Repetitive Nanosecond Pulses Research Articles

Cumulative effect of successive nanosecond repetitively pulsed discharges on the ignition of lean mixtures

This work aims to provide a better understanding of the cumulative effect of successive nanosecond repetitively pulsed (NRP) discharges on the ignition process. Fast chemiluminescence imaging of both the post-discharge and the following flame was used to analyze the ignition of a lean propane/air mixture (ϕ = 0.7) by a train of 82 NRP discharges at a pulse repetition frequency (PRF) of 30 kHz, in comparison with the traditional spark ignition. The transition from the non-equilibrium plasma to the ignition kernel has been imaged. It has been found that, differently from traditional spark ignition and despite similar experimental conditions, each NRP ignition develops in a unique way. NRP discharges generate both highly reactive species and thermal instabilities in the gap: the multi-pulse strategy produce a gas motion resulting in a jetting phenomenon, reported here for the first time. An algorithm within ImageJ software was used to quantitatively describe the observed jetting phenomenon. The results presented in this work suggest that jetting is the driver of the ignition initiated by NRP discharges, at least in the experimental conditions investigated here.

Measurements and kinetic modeling of atomic species in fuel-oxidizer mixtures excited by a repetitive nanosecond pulse discharge

This work presents the results of number density measurements of metastable Ar atoms and ground state H atoms in diluted mixtures of H2 and O2 with Ar, as well as ground state O atoms in diluted H2–O2–Ar, CH4–O2–Ar, C3H8–O2–Ar, and C2H4–O2–Ar mixtures excited by a repetitive nanosecond pulse discharge. The measurements have been made in a nanosecond pulse, double dielectric barrier discharge plasma sustained in a flow reactor between two plane electrodes encapsulated within dielectric material, at an initial temperature of 500 K and pressures ranging from 300 Torr to 700 Torr. Metastable Ar atom number density distribution in the afterglow is measured by tunable diode laser absorption spectroscopy, and used to characterize plasma uniformity. Temperature rise in the reacting flow is measured by Rayleigh scattering. H atom and O atom number densities are measured by two-photon absorption laser induced fluorescence. The results are compared with kinetic model predictions, showing good agreement, with the exception of extremely lean mixtures. O atoms and H atoms in the plasma are produced mainly during quenching of electronically excited Ar atoms generated by electron impact. In H2–Ar and O2–Ar mixtures, the atoms decay by three-body recombination. In H2–O2–Ar, CH4–O2–Ar, and C3H8–O2–Ar mixtures, O atoms decay in a reaction with OH, generated during H atom reaction with HO2, with the latter produced by three-body H atom recombination with O2. The net process of O atom decay is O + H → OH, such that the decay rate is controlled by the amount of H atoms produced in the discharge. In extra lean mixtures of propane and ethylene with O2–Ar the model underpredicts the O atom decay rate. At these conditions, when fuel is completely oxidized by the end of the discharge burst, the net process of O atom decay, O + O → O2, becomes nearly independent of H atom number density. Lack of agreement with the data at these conditions is likely due to diffusion of H atoms from the partially oxidized regions near the side walls of the reactor into the plasma. Although significant fractions of hydrogen and hydrocarbon fuels are oxidized by O atoms produced in the plasma, chain branching remains a minor effect at these relatively low temperature conditions.

Influence of residual charge on repetitively nanosecond pulsed dielectric barrier discharges in atmospheric air

The plane-to-plane dielectric barrier discharge within 5 mm air gap driven by repetitive nanosecond pulses is studied. A water resistance (WR) is connected in parallel with the discharge circuit. For the discharge without the WR, two reverse discharges occur in the falling front of the voltage pulse, and besides, the primary discharge occurs in the rising front. For the discharge with the WR, only the primary discharge takes place, and the voltage waveform is changed into the unipolar positive voltage pulse with the elevated fall time. Additionally, the discharge with the WR displays the intermediate between the traditional diffuse and filamentary modes. Moreover, the uniformity of the discharge with the WR is increased as the pulse repetition frequency decreases from 1200 Hz to 100 Hz. The above observations in the discharge with the WR are mainly attributed to the increase in the residual charge at the inception of the following pulsed discharge.

