A numerical study of high-pressure non-equilibrium streamers for combustion ignition application

Generation and propagation of the primary and secondary streamers, initiated by a positive pulsed surface dielectric barrier discharge under different pulse amplitude and different dielectric permittivity, are studied via experimental and numerical simulation. Species (positive ions, negative ions and oxygen atoms) generation and distribution in the primary and secondary streamers are discussed. Experimental and numerical results are qualitatively coincident in discharge morphology, voltage–current characteristics and deposited energy. From visual forms of the discharge, both the primary and secondary streamers are initiated from the edge of the high-voltage (HV) electrode and propagate along the dielectric barrier surface. However, the numerical results show that the primary streamer is not completely attached to the dielectric barrier surface, and the distance between the primary streamer head and the surface is the largest, which is about several microns. Moreover, the secondary streamer starts about tens of microns away from the HV electrode edge. Furthermore, when the secondary streamer occurs, the trailing phenomenon of the primary streamer can still be observed, which is sustained by the accumulated positive charge.

The presence of water molecules in air introduces complexities to residual charge transports and energy relaxations that may provoke streamer discharge instabilities under repetitive pulses. Evolutions of pulse-periodic positive streamer dynamics were investigated in humid air. Pulse-sequence and temporally resolved diagnostics were implemented to capture discharge evolutions. The streamer development and evolutions of charged species in humid air are qualitatively analyzed based on a 2D–0D combined simulation. Evolution features of streamer behaviors in humid air include the faster filamentation of the primary streamer, pronounced propagation selectivity to previous secondary streamers, more branches of both primary and secondary streamers, and accelerated secondary streamer into stagnation. The repetitively pulsed breakdown is proceeded by the progressive axial prolongation of secondary streamers with bright heads and faint tails. Nonintuitively, the withstanding capability does not illustrate dramatic differences in dry air and humid air especially at high pulse repetition frequency. High-density residing hydrated ions with high electron bound energies (impeding streamer propagation) and the enhanced thermal release to cause higher reduced electric field (facilitating streamer propagation) may contrarily affect evolutions and discharge instability developments of positive streamer in humid air.

Understanding the rapid dynamics of the primary streamer is crucial for comprehending the nanosecond pulsed discharge process. To reveal the fast primary streamer process, this study introduces a newly developed quadruple emICCD camera system capable of capturing a sequence of four discharge images in single pulse, coupled with self-customized software for data analysis. A nanosecond pulse power with its FWHM of 10.5 ns was applied to a coaxial reactor, focusing on the dynamics of the primary streamer. Our research clarifies the spatiotemporal variations of the primary streamer’s properties and examines their relation with inner electrode diameter (i.d. 0.2–2.0 mm). Results showed that in a pulse-powered coaxial electrode, there are three stages in the primary streamer process and that i.d. serves as an important factor influencing the formation and propagation of streamers. Interestingly, we found that streamer head velocity, streamer width, and streamer area for individual streamers remain constant prior to streamer channels reaching the outer electrode. Furthermore, we also observed an initial increase followed by a decrease in both streamer head velocity and streamer width with increasing i.d values. This study sheds light on the fundamental properties of the primary streamer during nanosecond pulsed discharge, contributing valuable insights for future plasma applications.

In pre-ionized pulsed streamer discharge, regulating the seed-electron density and its spatial distribution during the pre-ionization stage provides an effective approach to control streamer development and plasma chemical activity. Using electrical measurements, optical diagnostics, and a two-dimensional axisymmetric fluid model, this study systematically analyzes the behavior of pulsed streamer discharges under seed-electron pre-ionization and quantitatively clarifies how seed-electron density and spatial distribution govern the evolution of the primary streamer (PS), the secondary streamer (SS), as well as the generation of reactive species. The results show that UV pre-ionization enhances the discharge intensity and promotes streamer development. Under the same experimental conditions, simulations indicate that the minimum seed-electron densities required to initiate the development of PS and SS are 10 10 m -3 and 10 12 m -3 , respectively. When the seed-electron density exceeds 10 10 m -3 , PS development is enhanced, leading to increased formation of N 2 (C), N 2 + and O 2 + . Once the density exceeds 10 12 m -3 , SS development is accelerated, and the generation of N and O increases markedly. The spatial distribution of seed electrons is also critical to discharge development. Placing electrons near the anode region accelerates breakdown and promotes PS development, whereas electrons near the cathode region favor SS development, intensifies the radial electric field and electron density near the cathode region, and ultimately leads to a large discharge volume. These findings clarify how seed electrons regulate pulsed streamers and provide guidance for tailoring pre-ionization strategies to optimize plasma reactivity.

