Experimental study on nanosecond pulsed pin-to-plate discharge in supersonic air flow

Development of magnetohydrodynamic acceleration technology is expected to improve wind tunnel simulation capability and testing capability. The underlying premise is to produce uniform and stable plasma in supersonic air flow, and gas discharge is an effective way to achieve this. A nanosecond pulsed discharge experimental system under supersonic conditions was established, and a pin-to-plate nanosecond pulsed discharge experiment in Mach 2 air flow was performed to verify that the proposed method produced uniform and stable plasma under supersonic conditions. The results show that the discharge under supersonic conditions was stable overall, but uniformity was not as good as that under static conditions. Increasing the number of pins improved discharge uniformity, but reduced discharge intensity and hence plasma density. Under multi-pin conditions at 1000 Hz, the discharge was almost completely corona discharge, with the main current component being the displacement current, which was smaller than that under static conditions.     

Experimental and numerical investigation on the uniformity of nanosecond pulsed dielectric barrier discharge influenced by pulse parameters

Nanosecond(ns) pulsed dielectric barrier discharge(DBD) is considered as a promising method to produce controllable large-volume and high activity low-temperature plasma at atmospheric pressure, which makes it suitable for wide applications. In this work, the ns pulse power supply is used to excite Ar DBD and the influences of the pulse parameters(voltage amplitude, pulse width, pulse rise and fall times) on the DBD uniformity are investigated. The gas gap voltage(Ug) and conduct current(Ig) are...     

Effects of the transverse electric field on nanosecond pulsed dielectric barrier discharge in atmospheric airflow

In this paper, an asymmetric electrode geometry(the misalignment between the ends of highvoltage and grounded electrodes) is proposed in order to investigate the effects of the transverse electric field on nanosecond pulsed dielectric barrier discharge(DBD). The results show that diffuse discharge manifests in the misaligned region and the micro-discharge channel in the aligned region moves directionally. Moreover, the diffuse discharge area increases with the decrease of the discharge gap and pulse repetition frequency, which is consistent with the variation of the moving velocity of the micro-discharge channel. When airflow is introduced into the discharge gap in the same direction as the transverse electric field, the dense filamentary discharge region at the airflow inlet of asymmetric electrode geometry is larger than that of symmetric electrode geometry. However, when the direction of the airflow is opposite to that of the transverse electric field, the dense filamentary discharge region of asymmetric electrode geometry is reduced. The above phenomena are mainly attributed to the redistribution of the space charges induced by the transverse electric field.     

Spatiotemporal characteristics of nanosecond pulsed discharge in an extremely asymmetric electric field at atmospheric pressure

In this paper,high resolution temporal-spatial diagnostics are employed to research the optical characteristics of nanosecond pulsed dielectric barrier discharge in needle-plate electrode configuration.Temporal-spatial distributions of discharge images,the emission intensities of opticaI emission spectra,and plasma vibrational and rotational temperatures are investigated.By analyzing the evolution of vibrational and rotational temperatures in space and time dimensions,the energy distribution and energy transfer process in plasma are also discussed.It is found that a diffuse structure with high density plasma concentrated in the region near the needle tip can be presented in nanosecond pulsed discharge,and an obvious energy transfer from electronic energy to vibration energy can be observed in each discharge pulse.     

Development of a battery-operated floating-electrode dielectric barrier discharge plasma device and its characteristics

In this work, a portable floating-electrode dielectric barrier discharge(FE-DBD) device is designed with a rechargeable battery as the power supply. The characteristics of the FE-DBD with a metal electrode and human hand are studied and compared. The human contact safety is verified by calculating the current through the human body based on the equivalent circuit model. Escherichia coli inactivation experiments confirm the efficacy of the FE-DBD device in the envisaged applications.     

Experimental study of n-decane decomposition with microsecond pulsed discharge plasma

A highly-integrated experimental system for the plasma decomposition of fuels was built.Experiments were conducted in a flow reactor at atmospheric pressure and confirmed that n-decane could be cracked by large-gap dielectric barrier discharge under the excitation of a microsecond-pulse power supply. Alkanes and olefins with a C atom number that is smaller than10 as well as hydrogen were found in the cracked products of n-decane(n-C10 H22). The combination of preheating and plasma decomposition had strong selectivity for olefins. Under strong discharge conditions, small molecule olefins were found in the products. Moreover, there was a general tendency that small molecule olefins gradually accounted for higher percentage of products at higher temperature and discharge frequency.     

