Free-burning Argon Arc Research Articles

Comparative Studies on the Radiative Heat Transfer in Arc Plasma and Its Impact in a Model of a Free-Burning Arc

The radiative heat transfer in arc plasma models is considered from the point of view of its description in terms of a net emission coefficient, the method of spherical harmonics in its lowest order, and the discrete ordinate method. Net emission coefficients are computed, applying approximate analytical and numerical approaches and a multi-band representation of the spectral absorption coefficient with three kinds of its averaging and two datasets. Self-consistent access to the radiative heat transfer is applied to a two-dimensional axisymmetric model of a free-burning arc in argon at atmospheric pressure. The results obtained from the models employing the net emission coefficient, the method of spherical harmonics, and the discrete ordinate method are compared.

Pinching arc plasmas by high-frequency alternating longitudinal magnetic field

Arc plasmas have promising applications in many fields, and exploring their properties is of interest. This research paper presents detailed pressure-based finite volume simulations of an argon arc. Simulations of the free-burning argon arc show good agreement with experiment. We observe an interesting phenomenon that an argon arc concentrates intensively in a high-frequency alternating longitudinal magnetic field. This is different from existing constricting mechanisms, as here the arc is pinched through continuous dynamic transitions between shrinking and expansion. The underlying mechanism is that via working together with an arc's motion inertia, the applied high-frequency alternating magnetic field is able to effectively play a “plasma trap” role, which leads the arc plasma to be confined to a narrower space. This finding may provide a new approach to constrict arc plasmas.

Analyses on the nonequilibrium transport processes in a free-burning argon arc plasma under different operating conditions

During discharges of an arc plasma, complex mass, momentum and energy exchanges exist between the arc column and the surrounding cold gas, forming a nonequilibrium region deviating from both the local thermodynamic equilibrium and local chemical equilibrium states. The nonequilibrium synergistic transport plays a crucial role not only in controlling the characteristics of the arc plasmas theoretically, but also in optimizing the plasma material processing qualities in actual applications. In this paper, the nonequilibrium transport processes in free-burning argon arc plasmas under different operating pressures and arc currents are studied based on a complete nonequilibrium fluid model, and are also validated by comparing with measured data. The energy transfer processes under various operating conditions, especially Joule heating, elastic and inelastic collisions, conductive and convective heat transfer, and energy transfer related to the temperature ratio spatial gradient, are analyzed based on the concept of the ‘energy tree.’ The revealed major energy transfer channels in the high-pressure argon arc plasmas also provide some possibilities to control the characteristics of thermal plasmas in the future.

Spatially resolved LAAS diagnostics of a free-burning ar arc: Analysis of line broadening

Spatially resolved laser atomic absorption spectroscopy (LAAS) diagnostics are applied to a free-burning DC arc in argon at atmospheric pressure. The novel LAAS apparatus is capable of mode-hop free wavelength tuning over the larger range than in conventional systems (up to 300 GHz in the present work). It allows for the measurements of both the absorption coefficient and the spectral line profile continuously from the hot arc center to the non-equilibrium arc wings. The results of such measurements are presented in this work with the emphasize on the combined analysis of laser absorption, which gives densities of excited atoms, and line broadening, which delivers information on either electron density (in case of Stark broadening) or neutral atom density (in case of van der Waals broadening). Contributions by different broadening mechanisms (Stark, Doppler, van der Waals, Resonance) are separated and analyzed. Experimentally retrieved density and temperature profiles of electrons and neutral atoms are compared with the predictions of a self-consisted nonequilibrium arc model. Stark widths of Ar I lines 750.38, 751.46, and 763.51 nm are obtained in the temperature range of 11000–18000 K with the aim to extend the rather scarce data for those lines. The measured widths are compared with existing values from literature and theoretical predictions.

3D numerical investigation of a free-burning argon arc with metal electrodes using a novel sheath coupling procedure

A novel coupling procedure describing the complicated plasma-electrode interaction process has been developed and applied into the 3D finite volume simulation of a direct current tungsten inert gas welding system. This is achieved by making the space charge layer (sheath) incorporated into the computation domain and interact with both bulk plasma and cathode through the effective electrical conductivity. Both chemical and thermal nonequilibrium phenomena as well as the self-induced magnetic fields have been taken into consideration by the model to ensure a realistic numerical description of a non-thermal arc. The applicability of this coupling procedure is further improved by calculating the real electric potential, which is capable of accounting for the effects of the complicated drift and diffusion processes. Numerical results of both 100 and 200 A discharge currents are presented, field reversal is obtained at near-anode regions in both cases, which is followed by the negative anode sheath potential drop. The region of the strongest electron overpopulation appears at the intersection of plasma fringes and electrode surface. Numerical results of plasma temperature and voltage show good agreement with experimental measurements.

