Sergey B. Leonov

Near-Surface Electrical Discharge in Supersonic Airflow : Properties and Flow Control

The results of experimental and numerical investigations of the interaction between the near-wall electrical discharge and supersonic airflow in an aerodynamic channel with constant and variable cross sections are presented. Peculiar properties of the surface quasi-direct-current discharge generation in the flow are described. The mode with flow separation developing outside the discharge region is revealed as a specific feature of such a configuration. An interaction model is proposed on the basis of measurements and observations. A regime of gas-dynamic screening of a mechanical obstacle installed on the channel wall is studied. Variation of the main flow parameters caused by the surface discharge is quantified. The ability of the discharge to shift an oblique shock in an inlet is demonstrated experimentally. The influence of relaxation processes in nonequilibrium excited gas on the flow structure is analyzed. Comparison of the experimental data with the results of calculations based on the analytical model and on numerical simulations is presented.

Plasma-induced ignition and plasma-assisted combustion in high-speed flow

The suitability of the electrical discharge technique for application in plasma-induced ignition and plasma-assisted combustion in high-speed flow is reviewed. Nonequilibrium, unsteady and nonuniform modes are under analysis to demonstrate the advantage of such a technique over heating. A reduction in the required power deposition is possible due to unsteady operation and non-homogeneous spatial distribution. Mixing intensification in non-premixed flow could be achieved by nonuniform electrical discharges. The scheme of fuel ignition behind the wallstep and in the cavity is under consideration. Experimental results on multi-electrode discharge maintenance in the separation zone of supersonic flow are presented. The model test on hydrogen and ethylene ignition is demonstrated at direct fuel injection. An energetic threshold of fuel ignition under separation and in the shear layer is measured under the experimental conditions.

Plasma-Assisted Combustion of Gaseous Fuel in Supersonic Duct

The field of plasma-induced ignition and plasma-assisted combustion in high-speed flow is under consideration. Nonequilibrium, unsteady, and nonuniform modes are analyzed as the most promising in reducing a required extra power. Numerical simulations of uniform, nonequilibrium, continuous, and pulse discharge effect on the premixed hydrogen and ethylene-air mixtures in supersonic flow demonstrate an advantage of such a technique over heating. At the same time, the energetic price occurs rather large to be scheme practical. A reduction of the required power deposition and mixing intensification in nonpremixed flow could be achieved by nonuniform electrical discharges. Experimental results on multielectrode discharge maintenance behind wallstep and in the cavity of supersonic flow are presented. The model test on hydrogen and ethylene ignition is demonstrated at direct fuel injection

Mechanisms of Flow Control by Near-Surface Electrical Discharge Generation

The paper is devoted to the problem of flow management by the discharge plasma excited inflow mainly. Discharge plasma technology promises the advantages in a field of the boundary layer control, including stabilization of line of laminar-turbulent transition, guiding of separation processes, control of the local shocks position, and others. This paper presents and discusses results of recent experimental and analytic study of the problem in this field of interest. Two items are described here specifically: the effect of plasma-induced separation in high-speed flow and the boundary layer modification by HF DBD technique.

Effect of Electrical Discharge on Separation Processes and Shocks Position in Supersonic Airflow.

The paper analyses and discusses the results of resent experimental works on influence of non-uniform plasma formation on characteristics of near-wall layers and separated areas in high-speed airflow. Fresh experimental results are being presented, which are obtained at condition of small-scale model duct. Two different aerodynamic situations are investigated: surface electrical discharge in a free stream and surface discharge in separated zone behind the wall step. The discharge properties and structure are studied as well as the change of the gasdynamic structure of the flow due to electric discharge excitation. Images of the shocks interaction with the plasma overlayer observed by the Schlieren method are shown. The effect of the flow instability damping by the plasma layer is demonstrated. Application of the surface type of electric discharge for the flow modification in ducts, inlets and combustors is discussed. The work has been fulfilled in frames of Advance Flow/Flight Control (AFFC) concept.

