Aleksandr Rakitin

Nanosecond-Pulsed Discharges for Plasma-Assisted Combustion and Aerodynamics

The efficiency of nanosecond discharges as an active-particle generator for plasma-assisted combustion and ignition has been shown. The kinetics of alkane oxidation have been investigated from methane to decane in stoichiometric and lean mixtures with oxygen and air at room temperature under the action of high-voltage nanosecond unform discharge. The study of nanosecond barrier discharge influence on a flame propagation and flame blowoff velocity has been carried out. A significant increase of the flame blowoff velocity has been demonstrated. A decrease of 2-3 orders of magnitude of the plasma-assisted ignition delay time in comparison with the autoignition has been registered. Detonation initiating by high-voltage gas discharge has been demonstrated. The energy deposition in the discharge ranging from 70 mJ to 12 J for propane-oxygen-nitrogen mixtures leads to the transition to detonation at a distance of less than one diameter of the detonation tube. The influence of pulsed surface dielectric discharge on the flow separation for airfoils at a high angle of attack has been investigated within the velocity range from 20 to 110 m/s for the power consumption less than 1 W/cm of the wing span. The conclusion has been made that the main mechanism of plasma impact is the boundary-layer turbulization rather than acceleration.

Effect of High-Voltage Pulsed Discharges on Deflagration to Detonation Transition

An experimental sutdy of ignition and detonation initiation by two different kinds of high-voltage pulsed gas discharge has been performed in two smooth detonation tubes. The experiments were carried out at pressures ranging from 0.15 to 1 bar in various gaseous stoichiometric mixtures. In the first setup, a distributed nonequilibrium nanosecond discharge was used for mixture excitation and ignition. In the second setup, a localized microsecond pulsed spark discharge with a stored energy of 14 J developed. The electrical parameters of the discharges, ignition delay time, flame front, and shock wave velocities were measured in the experiments. ...

Plasma-assisted ignition and deflagration-to-detonation transition

Non-equilibrium plasma demonstrates great potential to control ultra-lean, ultra-fast, low-temperature flames and to become an extremely promising technology for a wide range of applications, including aviation gas turbine engines, piston engines, RAMjets, SCRAMjets and detonation initiation for pulsed detonation engines. The analysis of discharge processes shows that the discharge energy can be deposited into the desired internal degrees of freedom of molecules when varying the reduced electric field, E/n, at which the discharge is maintained. The amount of deposited energy is controlled by other discharge and gas parameters, including electric pulse duration, discharge current, gas number density, gas temperature, etc. As a rule, the dominant mechanism of the effect of non-equilibrium plasma on ignition and combustion is associated with the generation of active particles in the discharge plasma. For plasma-assisted ignition and combustion in mixtures containing air, the most promising active species are O atoms and, to a smaller extent, some other neutral atoms and radicals. These active particles are efficiently produced in high-voltage, nanosecond, pulse discharges owing to electron-impact dissociation of molecules and electron-impact excitation of N2 electronic states, followed by collisional quenching of these states to dissociate the molecules. Mechanisms of deflagration-to-detonation transition (DDT) initiation by non-equilibrium plasma were analysed. For longitudinal discharges with a high power density in a plasma channel, two fast DDT mechanisms have been observed. When initiated by a spark or a transient discharge, the mixture ignited simultaneously over the volume of the discharge channel, producing a shock wave with a Mach number greater than 2 and a flame. A gradient mechanism of DDT similar to that proposed by Zeldovich has been observed experimentally under streamer initiation.

Initiation of Detonation by Nanosecond Gas Discharge

An experimental study of ignition and detonation initiation by a non-equilibrium highvoltage nanosecond gas discharge has been performed. The experiments were carried out at different pressures ranging from 0.15 to 1 atm in various stoichiometric mixtures: C3H8 + 5O2, C3H8/C4H10 + 5O2 + xN2 (0≤x≤10), 0.5C6H14 + 5O2 + xN2 (0≤x≤3), and C3H8/C4H10 + air. The discharge was ignited by a 60 ns electrical pulse with amplitude up to 70 kV. The energy deposition ranged from 70 mJ to 14 J. Electrical parameters of the discharge, ignition delay time, the flame front and the shock wave velocities were measured in the experiments. The minimum ignition energy for stoichiometric propane/butane-air mixture at 1 atm amounted to 14 J. Three modes of flame front propagation were observed under the experimental conditions: deflagration, transient detonation, and Chapman-Jouguet detonation. Efficiency of the non-equilibrium high-voltage nanosecond discharge as a detonation initiator was shown: the length of the deflagration-to-detonation transition (DDT) amounted to 130 mm in C3H8 + 5O2 mixture at initial pressure of 0.3 atm under initiation energy of 70 mJ and to 300 mm in 0.5C6H14 + 5O2 + 3N2 mixture at 1 atm under 3 J of initiation energy. The DDT time for both these cases amounted to 0.6 ms.

Periodic Pulse Discharge Self-focusing and Streamer-to-Spark Transition in Under-critical Electric Field

The different regimes of nanosecond pulse-periodic discharge development in a point-toplane geometry were investigated. The development of a discharge burst at a frequency of 1 kHz was investigated at nanosecond temporal resolution. Kinetic and gasdynamic effects that control the transition from streamer to spark discharge were demonstrated. It was shown that subsequent streamers have higher velocities and smaller channel diameters than the initial streamer. Thus, the self-focusing of the periodic discharge due to the inhomogeneous excitation and heating of the gas in the previous discharge channel has been demonstrated. Discharge transitions into a short-circuit mode cause an increased release of energy. This energy release leads to a sharp increase in the intensity of gasdynamic perturbations, effective mixing of the recombining channel’s plasma with the surrounding air, loss of the symmetry of the initial conditions and generation of multiple channels. This process leads to the self-restriction of the energy release during the high-current phase of the discharge because of an effective decrease of inhomogeneities in the heat and concentration. Optimal conditions of the discharge pulse development and nonequilibrium plasma formation were determined from the point of view of selective nonequilibrium gas excitation.