High-spatial resolution measurements of NO density and temperature by Mid-IR QCLAS in open-air confined plasmas

Radial distributions of absolute nitric oxide (NO) density and gas temperature are measured in atmospheric confined plasmas using Mid-IR quantum cascade laser absorption spectroscopy (QCLAS). Two ro-vibrational transitions of the fundamental band are probed in the electronic ground state NO(X) at 1900.076 cm−1 and 1900.517 cm−1, respectively. Plasmas are generated using nanosecond repetitively pulsed (NRP) discharges in air at atmospheric pressure. The spatial measurements are radially performed halfway between the discharge electrodes with a resolution down to 0.3 mm by the Abel inversion technique. The gas temperature is determined using two methods: (i) based on the ratio of the two ro-vibrational absorption lines and (ii) based on the collisional broadening line shape. The local NO density is obtained from local absorption coefficients and temperature dependent line strengths. The results were found in good agreement. The time averaged gas temperature and density at the discharge center are found at 800(±100) K and 2(±0.2) × 1015 cm−3, respectively. The FWHM of the NO density radial profile is found at 6–7 mm. This is large compared to the discharge channel width, i.e. typically below 0.5 mm, which is explained mainly by hot reactive jets induced in the post-discharge.

Investigation of Gas Heating by Nanosecond Repetitively Pulsed Glow Discharges Used for Actuation of a Laminar Methane-Air Flame

ABSTRACTThis article reports on the quantification of the heating induced by nanosecond repetitively pulsed (NRP) glow discharges on a lean premixed methane-air flame. The flame, obtained at room temperature and atmospheric pressure, has an M-shape morphology. The equivalence ratio is 0.95 and the thermal power released by the flame is 113 W. The NRP glow discharges are produced by high voltage pulses of 10 ns duration, 7 kV amplitude, applied at a repetition frequency of 10 kHz. The average power of the plasma, determined from current and voltage measurements, is 1 W, i.e., about 0.9% of the thermal power of the flame. Broadband vibrational coherent anti-Stokes Raman spectroscopy of nitrogen is used to determine the temperature of the flame with and without plasma enhancement. The temperature evolution in the flame area shows that the thermal impact of NRP glow discharges is in the uncertainty range of the technique, i.e., ±40 K.

Investigation on the Characteristics of Dielectric Barrier Surface Discharge Driven by Repetitive Nanosecond Pulses in Airflows

Recently, more and more attention has been drawn to nanosecond pulsed surface dielectric barrier discharge (NPSDBD) due to its potential applications in flow control. A study of the interaction between high-speed airflows and NPSDBD generated by different nanosecond waveforms is presented in this paper. The effects of airflows on the discharge characteristics, such as plasma morphology, discharge current, the inception voltage of discharge, are investigated at different pulse rise times and flow velocities. The results show that the NPSDBD sustained by pulses with a fast rise time ( $\sim 20$ ns) is quasi-uniform, and relatively stable in the airflows. While the NPSDBD driven by pulses with a rise time more than 50 ns, it seems easier to be influenced by a flowing air of 60 m/s, with a filamentary pattern and a typical discharge current, which has two or more current peaks in the pulse front. In the case of pulses with slow rise time, the inception voltage of discharge increases with the flow velocity, but it almost remains constant in the case of fast rise time. In addition, the deposited energy of discharge generated by the pulses with slow rise time increases slightly with the influence of flowing air. The emission spectrum of NPSDBD is also examined. The variation of the emission spectrum with pulse waveforms is basically consistent with the different discharge behaviors driven by different pulse waveforms.

The discharge characteristics of surface dielectric barrier discharge sustained by repetitive nanosecond pulses in open air

A nanosecond pulsed surface dielectric barrier discharge (NPSDBD) is a promising method for flow control and combustion. We systematically investigated the influence of pulse parameters on the discharge characteristics of NPSDBD, especially on the conduction current of discharge and the energy deposition curves. Meanwhile, the differences of the characteristics of the discharge generated by positive pulses and negative pulses are focused in this paper. The underlying physics is also discussed. Four different discharge regimes of NPSDBD are presented, which can be distinguished by the temporal emission behaviors of discharge and the conduction current of discharge. The transitions of four discharge regimes were also investigated by changing the pulse amplitude, repetitive rate, and voltage polarity. It was found that it is easier to translate quasi-uniform discharge to filamentary discharge or transition mode for the repetitive pulses with a negative polarity. A phenomenological model was proposed to explain the differences between a positive repetitive pulse discharge and a negative repetitive pulse discharge.