Dynamics of energy coupling and thermalization in barrier discharges over dielectric and weakly conducting surfaces on µs to ms time scales

The paper presents experiments characterizing discharge development and energy coupling in a surface dielectric barrier discharge (SDBD), atmospheric air plasmas over dielectric and weakly conducting surfaces, over a wide range of time scales and electrical conductivities. The experiments are done using nanosecond pulse (NS) both single polarity and alternating polarity) and ac voltage waveforms. Discharge development and mechanisms of coupling with quiescent air are analysed using nanosecond gate camera imaging, schlieren imaging, and laser differential interferometry. It is shown that NS SDBD plasmas generate stochastic, localized, near-surface perturbations on a long time scale (>100 μs) after the discharge pulse. These perturbations, entirely different from compression waves generated on a short time scale (~1–10 μs), are caused by discharge contraction and originate from the ends of the filaments. Surface conductivity has almost no effect on discharge behaviour if RC time of the conducting surface layer is much longer compared to the characteristic time of NS or ac voltage waveforms. In the opposite limit of short RC time, the conducting layer acts as an extension of the high-voltage electrode. Discharge contraction significantly increases energy stored on the dielectric surface, which in this case exceeds energy dissipated as Joule heat. The stored energy is dissipated if the discharge pulse is followed by an opposite polarity pulse. In a single polarity discharge, on the other hand, surface charge accumulation limits energy coupled to the plasma by subsequent pulses. The results demonstrate that surface plasma actuator control authority may be significantly increased by using an alternating polarity pulse waveform, which is more effective than the removal of surface charge between the pulses using a weakly conducting surface.

The Effect of Plasma Induced Separation

This paper describes the results of experimental work for surface electrical discharge influence on separation processes in high-speed flow. It could be related to the fields of Weakly Ionized Plasma and Flow Control. Different aerodynamic situations are explored: plane wall, profiled plate, and backstep. In the first case the effect of plasma-induced separation is demonstrated. Energetic threshold is measured. In the second case a stabilization of the separation line was found as well as screening effect. In configuration with backstep the flow transformation to separation-less mode has been obtained.

Time-resolved measurements of plasma-induced momentum in air and nitrogen under dielectric barrier discharge actuation

There has been much recent interest in boundary layer (BL) actuation by offset surface dielectric barrier discharges (SDBD). These discharges either act directly on the gas momentum through the mechanism of charge separation or they increase the flow stability through the creation of disturbances to the BL at a particular frequency. The objective of the work reported here is to clarify the physical mechanism of plasma-flow interaction. Two problems are considered in detail: the exact spatial/temporal distribution of the plasma-related force, and the specific role of negative ions in the net force budget. The experiments were made with an offset electrode configuration of SDBD at voltage amplitude U≤12 kV and frequency f=0.02–2 kHz. The main data were obtained by time-resolved Pitot tube pressure measurements in air and nitrogen at atmospheric pressure. Three main features of SDBD behavior were considered. First, the strong inhomogeneity in the spatial distribution of the plasma-induced flow were detected....

Experiments on Electrically Controlled Flameholding on a Plane Wall in a Supersonic Airflow

We describe experiments on gaseous fuel ignition and flameholding controlled by an electrical discharge in high-speed airflow. The geometrical configuration does not include any mechanical or physical flameholder. The fuel is nonpremixed and injected directly into the air crossflow from the combustor bottom wall. A multi-electrode, nonuniform transversal electrical discharge is excited, also on the bottom wall, between flush-mounted electrodes. The initial gas temperature is lower than the value for autoignition of hydrogen and ethylene. Results are presented for a wide range of fuel mass flow rate and discharge power deposited into the flow. This coupling between the discharge and the flow presents a new type of flameholder over a plane wall for a high-speed combustor.

High-Speed Inlet Customization by Surface Electrical Discharge

Experimental and analytical study of near-surface transversal quasi-DC discharge effect on high-speed flow structure has been carried out for a model of supersonic inlet. Two main topics are discussed: the different discharge modes characteristics and the discharge influence on extrusive layer configuration downstream of the power deposition zone. The discharge properties control by external magnetic field has been demonstrated experimentally. Vibrationally non-equilibrium plasma effect on shock structure has been demonstrated by experimental data comparison for air (long relaxation time), carbon dioxide (short relaxation time), and argon (no vibrational excitation).