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.     

Modelling Study to Compare the Flow and Heat Transfer Characteristics of Low-Power Hydrogen, Nitrogen and Argon Arc-Heated Thrusters

A modelling study is performed to compare the plasma flow and heat transfer characteristics of low-power arc-heated thrusters (arcjets) for three different propellants: hydrogen, nitrogen and argon. The all-speed SIMPLE algorithm is employed to solve the governing equations, which take into account the effects of compressibility, Lorentz force and Joule heating, as well as the temperature- and pressure-dependence of the gas properties. The temperature, velocity and Mach number distributions calculated within the thruster nozzle obtained with different propellant gases are compared for the same thruster structure, dimensions, inlet-gas stagnant pressure and arc currents. The temperature distributions in the solid region of the anode-nozzle wall are also given. It is found that the flow and energy conversion processes in the thruster nozzle show many similar features for all three propellants. For example, the propellant is heated mainly in the near-cathode and constrictor region, with the highest plasma temperature appearing near the cathode tip; the flow transition from the subsonic to supersonic regime occurs within the constrictor region; the highest axial velocity appears inside the nozzle; and most of the input propellant flows towards the thruster exit through the cooler gas region near the anode-nozzle wall. However, since the properties of hydrogen, nitrogen and argon, especially their molecular weights, specific enthalpies and thermal conductivities, are different, there are appreciable differences in arcjet performance. For example, compared to the other two propellants, the hydrogen arcjet thruster shows a higher plasma temperature in the arc region, and higher axial velocity but lower temperature at the thruster exit. Correspondingly, the hydrogen arcjet thruster has the highest specific impulse and arc voltage for the same inlet stagnant pressure and arc current. The predictions of the modelling are compared favourably with available experimental results.     

Measurement of radial temperature distributions of the blown CO_2 arcs under different conditions

In this paper, the radial temperature distributions of the blown CO_2 arcs in a model gas circuit breaker were investigated by optical emission spectroscopy methods. The CO_2 flows with different flow rates(50, 100 and 150 1 min~(-1)) were created to axially blow the arcs burning in a polymethyl methacrylate(PMMA) nozzle. Discharges with different arc currents(200 and 400A) were conducted in the experiment. The absolute intensity method was applied for a carbon ionic line of 657.8 nm to obtain the radial temperature profiles of the arc columns at a cross-section 1 mm above the nozzle. The calibration for the intensity of the CⅡ 657.8 nm line was achieved by the Fowler–Milne method with the help of an oxygen atomic line of 777.2 nm.The highest temperature obtained in the arc center was up to 19 900 K when the arc current was 400 A and the CO_2 flow rate was 50 1 min~(-1), while the lowest temperature in the arc center was about 15 900 K when the arc current was 200 A and the CO_2 flow rate was 150 1min~(-1). The results indicate that as the arc current increases, the temperature in the arc center would also increase apparently, and a larger gas flow rate would lead to a lower central temperature in general. It can also be found that the influence of the CO_2 flow rate on the arc temperature was much less than that of the arc current under the present experimental conditions. In addition,higher temperature in the arc center would cause a sharper temperature decrease from the central region towards the edge.     

Safety of fusion magnets: Model experiments to high current arcs at ITER busbars and feeder lines

Electric arcs moving along the power cables (the so-called busbars) of the toroidal field (TF) coils of ITER may reach and penetrate the cryostat wall. Model experiments with the VACARC (VACuum ARC) device were initiated to investigate the propagation and destruction mechanisms of busbar arcs in small scale. These experiments are intended to support the development and validation of a numerical model. The present setup simulates the fact that the busbar is contained by an inner and an outer feeder tube which absorb radiated energy and will be in contact with hot metal melt from the busbar. The tubes may also direct the hot plasma jet flow of the arc as long as they are intact. Experiments at scaled values for tube diameter, wall thickness and expected power density suggest serious damage to the inner feeder tube by the arc. This asks for further investigations that include the outer tube, too. The extent of damage to the tube samples depends on arc power, as expected. At high currents an additional luminous effect takes place, which is attributed to a reaction of oxygen and hydrogen probably set free from the dissociation of the insulation resin by the heat of the arc.     

