature and plasma parameters in a microwave discharge not at thermal equilibrium are presented. The investigation is concerned with high-pressure hydrogen under conditions such that radiation is emitted both by atomic and by molecular components of the plasma are primarily governed by the interaction of the excited atoms and molecules with heavy particles. Because this takes place the approximate models which are successfully applied to analyse data from spectroscopic diagnostics cannot be used to describe the radiative properties of the discharge. One of the main objectives of research consists of making headway in spectral methods of gas-temperature determination in the high pressure range by invoking for interpretation of experimental data calculations performed in the framework of the kinetic scheme of thermal non-equilibrium discharge in hydrogen as well as general physical reasoning relating to the role of interactions between heavy particles in non-equilibrium plasma radiation. The following measurement techniques have been used: spectral measurements of Balmer-series radiation; spectral measurement of the gas temperature by recording the Doppler broadening of the ; and spectral measurements of by recording the Fulcher band. In addition the electron concentration has been measured with the help of a microwave interferometer and pyrometric measurements of the temperature of a test solid body placed in a microwave-discharge plasma have been performed. Measurements of the gas temperature agree satisfactorily under the assumption that collisions of excited particles with heavy components of a gas-discharge medium are of considerable importance. Conclusions regarding the ion composition and degree of dissociation of hydrogen molecules can be drawn.

Ignition of a combustible gas mixture by a laser spark excited in the reactor volume

Ignition of a stoichiometric CH4: O2 mixture by a laser spark excited in the reactor volume is studied experimentally. It is found that the spark initiates a feebly radiating incomplete-combustion wave, which is much faster than the combustion wave, but is substantially slower than the detonation wave. With a time delay of 500–700 μs, a bright optical flash occupying the entire chamber volume is observed, which indicates fast (involving branching chain reactions) ignition of the gas mixture. A conclusion is drawn regarding the common nature of the process of ignition of a combustible gas mixture by a laser spark excited in the reactor volume and the previously investigated initiation of combustion by laser sparks excited at solid targets, high-power microwave discharges, and high-current gliding discharges.

A nonequilibrium plasma accompanying the ignition of methane-oxygen mixtures

Measurements of electron density and electron‐neutral collision frequency in combustion-produced plasmas in methane‐oxygen stoichiometric mixtures are presented. The electron density in the developed flame was found to be at a level of ne ≈ (2‐6) × 10 12 cm −3 , which exceeds the values obtained or predicted in the previous experimental and theoretical works. The collision frequency between electron and neutral components is νen ≈ 10 12 s −1 , thus exceeding that in the preflame mixture by more than an order of magnitude. Based on the calculations performed within the framework of a multicomponent kinetic scheme the most important processes of chemi-ionization in methane‐oxygen flames are determined. It is shown that the maximum electron density in the developed flame is close to ∼3 × 10 12 cm −3 , which is in reasonable agreement with the measured values. (Some figures may appear in colour only in the online journal)

Ignition of a combustible gas mixture by a high-current electric discharge in a closed volume

Results are presented from experimental studies and numerical calculations of the ignition of a stoichiometric CH4: O2 gas mixture by a high-current gliding discharge. It is shown that this type of discharge generates an axially propagating thermal wave (precursor) that penetrates into the gas medium and leads to fast gas heating. This process is followed by an almost simultaneous ignition of the gas mixture over the entire reactor volume.

Long-lived plasmoids generated by surface laser sparks in combustible gas mixtures

Results are presented from experimental studies of laser sparks generated at the surface of a metal target in a chemically active (combustible) gas mixture. It is shown that microplasmoids produced on the target surface penetrate into the ambient gas and initiate its combustion. The lifetime of microplasmoids is found to be anomalously long, up to 1–1.5 ms, which substantially exceeds their lifetime in a chemically inactive medium.

