Nature of convection-stabilized DC arcs in dual-flow nozzlegeometry. II. Optical diagnostics and theory

For pt.I see ibid., vol.18, no.1, p.91-101, 1990. A supersonic flow field with a 5.5-cm-long and ≈2.2-mm-thick cylindrical arc plasma column was observed with a four-mirror Schlieren optical system in dual-flow nozzle geometries. For both the orifice-type nozzle and the two dimensional convergent-divergent nozzle, the arc current was varied from 45 to 110 A. The optical cold-flow-plasma boundary displayed a sharp and laminar character in both nozzles, and a sharply defined, almost-constant-diameter, quiet arc is observed between the nozzles. Downstream of the nozzle throat the arc expands and assumes a conical shape. In this region, the fringe formation inside the arc is still clear, which is an indication of the laminar nature of the plasma. However, the arc boundary is not as distinct. A cooler arc is observed downstream of the nozzle throat. Using the experimentally determined axial static pressure and cold-flow mass flux rate distributions of pt.I and the channel-flow model with constant arc temperature, the energy integral was solved for the arc radius as a function of the axial distance. From this, the arc electric field strength, voltage, resistance, and power were determined, and the total heat transfer was related to the arc power. Good agreement between the calculated values and experimental data was observed     

The short vacuum arc. II. Model and theoretical aspects

For pt.I see ibid., vol.16, no.3, p.342-7, June 1988. A physical model is presented of the development of a short cathode-anode vacuum arc, from the moment of electrode separation to the final state, a cathode-autonomous vacuum arc. Although the highly nonstationary initial phase is accessible only to qualitative reasoning, the analysis of the stationary arc is treated quantitatively, accounting for anode influence using an existence diagram. The cathode-autonomous phase is previously known in the literature. Results for the phase transitions, arc appearance, and arc motion agree qualitatively with previous observations (reported in Part I)     

Experimental investigation of the anode region of a free-burning atmospheric-pressure inert-gas arc I. General characteristics of the discharge. Low-current regime

This paper reports the experimental investigation of the anode region of a free-burning inert-gas arc at atmospheric pressure in the current range from a few amperes to hundreds of amperes. The tungsten thermionic-emission cathode and the large-diameter water-cooled copper anode that were used permitted the anode arc root to assume its natural form. The general characteristics of the discharge are given and results are presented from investigations of the anode region at low currents, where the anode arc root is single and constricted, but erosion-free. Measurements of the plasma parameters as well as the current density in the arc root are reported, and a comparison is made between the values obtained and those characteristic of the region of the cathode arc root. Zh. Tekh. Fiz. 67, 35–40 (January 1997)     

Study of a circuit-breaker arc with self-generated flow. I. Energytransfer in the high-current phase

This study deals with the first phase of operation, corresponding to the energy transfer between arc and gas and to the pressure rise. The experimental study is devoted to measurements of current, arc voltage, and pressure variations in N₂ and SF₆. For currents of the order of 10 kA the mean measured electric field was about 32 V/cm in SF₆ and 36 V/cm in N₂. Through a bibliographical study and a modeling approach of the interruption arc, an analysis of the role of the different mechanisms of energy transfer between the arc and SF₆ was conducted. With a 10 kA pulse, about 80% of the transfer is found to be due to convection and the rest to radiation from the arc. This transfer results in an overpressure of nearly 1 bar in SF₆ and 2 bar in N₂. It appears that the operation of this type of circuit breaker is limited to high currents: for currents below 7 kA the overpressure is lower than 0.5 bar, which does not provide efficient blowing at current zero     

Numerical modelling of unsteady heating and melting of the anode by electric arc. Part 1. Mathematical model and computed characteristics of the arc column

The two-dimensional mathematical model in the approximation of a partial local thermodynamic equilibrium of plasma and the technique for numerical computation of the unsteady electric arc characteristics, including the conjugate heat exchange of the electric-arc plasma flow with the treated product (anode) are considered. The results of overall testing of the numerical algorithm point to the correctness of the model and computational technique. The results of the computation of the arc characteristics from the conventional ignition moment until the passage to stationary regime are presented. It is found that a single thermal torus, which scatters fairly quickly in the ambient medium, forms around the arc column.     

Cover Picture: Contrib. Plasma Phys. 9/2019

Typical discharge images of multi‐arcs in the arc chamber (for every arc, gas flow rate: 10 slm, discharge power: 450 W). Note that timing starts from forming three independent arcs in the arc chamber. Fig. 3 of the paper by Q.F. Lin et al.     