The earlier experimental setup for the study of a multi-streamer single pulse sliding discharge was supplemented by a photoelectric system for recording streamer radiation with a time resolution of 1.5 ns. Measurements of streamer delay and propagation velocity in Ne of atmospheric pressure at two supply voltage polarities were performed. The results showed that during each discharge pulse, regardless of its polarity, streamers of two types propagate across the gap in opposite directions. The first ones, primary streamers, start from a high-voltage electrode and move to a grounded electrode. The other type of streamers, reverse streamers, propagate in the opposite direction from a grounded electrode only after primary streamers have reached it. The experiments were carried out using voltage amplitude of 3 and 3.5 kV applied to plane-parallel electrodes on an alumina ceramic substrate with a rectangular discharge area of 9.5×11.6 cm2 in size. The total delay time range of the streamers was up to 2500–2700 ns and their motion velocity up to (3.5−5.6)×107 cm/s, depending on the discharge voltage and polarity. The dependence of delay time and velocity of the streamers' head motion on their position in the gap is presented. A qualitative explanation of the primary streamer characteristics is proposed, while the obtained experimental data are not sufficient for a similar explanation with respect to reverse streamers. The presented results are significant for studying the physical mechanisms of counterpropagating surface streamer/ionization wave dynamics, while the experimental methodology allows recording their spatiotemporal evolution in the gap.

The non-thermal, chemically active plasma is one of the promising technologies for the removal of the hazardous environmental pollutants. Pulsed streamer discharges in atmospheric pressure gases have received a great deal of interest for many years. It is very important to know the propagation mechanism of streamer discharges in order to improve the energy efficiency of pulsed discharge systems. In the present work, a Blumlein generator that has a pulse width of 100 ns was used to investigate the streamer discharge propagation. The propagation images in a coaxial reactor were taken by a streak camera for both positive and negative polarities of the applied voltage. From the results, primary and secondary streamers were observed propagating from the inner wire electrode to the outer cylinder electrode, and the maximum propagation velocity of the streamer head were 0.6 - 1.2 mm/ns in the range of 43 - 93 kV of the absolute value of the applied voltage. The propagation velocity of the positive streamer was faster than that of the negative polarity. The electric field at the wire electrode for streamer onset was 12 MV/m for the positive streamer and 20 MV/m for the negative streamer.

DUE TO COPYRIGHT RESTRICTIONS ONLY AVAILABLE FOR CONSULTATION AT ASTON UNIVERSITY LIBRARY AND INFORMATION SERVICES WITH PRIOR ARRANGEMENT Far-reaching similarities between the mechanism of electrical breakdown in the three insulating media - gaseous, liquid and solid - have been found , In all media, discharge channels of a similar type, generally referred to as streamers, are formed which differ for each electrode configuration. In a divergent field of a point-plane electrode system, streamers are generated in the vicinity of the point-electrode, They are subsequently promoted by direct ionization and photoelectric ionization processes in the direction of the plane electrode. The charge carriers remaining behind in the streamer tail, due to their ionized state and space charge, affect the subsequent processes. A model for streamer propagation in gaseous dielectrics i s presented and its relationship to the characteristics of discharge growth is discussed. The initiation phase is considered first , leading to primary, secondary and return streamers, and finally to possible transition to arc - over. It is suggested that the same pattern of events applies to electrical breakdown of liquids and solids based on the many points of similarity observed in the growth of discharge in the three media.