Research on the characteristics of atmospheric air dielectric barrier discharge under different square wave pulse polarities

Energy efficiency limits the application of atmospheric pressure dielectric barrier discharge(DBD),such as air purification,water treatment and material surface modification.This article focuses on the electrical and optical effects of the DBD under three square wave pulses polarities-positive,negative and bipolar.The result shows that under the same voltage with the quartz glass medium,the discharge efficiency of bipolar polarity pulse is the highest due to the influence of deposited charge.With the increase of air gap distance from 0.5 to 1.5 mm,average power consumed by the discharge air gap and discharge efficiency decrease obviously under alumina,and increase,and then decrease under quartz glass and polymethyl methacrylate(PMMA).Through spectrum diagnosis,in the quartz glass medium,the vibration temperature is the highest under negative polarity pulse excitation.Under bipolar pulse,the vibration temperature does not change significantly with the change of air gap distance.For the three dielectric materials of quartz glass,alumina and PMMA,the molecular vibration temperature is the highest under the quartz glass medium with the same voltage.When the gap spacing,pulse polarity or dielectric material are changed,the rotational temperature does not change significantly.     

Investigation on discharge characteristics of a coaxial dielectric barrier discharge reactor driven by AC and ns power sources

A coaxial dielectric barrier discharge(DBD) reactor with double layer dielectric barriers has been developed for exhaust gas treatment and excited either by AC power or nanosecond(ns)pulse to generate atmospheric pressure plasma. The comparative study on the discharge characteristics of the discharge uniformity, power deposition, energy efficiency, and operation temperature between AC and ns pulsed coaxial DBD is carried out in terms of optical and electrical characteristics and operation temperature for optimizing the coaxial DBD reactor performance. The voltages across the air gap and dielectric layer and the conduction and displacement currents are extracted from the applied voltages and measured currents of AC and ns pulsed coaxial DBDs for the calculation of the power depositions and energy efficiencies through an equivalent electrical model. The discharge uniformity and operating temperature of the coaxial DBD reactor are monitored and analyzed by optical images and infrared camera. A heat conduction model is used to calculate the temperature of the internal quartz tube. It is found that the ns pulsed coaxial DBD has a much higher instantaneous power deposition in plasma, a lower total power consumption, and a higher energy efficiency compared with that excited by AC power and is more homogeneous and stable. The temperature of the outside wall of the AC and ns pulse excited coaxial DBD reaches 158 ℃ and 64.3 ℃ after 900 s operation, respectively.The experimental results on the comparison of the discharge characteristics of coaxial DBDs excited by different powers are significant for understanding of the mechanism of DBDs,reducing energy loss, and optimizing the performance of coaxial DBD in industrial applications.     

Dynamics of plasma bullets by nanosecond pulsed micro-hollow cathode discharge within air

In this paper, the air plasma jet produced by micro-hollow cathode discharge(MHCD) is investigated. The discharge is powered by a positive nanosecond pulse high voltage supply. The waveforms of the discharge, the images of the jet, the evolution of the plasma bullet and the reactive species are obtained to analyze the characteristics of the MHCD plasma jet. It is found that the length of the plasma jet is almost proportional to the air flow rate of 2–6 slm. Two plasma bullets appear one after another during a single period of the voltage waveform, and both of the two plasma bullets are formed during the positive pulse voltage off. The propagation velocity of the two plasma bullets is on the order of several hundred m/s, which is approximate to that of the air flow. These results indicate that the gas flow has an important influence on the formation of this MHCD plasma jet.     

Investigation of nanosecond-pulsed dielectric barrier discharge actuators with powered electrodes of different exposures

Nanosecond-pulsed dielectric barrier discharge actuators with powered electrodes of different exposures were investigated numerically by using a newly proposed plasma kinetic model. The governing equations include the coupled continuity plasma discharge equation, drift-diffusion equation, electron energy equation, Poisson's equation, and the Navier–Stokes equations.Powered electrodes of three different exposures were simulated to understand the effect of surface exposure on plasma discharge and surrounding flow field. Our study showed that the fully exposed powered electrode resulted in earlier reduced electric field breakdown and more intensive discharge characteristics than partially exposed and rounded-exposed ones. Our study also showed that the reduced electric field and heat release concentrated near the right upper tip of the powered electrode. The fully exposed electrode also led to stronger shock wave, higher heating temperature, and larger heated area.     

Study of vortex in flow fields induced by surface dielectric barrier discharge actuator at low pressure based on Q criterion

Flow fields induced by a surface dielectric barrier discharge actuator at low pressure of 7 kPa are measured by particle image velocimetry.The distribution of local vortices in the flow field is revealed by the Q criterion.The reason for the generation of vortices is analyzed and the influence of pulse frequency and duty cycle on vortices is studied.The results show that the Q criterion can reveal the small-scale vortices,which cannot be indicated by the streamline.The direction transition zone where the induced jet moves from the vertical to the tangential and the shear layer between the jet and stationary air are prone to the generation of strong vortices.The influence of pulse frequency on vortices is not obvious,but the variation of duty cycle can significantly affect the strength and distribution of vortices.     

