Free-burning Arc Research Articles

Overview spectra and axial distribution of spectral line intensities in a high-current vacuum arc with CuCr electrodes

Spectroscopic investigations of free-burning vacuum arcs in diffuse mode with CuCr electrodes are presented. The experimental conditions of the investigated arc correspond to the typical system for vacuum circuit breakers. Spectra of six species Cu I, Cu II, Cu III, Cr I, Cr II, and Cr III have been analyzed in the wavelength range 350–810 nm. The axial intensity distributions were found to be strongly dependent on the ionization stage of radiating species. Emission distributions of Cr II and Cu II can be distinguished as well as the distributions of Cr III and Cu III. Information on the axial distribution was used to identify the spectra and for identification of overlapping spectral lines. The overview spectra and some spectral windows recorded with high resolution are presented. Analysis of axial distributions of emitted light, which originates from different ionization states, is presented and discussed.

Thermal and Chemical Nonequilibrium Effects in Free-Burning Arcs

Effects caused by thermal and chemical nonequilibrium in free-burning arcs in argon are presented and discussed. The results cover a range of arc currents between 100 and 200 A, interelectrode distance of 5–10 mm, and a variation of the electrode material and the shape of the cathode tip. The results obtained show that nonequilibrium in the near-electrode regions has a strong impact on the distributions of temperatures, electric conductivity and current density, and the arc voltage. Departure from chemical nonequilibrium in the vicinity of the cathode as a result of transport processes is indicated. Moreover, overpopulation of the atomic ground state close to the arc axis, under population of the excited states in the arc periphery with respect to the equilibrium values, axial dependence of the off-axis maximum of the radial emission coefficient of Ar I at 696.5 nm, and enhanced population of the Ar (1s4) resonance level due to trapping of resonance radiation in the arc periphery are considered.

Effect of trapping of resonance radiation in a free-burning Ar arc

A modified method for the solution of the Holstein-Biberman transport equation is proposed for cylindrical geometry, arbitrary absorption coefficient accounting for plasma inhomogeneity and a Lorentz spectral lineshape. The method is applied to the plasma generated by a free-burning arc in argon at atmospheric pressure in order to study the effect of trapping of resonance radiation on the population of the resonance state Ar(1s4). The results obtained are compared with those of a self-consistent nonequilibrium model of the arc including excited atomic levels. They show an enhanced (compared to the results of the nonequilibrium model) population of the resonance state outside of the arc core as an effect of the trapping of resonance radiation and almost negligible collisional de-excitation. In the arc core, the effect is negligible so that the calculated population of the Ar(1s4) state confirms the results obtained with the nonequilibrium model.

Derivation of a Continuum Model for Low Temperature Plasma Flow and its Application on Arc Discharge

A set of governing equations for non-equilibrium plasma flow is derived based on the Boltzmann equation and existing models. This continuum model considers both thermal and chemical non-equilibrium effects and can be applied in a wide range of flow and discharge conditions. The model is verified using a free-burning arc problem, and is applied to investigate the flow and discharge characteristics of an arcjet. It shows that the arc attachment point in arcjet moves downstream with the increasing flow rate due to the arc and flow interaction.

Study of the Influence of External Environment to Arc Channel in Argon with Laminar Flow

A 3D model is used to simulate free burning arc, and the influence of external environment to arc channel are discussed. The commercial CFD code FLUENT is used to model the plasma and the solid anode part. The cross-flow (2m/s, 5m/s, and 10m/s) and external magnetic fields (2mT, 5mT, and 10 mT) are applied to deflect arc. And then the condition they work together have also be discussed. Because the mechanisms of the influence are different, they only influences are reflect on the arc root displacement. The influence of arc root displacement to potential drop has been explained.

Study on temperature measurement of two-electrode TIG arc plasma

Measurements of arc plasma are important for determining the associated physical properties. Such measurements usually involve the use of spectroscopic techniques to measure temperature distribution in free-burning arcs. Most studies have reported the temperature of axially symmetric arc plasma using Abel inversion. This method cannot be used for axially asymmetric arc plasma such as two-electrode TIG arc plasma. It is a complex phenomenon because the arc plasma generated from each electrode is affected by the other due to electromagnetic force. We measured the temperature distribution of tandem TIG arc plasma three dimensionally with a one camera rotation system. The measurement method includes the multi-directional detections and the image reconstruction technique: maximum likelihood–expectation maximization. We evaluated the influence of the number of detection directions on the reconstruction by virtual intensity distribution and found that the sufficient direction number is 6. As a result, we could measure the temperature distribution of the inclined, drawn and coupled arc plasma provided by a two-electrode TIG arc.

