In recent decades particular interest in applications of nonequilibrium plasma for the problems of plasma-assisted ignition and plasma-assisted combustion has been observed. A great amount of experimental data has been accumulated during this period which provided the grounds for using low temperature plasma of nonequilibrium gas discharges for a number of applications at conditions of high speed flows and also at conditions similar to automotive engines. The paper is aimed at reviewing the data obtained and discusses their treatment. Basic possibilities of low temperature plasma to ignite gas mixtures are evaluated and historical references highlighting pioneering works in the area are presented. The first part of the review discusses plasmas applied to plasma-assisted ignition and combustion. The paper pays special attention to experimental and theoretical analysis of some plasma parameters, such as reduced electric field, electron density and energy branching for different gas discharges. Streamers, pulsed nanosecond discharges, dielectric barrier discharges, radio frequency discharges and atmospheric pressure glow discharges are considered. The second part depicts applications of discharges to reduce the ignition delay time of combustible mixtures, to ignite transonic and supersonic flows, to intensify ignition and to sustain combustion of lean mixtures. The results obtained by different authors are cited, and ways of numerical modelling are discussed. Finally, the paper draws some conclusions on the main achievements and prospects of future investigations in the field.

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Plasma assisted ignition and combustion

Plasma-assisted ignition and combustion

The results are given of an experimental investigation of a cathode-directed streamer discharge in synthetic air in the pressure range from 760 to 300 torr and their comparison with the results of direct numerical simulation in a 2D hydrodynamic approximation. The pattern of discharge branching upon variation of pressure is investigated experimentally. The results are given of comparison of the predicted and measured values of anode current, streamer propagation velocity, and channel diameter. It has been demonstrated that the electric field in the streamer head is hardly affected by the pressure decrease, while the electron concentration decreases with pressure by an order of magnitude. At the same time, production of chemical species in a cathode-directed streamer discharge varies at a rate of at least the second power of inverse pressure.

Development of a cathode-directed streamer discharge in air at different pressures: experiment and comparison with direct numerical simulation.

The paper reviews recent progress in two rapidly developing engineering applications of plasmas, plasma assisted combustion and plasma assisted high-speed flow control. Experimental and kinetic modeling results demonstrate the key role of non-thermal plasma chemistry in hydrocarbon ignition by uniform, repetitively pulsed, nanosecond pulse duration, low-temperature plasmas. Ignition delay time in premixed ethylene‐air flows excited by the plasma has been measured in a wide range of pulse repetition rates and equivalence ratios and compared with kinetic modeling calculations, showing good agreement. Comparing ignition delay time predicted by the model for plasma assisted ignition and for ignition by equilibrium heating demonstrated that chain reactions of radicals generated by the plasma reduce ignition time by up to two orders of magnitude and ignition temperature by up to 300K. These results provide additional evidence of the non-thermal nature of low-temperature plasma assisted ignition. Experiments and flow modeling show that the dominant mechanism of high-speed plasma flow control is thermal, due to heating of the flow by the plasma. Development and characterization of pulsed dc and pulsed RF localized arc filament plasma actuator arrays for control of high-speed atmospheric pressure jet flows are discussed. Actuator power is quite low, ∼10W at 10% duty cycle. Plasma emission spectra show that a greater fraction of the pulsed RF discharge power goes to heat the flow (up to 2500 ◦ C), while a significant fraction of the pulsed dc discharge power is spent on electrode and wall heating, resulting in their erosion. Rapid localized heating of the flow by the pulsed arc filaments, at a rate of ∼1000K/10 µs, results in the formation of strong compression/shock waves, detected by schlieren imaging. Effect of flow forcing by repetitively pulsed RF actuators is demonstrated in a M = 1.3 axisymmetric jet. These two case studies provide illustrative examples of isolating non-thermal (non-equilibrium plasma chemistry) and thermal (Joule heating) effects in plasmas and adapting them to develop efficient large-volume plasma igniters and high-speed flow actuators. (Some figures in this article are in colour only in the electronic version)

https://iopscience.iop.org/article/10.1088/0963-0252/18/3/034018/pdf

Plasma assisted ignition and high-speed flow control: non-thermal and thermal effects