Long-lived plasmoids as initiators of combustion of gas mixtures

A slipping surface discharge excited along a multielectrode metal-dielectric system in a chemically active (combustible) CH4-O2 mixture has been studied experimentally. Results obtained with the help of high-speed hotography, shadowgraphy, optical spectroscopy, and piezoelectric measurements are presented. It is hown that the slipping surface discharge is a source of high-temperature, thermally equilibrium metal plasmoids hose lifetime is much longer than the plasma recombination time (long-lived plasmoids). The experimental investigation of the evolution of plasmoids introduced into a combustible gaseous medium allows one to conclude that the medium significantly increases the lifetime of plasmoids and that plasmoids, in turn, play an important role in the initiation of gas-mixture combustion.

Combustion of methane-oxygen and methane-oxygen-CFC mixtures initiated by a high-current slipping surface discharge

Results are presented from experimental studies of the destruction of chlorofluorocarbon (CF2Cl2) molecules in a methane-oxygen (air) gas mixture whose combustion is initiated by a high-current slipping surface discharge. It is found that a three-component CH4+O2(air)+CF2Cl2 gas mixture (even with a considerable amount of the third component) demonstrates properties of explosive combustion involving chain reactions that are typical of two-component CH4+O2 mixtures. Experiments show the high degree of destruction (almost complete decomposition) of chlorofluorocarbons contained in the mixture during one combustion event. The combustion dynamics is studied. It is shown that the combustion initiated by a slipping surface discharge has a number of characteristic features that make it impossible to identify the combustion dynamics with the formation of a combustion or detonation wave. The features of the effects observed can be related to intense UV radiation produced by a pulsed high-current surface discharge.

An Axisymmetric Electric Discharge as a Means for Remote Heating of Gas and for Ignition of Combustible Gas Mixture

Experimental and theoretical investigations are performed of gasdynamic phenomena accompanying a ring electric discharge excited in air at atmospheric pressure. It is experimentally demonstrated that the discharge generates a toroidal (three-dimensional) shock wave which cumulatively converges toward the axis. A mathematical model is constructed, which describes the process of focusing of the toroidal shock wave. The results of measurements of shock-wave processes accompanying the discharge are in good agreement with the calculation results; this enables one to estimate the gas temperature which may be attained in the region of cumulative convergence at some distance from the ring center. Results are given of first experiments in the excitation of a ring shock wave in a combustible gas mixture (stoichiometric mixture of CH4:O2).

Processes of chemoionization in the course of inflammation of a methane-oxygen mixture by a high-current gliding surface discharge in a closed chamber

Results are presented from experiments on the inflammation of a stoichiometric methane-oxygen mixture by a high-current multielectrode spark-gap in a closed cylindrical chamber. It is shown that, in both the preflame and well-developed flame stages, the gas medium is characterized by a high degree of ionization (ne ≈ 1012 cm−3) due to chemoionization processes and a high electron-neutral collision frequency (νe0 ≈ 1012 s−1).

Fast combustion waves and chemi-ionization processes in a flame initiated by a powerful local plasma source in a closed reactor

Results are presented from experimental studies of the initiation of combustion in a stoichiometric methane–oxygen mixture by a freely localized laser spark and by a high-current multispark discharge in a closed chamber. It is shown that, preceding the stage of ‘explosive’ inflammation of a gas mixture, there appear two luminous objects moving away from the initiator along an axis: a relatively fast and uniform wave of ‘incomplete combustion’ under laser spark ignition and a wave with a brightly glowing plasmoid behind under ignition from high-current slipping surface discharge. The gas mixtures in both the ‘preflame’ and developed-flame states are characterized by a high degree of ionization as the result of chemical ionization (plasma density ne≈1012 cm−3) and a high frequency of electron–neutral collisions (νen≈1012 s−1). The role of chemical ionization in constructing an adequate theory for the ignition of a gas mixture is discussed. The feasibility of the microwave heating of both the preflame and developed-flame plasma, supplementary to a chemical energy source, is also discussed.