Interaction of a free burning arc with regenerative protective layers

The possible use of protective layers made of ceramic powders for walls in thermal plasma applications is studied. A stable free burning arc of currents up to 5 kA between copper- tungsten electrodes is used to analyse the arc interaction with samples coated by mixtures of CaCO3, MgCO3, and Mg(OH)2 with plaster. By means of optical emission spectroscopy the maximum arc temperature and the radiation impact on the surfaces are estimated to be around 15000 K and 20 MWm−2, respectively. Thermographic measurements confirm the efficient protection of substrates by all layer materials. Layers containing CaCO3 lead to the lowest heating of ceramic samples which may be caused by a strong evaporation of the layer material.

Thermal plasma source for processing of MoO3 crystals

Simple original technique for fabrication of molybdenum trioxide MoO3 crystals during electric arc discharge directly from metallic molybdenum was proposed. The vertically oriented free-burning arc was ignited between the end surfaces of metallic molybdenum non- cooled electrodes. Molybdenum oxide crystals were deposited on side surface of bottom electrode (anode). Observation of crystals formation zone was used for determination of main process stages. The resulting products were studied by X-rays diffraction and optical microscopy methods. Investigations indicate, that self-organizing vapor-deposition process of MoO3 crystals formation has place. The resulting product is colorless sparkling prisms and platelets, which mainly consist of orthorhombic a-MoO3 phase. Optical microscopy indicates formation of closely packed feather-like pins structures by vapor-solid process.

Electrode patterns in arc discharge simulations: effect of anode cooling

Self-organized electrode patterns are often observed experimentally in diverse types of electrical discharges, including atmospheric-pressure electric arcs, but rarely captured in general-purpose computational plasma dynamics simulations. Time-dependent three-dimensional thermodynamic non-equilibrium (two-temperature) simulations reveal the spontaneous formation of self-organized anode attachment spot patterns in the free-burning arc, a canonical direct-current (dc) discharge with an axisymmetric electrode configuration and the absence of external forcing. The simulations are based on a monolithic fluid-electromagnetic plasma flow model numerically implemented within a second-order-accurate in space and time variational multiscale finite element framework. Simulation results show the gradual emergence of spot patterns with increasing levels of anode cooling: from a single diffuse spot for low cooling levels to the eventual coverage of the anode region by small spots for intense cooling. The characteristics of the patterns, such as the number, size and location of the spots, markedly depend on the imposed total current. Furthermore, the patterns transition from steady to dynamic with decreasing total current for high cooling levels. The pattern dynamics show the formation of new spots by the splitting of old ones occurring in the center of the plasma, as well as the movement and eventual extinction of spots at the plasma boundaries. The different types of anode patterns (from diffuse to self-organized spots) have a significant effect on the total voltage drop across the plasma column, but a minor effect on other plasma characteristics away from the anode region. The results indicate that thermal instability together with equilibration between heavy-species and electron energy have a dominant role in the formation of anode patterns in arc discharges.

Variational multiscale method for nonequilibrium plasma flows

Plasmas at atmospheric or higher pressures, typically denoted as thermal plasmas, are partially ionized gases in which the high collision frequency among its constituents (molecules, atoms, ions, and electrons) causes intense transfer of electromagnetic to thermal energy. Thermal plasma flows, often generated by electric arcs, are at the core of diverse technologies, such as plasma cutting, spraying, circuit breakers, lighting, fuel reforming, and gasification. A computational nonequilibrium thermal plasma flow model based on the Variational Multiscale (VMS) paradigm is presented. The plasma is described as a compressible reactive electromagnetic fluid in chemical equilibrium and thermodynamic nonequilibrium. Two energy conservation equations, one for electrons and the other for the heavy-species, are used to describe deviations from Local Thermodynamic Equilibrium. Material properties (e.g., mass density, enthalpy, viscosity, and electrical conductivity) vary by several orders of magnitude in a strongly nonlinear manner within these flows, which severely increases the stiffness of the model. The equations describing the plasma flow are treated in a monolithic approach as a transient–advective–diffusive–reactive (TADR) transport system. An algebraic VMS Finite Element Method appropriate for the treatment of general TADR problems is presented. The method is complemented with an intrinsic time-scales matrix definition to model the fluid–electromagnetic sub-grid scales inexpensively and a discontinuity-capturing operator to increase its robustness in the handling of large gradients. The resulting discrete system is solved by a generalized-alpha time-stepper together with a globalized inexact Newton–Krylov nonlinear solver. The VMS method is verified with incompressible, compressible, and magnetohydrodynamic (MHD) benchmark flow problems, and the VMS plasma model is validated with three canonical and industrially-relevant flows: the free-burning arc, and the transferred and non-transferred arc flows in a plasma torch.