A 3-D DNS and experimental study of the effect of the recirculating flow pattern inside a reactive kernel produced by nanosecond plasma discharges in a methane-air mixture

This paper presents 3-D DNS and experimental schlieren results used to study a non-equilibrium plasma discharge in a lean methane-air mixture. A detailed combustion mechanism and a plasma model, developed in our previous work, are used in the 3-D computations in order to study the impact of gas flow recirculation on the temporal evolution of species and gas temperature in the vicinity of the discharge zone. The results show that the formation of a fresh gas counterflow, with stagnation plane at the centre of the discharge and parallel to the electrodes, changes the topology of the hot kernel from an initial cylindrical shape to a toroidal one. This phenomenon leads to an increase of the area/volume ratio of the reactive kernel, that may result under certain conditions in kernel extinction. The results also show the importance of considering this 3-D gas flow recirculation to correctly predict the temporal evolution of the hot kernel. In particular, we show that the temperature and species concentrations in the central region of the discharge return to fresh gas conditions shortly after the end of the pulse. In the experimental case investigated here, this time is of the order of 150 µs. This result is particularly important for ignition by Nanosecond Repetitively Pulsed (NRP) discharges, because the gas conditions at the beginning of each successive pulse depend strongly on the time interval between pulses, thus on the pulse frequency

Transfer functions of laminar premixed flames subjected to forcing by acoustic waves, AC electric fields, and non-thermal plasma discharges

The responses of laminar methane–air flames to forcing by acoustic waves, AC electric fields, and nanosecond repetitively pulsed (NRP) glow discharges are reported here. The experimental setup consists of an axisymmetric burner with a nozzle made from a quartz tube. Three different flame geometries have been studied: conical, M-shaped and V-shaped flames. A central stainless steel rod is used as a cathode for the electric field and plasma excitations. The acoustic forcing is obtained with a loudspeaker located at the bottom part of the burner. For forcing by AC electric fields, a metallic grid is placed above the rod and connected to an AC power supply. Plasma forcing is obtained by applying high-voltage pulses of 10-ns duration applied at 10 kHz, between the rod and an annular stainless steel ring, placed at the outlet of the quartz tube. The chemiluminescence of CH* is used to determine the heat release rate fluctuations. For forcing by acoustic waves and plasma, the geometry of the flame plays a key role in the response of the combustion, while the flame shape does not affect the response of the combustion to electric field forcing. The flame response to acoustic forcing of about 10% of the incoming flow is similar to those obtained in the literature. The flames are found to be responsive to an AC electric field across the whole range of frequencies studied. A forcing mechanism, based on the generation of ionic wind, is proposed. The gain of the transfer function obtained for plasma forcing is found to be up to 5 times higher than for acoustic forcing. A possible mechanism of plasma forcing is introduced.

Study on the characteristics of barrier free surface discharge driven by repetitive nanosecond pulses at atmospheric pressure

Nanosecond pulsed plasma has an enormous potential in many applications. In this paper, the characteristics of barrier free nanosecond pulsed surface discharge are investigated by the use of an actuator with a strip-strip film electrode configuration, including the effect of electrode width and the gap distance on the plasma morphology and electrical characteristics at atmospheric pressure. It was found that it is relative easier to generate a quasi uniform discharge with a thinner electrode width and a smaller gap distance. The underlying physical mechanism was also discussed. Besides that, the influence of airflow on repetitive pulsed surface discharge was examined. By comparing to the discharge produced by two different pulse waveforms in airflows, we found that the discharge driven by a faster pulse behaves more stable. Finally, a model was developed to analyze the interaction of the airflow and the discharge channels.

Influence of air flow parameters on nanosecond repetitively pulsed discharges in a pin-annular electrode configuration

The effect of various air flow parameters on the plasma regimes of nanosecond repetitively pulsed (NRP) discharges is investigated at atmospheric pressure. The two electrodes are in a pin-annular configuration, transverse to the mean flow. The voltage pulses have amplitudes up to 15 kV, a duration of 10 ns and a repetition frequency ranging from 15 to 30 kHz. The NRP corona to NRP spark (C-S) regime transition and the NRP spark to NRP corona (S-C) regime transition are investigated for different steady and harmonically oscillating flows. First, the strong effect of a transverse flow on the C-S and S-C transitions, as reported in previous studies, is verified. Second, it is shown that the azimuthal flow imparted by a swirler does not affect the regime transition voltages. Finally, the influence of low frequency harmonic oscillations of the air flow, generated by a loudspeaker, is studied. A strong effect of frequency and amplitude of the incoming flow modulation on the NRP plasma regime is observed. Results are interpreted based on the cumulative effect of the NRP discharges and an analysis of the residence times of fluid particles in the inter-electrode region.