Positive and negative streamers are studied in ambient air at 1 bar; they emerge from a needle electrode placed 40mm above a planar electrode. The amplitudes of the applied voltage pulses range from 5 to 96 kV; most pulses have rise times of 30 ns or shorter. Diameters, velocities and energies of the streamers are measured. Two regimes are identified; a low voltage regime where only positive streamers appear and a high voltage regime where both positive and
negative streamers exist. Below 5 kV, no streamers emerge. In the range from 5 to 40 kV, positive streamers form, while the negative discharges only form a glowing cloud at the
electrode tip, but no streamers. For 5–20 kV, diameters and velocities of the positive streamers have the minimal values of d = 0.2mm and v ≈ 10^5 ms^(−1). For 20–40 kV, their diameters increase by a factor of 6 while the voltage increases only by a factor of 2. Above the transition value of 40 kV, streamers of both polarities form; they strongly resemble each other, though the positive ones propagate further; their diameters continue to increase with applied voltage. For 96 kV, positive streamers attain diameters of 3mm and velocities of 4 × 10^6 ms^(−1); negative streamers are about 20% slower and thinner. An empirical fit formula for the relation between velocity v and diameter d is v = 0.5d^2 mm^(−1) ns^(−1) for both polarities. Streamers of both polarities dissipate energies of the order of several millijoules per streamer while crossing the gap.

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Positive and negative streamers in ambient air: measuring diameter, velocity and dissipated energy

The emission spectroscopy technique is used to analyze a cathode-directed streamer discharge in air at atmospheric pressure in point-plane geometry at interelectrode distances of up to 100 mm and a high-voltage pulse amplitude of 18 kV. The densities of molecules in the N2(C3Πu, v=0), N2+(B2Σu+, v=0) and NO(A2Σ+, v=0) states are determined, and the reduced electric field in the streamer head is estimated. It is shown that the increase in the average electric field in the discharge gap substantially intensifies the production of active particles in the discharge plasma and makes the plasma more homogeneous. This effect is only related to the increase in the fraction of regions with a high electric field in the discharge gap and, as a result, the reduction of the discharge energy losses via rapidly thermalized degrees of freedom. The active particles are only produced in the streamer head, including the case in which the interelectrode gap is bridged by the streamer channel.

Discharge dynamics and the production of active particles in a cathode-directed streamer

Measurements of the rates of quenching of N 2 (C 3 Π u ) and N 2 + (B 2 Σ u + ) states by N 2 , O 2 , and CO molecules in the afterglow plasma of a nanosecond discharge

The kinetics of the nonthermal decomposition of nitrous oxide (N2O) in a nonequilibrium plasma is investigated experimentally. A numerical model of the process is constructed and used to simulate the decomposition of N2O in a high-current pulsed discharge. The most important channels for decomposition are revealed by analyzing the results obtained. The role of the charged, electronically excited, and vibrationally excited components is examined. It is shown that the mechanism for the thermally nonequilibrium decomposition of N2O in a high-current pulsed discharge is governed by the reactions involving ions and electronically excited molecules.

S. M. Starikovskaya

Papers

Discharge dynamics and the production of active particles in a cathode-directed streamer

Measurements of the rates of quenching of N 2 (C 3 Π u ) and N 2 + (B 2 Σ u + ) states by N 2 , O 2 , and CO molecules in the afterglow plasma of a nanosecond discharge

Nonthermal decomposition of nitrous oxide in a high-current pulsed discharge

Dynamics of the excitation of electronic states of H 2 molecules in a fast ionization wave

Dynamics of the population of the electronic states of molecular nitrogen and the structure of a fast ionization wave