Investigation of flame structure in plasma-assisted turbulent premixed methane-air flame

The mechanism of plasma-assisted combustion at increasing discharge voltage is investigated in detail at two distinctive system schemes (pretreatment of reactants and direct in situ discharge).OH-planar laser-induced fluorescence (PLIF) technique is used to diagnose the turbulent structure methane-air flame,and the experimental apparatus consists of dump burner,plasma-generating system,gas supply system and OH-PLIF system.Results have shown that the effect of pretreatment of reactants on flame can be categorized into three regimes:regime Ⅰ for voltage lower than 6.6 kV;regime Ⅱ for voltage between 6.6 and 11.1 kV;and regime Ⅲ for voltage between 11.1 and 12.5 kV.In regime Ⅰ,aerodynamic effect and slower oxidation of higher hydrocarbons generated around the inner electrode tip plays a dominate role,while in regime Ⅲ,the temperature rising effect will probably superimpose on the chemical effect and amplify it.For wire-cylinder dielectric barrier discharge reactor with spatially uneven electric field,the amount of radicals and hydrocarbons are decreased monotonically in radial direction which affects the flame shape.With regard to in situ plasma discharge in flames,the discharge pattern changes from streamer type to glow type.Compared with the case of reactants pretreatment,the flame propagates further in the upstream direction.In the discharge region,the OH intensity is highest for in situ plasma assisted combustion,indicating that the plasma energy is coupled into flame reaction zone.     

1d3v PIC/MCC simulation of dielectric barrier discharge dynamics in hydrogen sulfide

In this study, we computationally examined the dynamics of dielectric barrier discharge in hydrogen sulfide. The simulations were performed with a 1d3v particle-in-cell/Monte Carlo collision model in which a parallel-plate electrode geometry with dielectrics was used. Particle recombination process is represented in the model. The discharge mode was found to be initially Townsend discharge developing from the cathode to the anode, and at the peak of the current, a more stable glow discharge develops from the anode to the cathode. A higher applied voltage results in sufficient secondary electrons to trigger a second current peak, and then the current amplitude increases. As the frequency is increased, it leads to the advance of the phase and an increase in the amplitude of the current peak. A higher dielectric permittivity also makes the discharge occur earlier and more violently in the gap.     

Study on the Enhancement Effect of Dielectric Barrier Discharge on the Premixed Methane/Oxygen/Helium Flame Velocity

Recently,plasma-assisted combustion has become a potentially applicable technology in many combustion scenarios.In this paper,a dielectric barrier discharge(DBD) plasma generator is designed to explore the effect of plasma on the CH4 oxidation process,and several properties of combustion are considered.First,in the presence or absence of plasma discharge,physical appearance of the flame is examined and analyzed.Second,the flame propagation velocity is calculated by the flame front extracted from the imaging data with the Bunsen burner method.Finally,the main molecular components and their intensity variation in the flame and the plasma zones are identified with an emission spectrograph to analyze the effect of active species on the combustion process.We also discuss the possible kinetic regime of plasma-assisted combustion.Experimental results imply that plasma discharge applied to the premixed CH_4/O_2/He mixture significantly raises the flame speed with equivalence ratios ranging from 0.85 to 1.10,with the flame speed improved by 17%to 35%.It can be seen that plasma can improve methane oxidation efficiency in the premixed fuel/oxidizer,especially at a low equivalence ratio.     

Particle-in-cell/Monte Carlo simulation of filamentary barrier discharges

The plasma behavior of filamentary barrier discharges in helium is simulated using a twodimensional (2D) particle-in-cell/Monte Carlo model.Four different phases have been suggested in terms of the development of the discharge:the Townsend phase;the space-charge dominated phase;the formation of the cathode layer,and the extinguishing phase.The spatialtemporal evolution of the particle densities,velocities of the charged particles,electric fields,and surface charges has been demonstrated.Our simulation provides insights into the underlying mechanism of the discharge and explains many dynamical behaviors of dielectric barrier discharge (DBD) filaments.     

Numerical investigation of Ar–NH_3 mixture in homogenous DBDs

In this paper, a study of homogenous dielectric barrier discharge Ar–NH_3 was made in order to investigate the electrical and the physical characteristics of homogenous discharge at atmospheric pressure. The discharge model includes the electrical module and the chemical kinetic module. The results obtained by the present model were compared with experimental work. The evolution of the plasmas voltage, the species densities, and the concentration of charged species are analyzed and discussed.     