Effect of Airflows on Repetitive Nanosecond Volume Discharges**supported by National Natural Science Foundation of China (Nos. 51006027, 51437002, and 51477035)

Atmospheric pressure discharges excited by repetitive nanosecond pulses have attracted significant attention for various applications. In this paper, a plate-plate discharge with airflows is excited by a repetitive nanosecond pulse generator. Under different experiment conditions, the applied voltages, discharge currents, and discharge images are recorded. The plasma images presented here indicate that the volume discharge modes vary with airflow speeds, and a diffuse and homogeneous volume discharge occurs at the speed of more than 35 m/s. The role of airflows provides different effects on the 2-stage pulse discharges. The 1st pulse currents nearly maintain consistency for different airflow speeds. However, the 2nd pulse current has a change trend of first decreasing and then rapidly increasing, and the value difference for 2nd pulse currents is about 20 A under different airflows. In addition, the experimental results are discussed according to the electrical parameters and discharge images.

Focus on test and measurement

Focus on test and measurement

Ignition of Quiescent Lean Propane–Air Mixtures at High Pressure by Nanosecond Repetitively Pulsed Discharges

We present an experimental study of lean mixture ignition by nanosecond repetitively pulsed (NRP) discharges. The plasma is created in a lean propane/air mixture at pressure up to 10 bar and equivalence ratio 0.7, premixed in a constant volume vessel. We characterize the initial spark radius, the ignition kernel development and the flame propagation as a function of pressure (up to 10 bar) and the pulse energy (1–6 mJ per pulse). Comparisons with a conventional igniter show that better results are obtained with NRP discharges in terms of flame propagation speed, in particular at high pressure, due to the increased wrinkling of the flame front that is induced by NRP discharges.

Surface charge dynamics and OH and H number density distributions in near-surface nanosecond pulse discharges at a liquid / vapor interface

The present work provides insight into surface charge dynamics and kinetics of radical species reactions in nanosecond pulse discharges sustained at a liquid-vapor interface, above a distilled water surface. The near-surface plasma is sustained using two different discharge configurations, a surface ionization wave discharge between two exposed metal electrodes and a double dielectric barrier discharge. At low discharge pulse repetition rates (~100 Hz), residual surface charge deposition after the discharge pulse is a minor effect. At high pulse repetition rates (~10 kHz), significant negative surface charge accumulation over multiple discharge pulses is detected, both during alternating polarity and negative polarity pulse trains. Laser induced fluorescence (LIF) and two-photon absorption LIF (TALIF) line imaging are used for in situ measurements of spatial distributions of absolute OH and H atom number densities in near-surface, repetitive nanosecond pulse discharge plasmas.Both in a surface ionization wave discharge and in a double dielectric barrier discharge, peak measured H atom number density, [H] is much higher compared to peak OH number density, due to more rapid OH decay in the afterglow between the discharge pulses. Higher OH number density was measured near the regions with higher plasma emission intensity. Both OH and especially H atoms diffuse out of the surface ionization wave plasma volume, up to several mm from the liquid surface. Kinetic modeling calculations using a quasi-zero-dimensional H2O vapor / Ar plasma model are in qualitative agreement with the experimental data. The results demonstrate the experimental capability of in situ radical species number density distribution measurements in liquid-vapor interface plasmas, in a simple canonical geometry that lends itself to the validation of kinetic models.

Optimization Design of a Repetitive Nanosecond Pulse Generator Based on Saturable Pulse Transformer and Magnetic Switch

Saturable pulse transformer (SPT) and magnetic switch (MS) are widely used together in pulse generators (PGs). It is easy to obtain a desirable output with a high voltage and low rise time when choosing the suitable PG topology. A compact, unipolar, and repetitive high-voltage nanosecond PG, which is based on SPT and MS, is designed and constructed. The operation principles of the resonant charging based on SPT and MS are described. The design and performance optimization of the magnetic components including SPT and MS are presented. The minimum volume of the SPT is derived and verified in the experiment. The core volume and turns of the MS are determined both considering the charging process and the output characteristic. The saturation times of the pulse transformer and MS can be optimally determined based on the $B$ – $H$ curve, which is obtained under the same working conditions. It reveals that the design and performance optimization methods of the SPT and MS are reasonable and can be used to guide the design of pulse transformers and MSs in pulsed power, power electronics, and industrial electronics.