Electrical and aerodynamic characteristics of sliding discharge based on a microsecond pulsed plasma supply

Plasma flow control technology has broad prospects for application. Compared with conventional dielectric barrier discharge plasma actuators (DBD-PA), the sliding discharge plasma actuator (SD-PA) has the advantages of a large discharge area and a deflectable induced jet. To achieve the basic performance requirements of light weight, low cost, and high reliability required for UAV (Unmanned Aerial Vehicle) plasma flight experiments, this work designed a microsecond pulse plasma supply that can be used for sliding discharge plasma actuators. In this study, the topology of the primary circuit of the microsecond pulse supply is determined, the waveform of the output terminal of the microsecond pulse plasma supply is detected using the Simulink simulation platform, and the design of the actuation voltage, the pulse frequency modulation function and the construction of the hardware circuit are achieved. Using electrical diagnosis and flow field analysis, the actuation characteristics and flow characteristics of sliding discharge plasma under microsecond pulse actuation are studied, the optimal electrical actuation parameters and flow field characteristics are described.     

Characteristic studies on positive and negative streamers of double-sided pulsed surface dielectric barrier discharge

The mechanisms of streamer generation and propagation in double-sided pulsed surface dielectric barrier discharge (SDBD) on both sides have been analyzed and investigated by experiment and numerical simulation. The fully exposed asymmetric SDBD has two discharge processes located on the high voltage electrode (HVE) side and the ground electrode (GE) side. Discharge images of the HVE side and GE side are taken by a digital camera under continuous pulse and ICCD (Intensified Charge Coupled Device) is utilized to diagnose the generation and propagation of streamers in single pulse discharge. In order to understand the physical mechanisms of streamer evolution more deeply, we establish a 2D simulation model and analyze it from the aspects of electron density, ion density, reduced electric field and electron impact ionization source term. The results show that the primary and secondary discharges on the HVE side and the GE side of the double-sided SDBD are composed of positive streamer and negative streamer, respectively. On the HVE side, the accumulation of positive charges on the dielectric surface causes the direction of the electric field to reverse, which is the principal factor for the polarity reversal of the streamer. On the GE side, both the negative charges accumulated on the dielectric surface and the falling voltage are the key factors for the streamer polarity switch.     

Influences of frequency on nitrogen fixation of dielectric barrier discharge in air

The influences of frequency on nitrogen fixation of dielectric barrier discharge in air were studied by electrical diagnostics, gas detection and infrared detection methods. The system power, nitrogen oxide concentration, voltage–current waveform, dielectric surface temperature distribution and filamentous discharge pictures were measured, and then the energy yield was calculated; paper studied their changing tendencies in the presence of frequency. Results show that frequency has strong influences on nitrogen fixation. When the parameters of reaction chamber and amplitude of applied voltage is fixed, with the increasing of frequency, the system power increases; in 5-10 kHz, nitrogen oxide gas concentration up to 1113.7 mg m~(-3), and 7 k Hz is the optimal nitrogen fixation frequency whose energy yield is 20.5 mg(m~3 W)~(-1).     

Numerical study of the influence of dielectric tube on propagation of atmospheric pressure plasma jet based on coplanar dielectric barrier discharge

A 2D fluid model was employed to simulate the influence of dielectric on the propagation of atmospheric pressure helium plasma jet based on coplanar dielectric barrier discharge (DBD). The spatio-temporal distributions of electron density, ionization rate, electrical field, spatial charge and the spatial structure were obtained for different dielectric tubes that limit the helium flow. The results show that the change of the relative permittivity of the dielectric tube where the plasma jet travels inside has no influence on the formation of DBD itself, but has great impact on the jet propagation. The velocity of the plasma jet changes drastically when the jet passes from a tube of higher permittivity to one of lower permittivity, resulting in an increase in jet length,ionization rate and electric field, as well as a change in the distribution of space charges and discharge states. The radius of the dielectric tube has a great influence on the ring-shaped or solid bullet structure. These results can well explain the behavior of the plasma jet from the dielectric tube into the ambient air and the hollow bullet in experiments.     