To describe the beginning of the evolution of the cathode-directed streamer channel after it has bridged the discharge gap, a simplified model is considered, which considers ionization, heating, and heat removal through thermal conductivity. The spatial picture obtained from it and the estimates of the values of the process characteristics correspond to those observed in secondary streamers. Factors are indicated that practically make it impossible to stop the propagation of the secondary streamer in the part of the channel nearer to the cathode.

Streamer properties such as their velocity, diameter, intensity and density, can be obtained by analysis of temporal and spatial resolved ICCD imaging. In this paper, experimental results on streamer generation and propagation as a function of several high-voltage pulse and reactor parameters are described. Experiments were performed on a large scale wire–plate reactor in ambient air. The set-up allows for independent variation of the parameters over wide ranges. The minimum gate time of the ICCD camera is 5 ns, allowing for a high temporal resolution. The camera can be triggered with a precision of 1 ns. Both negative and positive polarity pulses are investigated. The most important conclusions are as follows. (1) The streamer velocity ((0.5–2.5) × 106 m s−1) increases if the applied electric field and/or the voltage rise rate is increased. (2) The same is true regarding the velocity ((0.2–1.2) × 105 m s−1) with which the streamer diameter (0.7–3.0 mm) increases during propagation. (3) Typical properties (velocity, diameter, etc) of negative and positive polarity streamers vary less than 25%, especially when the applied electric field is high. (4) As long as the dc bias voltage is below the dc corona onset value it does not have a separate effect on the visual streamer properties. Only the total voltage (peak voltage + dc) is of importance. (5) A simple model was used to determine the electric field in the secondary streamer channel. It was found that in the light emitting part of the secondary streamer the electric field is approximately 21.5 kV cm−1. In the remainder (dark part) of the channel the electric field is around 6.5 kV cm−1. This paper shows mainly experimental findings. Not all observed relations and phenomena could be explained. This is partly caused by the fact that current theoretical and numerical models are not yet able to describe the experimental situation as used during this study.

The paper presents results of singlet oxygen generation experiments in a high-pressure, non-selfsustained crossed discharge. The discharge consists of high-voltage, short pulse duration, high repetition rate pulsed discharge, which produces ionization in the flow, and a low-voltage DC discharge which sustains current in a decaying plasma between the pulses. The sustainer voltage can be independently varied to maximize the energy input into electron impact excitation of singlet delta oxygen. The results demonstrate operation of a stable and diffuse crossed discharge in O2-He mixtures at static pressures of at least up to P0=380 torr and sustainer discharge powers of at least up to 1200 W, achieved at P0=120 torr. The reduced electric field in the positive column of the sustainer discharge varies from E/N=0.3·10 -16 V·cm 2 to E/N=0.65·10 -16 V·cm 2 , which is significantly lower than E/N in self-sustained discharges and close to the theoretically predicted optimum value for O2(a 1 ∆) excitation. Measurements of visible emission spectra O2(b 1 Σ→X 3 Σ) in the discharge afterglow show the O2(b 1 Σ) concentration to increase with the sustainer discharge power and to decrease as the O2 fraction in the flow is increased. Rotational temperatures inferred from these spectra in 10% O2 – 90% He flows at P0=120 torr and mass flow rates of m& =0.73 g/sec to 2.2 g/sec are 365 K to 465 K. Singlet delta oxygen yield at these conditions, 1.7% to 4.4%, was inferred from the integrated intensity of the (0,0) band of the O2(a 1 ∆→X 3 Σ) infrared emission spectra calibrated using a blackbody source. The yield remains nearly constant in the discharge afterglow, up to at least 15 cm distance from the discharge. Kinetic modeling calculations using a quasi-one-dimensional nonequilibrium pulser-sustainer discharge model coupled with the Boltzmann equation for plasma electrons predict gas temperature rise in the discharge in satisfactory agreement with the experimental measurements. However, the model overpredicts the O2(a 1 ∆) yield by a factor of 2-2.5, which suggests that the model’s description of nonequilibrium O2-He plasma kinetics at high pressures is not quite adequate.

