Andrey Nikipelov

SDBD plasma actuator with nanosecond pulse-periodic discharge

This paper presents a detailed explanation of the physical mechanism of the nanosecond pulsed surface dielectric barrier discharge (SDBD) effect on the flow. Actuator-induced gas velocities show near-zero values for nanosecond pulses. The measurements performed show overheating in the discharge region on fast (? 1??s) thermalization of the plasma input energy. The mean values of such heating of the plasma layer can reach 70?K, 200?K and even 400?K for 7?ns, 12?ns and 50?ns pulse durations, respectively. The emerging shock wave together with the secondary vortex flows disturbs the main flow. The resulting pulsed-periodic disturbance causes an efficient transversal momentum transfer into the boundary layer and further flow attachment to the airfoil surface. Thus, for periodic pulsed nanosecond dielectric barrier discharge, the main mechanism of impact is the energy transfer and heating of the near-surface gas layer. The following pulse-periodic vortex movement stimulates redistribution of the main flow momentum.

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.

Experimental Study and Numerical Simulation of Flow Separation Control with Pulsed Nanosecond Discharge Actuator

Active flow separation control with a nanosecond pulse plasma actuator, which is essentially a simple electrode system on the surface of an airfoil, introducing lowenergy gas discharge into the boundary layer, with little extra weight and no mechanical parts, was performed in wind-tunnel experiments on various airfoil models. In stall conditions the significant lift increase up to 30% accompanied by drag reduction (up to 3 times) was observed. The critical angle of attack shifted up to 5–7 degrees. Schlieren imaging show the shock wave propagation and formation of large-scale vortex structure in the separation zone, which led to separation elimination. The experimental work is supported by numerical simulations of the phenomena. The formation of vortex similar to that observed in experiments was simulated in the case of laminar leading edge separation. Model simulations of free shear layer show intensification of shear layer instabilities due to shock wave to shear layer interaction. The mechanism of flow control by nanosecond plasma discharge is based on extra vorticity created by the shock wave, which is produced from the layer of the hot gas. This hot gas in generated during the fast thermalisation process, in which up to 60% of the discharge energy is converted to heat in less than 1 µs [1]. This phenomenon gives an opportunity for nanosecond discharge actuator to be effective at high velocities [2, 3]. The current work continues studying the performance of nanosecond plasma actuator. A series of wind tunnel experiments was carried out with different actuator layouts at flow velocities up 80 m/s at various airfoils with chords up to 1.5 m and spans up to 5 m. A numerical model was developed to prove the shock wave mechanism of actuator operation. 2. Experiment In the present work, a linear actuator was used [4]. The actuator consisted of a base layer of insulator attached onto the surface of the airfoil, a covered electrode, an interelectrode layer of insulation and an exposed electrode. In the majority of the cases, exposed electrode was ground, and the high-voltage electrode was covered one. High-voltage nanosecond pulses were provided by three different nanosecond pulsers, which were capable of producing pulses of up to 50 kV with rising time of 3-15 ns and duration from 10 to 50 ns at repetition frequencies up to 10 kHz. Low-speed experiments was carried out in open jet wind tunnel using the NACA0015 airfoil with the chord of 20 cm and span about 75 cm. The tunnel was equipped with an

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.

Compact Catalyst-Free Liquid Fuel to Syngas Reformer with Plasma-Assisted Flame Stabilization

The paper presents the results of development of a compact prototype of a catalyst–free liquid fuel reformer with 10 kW of full burning power. The produced syngas contains 20% of H2 and 22% of CO with both gasoline and diesel fuel. This hydrogen yield corresponds to 80% of the theoretical maximum. Electrical discharge in the form of repetitive nanosecond sparks is used to stabilize combustion in thansient modes, including cold start–up. A self–cleaning mode is implemented to reduce soot emissions. The results show that plasma stabilization of the reforming process is required for cold start of the reformer, for lean flame stabilization during the cleaning phase, and during normal operation with rich mixture to initiate the first, oxygen–fuel stage of the conversion process. The second stage of the fuel conversion via partial oxidation is the reaction of water vapour with hydrocarbons. During this stage, the plasma–assisted oxidation becomes inefficient due to non–chain mechanism of the process. Thus, the thermal mechanism of chemical processes becomes most important and the high hydrogen yield is due to optimal burner and reaction chamber geometry and to efficient thermal insulation.

Low-Temperature Plasma Chemistry and Plasma Assisted Partial Oxidation

In this paper a study of plasma-assisted combustion and fuel reforming was performed. A plasmatron based on nanosecond high-voltage discharge was constructed for application in down-scaled reformer. Investigations were performed in a wide range of discharge parameters and for different mixture composition while studying propane-air conversion to syngas. Measurements of hydrogen yield in different regimes were performed. Dependencies of hydrogen concentration on such parameters as equivalence ratio, discharge frequency and amplitude were found. A plasmatron based on nanosecond high-voltage discharge was constructed for igniting and flame stabilizing of propane-air mixture in reformer. Also a study of strimer action on preheated propane-air mixture was performed via LIF to enhance our knowledge of nanosecond voltage pulse discharge driven ignition and flame stabilization. It was shown that in the channel of positive streamer propagating through combustible mixture of methane and air of atmospherical pressure hydroxyl radicals are produced. Absolute OH concentration value over the time after the discharge profiles have been obtained with resolution in time being up to 100 ns.