Unsteady state analysis of free-burning arcs in a 3-Phase AC plasma torch: comparison between parallel and coplanar electrode configurations

An MHD model with parallel electrode configuration was developed to determine the influence of the electrode configuration on 3-Phase arc discharge behavior. The results obtained with this model were compared to previous results obtained with coplanar graphite electrode configuration. Results show a different arc behavior in both cases. In a parallel electrode configuration, the arcs are not confined within the inter-electrode gap as in coplanar electrode configuration. These results were compared to the arc behavior observed with a high-speed video-camera in a nitrogen atmosphere with a high correlation. The influence of frequency and current was also investigated. By adjusting the electrode configuration, the arc elongation and the dissipated power could be controlled. According to the application, such as plasma-assisted combustion, plasma gasification, plasma spray or for different plasma gases, the electrode configuration could play a key role to improve the discharge or operation. This study gives us a better understanding of 3-Phase AC arc plasma discharge.

Cathode fall voltage of TIG arcs from a non-equilibrium arc model

This work presents modelling results concerning a tungsten inert gas (TIG) welding arc. The model provides a consistent description of the free burning arc, the arc attachment and the electrodes. Thermal and chemical non-equilibrium is considered in the whole arc area, and a detailed model of the cathode space-charge sheath is included. The mechanisms in the cathode pre-sheath are treated in the framework of a non-equilibrium approach which is based on a two-fluid description of electrons and heavy particles and a simplified plasma chemistry of argon. A consistent determination of the electrode fall voltages and temperature distributions is achieved. The model is applied to arcs in pure argon at currents up to 250 A, whereby welding of a workpiece made of mild steel with a fixed burner is considered. Arc voltages in the range from 12 to 17 V are obtained at 50 at 250 A, respectively. The space-charge sheath voltage is found to be about 7 V and almost independent of the current. The corresponding temperatures of the cathode tip are in the range from 3,000 K to about 3,800 K. The results obtained are in a good agreement with measurements.

Modeling of Free-Burning Arc and Effects of Boundary Conditions on the Anode Temperature Field

A two dimensional numerical model of the free-burning arc and its interaction with anode are given. The commercial CFD code FLUENT is used to model the plasma and the solid anode part. The anode sheath is considered, as well as the heat transfer mechanism of the anode surface. The second boundary condition of heat conduction is introduced to give a more reasonable cooling boundary, so that the temperature distribution in the anode plate is more realistic. Through iteration calculation of steady MHD equations, the temperature field, pressure field and velocity field profiles are given. The result is of significant use to analysis thermal and control the electric power in the arc industrial applications.

The physical processes of current cutoff in lightning leaders

Current cutoff in lightning channels, which takes place in the development of both intracloud and cloud-to-ground flashes, is still poorly understood. A new evaluation of the conditions leading to current cutoff, and also of the two existing hypotheses of the cutoff mechanism, is the main objective of the paper. We reviewed the literature with results of measurements and modeling of free-burning arcs in a laboratory (the closest analogs of lightning leaders) focusing on the relationship between the internal electric field and current. This relationship governs the leader's behavior in the current cutoff. In our analysis of the mechanisms leading to current cutoff, we identify the two types of current cutoff in lightning channels: the current cutoff in a single, unbranched leader channel, which occurs as the result of reaching the threshold conditions for leader propagation; and the current cutoff in branched leaders, when screening by the leader branches alters the ambient electrical environment, thus diminishing the leader current and causing cutoff at a branching point or at the base of the straight channel that preceded branching. We advance the electrostatic model of the screening effect of branching on current cutoff, introduced by Mazur and Ruhnke (1993), and we provide the evidence of this mechanism from lightning observations. We also critically evaluate the concept of lightning-channel instability, proposed by Heckman (1992), as a suggested mechanism leading to current cutoff. We show that the fundamentals of this concept and therefore the concept in its entirety are invalid.

Temperature determination in copper-dominated free-burning arcs

Plasma temperatures of free-burning arcs dominated by Cu vapour are presented. It is shown that with increasing optical thickness the methods for optical thin evaluation of spectral lines fail and even methods for temperature determination in optically thick plasmas are limited. It reveals that a strongly simplified radiation model leads to more realistic plasma temperatures in the investigated range of plasma parameters. Temperatures between 10 000 and 20 000 K and Cu partial pressures between 0.1 and 0.5 bar are found at currents between 1 and 4 kA. Corresponding current densities range from 35 to 166 A mm−2.