Nature of convection-stabilized DC arcs in dual-flow nozzlegeometry. I. The cold flow field and DC arc characteristics

An experimental investigation of the steady-state low current air arcs in a dual-flow nozzle system is presented. The cold flow field with no arc was determined for various nozzle geometries, i.e. two- and three-dimensional and orifice nozzles, and nozzle pressure ratios. Supersonic flow separation and oblique and detached shock waves were observed in the flow field. Using a finite-element computer program, the Mach number contours were determined in the flow field for various nozzle-gap spacings and pressure ratios. In addition, the DC arc voltage and current measurements were made for an electrode gap spacing of ≈5.5 cm and current levels of I≈25, 50, and 100 A for the three nozzle geometries. The arc voltage and arc power increased rapidly as the flow speed increased from zero to sonic velocity at the nozzle throat. The shock waves in the converging-diverging nozzles resulted in a decrease in the overall resistance by about 15%     

Experimental and theoretical investigation of high-pressure arcs.II. The magnetically deflected arc (three-dimensional modeling)

While Part I deals with cylindrical arcs, Part II studies the influence of transverse magnetic fields on the arc column for ambient pressures of 0.1-5.0 MPa. If exposed to a magnetic induction of several millitesla, the column of an arc is deflected by the Lorentz forces. In this paper, heat transfer and fluid flow with coupled electromagnetic forces are modeled for the magnetically deflected arc. To verify the predictions, the three-dimensional temperature distributions of the arc column are determined by line and continuum radiation measurements using tomographic methods. These temperature maps are compared with the results of the numerical simulations. To gain insight into the physical professes of the discharge and to make the arc properties available which are not readily measured, a self-consistent numerical model of the arc column is applied to the time-dependent and three-dimensional case. The temperature, velocity, pressure, and current densities are predicted by solving the conservation equations for mass, momentum, and energy, and Ohm's and Biot-Savart's law using material functions of the plasma. A control volume approach facilitates a numerically conservative scheme for solving the coupled partial differential equations. The predictions are in fair agreement with experimental results. A time-dependent fully implicit simulation of the arc was used to investigate the arc instabilities for large magnetic inductions     

Perturbed Arcs in Turbulent Axial Gas Flow. III. Numerical Simulation of Stochastically Disturbed Arc Discharges

Stochastically perturbed arcs are modelled by means of a Mayr type differential equation, the thermal constant τ being stochastically controlled by Gaussian fluctuations of given dispersion and correlation time: 1/τ = 1/τ₀ (1 + ζ). The ζ-signals are taken from an electronic noise source by an appropriately equipped intelligent CAMAC crate, which further processes the whole simulation program. Special cases treated are 1. stationary (in the mean) arc, 2. freely decaying arc, and additionally, 3. thermal reignition of a decaying arc with stiff recovery voltage.     

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讨论了在相距25.0mm长度40.0mm的平行电极间的由强度可达6040Gs的外磁场驱动的滑移弧功率变化过程,分析了滑移弧等离子体所处的状态。当电弧运行在电极平行段时,电弧功率在0.65kW附近剧烈振荡;当电弧接近平行电极顶端时,电弧功率由约0.65kW迅速增加到约2.0kW。在一个放电周期内,电弧功率基本上是从小到大持续增加的,由弧电源提供能量来维持滑移弧等离子体的平衡状态。根据Ozlem Mutaf-Yardimici等人的理论,此时滑移弧等离子体仍处于平衡态。     

Study of a circuit-breaker arc with self-generated flow. II. Theflow phase

For pt.I see ibid., vol.16, no.6, p.606-14 (1988). The experimental results presented concern the variations of the mean electric field and gas mass flow during this phase. The most important part consists of a modeling of the evolution of the interruption arc during the decrease of the current from 1000 A to 0. In this modeling, based on the conservation equations of mass and energy, the boundary conditions are determined by an approximate separate modeling of the arc whereas turbulence is treated through Prandtl's approximation. This theoretical study has been developed in the case of SF₆ and nitrogen. The computed values of the electric field and temperature show that the arc has a quasi-stationary behaviour as long as the current intensity is greater than a few tens of amperes, for a decay rate of 1.35 A/μs. The energy losses are governed by radiation at high current and by turbulence conduction at low current. The most important results concern the conductance, whose evolution time constant, immediately prior to current zero, is 3.5 μs in SF₆ and 15 μs in nitrogen. The difference is essentially due to variations with temperature of thermal conductivity and specific heat in two gases     