Study of double-chamber air arc plasma torch and the application in solid-waste disposal

Arc plasma can be applied in hazardous solid waste disposal for higher temperature than common heating methods, but some practical issues exist in practical engineering application. In this study, an air arc plasma torch with double chambers and magnetic controlling is designed to realize wide variable power and long electrode life. The detailed characteristics and laws of the air arc are studied. The condition parameters of arc current(I), air flow rate(G) and the structure parameters of inlet area ratios and electrode diameters influence both the arc voltage and arc root positions. The arc rotating driven by magnetic field effectively lengthens the electrode life. The gasification process and product of organic wastes by air plasma are influenced largely by the waste compositions and the air flow rate. A furnace structure with more even atmosphere and longer residence time should be considered for better gasification. Oxygen-deficient environment is important to suppress NOxformation during the application of air plasma. Inorganic solid wastes can be melt by the air plasma and cooled down to form compact vitreous structures in which heavy metals can be locked and the leaching rates significantly decrease down.     

Dynamic characteristics of multi-arc thermal plasma in four types of electrode configurations

The enhanced volume of thermal plasma is produced by a multi-arc thermal plasma generator with three pairs of discharge electrodes driven by three directed current power suppliers. Combined with a high-speed camera and an oscilloscope, which acquire optical and electric signals synchronously, the dynamic behavior of different kinds of multi-arc discharge adjusted by the electrode arrangement is investigated. Also, the spatial distributions and instability of the arc discharge are analyzed in four electrode configurations using the gray value statistical method. It is found that the cathodic arcs mainly show a contracting state, while the anodic arcs have a trend of transition from shrinkage to a diffusion-like state with the increase of the discharge current. As a result of the adjustment of the electrode configuration, a high temperature region formed in the center of the discharge region in configurations of adjacent electrodes with opposite flow distribution and opposite electrodes with swirl flow distribution due to severe fluctuation of arcs. The discharge voltage rises with increased discharge current in this novel multi-arc plasma generator. It is also found that anode ablation mainly occurs on the conical surface at the copper electrode tip, while cathode erosion mainly occurs on the surface of the inserted tungsten and the nearby copper.     

Behavior of an indigenously fabricated transferred arc plasma furnace for smelting studies

The utilization of industrial solid waste for metal recovery requires high-temperature tools due to the presence of silica and alumina, which is reducible at high temperature. In a plasma arc furnace, transferred arc plasma furnace(TAP) can meet all requirements, but the disadvantage of this technology is the high cost. For performing experiments in the laboratory, the TAP was fabricated indigenously in a laboratory based on the different inputs provided in the literature for the furnace design and fabrication. The observed parameters such as arc length, energy consumption, graphite electrode consumption, noise level as well as lining erosion were characterized for this fabricated furnace. The nitrogen plasma increased by around 200 K(200 ℃) melt temperature and noise levels decreased by ~10 d B compared to a normal arc.Hydrogen plasma offered 100 K(100 ℃) higher melt temperature with ~5 d B higher sound level than nitrogen plasma. Nitrogen plasma arc melting showed lower electrode and energy consumption than normal arc melting, whereas hydrogen plasma showed lower energy consumption and higher electrode consumption in comparison to nitrogen plasma. The higher plasma arc temperature resulted in a shorter meltdown time than normal arc with smoother arcing. Hydrogen plasma permitted more heats, reduced meltdown time, and lower energy consumption, but with increased graphite consumption and crucible wear. The present study showed that the fabricated arc plasma is better than the normal arc furnace with respect to temperature generation, energy consumption, and environmental friendliness. Therefore, it could be used effectively for smelting-reduction studies.     

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.     

Nonequilibrium modeling study on plasma flow features in a low-power nitrogen/hydrogen arcjet thruster

Gasdynamic flow features in an electrothermal arcjet thruster with a mixture of 1:2 nitrogen/ hydrogen as the working gas have been studied by a two-temperature numerical simulation.Seven species and 17 kinetic processes are included in the chemical kinetic model used to represent dissociation,ionization,and the corresponding recombination reactions in this nitrogen/hydrogen mixture system.Based on the gas flow characteristics inside the arcjet nozzle,a new method is introduced to define the edge of the cold boundary layer,which is more convenient to analyze the evolution and development of plasma flow in an arcjet thruster.The results show that the arcjet thruster performance is determined largely by the exchange of energy and momentum between the low-density,high-temperature arc region and the high-density,coolttow region near the nozzle wall.A significant thermal nonequilibrium is found in the cold boundary layer in the expansion portion of the nozzle.The important chemical kinetic processes determining the distribution of hydrogen and nitrogen species in different flow regions are presented.It has been shown that the reaction rate of hydrogen species ionization impacted by electrons is much higher than that of nitrogen species ionization in the center of the constrictor of the arcjet thruster.This indicates that hydrogen species is very important in the conversion of applied electric energy into thermal energy in the constrictor region of the arcjet thruster.     

