Two-dimensional self-consistent numerical simulation of the whole discharge region in an atmospheric argon arc

A 2D self-consistent numerical model of the whole argon-arc discharge region that includes electrodes is developed in this work to facilitate analysis of the physical processes occurring in atmospheric arc plasma. The 2D arc column model contains the ionization and thermal non-equilibrium, which is coupled with a 1D electrode sheath model. The influence of plasma-species diffusion near the electrode region is investigated based on Maxwell–Stefan equations and the generalized Ohm's law. The numerical results of argon free-burning arcs at atmospheric pressure are then investigated. The simulation shows that the plasma is obviously in the state of thermal and ionization equilibrium in the arc core region, while it deviates from thermal and ionization equilibrium in the arc fringe region. The actual electron density decreases rapidly in the near-anode and near-cathode regions due to non-equilibrium ionization, resulting in a large electron number gradient in these regions. The results indicate that electron diffusion has an important role in the near-cathode and near-anode regions. When the anode arc root gradually contracts, it is easy to obtain a positive voltage drop of the anode sheath (I = 50 A), while it remains difficult to acquire a positive anode sheath voltage drop (I = 150 A). The current–voltage characteristics predicted by our model are found to be identical to the experimental values.     

Three-dimensional non-equilibrium modeling of a DC multi-cathode arc plasma torch

In this paper, a three-dimensional non-equilibrium steady arc model is used to investigate the temperature, velocity and electromagnetic field in multi-cathode arc torch, and the formation mechanism of a large-area, uniform and diffused arc plasma is analyzed. The numerical simulation results show that a large volume plasma region can be formed in the central region of the generator during discharge. During this process, the maximum electron temperature appears near the cathode and in the central convergence region, while the maximum heavy particle temperature only appears in the central convergence region. This phenomenon is consistent with the experimental arc images. Near the cathode tip, the arc column is in a contraction state. In the area slightly away from the cathode, the six arc columns begin to join together. In the plasma generator, there is a large-scale current distribution in all directions of X, Y and Z, forming a stable arc plasma with a wide range of diffusion. The calculated electron temperature distribution is in good agreement with the measured electron temperature. The results suggest that the largearea diffused arc plasma in the multi-cathode arc torch is the combined effect of current distribution, convection heat transfer and heat conduction.     

Self-organized pattern on the surface of a metal anode in low-pressure DC discharge

Self-organization phenomena on the surface of a metal electrode in low-pressure DC discharge is studied. In this paper, we carry out laboratory investigations of self-organization in a lowpressure test platform for 100–200 mm rod-plane gaps with a needle tip, conical tip and hemispherical tip within 1–10 k Pa. The factors influencing the pattern profile are the pressure value, gap length and shape of the electrode, and a variety of pattern structures are observed by changing these factors. With increasing pressure, first the pattern diameter increases and then decreases. With the needle tip, layer structure, single-ring structure and double-ring structure are displayed successively with increasing pressure. With the conical tip, the ring-like structure gradually forms separate spots with increasing pressure. With the hemispherical tip, there are anode spots inside the ring structure. With the increase of gap length, the diameter of the selforganized pattern increases and the profile of the pattern changes. The development process of the pattern contains three key stages: pattern enlargement, pattern stabilization and pattern shrink.     

第二阳极作用改善阴极真空弧沉积中弧放电稳定性的研究

研究了阴极弧等离子体沉积中第二阳极现象改善弧放电稳定性的作用。结果表明：由于弧放电规模增大,等离子体电阻降低,第二阳极现象的存在可大幅度提高造成阴极弧放电不稳定的聚焦磁场阈值。     

Numerical simulation of constricted and diffusive arc–anode attachments in wall-stabilized transferred argon arcs

A numerical simulation is conducted to investigate arc–anode attachment behavior, especially the formation mechanism of the constricted arc attachment mode for the water-cooled anode of wall-stabilized transferred argon arcs. Argon molecular ions and the corresponding kinetic processes are included to the finite-rate chemistry model in order to capture the chemical nonequilibrium characteristics of the arc near the anode region. Modeling results show that constricted and diffusive arc–anode attachments can be self-consistently obtained at different arc currents while keeping other parameters unchanged. The dominant kinetic processes contributing to ionization and recombination in the arc center and fringes are presented. The results show that in arc fringes and the arc attachment region, molecular ion recombination plays an important role which leads to the rapid loss of electrons. The radial evolution of the production, loss and transport processes of electrons is further analyzed. It is found that for the constricted arc attachment mode, both the recombination and convection transport caused by the anode jet result in the loss of electrons at the arc fringes, which leads to the shrinkage of the arc column at the anode. The formation of the anode jet is due to the combined action of radial and axial Lorentz forces in the anode region.     