Investigation of operating parameters on CO2 splitting by dielectric barrier discharge plasma

Experiments of CO2 splitting by dielectric barrier discharge (DBD) plasma were carried out,and the influence of CO2 flow rate,plasma power,discharge voltage,discharge frequency on CO2 conversion and process energy efficiency were investigated.It was shown that the absolute quantity of CO2 decomposed was only proportional to the amount of conductive electrons across the discharge gap,and the electron amount was proportional to the discharge power;the energy efficiency of CO2 conversion was almost a constant at a lower level,which was limited by CO2 inherent discharge character that determined a constant gap electric field strength.This was the main reason why CO2 conversion rate decreased as the CO2 flow rate increase and process energy efficiency was decreased a little as applied frequency increased.Therefore,one can improve the CO2 conversion by less feed flow rate or larger discharge power in DBD plasma,but the energy efficiency is difficult to improve.     

Numerical investigation on the effect of gas parameters on ozone generation in pulsed dielectric barrier discharge

Pulsed dielectric barrier discharge is a promising technology for ozone generation and is drawing increasing interest. To overcome the drawback of experimental investigation, a kinetic model is applied to numerically investigate the effect of gas parameters including inlet gas temperature, gas pressure, and gas flow rate on ozone generation using pulsed dielectric barrier discharge. The results show that ozone concentration and ozone yield increase with decreasing inlet gas temperature, gas pressure, and gas flow rate. The highest ozone concentration and ozone yield in oxygen are about 1.8 and 2.5 times higher than those in air, respectively. A very interesting phenomenon is observed: the peak ozone yield occurs at a lower ozone concentration when the inlet gas temperature and gas pressure are higher because of the increasing average gas temperature in the discharge gap as well as the decreasing reduced electric field and electron density in the microdischarge channel. Furthermore, the sensitivity and rate of production analysis based on the specific input energy (SIE) for the four most important species O₃, O, O(1D), and O₂(b¹∑) are executed to quantitatively understand the effects of every reaction on them, and to determine the contribution of individual reactions to their net production or destruction rates. A reasonable increase in SIE is beneficial to ozone generation. However, excessively high SIE is not favorable for ozone production.     

Conversion of NO with a catalytic packed-bed dielectric barrier discharge reactor

This paper discusses the conversion of nitric oxide (NO) with a low-temperature plasma induced by a catalytic packed-bed dielectric barrier discharge (DBD) reactor.Alumina oxide (Al2O3),glass (SiO2) and zirconium oxide (ZrO2),three different spherical packed materials of the same size,were each present in the DBD reactor.The NO conversion under varying input voltage and specific energy density,and the effects of catalysts (titanium dioxide (TiO2) and manganese oxide (MnOx) coated on Al2O3) on NO conversion were investigated.The experimental results showed that NO conversion was greatly enhanced in the presence of packed materials in the reactor,and the catalytic packed bed of MnOx/Al2O3 showed better performance than that of TiO2/Al2O3.The surface and crystal structures of the materials and catalysts were characterized through scanning electron microscopy analysis.The final products were clearly observed by a Fourier transform infrared spectrometer and provided a better understanding of NO conversion.     

Experimental study on surface arc plasma actuation-based hypersonic boundary layer transition flow control

Effective control of hypersonic transition is essential. In order to avoid affecting the structural profile of the aircraft, as well as reducing power consumption and electromagnetic interference, a low-frequency surface arc plasma disturbance experiment to promote hypersonic transition was carried out in the Φ0.25 m double-throat Ludwieg tube wind tunnel at Huazhong University of Science and Technology. Contacting printed circuit board sensors and non-contact focused laser differential interferometry testing technology were used in combination. Experimental results showed that the low-frequency surface arc plasma actuation had obvious stimulation effects on the second-mode unstable wave and could promote boundary layer transition by changing the spectral characteristics of the second-mode unstable wave. At the same time, the plasma actuation could promote energy exchange between the second-mode unstable wave and other unstable waves. Finally, the corresponding control mechanism is discussed.     

Experimental study on characteristics of sodium fire in a columnar flow with discontinuous injections

In the operation of the sodium-cooled fast reactor, the accident caused by the leakage and combustion of liquid sodium is common and frequent in sodium-related facilities. In this paper, an experiment Harbin Engineering University-Sodium Columnar Fire 4 (HEU-SCF4) is carried out to focus on the combustion characteristics in a discontinuous columnar flow. Liquid sodium (250°C) with the flux of about 1.0 m³/h was injected into a 7.9 m³ stainless steel cylindrical test chamber twice to shape the sodium fire in a columnar flow with discontinuous injections. The temperature, pressure data, and heat release rate in the test chamber were acquired and analyzed. It is found that each injection would cause a peak of space temperature, and it takes much shorter time to reach temperature peak in the second injection than the first injection. The peaks of pressure and heat release rate caused by the second injection are much higher than those caused by the first injection. This study is promising to evaluate the combustion characteristics of sodium fire in a columnar flow with different injection forms in the sodium-related facilities.