Characteristic analysis of CO2 switching arcs under a DC current

The current interruption capability of a gas,when used in high voltage gas-blast circuit breakers,depends not only on its material properties but also the flow field since turbulence plays a dominant role in arc cooling during the interruption process.Based on available experimental results,a study of CO2 switching arcs under a DC (direct current) current in the model circuit breaker has been conducted to calibrate CO2 arc model and to analyse its electric and thermal property.Through detailed analysis of the results mechanisms responsible for the temperature distribution are identified and the domain energy transportation process of different region discussed.The present work provides significant coefficients for CO2 switching arc simulation and gives a better understanding of CO2 arc burning mechanisms.     

等离子体切割的弧特性研究

基于弧柱双区域近似的简单理论,研究了空气等离了体切割产生的等离子体弧特性,给出了不同工作条件（空气流速及喷嘴直径）下,弧的半径、电压及喷嘴出口处压强随弧电流变化的计算结果。研究结果表明,计算与实验结果符合得好。同时讨论了空气流速及喷嘴直径对等离子体功率及作用力的影响。     

Investigation on Plasma Jet Flow Phenomena During DC Air Arc Motion in Bridge-Type Contacts

Arc plasma jet flow in the air was investigated under a bridge-type contacts in a DC 270 V resistive circuit.We characterized the arc plasma jet flow appearance at different currents by using high-speed photography,and two polished contacts were used to search for the relationship between roughness and plasma jet flow.Then,to make the nature of arc plasma jet flow phenomena clear,a simplified model based on magnetohydrodynamic (MHD) theory was established and calculated.The simulated DC arc plasma was presented with the temperature distribution and the current density distribution.Furthermore,the calculated arc flow vclocity field showed that the circular vortex was an embodiment of the arc plasma jet flow progress.The combined action of volume force and contact surface was the main reason of the arc jet flow.     

Interruption Phenomenon in Intermediate-Frequency Vacuum Arc

In the condition of the 3 mm gap, experiments for 360 Hz intermediate-frequency vacuum arc are carried out in interrupters with the diameters being 41 mm and with the contact materials being CuCr50 and Cu-W-WC alloy respectively. The results indicate that the contacts material is closely related to the breaking capacity of the vacuum interrupters and characteristics of an intermediate-frequency vacuum arc. For contacts with the same diameter, the breaking capacity of CuCr50 is better than that of Cu-W-WC. When the current fails to be interrupted, the arcs overflow the gap and present irregular performances in the first half wave. Consequently a voltage spike appears. More macroscopic metal droplets can be seen in the arc column between CuCr50 contacts because of the lower melting point. It is observed that the droplet emission is much more severe during arc reignition than that in the first half wave. It is much more conspicuous that the high frequency arc voltage noises appear in Cu-W-WC contacts when the vacuum arcs reignite, for higher temperature and stronger electronic emission ability of Cu-W-WC contacts.     

Experimental Analysis on Mitigation of Thermal Stratification in the Suppression Pool of a Water Wall Type Passive Containment Cooling System

A water wall type passive containment cooling system with an outer pool surrounding the suppression pool is one passive containment cooling system. In the system, a baffle plate in the suppression pool mitigates thermal stratification formed at the vent tube outlet level and enlarges the heat transfer area. The effectiveness of the baffle plate in mitigating thermal stratification was experimentally confirmed; the heat transferred to the outer pool increased about 50% due to a larger high temperature region and a longer effective heat transfer length. The experimental analysis was performed using a three-dimensional thermal-hydraulic analysis program. In the analysis, a laminar flow model and slip conditions on structural walls were used, and the calculated temperature profiles and natural circulation flow rates along the baffle plate agreed with measurements. The model was then judged as a valid and practical tool to evaluate global natural circulation and temperature distributions in a large pool. And it was analytically corn- firmed that the thermal resistance of the PCV wall and the heat flux to the outer pool affected the performance of the baffle plate.     