Comparing two non-equilibrium approaches to modelling of a free-burning arc

Two models of high-pressure arc discharges are compared with each other and with experimental data for an atmospheric-pressure free-burning arc in argon for arc currents of 20–200 A. The models account for space-charge effects and thermal and ionization non-equilibrium in somewhat different ways. One model considers space-charge effects, thermal and ionization non-equilibrium in the near-cathode region and thermal non-equilibrium in the bulk plasma. The other model considers thermal and ionization non-equilibrium in the entire arc plasma and space-charge effects in the near-cathode region. Both models are capable of predicting the arc voltage in fair agreement with experimental data. Differences are observed in the arc attachment to the cathode, which do not strongly affect the near-cathode voltage drop and the total arc voltage for arc currents exceeding 75 A. For lower arc currents the difference is significant but the arc column structure is quite similar and the predicted bulk plasma characteristics are relatively close to each other.

Large Eddy Simulation of a Free-Burning Arc Discharge in Argon with a Turbulent Cross Flow and External Field

In most of the numerical approaches proposed for modeling high-intensity plasma-arcs, the effects of turbulence on the arc structure are often excluded because of the intricate physics originating from the interaction of turbulent scales, high-temperature gas dynamics, magnetohydrodynamics (MHD) and chemical kinetics. The goal of this study is threefold: to develop a generic turbulent MHD model to simulate free-burning arc discharges, to validate the code with available experimental data, and to investigate the effect of an external field and turbulent cross flow on the free-burning arc configuration. The governing equations are solved in conservative form using a hybrid scheme that combines a high-order monotonic upwind scheme with a second-order central scheme. The fluid and MHD turbulence are resolved using a large eddy simulation (LES) approach with a recently developed sub-grid closure model. An implicit scheme is used to compute the magnetic diffusion term appearing in the magnetic induction equation to alleviate the severe time-step constraint. The comparison of the model prediction with experimental data for Argon arcs at different current intensities shows generally good agreement. When an external field is applied, the overall shape of the free-burning arc drastically changes. The straightening of the arc indicates the potential for stabilization of a free-burning arc by magnetic forces. Even though the turbulence is significantly attenuated as a result of the thermal expansion near the cathode, it adds an unsteady characteristic to the arc and, in general, has a negative impact on the stabilization of the electrical discharge.

Plasma chemistry in the free-burning Ar arc

Recent ambitions have been aimed at modelling of a free-burning arc in argon by means of a self-consistent description of the arc plasma and the electrodes avoiding assumptions of equilibrium. As a next step, the present work deals with the impact of the reaction chemistry used in non-equilibrium modelling. A set of reactions accounts for elastic scattering, excitation and de-excitation in collisions with electrons and atoms, step-wise ionization, recombination and spontaneous emission. A simple chemistry model based on the approximation of prompt ionization, a model with an effective excited level, and a model implying higher excited levels of argon atoms are considered. The results obtained show that all three models are capable of predicting the arc characteristics. They deliver values of electric current density, electric conductivity, arc voltage, electron and heavy particle temperature which are close to each another. The simple model can serve as a fast-track evaluation of the arc characteristics, and for sensitive studies of the impact of model parameters. The extended model allows obtaining the populations of individual and effective levels giving rise to important radiative transitions.

3D temperature measurement of tandem TIG arc plasma

Measurements of arc plasma are important for determining the associated physical properties. Such measurements usually involve spectroscopic techniques that measure the temperature distribution in free-burning arcs. Most existing studies have reported the temperature of axially symmetric arc plasma using Abel inversion. This method cannot be used for axially asymmetric arc plasma such as tandem tungsten inert gas (TIG) arc plasma. It is a complex phenomenon because the arc plasma generated from each electrode is affected by the plasma of the other due to electromagnetic force. We measured the temperature distribution of tandem TIG arc plasma in 3D with one camera rotation. The measurement method is based on computed tomography consisting of multidirectional detections and image reconstruction. As a result, we could measure the temperature distribution of the coupled arc plasma provided by the two-electrode TIG arc. In addition, we evaluated the influence of detection directions and numbers on the reconstructed emission intensity distribution.

Numerical analysis of free-burning argon arcs based on the local thermodynamic equilibrium model at various electrical currents

Free-burning arcs where the work piece acts as an anode were numerically analyzed using a computational domain including the arc itself and its anode region based on the local thermodynamic equilibrium model. Because the major arc parameters such as temperature, axial velocity, electric potential difference and pressure-rise from ambient atmospheric pressure are much dependent on the working current, our investigation was concerned with developing a capability to model free-burning argon arcs and considering the energy flux going into the anode at various values of the electrical current (I=50, 100 and 200A) by computational fluid dynamics analysis. Predicted temperatures along the z-axis between the electrodes were in fair agreement with existing experimental results. Particularly, reasonable relationships between the maximum velocity or temperature and the applied current were predicted, which matched well with other theoretical results. In addition, some discrepancies with other predictions were shown in the results about electric potential and pressure-rise. It should be related to the omission of the space-charge effect near the electrodes for a simplified unified model and the application of a turbulence model for the steep temperature gradient at the arc edges.