Effects of Oxygen Concentration on Pulsed Dielectric Barrier Discharge in Helium-Oxygen Mixture at Atmospheric Pressure

In this work the effects of O_2 concentration on the pulsed dielectric barrier discharge in helium-oxygen mixture at atmospheric pressure have been numerically researched by using a one-dimensional fluid model in conjunction with the chosen key species and chemical reactions.The reliability of the used model has been examined by comparing the calculated discharge current with the reported experiments. The present work presents the following significant results. The dominative positive and negative particles are He_2~+ and O_2~-, respectively, the densities of the reactive oxygen species(ROS) get their maxima nearly at the central position of the gap, and the density of the ground state O is highest in the ROS. The increase of O_2 concentration results in increasingly weak discharge and the time lag of the ignition. For O_2 concentrations below 1.1%,the density of O is much higher than other species, the averaged dissipated power density presents an evident increase for small O_2 concentration and then the increase becomes weak. In particular,the total density of the reactive oxygen species reaches its maximums at the O_2 concentration of about 0.5%. This characteristic further convinces the experimental observation that the O_2 concentration of 0.5% is an optimal O_2/He ratio in the inactivation of bacteria and biomolecules when radiated by using the plasmas produced in a helium oxygen mixture.     

Numerical simulation of atmospheric pulse-modulated radio-frequency glow discharge ignition characteristics assisted by a pulsed discharge

A one-dimensional self-consistent fluid numerical model was developed to study the ignition characteristics of a pulse-modulated(PM) radio-frequency(RF) glow discharge in atmospheric helium assisted by a sub-microsecond voltage excited pulsed discharge. The temporal evolution of discharge current density and electron density during PM RF discharge burst was investigated to demonstrate the discharge ignition characteristics with or without the pulsed discharge. Under the assistance of pulsed discharge, the electron density in RF discharge burst reaches the magnitude of 1.87 × 10~(17) m~(-3) within 10 RF cycles, accompanied by the formation of sheath structure. It proposes that the pulsed discharge plays an important role in the ignition of PM RF discharge burst. Furthermore, the dynamics of PM RF glow discharge are demonstrated by the spatiotemporal evolution of the electron density with and without pulsed discharge. The spatial profiles of electron density, electron energy and electric field at specific time instants are given to explain the assistive role of the pulsed discharge on PM RF discharge ignition.     

Comparative research of plasma-assisted milling and traditional milling in synthesizing AIN

In this paper,traditional milling and discharge plasma-assisted milling are employed to synthesize aluminum nitride (AlN) powder at nanometer scale by milling the mixture of aluminum and lithium hydroxide monohydrate.AlN powders can be generated in traditional milling and plasma-assisted milling in an hour milling time.Differential thermal analysis curves show that the reaction temperature of the powders treated by plasma-assisted milling is lower than that of traditional milling.These results indicate that plasma-assisted milling has higher efficiency in the synthesis of AlN,getting smaller crystallite size and activating powder.Moreover,an optical emission spectrum is employed to demonstrate the active species in plasma.The different formation process of AlN in the two-milling process,and the promotion effects of plasma in the milling process are discussed.     

A Simplified Numerical Study of the Kr/Cl2 Plasma Chemistry in Dielectric Barrier Discharge

In this paper, the generation of excimers and exciplexe radiation in mixtures of rare gas with halogen by homogeneous dielectric barrier discharge （DBD） is investigated. The typical characteristics of an excilamp based on KrCl exciplexe molecules and the kinetic processes for the formation and the decay of this molecules in the Kr/Cl₂ mixture are studied. The computer model developed is based on the Kr/Cl₂ mixture chemistry, the equivalent electric circuit and the Boltzmann equations. The importance in the kinetic processes of some species such as the metastable state of Krypton （Kr（3P0,2）） and the negative ion of chloride （Cl） is considered. The results illustrate the time variations of charged species （ne,Kr+,Cl,Cl+,Cl+2,Kr+2）, excited atoms and molecules （Kr（3P0,2）, Kr（3P1）, Cl, Cl2）, the excimers （Kr2,KrCl（B）,KrCl（C）,Kr2Cl） and the UV photon concentrations （in 222nm,235nm,258nm and 325nm range）. The effects of chlorine concentration and the total gas pressure in the Kr-Cl₂ discharge on the electric parameters and radiation emissions are investigated.     