The short vacuum arc. I. Experimental investigations

High-current vacuum arcs drawn between commercial radial-magnetic field, chromium-copper contacts were studied by high-speed photography. The aim of the investigation was to study phenomena of relevance to high-current interruption, such as arc constriction and arc motion. The arcs were drawn at contact opening speeds typical of commercial devices, are duration being 10 ms or less. The arcs were `short' for much of their lifetime, and strong electrode-discharge coupling could be expected. Such arcs are also of principal interest. Arc behavior was found to be strongly influenced by the gap length d and the arc current I. No diffuse mode was observed at d less the d_min≈4 mm and at instantaneous currents I above I_max±25 kA. The diffuse discharge mode was assumed more readily when d was large. At d below 2 mm, the arcs could not be moved by a magnetic field. Increasing both I and d resulted in an increase of the probability and duration of arc motion and of the arc path length. Arc speed was often irregular, showing that arc motion is also affected by parameters other than the purely electrodynamic ones     

Experimental and theoretical investigation of high-pressure arcs.I. The cylindrical arc column (two-dimensional modeling)

The properties of the free-burning arc column are studied for ambient pressures of 0.1 MPa (i.e., atmospheric) to 10 MPa for applications in underwater welding and cutting as well as arc discharge lamps. The influence of transverse magnetic fields is studied in Part II. A DC current of 50-100 A is applied to an argon discharge with a conical tungsten cathode and a plane water-cooled anode which are separated by several millimeters. The electrical properties are measured, and the temperature distribution is determined by spectroscopic means utilizing a two-dimensional charge-coupled device (CCD) sensor. A self-consistent numerical solution of the conservation equations yields the temperature, velocity, pressure, and current distributions. The predicted arc temperatures agree well with the measured temperature distributions. An analysis of the conservation equations shows that the arc column becomes radiation dominated with increasing pressures resulting in small temperature gradients within the column and large gradients at the boundaries. It is found that a net emission coefficient might be used to account for the radiative heat transfer in the investigated parameter range. The arc constricts due to increased convective cooling especially at the cathode, while temperatures and velocities are decreasing. The power expended in the column scales approximately with the square root of the ambient pressure in line with the radiation dominance, whereas the voltage drop across the electrode sheaths exhibits no pressure dependence for a given current     

The influence of vaporization upon the roots of a high current arc. I. Different forms of vaporization in the arc roots

Arc roots at which marked vaporization occur are unstable and therefore in general non-stationary. High speed photography of arc roots using various electrode materials in Ar, SF₆, and N₂ shows that the appearance of the arc roots results from the superposition of several phenomena. On graphite cathodes in Ar it was possible to observe these phenomena separately by varying the cooling of the electrodes. It is shown that distinctions must be drawn between the real current transfer region with relatively weak vaporization, vaporization from the electrode surface outside this region and vaporization from within the body of the electrode. The energy-balance determined experimentally confirms the theoretical prediction that the energy for strong vaporization of the cathode is supplied by an ioncurrent.     

Decay of inert-gas-discharge plasma in the expansion mode. Part 1: Dynamics of the plasma ionization state,criterion of recombination nonequilibrium,and experimental study of argon plasma decay

The influence of expansion of a pulsed arc plasma on its decay is studied theoretically and experimentally with allowance for deviation from the equilibrium state. The dynamics of the ionization state during the decay of a dense expanding plasma is traced, and a criterion of its recombination nonequilibrium is determined. The results of calculation are compared with experimental data by the example of a pulsed arc in argon.     

Perturbation analysis of an energy-balance model: time constants oflow-current cascade arcs in argon

A time-constant formula is derived from a perturbation analysis of an equilibrium energy-balance equation of arc plasmas. The formula gives the time constant associated with changes in arc quantities when the arc current is perturbed from its steady-state. The current perturbation is assumed to be very small and the mathematical approach assumes that the arc behavior is linear. This paper investigates how well a linear model describes the small-signal behavior of the arc. The derived formula gives an insight into which are properties are responsible for the dynamic behavior of the arc about its steady-state operating point. Application of the method has been illustrated with arcs in argon. Results calculated with the formula are compared to time-constants deduced from equilibrium model predictions of the decay in the electric field when the current is stepped     