Schlieren imaging and pulsed detonation engine testing of ignition by a nanosecond repetitively pulsed discharge

A nanosecond repetitively pulsed (NRP) discharge in the spark regime has been investigated as to its effectiveness in reducing ignition time, both in a flow tube and a pulsed detonation engine (PDE). The flame-development time for methane–air mixtures in the flow tube is found to be a function of the total ignition energy and the pulse repetition frequency. Schlieren imaging revealed that at low pulse-repetition frequency (0–5kHz), ignition kernels formed by the discharge are each transported away from the discharge gap before the following pulse arrives. At higher pulse-repetition frequencies (⩾10kHz), multiple pulses are all coupled into a single ignition kernel, thus the resulting ignition kernel size and the total energy deposition into the kernel are increased, resulting in a faster transition into a self-propagating flame. Imaging of the NRP discharge in air revealed that at high pulse frequencies (>10kHz) and peak pulse amplitude (>9kV), the plasma emission is not quenched in-between pulses, resulting in a building up of heat and radicals in the center of the ignition kernel. Optical emission spectra revealed the presence of electronically excited N2, O, and N, as well as O+ and N+, during and between the discharge pulses. Numerical modeling of the plasma indicated that reactions of excited species mainly lead to the production of O atoms and the increase of gas temperature, which shortens induction chemistry timescales, and thus reduces the flame-development time through both kinetic and thermal mechanisms. Ignition of aviation gasoline–air mixtures by NRP discharge in a PDE also demonstrated a noticeable reduction in ignition time as compared to an automotive aftermarket multiple capacitive-discharge ignition system.

Optical and electrical investigation of a cylindrical diffuse-discharge chamber

More and more attention has been attached to atmospheric-pressure air diffuse plasma due to its enormous potential applications. In this paper, we designed a large-scale, cylindrical diffuse-plasma chamber using wire electrodes and a repetitive nanosecond pulse generator. The plasma chamber can be completely exposed in the open air without any barrier dielectric, and the length of cylindrical plasma chamber was extensible. Using optical and electrical measurements, we investigated the effects of electrode distance, electrode length, pulse repetition frequency, and electrode angle on the uniformity of discharge space. Four discharge regions were distinguished based on different spectral characteristics. Additionally, it was found that the discharge uniformity was improved as the electrode distance decreases, but remained almost constant with the variations of electrode length and pulse repetition frequency. Both of the plasma uniformity and the power density increased significantly as the electrode angle reduced.

Repetitive nanosecond pulse generator based on Tesla transformer

The combination of pulse transformer and sharpening switch is a relatively mature way to generate nanosecond pulse. In this way, a compact high voltage repetitive nanosecond pulse generator (HRNPG) based on Tesla transformer and spark switch is developed in this paper. The pulse generator mainly consists of a repetitive charging module, a Tesla transformer and a sharpening switch. Repetitive charging module is mainly implemented by temporal coordination of two thyristors, Tesla transformer is the core of the pulse generator and main booster module, sharpening switch is a three electrode self-breakdown type gas switch, which is used to sharpen the transformer secondary voltage into nanosecond pulse waveform. Each part of the HRNPG is presented and tested in detail. Applied to a 6 k resistor, the HRNPG prototype is capable of generating a pulse with 100 kV peak voltage and 30 ns rise time at the maximum repetition rate of 500 Hz.

0616 ナノ秒パルスプラズマアクチュエータを用いた翼上の流れ制御のための数値モデル

The significant effects of dielectric barrier discharge (DBD) driven by repetitive nanosecond (NS) pulse voltage on lift enhancement on stalled airfoils have been demonstrated. The essential effect of NS plasma is considered as thermal effect. The detailed effect of the thermal excitation on the vortex structures is, however, not fully understood. In the present work, a simple momentum model for incompressible Unsteady Reynolds-Averaged Navier-Stokes simulation was proposed, in which the effect of plasma was implemented as a wall velocity boundary condition on the airfoil. Although this simplified simulation does not take account of thermalization process and shock dynamics, the results showed discrete vortices generated by rolling-up shear layer over the leading edge. It qualitatively consisted with the trend of time-averaged pressure distribution obtained by the corresponding experiment.