A numerical simulation study on active species production in dense methane-air plasma discharge

Recently, low-temperature atmospheric pressure plasmas have been proposed as a potential type of ‘reaction carrier' for the conversion of methane into value-added chemicals. In this paper, the multi-physics field coupling software of COMSOL is used to simulate the detailed discharge characteristics of atmospheric pressure methane-air plasma. A two-dimensional axisymmetric fluid model is constructed, in which 77 plasma chemical reactions and 32 different species are taken into account. The spatial density distributions of dominant charged ions and reactive radical species, such as CH_4~+CH_3~+N_2~+O_2~+H, O, CH_3, and CH_2, are presented, which is due to plasma chemical reactions of methane/air dissociation(or ionization) and reforming of small fragment radical species. The physicochemical mechanisms of methane dissociation and radical species recombination are also discussed and analyzed.     

Numerical simulation of carbon arc discharge for graphene synthesis without catalyst

In this study, graphene sheets are prepared under a hydrogen atmosphere without a catalyst, and the growth mechanism of graphene by direct current arc discharge is investigated experimentally and numerically. The size and layer numbers of graphene sheets increase with the arc current.Distributions of temperature, velocity, and mass fraction of carbon are obtained through numerical simulations. A high current corresponds to a high saturation temperature, evaporation rate, and mass density of carbon clusters. When the carbon vapor is saturated, the saturation temperatures are 3274.9, 3313.9, and 3363.6 K, and the mass densities are 6.4×10~(22),8.42×10~(22), and 1.23×10~(23) m~(-3) under currents of 150, 200, and 250 A, respectively. A hydrogen-induced marginal growth model is used to explain the growth mechanism. Under a high current, the condensation coefficient and van der Waals force increase owing to the higher saturation temperature and mass density of carbon clusters, which is consistent with experimental results.     

Numerical Study of Pulsed Dielectric Barrier Discharge at Atmospheric Pressure Under the Needle-Plate Electrode Configuration

In this paper,we study the characteristics of atmospheric-pressure pulsed dielectric barrier discharge (DBD) under the needle-plate electrode configuration using a one-dimensional self-consistent fluid model.The results show that,the DBDs driven by positive pulse,negative pulse and bipolar pulse possess different behaviors.Moreover,the two discharges appearing at the rising and the falling phases of per voltage pulse also have different discharge regimes.For the case of the positive pulse,the breakdown field is much lower than that of the negative pulse,and its propagation characteristic is different from the negative pulse DBD.When the DBD is driven by a bipolar pulse voltage,there exists the interaction between the positive and negative pulses,resulting in the decrease of the breakdown field of the negative pulse DBD and causing the change of the discharge behaviors.In addition,the effects of the discharge parameters on the behaviors of pulsed DBD in the needle-plate electrode configuration are also studied.     

Effects of magnetic field on electron power absorption in helicon fluid simulation

In this study, a code, named Peking University Helicon Discharge(PHD), which can simulate helicon discharge processes under both a background magnetic field greater than 500 G and a pressure less than 1 Pa, is developed. In the code, two fluid equations are used. The PHD simulations led to two important findings:(1) the temporal evolution of plasma density with the background magnetic field exhibits a second rapid increase(termed as the second density jump),similar to the transition of modes in helicon plasmas;(2) in the presence of a magnetic field, the peak positions of electron power absorption appeared near the central axis, unlike in the case of no magnetic field. These results may lead to an enhanced understanding of the discharge mechanism.     

Numerical simulation of the start-up process of a miniature ion thruster

A particle-in-cell Monte Carlo collision model of a discharge chamber is established to investigate the start-up process of a miniature ion thruster. We present the discharge characteristics at different stages (the initial stage, development stage, and stable stage) according to the trend of the discharge current with time. The discharge current is the sum of the sidewall current and the backplate current. During the start-up process, the sidewall current lags behind the backplate current. The variation and distribution characteristics of the discharge current over time are determined by the electron density distribution and electric potential distribution.     