Non-equilibrium plasma ignition for internal combustion engines

High-voltage nanosecond gas discharge has been shown to be an efficient way to ignite ultra-lean fuel air mixtures in a bulk volume, thanks to its ability to produce both high temperature and radical concentration in a large discharge zone. Recently, a feasibility study has been carried out to study plasma-assisted ignition under high-pressure high-temperature conditions similar to those inside an internal combustion engine. Ignition delay times were measured during the tests, and were shown to be decreasing under high-voltage plasma excitation. The discharge allowed instant control of ignition, and specific electrode geometry designs enabled volumetric ignition even at high-pressure conditions.

Plasmatrons Powered By Pulsed High-Voltage Nanosecond Discharge For Ultra-Lean Flames Stabilization

A study of pulsed high–voltage nanosecond discharge development in a series of plasmatrons has been conducted. The discharge exhibited three modes of development depending on frequency, voltage and mass flow rate: surface streamer, localized spark, and distributed nonequilibrium transient spark. The current study focuses on the conditions of mode switching and the suggested mechanism for this phenomena. The developed plasmatrons have been used to stabilize ultra–lean (ER>0.06) flames in a wide range of equivalence ratios and temperatures for methane and diesel vapour at 1 bar. The optimal configurations and discharge parameters for flame stabilization at these conditions have been found experimentally. The plasmatrons demonstrate exceptional flame stability with an average discharge power less than 20 W for a total power of the burner higher than 1 kW for ultra–lean flame conditions.

Gradient Mechanism of Detonation Initiation for PDE Applications

An experimental study of detonation initiation by high–voltage nanosecond gas discharges has been performed in a smooth detonation tube with four–cell discharge chamber designed to realize a gradient initiation mechanism. The chambers were constructed on the basis of our previous studies and introduced analogous cell geometries. The discharge study performed in various chambers has shown that three modes of discharge development are realized under the experimental conditions: a spark mode with high–temperature channel formation, a streamer mode with non–uniform gas excitation, and a transient mode. The mechanisms of deflagration to detonation transition (DDT) under different discharge modes have been proposed and confirmed experimentally. Under spark and transient initiation, simultaneous ignition inside the discharge channel occurs, forming a shock wave and leading to a conventional deflagration to detonation transition (DDT) via an adiabatic explosion. The governing parameters have been established and a significantly higher efficiency in terms of detonation initiation has been achieved due to the enhanced geometry. Successful DDTs have been observed in a stoichiometric propane–oxygen mixture diluted with 40% of nitrogen under energy inputs as low as 200 mJ at initial pressures of 0.8 bar and higher. The run–up distance is within 80 mm, the DDT time is below 0.5 ms. A technique for detonation initiation in fuel–air mixtures in smooth detonation tubes can now be elaborated.

Streamer Breakdown Development in Undercritical Electric Field

The different regimes of nanosecond pulsed-periodic discharge development in a point-to-plane geometry are investigated. The development of a discharge burst at a frequency of 1 kHz is investigated with nanosecond temporal resolution. Kinetic and gasdynamic effects that control the transition from streamer to spark discharge are demonstrated. It was shown that subsequent streamers have higher velocities and smaller channel diameters than the initial streamer. Thus, the self-focusing of the periodic discharge due to the inhomogeneous excitation and heating of the gas in the previous discharge channel has been demonstrated. Discharge transitions into a short-circuit mode cause an increased release of energy. This energy release leads to a sharp increase in the intensity of gasdynamic perturbations, effective mixing of the recombining channels plasma with the surrounding air, loss of the symmetry of the initial conditions, and generation of multiple channels. This process leads to the self-restriction of the energy release during the high-current phase of the discharge because of an effective decrease of inhomogeneities in the heat and concentration.

On-Board Plasma Assisted Fuel Reforming

It is well known that the addition of gaseous fuels to the intake manifold of diesel engines can have significant benefits in terms of both reducing emissions of hazardous gases and soot and improving fuel economy. Particularly, the addition of LPG has been investigated in numerous studies. Drawbacks, however, of such dual fuel strategies can be found in storage complexity and end-user inconvenience. It is for this reason that on-board refining of a single fuel (for example, diesel) could be an interesting alternative. A second-generation fuel reformer has been engineered and successfully tested. The reformer can work with both gaseous and liquid fuels and by means of partial oxidation of a rich fuel-air mix, converts these into syngas: a mixture of H2 and CO. The process occurs as partial oxidation takes place in an adiabatic ceramic reaction chamber. High efficiency is ensured by the high temperature inside the chamber due to heat release. Thus, efficient thermal insulation is crucial to maintain said temperature. Heat recuperation from the reformer exhaust also improves the thermal efficiency. The prototype yields up to 20% of H2 (80% of the theoretical maximum) and 22% of CO with all kinds of fuels tested, including automotive diesel fuel. Efficient thermal insulation allows to keep the dimensions below 40 cm in any direction for a full burning power of 10-30 kW while outer wall of the reformer is exposed to air at normal temperature.