磁过滤真空弧沉积中过滤管道的第二阳极作用研究

利用一台磁过滤真空弧沉积装置,初步研究了磁过滤管道对系统弧放电的影响。实验证明,磁过滤管道在阴极真空弧沉积中不仅起到消除大颗粒的作用,还作为阴极真空弧放电的第二阳极对弧放电产生影响。给出了磁过滤阴极真空弧放电的等效电路,并用此电路对磁过滤阴极真空弧放电中的部分实验现象进行了解释。     

阴极端面旋转弧根的数值分析

辅助电弧的阴极弧根在管状钨阴极端面旋转时，随着阴极弧根旋转频率的增加，在阴极端面形成一圈温度较高的区域；区域中各点的温度（除弧根处温度）趋于均匀，从而为主电弧的阴极弧根在这个区域中分裂成多弧根或形成扩散型弧根模式提供了必要的温度条件；弧根处的阴极端面温度随旋转频率的增加而下降并趋于一个稳定的值。本文利用三维热传导方程对此进行了分析。     

Comparison of Reynolds average Navier-Stokes turbulence models in numerical simulations of the DC arc plasma torch

Five turbulence models of Reynolds average Navier-Stokes(RANS),including the standard k-ω model,the RNG k-e model taking into account the low Reynolds number effect,the realizable k-ω model,the SST k-ω model,and the Reynolds stress model(RSM),are employed in the numerical simulations of direct current(DC)arc plasma torches in the range of arc current from 80 A to 240 A and air gas flow rate from 10 m^3 h^-1 to 50 m^3 h^-1.The calculated voltage,electric field intensity,and the heat loss in the arc chamber are compared with the experiments.The results indicate that the arc voltage,the electric field,and the heat loss in the arc chamber calculated by using the standard k-ω model,the RNG k-ωmodel taking into account the low Reynolds number effect,and the realizable k-ω model are much larger than those in the experiments.The RSM predicts relatively close results to the experiments,but fails in the trend of heat loss varying with the gas flow rate.The calculated results of the SST k-ω model are in the best agreement with the experiments,which may be attributed to the reasonable predictions of the turbulence as well as its distribution.     

Comparative analysis of the arc characteristics inside the converging-diverging and cylindrical plasma torches

A two-temperature thermal non-equilibrium model is used to simulate and compare the arc characteristics within the converging-diverging and traditional cylindrical plasma torches.The modeling results show that the presence of the constrictor within the converging-diverging torch makes the evolution characteristics of the arc significantly different from that of cylindrical torch.Compared with a cylindrical geometrical torch,a much higher plasma flow velocity and relatively longer high temperature region can be generated and maintained inside the converging-diverging torch.In the constrictor of converging-diverging torch,the normalized radius of arc column increases and the degree of thermodynamic equilibrium of the plasma is significantly improved with the increase of axial distance.The radial momentum balance analysis shows that for the cylindrical torch,the pressure gradient that drives the arc expansion and the Lorentz force that drives the arc contraction dominate the radial evolution of the arc.While at the converging and constrictor region of a converging-diverging plasma torch,the radial gas dynamic forces in arc fringes pointing toward the arc center enhance the mixing of the cold gas of boundary layer with the high temperature gas of the arc center,increasing the average gas temperature and decreasing the thickness of cold boundary layer,thereby facilitating the formation of diffusion type arc anode attachment at the diverging section of torch.     

Influence of arc current and pressure on non-chemical equilibrium air arc behavior

The influence of arc current and pressure on the non-chemical equilibrium(non-CE) air arc behavior of a nozzle structure was investigated based on the self-consistent non-chemical equilibrium model. The arc behavior during both the arc burning and arc decay phases were discussed at different currents and different pressures. We also devised the concept of a nonequilibrium parameter for a better understanding of non-CE effects. During the arc burning phase, the increasing current leads to a decrease of the non-equilibrium parameter of the particles in the arc core, while the increasing pressure leads to an increase of the non-equilibrium parameter of the particles in the arc core. During the arc decay phase, the non-CE effect will decrease by increasing the arc burning current and the nozzle pressure. Three factors together—convection, diffusion and chemical reactions—influence non-CE behavior.     