Plasma jet normally operates in a regime of positive or negative streamers. Different propagating behaviors have been investigated for positive streamers. However, there are few studies on negative streamers. In this paper, a two-dimensional fluid model is used to simulate the behavior of a negative streamer encountered with a cloud of positive ions. Simulation results indicate that the negative streamer behaves like a free streamer when the ion cloud has a low charge density. After initiation near the needle tip, the streamer propagates along the axis, passes through the ion cloud, and finally reaches the anode vicinity along a straight line. When the streamer reaches the ion cloud, its head radius (R0) is investigated as a function of parameters, including the voltage amplitude and the gap width, as well as the charge density, axial position, and spatial size of the ion cloud. At a given moment, electric field strength and electron density near the streamer head increase with increasing charge density of the ion cloud. In the case of a high charge-density ion cloud, after a primary streamer propagates some distance along the axis, a positive secondary streamer, and a negative secondary streamer start to propagate from the cloud to the cathode and the anode, respectively.

The discharge and surface charge dynamics, as well as the local streamer-to-filament transition of a three-electrode surface discharge under repetitive nanosecond pulses with a frequency of 1 kHz at atmospheric pressure, are studied through experiments and simulations. Evolutions of plasma morphology and electric field vector, as obtained by an ICCD camera and an improved E-FISH method, respectively, are utilized to analyze discharge dynamics. A 2D fluid model combined with a 0D kinetic model is established to study the accumulation behavior of surface charge and mechanism of the local streamer-to-filament transition. The simulation results demonstrate a qualitative agreement with the experimental measurements on the discharge evolution, waveforms of discharge current and key features of the electric field. The results show that a three-electrode surface discharge includes three discharge phases: the primary streamer, local enhanced discharge and reverse breakdown. The local enhanced discharge occurs near the high-voltage (HV) electrode, characterized by a local streamer-to-filament transition and subsequent emission belt parallel to the edge of HV electrode, after the primary streamer bridges the two electrodes. The local streamer-to-filament transition is attributed to the local accumulation of active species with a lower threshold energy in the high field region (~ 25 kV/cm) similar to that of the secondary streamer in pin-plane discharges. The emission belt is formed by the high-density charge spot at the filament head, a phenomenon attributable to charge migration under the influence of an applied electric field. The spatial non-uniformity of plasma channel is a general feature in non-uniform field discharges and is a key process that induces the discharge mode transition.

Two electrodes in one are obtained based on a bottom-up approach leading to a fully integrated microelectrochemical cell with independently addressable coaxial electrodes, as described by A. Kuhn and co-workers in article 1500192. The inherent macroporosity ensures tunable high surface areas for both cylindrical electrodes, thus allowing efficient transport and conversion of electroactive molecules. Such coaxial architectures are desirable for miniaturized electrochemical devices such as (bio)fuel cells, supercapacitors or biosensors.

In this paper, Schlieren photography technique has been applied for the visualization of the pressure field of a single-shot underwater pulsed discharge. A needle-to-plane electrode configuration submerged in distilled water was used. The detailed time- and space-resolved images of both streamers and pressure waves were captured. As a result, several phenomena, such as the phase change prior to the initiation of the discharge, primary and secondary streamers propagation, shock wave generation, and the bubble formation, were observed. From these observations, a scenario of multiple events from prebreakdown to post-discharge was proposed. The gas bubbles generated in discharge were used to visualize the electrohydrodynamic flow induced by the discharge.

Summary: The primary streamer propagation is presented under a high over voltage biexponential pulse. The positive corona current pulse and the streak emission in the visible band are compared with classical ones obtained under continuous voltages. A cathode spot formation is observed and secondary emission processes are expected to play a major role in the development of this zone. The mean velocity of primary streamers increases with the order of corona under a Heaviside voltage pulse. This increase may be due to neutral depopulation of the canal.Photograph of corona under an applied voltage.magnified imagePhotograph of corona under an applied voltage.