Electrical Characteristics of Pseudoglow Discharges in Helium under Atmospheric Pressure

A pseudoglow discharge behaviour is achieved at a 2.0-mm dielectric-dielectric electrode gap in pure helium under atmospheric pressure. An experimental study of the pseudoglow discharges is presented. The electrical characteristics and the discharge photos of the pseudoglow discharges are analyzed and discussed. The current-voltage parameters of the pseudoglow dis- charges are considered in regard to the influence on their behaviour.     

高温气冷堆热气联箱内部流场数值模拟

以高温气冷堆热气联箱为研究对象，在实验研究基础上，采用流体力学计算程序CFX5对热气联箱和热气导管内部流场进行数值模拟，以获得热气联箱和热气导管内的速度场、压力场和温度场，为高温气冷堆热气联箱的设计和实验研究提供参考。数值计算结果表明：热气联箱内气流发生剧烈搅混，加速了不同温度气流间的热传递，有利于高温和低温气流间的温度混合，存在肋片的区域未发生剧烈的气流搅混，不利于气流间的热传递；热气导管内温度混合率随其长度的增加逐渐增大，当热气导管长度为2.5m以上时，温度混合率达到99％以上。     

Experimental investigation on upstream and downstream discharges in airflows

Dielectric barrier discharge has widely used in airflow control, ignition and combustion, and other applications; the influence of airflow on dielectric barrier discharge is of extensive concern. Previous studies demonstrate that the discharge becomes more uniform and the discharge intensity decreases with increasing of airflow velocity. In this study, we adopt a discharge cell construction with upstream and downstream structure and study the discharge states and intensities. The experimental results demonstrate that within a specific range of airflow speed, the upstream discharge intensity is decreased, and the downstream discharge intensity is enhanced. The physical basis for this phenomenon is proposed as follows: Within a pulse interval time, some particles, such as charged and metastable particles produced by the upstream discharge, could be transported to the downstream region. The concentration of particles in the downstream region is increased, and these particles play a pre-ionization role in the downstream discharge, the intensity of the downstream discharge is enhanced. Further, factors such as the pulse frequency and the distance between electrodes are discussed in detail, along with the conditions for enhancing downstream discharge intensity.     

Atmospheric Pressure Radio Frequency Dielectric Barrier Discharges in Nitrogen/Argon

This work reports the experimental results on the characteristics of radio frequency dielectric barrier N2 /Ar discharges.Depending on the nitrogen content in the feed gas and the input power,the discharge can operate in two diferent modes: a homogeneous glow discharge and a constricted discharge.With increasing input power,the number of discharge columns increases.The discharge columns have starlike structures and exhibit symmetric self-organized arrangement.Optical emission spectroscopy was performed to estimate the plasma temperature.Spatially resolved gas temperature measurements,determined from NO emission rotational spectroscopy were taken across the 4.4 mm gap filled by the discharge.Gas temperature in the middle of the gas gap is lower than that close to the electrodes.     

Comparative research of plasma-assisted milling and traditional milling in synthesizing AlN

In this paper, traditional milling and discharge plasma-assisted milling are employed to synthesize aluminum nitride (AlN) powder at nanometer scale by milling the mixture of aluminum and lithium hydroxide monohydrate. AlN powders can be generated in traditional milling and plasma-assisted milling in an hour milling time. Differential thermal analysis curves show that the reaction temperature of the powders treated by plasma-assisted milling is lower than that of traditional milling. These results indicate that plasma-assisted milling has higher efficiency in the synthesis of AlN, getting smaller crystallite size and activating powder. Moreover, an optical emission spectrum is employed to demonstrate the active species in plasma. The different formation process of AlN in the two-milling process, and the promotion effects of plasma in the milling process are discussed.     

Electrical characters and optical emission spectra of VBD coupled SBD excited by sine AC voltage in atmospheric air

In this paper,volume coupled surface barrier discharge(V-SBD) with three structures possessing different volumes is excited by sine AC power in atmospheric air.Discharge images,waveforms of applied voltage and discharge current,and optical emission spectra simulating rotational and vibrational temperatures are recorded and analyzed.The effects of applied voltage on emission intensities of N_2(C~3Π_u→ B~3Π_g) and N_2~+(B~2∑_u~+ → X~2E_g~+),and rotational and vibrational temperatures are investigated.The results show that as applied voltage rises,emission intensities and rotational temperatures increase while vibrational temperatures decrease.In addition it is found that,as applied voltage varies,the rotational temperature of surface discharge changes faster than that of volume discharge.