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 simulation and analysis of lithium plasma during low-pressure DC arc discharge

Alkali metal DC arc discharge has the characteristics of easy ionization, low power consumption, high plasma temperature and ionization degree, etc, which can be applied in aerospace vehicles in various ways. In this paper, we calculate the physical property parameters of lithium vapor, one of the major alkali metals, and analyze the discharge characteristics of lithium plasma with the magnetohydrodynamic (MHD) model. The discharge effects between constant current and voltage sources are also compared. It is shown that the lithium plasma of DC arc discharge has relatively high temperature and current density. The peak temperature can reach tens of thousands of K, and the current density reaches 6×10 ⁷ A⁻² . Under the same rated power, the plasma parameters of the constant voltage source discharge are much higher than those of the constant current source discharge, which can be used as the preferred discharge mode for aerospace applications.     

Numerical simulation of the plasma acceleration process in a magnetically enhanced micro-cathode vacuum arc thruster

A particle-in-cell simulation is conducted to investigate the plasma acceleration process in a micro-cathode vacuum arc thruster. A coaxial electrode structure thruster with an applied magnetic field configuration is used to investigate the effects of the distribution of the magnetic field on the acceleration process and the mechanism of electrons and ions. The modeling results show that due to the small Larmor radius of electrons, they are magnetized and bound by the magnetic field lines to form a narrow electron channel. Heavy ions with a large Larmor radius take a long time to keep up with the electron movement. The presence of a magnetic field strengthens the charge separation phenomenon. The electric field caused by the charge separation is mainly responsible for the ion acceleration downstream of the computation. The impact of variations in the distribution of the magnetic field on the acceleration of the plasma is also investigated in this study, and it is found that the position of the magnetic coil relative to the thruster exit has an important impact on the acceleration of ions. In order to increase the axial velocity of heavy ions, the design should be considered to reduce the confinement of the magnetic field on the electrons in the downstream divergent part of the applied magnetic field.     

Numerical simulation of low-current vacuumarc jet considering anode evaporation in different axial magnetic fields

As the main source of the vacuum arc plasma, cathode spots (CSs) play an important role on the behaviors of the vacuum arc. Their characteristics are affected by many factors, especially by the magnetic field. In this paper, the characteristics of the plasma jet from a single CS in vacuum arc under external axial magnetic field (AMF) are studied. A multi-species magneto-hydro-dynamic (MHD) model is established to describe the vacuum arc. The anode temperature is calculated by the anode activity model based on the energy flux obtained from the MHD model. The simulation results indicate that the external AMF has a significant effect on the characteristic of the plasma jet. When the external AMF is high enough, a bright spot appears on the anode surface. This is because with a higher AMF, the contraction of the diffused arc becomes more obvious, leading to a higher energy flux to the anode and thus a higher anode temperature. Then more secondary plasma can be generated near the anode, and the brightness of the ‘anode spot’ increases. During this process, the arc appearance gradually changes from a cone to a dumbbell shape. In this condition, the arc is in the diffuse mode. The appearance of the plasma jet calculated in the model is consistent with the experimental results.     

典型DC/DC电源变换器电离辐射效应研究

研究了典型DC/DC电源变换器辐照后不同输出负载、不同输入电压等测试条件下,DC/DC电源变换器输出电压等参数变化情况与辐照剂量之间的关系.研究结果表明,DC/DC电源变换器输出电压衰减程度随输出负载功率增大;输入电压变化对输出电压特性的影响程度较小.     

Numerical modelling of a low power non-transferred arc plasma reactor for methane conversion

Thermal flow characteristics and the methane conversion reaction in a low power arc plasma reactor for efficient storage and transport of methane, which is the main component of shale gas, were simulated. The temperature and velocity distributions were calculated according to the type of discharge gases and arc current level by a self-developed magnetohydrodynamics (MHD) code and a commercial ANSYS-FLUENT code; the transport of chemical species was analyzed as including the chemical reactions of methane conversion. The simulated results were verified by the comparison of calculated and measured arc voltages with permissible low error as under 4%. Three C₂ hydrocarbon gases with ethane (C₂H₆), ethylene (C₂H₄), and acetylene (C₂H₂) were selected as the converted species of methane from experimental data. The mass fraction of C₂ hydrocarbons and hydrogen as the product of the conversion reaction at the reactor was also calculated. Those values show good agreement with the actual experimental results in that the major conversion reaction occurred in C₂H₂ and hydrogen, and the conversions to C₂H₆, C₂H₄, and hydrogen were minor reactions of methane pyrolysis conversion.     