Observation of arc modes in a magnetically rotating arc plasma generator

An arc plasma can present various forms under the influence of an external magnetic field. In this study, a magnetically rotating arc plasma generator has been developed to produce three arc modes, namely rigid arc, distorted arc, and diffuse arc, which are obtained by controlling the gas flow rate. The evolution of these arc modes are experimentally studied and discussed. Results show that, as the gas flow rate increases, the arc mode is first transformed from the rigid to the distorted mode, and then to the diffuse mode. Comparisons show that the location of the arc attachment is a key factor in determining the rigid and distorted modes. The diffuse arc is observed under larger gas flow rates, but the completely diffuse arc can exist only within a narrow range of gas flow rates. Compared to the distorted arc, the diffuse arc has not only better stability but also a wider high‐temperature plasma zone, which indicates that the diffuse mode may be more useful in industry.     

Fluctuation of arc plasma in arc plasma torch with multiple cathodes

Fluctuation phenomena commonly exist in arc plasmas, limiting the application of this technology.In this paper,we report an investigation of fluctuations of arc plasmas in an arc plasma torch with multiple cathodes.Time-resolved images of the plasma column and anode arc roots are captured.Variations of the arc voltage, plasma column diameter, and pressure are also revealed.The results indicate that two well-separated fluctuations exist in the arc plasma torch.One is the high-frequency fluctuation(of several thousand Hz), which arises from transferring of the anode arc root.The other is the low-frequency fluctuation(of several hundred Hz), which may come from the pressure variation in the arc plasma torch.Initial analysis reveals that as the gas flow rate changes, the low-frequency fluctuation shows a similar variation trend with the Helmholtz oscillation.This oscillation leads to the shrinking and expanding of the plasma column.As a result, the arc voltage shows a sinusoidal fluctuation.     

Unsteady three-dimensional model of electric arc. Part 2. Verification

Numerical computations of the electric arc in the external magnetic field have been done for testing the unsteady three-dimensional mathematical model. The arc of a conical and helical shape has been implemented numerically. A satisfactory agreement of the computational results with experimental data points overall to the correctness of the mathematical model and of the computational algorithm.     

药芯焊丝脉冲TIG电弧增材制造电弧特性研究

对药芯焊丝脉冲TIG电弧增材制造电弧特性展开研究。利用高速摄像拍摄不同熔敷层脉冲电流条件下的电弧与熔滴过渡图片,对高速摄像图片进行分析,发现焊丝熔化过程存在“滞熔”现象,导致熔滴过渡存在渣桥过渡与液桥过渡两种接触过渡方式,在脉冲峰值电流较小的50/100 A电流参数下,出现熔滴断续的渣桥过渡的频率最高。熔滴过渡影响电弧温度场与药粉成分在电弧中的分布,利用光谱诊断分析熔敷过程中在不同脉冲峰值电流与脉冲基值电流条件下电弧温度场及药粉成分在电弧中的分布。利用点阵法测量得到各点光谱数据,根据Boltzmann图法计算各点温度,将各点温度拟合得到完整电弧温度场,结果表明,焊丝从钨极轴线前（左）侧送入,吸收电弧热量并且对电弧有扰动作用,电弧前侧温度低于电弧后（右）侧,电弧前侧尺寸稍小于后侧;随着熔敷层数增加,降低峰值电流,电弧收缩,高温区面积相对减小,低温区面积相对增大。电弧最高温度区域出现在钨极下方1~2 mm的范围,大约为13 000~15 000 K,脉冲峰值电流越大则最高温度区域面积越大。在脉冲基值电流时期,由于电流小,电弧面积相比于峰值时期要小得多,焊丝与电弧相互作用减弱,电弧温度场基本关于钨极轴线对称分布。选择药芯焊丝中特有的Na元素的NaⅠ589.6 nm谱线对其分布点进行标记,拟合绘出不同脉冲峰值电流与基值电流下药粉元素在电弧中的分布情况,结果表明,电流越小,药粉运动高度越低,在不同的脉冲峰值电流下药粉均没有沾染到钨极上,在不同的脉冲峰值电流与脉冲基值电流下 Na元素均偏电弧后侧分布,说明焊丝自电弧前侧送入熔池后,在电弧前侧的电弧中没有出现药粉强烈的喷发现象,而是进入熔池进行冶金反应。接触过渡解决了碱性焊丝工艺性差的问题,电弧较为稳定,避免药粉喷发损伤钨极,熔敷过程稳定进行。