直流电弧等离子体点火器射流特性研究

采用光谱仪测量了等离子体点火器出口射流的发射光谱,利用玻尔兹曼曲线斜率法计算了射流的电子温度,并通过电离平衡方程计算了射流气体温度,获得点火器出口射流长度、射流速度、电子温度和射流温度随弧电流及进口氩气流量的变化规律。并分析了航空等离子体电弧射流中是否可使用电子温度来代替射流气体温度。实验表明：弧电流随着进口氩气流量的增大而减小;出口射流长度和速度随弧电流的增大而增大,随进口氩气流量的增大先增大后减小;出口电子温度、电子密度和射流温度随弧电流的增大而升高,随氩气流量的增大而降低。     

Optical signature of RF arcs in the ICRH frequency range

RF arc detection is a key operational and safety issue for ICRF systems. Dedicated measurements on a RF test-stand were made in order to characterize the optical signature of RF arcs (time and spectrum) to assess the potential of optical arc detection on ICRF systems. Time-resolved intensity measurements were carried out using high-speed Si photodetectors with different bandpass filters. The rise time of the arc emission, the light intensity and the time evolution of the arc under different pressure conditions are discussed. Spectral signature of the arcs was obtained using spectrometers in the visible and UV range. It is shown that the arc emission spectrum is independent of the gas pressure and that it is mainly dominated by the stainless steel components of the test bench. The results of the RF arc optical characterization is discussed with a view on the implementation of an optical arc detection system for an ICRF antenna on a fusion machine, both for sections viewing the plasma light as for private vacuum sections.     

Enthalpy Probe Technique for Thermal Plasma Diagnostics

The measuring principle and experimental results of the enthalpy probe technique for thermal plasma diagnostics are presented. Its calibration and errors are discussed. Typical results are presented for the system operation in an Ar/H2(5 % H2) plasma arc jet under a reactor chamber pressure of 101.3 kPa. The plasma temperature and velocity profiles are measured. The center temperature and velocity are 6600 K and 850 m/s for plasma power 9 kW at axial locationof 17 mm.     

Numerical Simulation of Fluid Flow and Heat Transfer in a DC Non-Transferred Arc Plasma Torch Operating Under Laminar and Turbulent Conditions

In this work, a magnetic fluid dynamics (MHD) model is used to simulate the electromagnetic field, heat transfer and fluid flow in a DC non-transferred arc plasma torch under laminar and turbulent conditions. The electric current density, temperature and velocity distributions in the torch are obtained through the coupled iterative calculation about the electromagnetic equations described in a magnetic vector potential format and the modified fluid dynamics equations. The fluid-solid coupled calculation method is applied to guarantee the continuity of the electric current and heat transfer at the interface between the electrodes and fluid. The predicted location of the anodic arc root attachment and the arc voltage of the torch are consistent with corresponding experimental results. Through a specific analysis of the influence of mass flow rates and electric current on the torch outlet parameters, the total thermal efficiency, thermal loss of each part, and the laws of the variation of outlet parameters with the variation of mass flow rates and electric current was obtained. It is found that operation under a laminar condition with a limited area of the anode could increase the total thermal efficiency of the torch.     

Design and characteristics of a triple-cathode cascade plasma torch for spheroidization of metallic powders

Arc plasma torch is an effective tool for spheroidization of metallic powders. However, as most conventional plasma torches were not specifically designed for plasma spheroidization, they may exhibit the disadvantages of the radial injection of powders, large fluctuations in the arc voltage, large gas flow rate, and disequilibrium between multiple plasma jets during the spheroidization process. Therefore, this paper presents a triple-cathode cascade plasma torch (TCCPT) for plasma spheroidization. Its structural design, including three cathodes, a common anode, and three sets of inter-electrodes, are detailed to ensure that powders can be inserted into the plasma jet by axial injection, the arc voltage fluctuations are easily maintained at a low level, and the plasma torches can work at a relatively small gas flow rate. Experimental results showed that the proposed TCCPT exhibits the following characteristics: (1) a relatively small arc voltage fluctuation within 5.3%; (2) a relatively high arc voltage of 75 V and low gas flow rate range of 10–30 SLM; (3) easy to be maintained at the equilibrium state with the equilibrium index of the three plasma jets within 3.5 V. Furthermore, plasma spheroidization experiments of SUS304 stainless steel powers were carried out using the proposed TCCPT. Results verified that the proposed TCCPT is applicable and effective for the spheroidization of metallic powders with wide size distribution.     

Theory and Experiment with a Gas Stabilized Constricted Arc

Measurements of voltage gradient and static pressure distribution in the constrictor of a gas stabilized constricted arc discharge are reported. These and previously obtained data yielding the column diameter are used to compare the behavior of this discharge with laminar column theories. Significant disagreement are found, and it is concluded that the discharge represents an inlet flow region. A qualitative model is presented of the physical processes occuring in this inlet region. The implications for arc heater efficiency of this model and of the data are discussed.