Simulation of arcs for DC relay considering different impacts

Recently DC relay has been concerned as a key component in DC power distribution,management and control systems like aircraft,new energy vehicle,IT and communication industries.Ordinarily,magnetic force and contact moving speed have great influence on arc behaviours in the breaking process.This paper focuses on the numerical investigation of arc during the contact opening process in a real 400V/20 A DC relay product coupling with an inductive load circuit.A 3D air arc model based on the magneto-hydrodynamic theory was built and calculated.A method coupling different computational software was used to take the nonlinear permanent magnet and contact opening process into consideration simultaneously.Arc behaviours under different magnetic field and contact opening speed were presented and discussed carefully.It has been found that the increase of the magnetic field is beneficial to the quick build-up of arc length and voltage.Arc breaking duration becomes shorter with the increase in contact opening speed from 63.5 rad s^-1 to 94.5 rad s^-1,such reduction is less significant with an increase of opening speed from 94.5 rad s^-l to 118.5 rad s^-1.     

Numerical analysis on the effect of process parameters on deposition geometry in wire arc additive manufacturing

Here we develop a two-dimensional numerical model of wire and arc additive manufacturing (WAAM) to determine the relationship between process parameters and deposition geometry, and to reveal the influence mechanism of process parameters on deposition geometry. From the predictive results, a higher wire feed rate matched with a higher current could generate a larger and hotter droplet, and thus transfer more thermal and kinetic energy into melt pool, which results in a wider and lower deposited layer with deeper penetration. Moreover, a higher preheat temperature could enlarge melt pool volume and thus enhance heat and mass convection along both axial and radial directions, which gives rise to a wider and higher deposited layer with deeper penetration. These findings offer theoretical guidelines for the acquirement of acceptable deposition shape and optimal deposition quality through adjusting process parameters in fabricating WAAM components.     

Formation of a 40 A DC Current Arc During the Opening of Silver Contacts

The erosion of silver contacts due to break arcs with length proportional to time and of variable duration has been measured by weighing the contacts following 5000 openings at a constant current equal to 40 A.The experimental results show that,for arc durations shorter than 60 μs,the transfer of metal from the anode to the cathode occurs,but after passing this stage,when the two electrodes are separated by greater distances,each will display erosion.This is the result of the diffusion of material outside the space between the two electrodes.In order to interpret these results,we have applied a classical model of the physical phenomena occurring at the root of the arc.Analysis of the experimental results shows that for an arc duration of less than15 μs,no distinct cathode root is seen to exist,but beyond this,several spots appear gradually on the cathode for arc duration up to 50 μs,after which they merge into a single spot.The comparison between experiment results and theoretical interpretation is reasonable up to 60 μs.     

Particle-in-cell simulation for effect of anode temperature on discharge characteristics of a Hall effect thruster

Propellant gas flow has an important impact on the ionization and acceleration process of Hall effect thrusters (HETs). In this paper, a particle-in-cell numerical method is used to study the effect of the anode temperature, i.e., the flow speed of the propellant gas, on the discharge characteristics of a HET. The simulation results show that, no matter the magnitude of the discharge voltage, the calculated variation trends of performance parameters with the anode temperature are in good agreement with the experimental ones presented in the literature. Further mechanism analysis indicates that the magnitude of the electron temperature is responsible for the two opposing variation laws found under different discharge voltages. When the discharge voltage is low, the electron temperature is low, and so is the intensity of the propellant ionization; the variation of the thruster performance with the anode temperature is thereby determined by the variation of the neutral density that affects the propellant utilization efficiency. When the discharge voltage is high, the electron temperature is large enough to guarantee a high degree of the propellant utilization no matter the magnitude of the anode temperature. The change of the thruster performance with the anode temperature is thus dominated by the change of the electron temperature and consequently the electron-neutral collisions as well as the electron cross-field mobility that affect the current utilization efficiency.