Non-equilibrium modelling of free-burning argon arc in different anode sheath regimes

In this paper, an anode sheath model coupled with two-dimension arc column model is performed to study the physical process in the near-anode region. In the anode sheath model, two different physical mechanisms of ion and electron dynamics, for positive and negative anode sheath voltages, are considered. The effect of the arc column on the anode is also considered. The arc column model accounts for the thermal non-equilibrium and ionization non-equilibrium. The diffusion equations and generalized Ohm’s law are used to analyze the effect of diffusion of plasma species near the anode region. The simulation results from this model are presented for an atmospheric free-burning argon arc with arc currents of 50 A and 150 A. The simulation shows that: (1) both positive and negative anode sheath voltages are obtained for I = 50 and 150 A. The distributions of electric potential in front of the anode surface predicted by the model are in good agreement with the experimental data. (2) In the negative anode sheath regime, the diffusion current density exceeds the total current density, which makes the electric field reverse in front of anode surface. In the positive anode sheath regime, the diffusion current becomes negative in the near-anode region, which makes the ohmic current increase rapidly in the near-anode region. (3) There is a significant interaction, which is positive feedback, between the diffusion current and anode sheath voltage in both positive and negative anode sheath modes. (4) The thermal conduction from the arc column plays a major role in the heat transfer to the anode for the negative anode sheath mode while it becomes much smaller for the positive anode sheath mode.

Spatially resolved LAAS/OES diagnostics of a free-burning Ar arc: measurements of excited atom densities

Spatially resolved optical diagnostics are applied to a free-burning DC arc in argon at atmospheric pressure. Densities of metastable and resonance -atoms are measured by laser atomic absorption spectroscopy (LAAS). The novel LAAS design allowed for an extended range of wavelength scan covering the complete Stark broadened line profile (∼ nm). High spatial resolution of 0.2 mm allowed measurements of atom densities and line profiles in the regions of high-temperature gradients. Besides absorbing -states, the radiating -states were measured using optical emission spectroscopy (OES). Results of the LAAS and OES measurements are compared with the nonequilibrium collisional-radiative model. Measurement of metastable states in the non-radiating regions near the cathode reveals the existence of a ‘collar’ effect. The interpretation of the effect with a focus on transport of resonance radiation is given, quantitative agreement between simulations and experiment is reached.

Tomographic measurement of temperature in non-axisymmetric arc plasma by single camera multi-view imaging system.

In this paper, we introduce a method for dynamic and three-dimensional (3D) arc temperature measurement in non-axisymmetric arc plasma. Optical filters with narrow spectral bands are used to gain 696.5 nm line spectral information of a free-burning Argon (Ar) arc. A measuring system with a setup of mirrors allows a single camera chip to image multi-views of identical welding arc onto one monochromatic image. And based on the underlying characteristics of the arc plasma, we present a modified maximum likelihood expectation maximum (ML-EM) algorithm to reconstruct the emission coefficients from the line intensities. To verify the accuracy of our algorithm, two models of arc plasma have been established to simulate the reconstruction process. Simulation results demonstrated that the modified ML-EM can be utilized to improve the performance of 3D reconstruction compared to common ML-EM methods. To obtain the optimal scheme of the measuring system with limited view directions, the reconstruction quality with different directions is presented and discussed. In the experiment, asymmetric 3D plasma temperature distribution was determined at a direct current (DC) of 80 A and 100A. The results proved the feasibility of the 3D temperature measure method to characterize an asymmetric arc.

Non-equilibrium synergistic effects in atmospheric pressure plasmas

Non-equilibrium is one of the important features of an atmospheric gas discharge plasma. It involves complicated physical-chemical processes and plays a key role in various actual plasma processing. In this report, a novel complete non-equilibrium model is developed to reveal the non-equilibrium synergistic effects for the atmospheric-pressure low-temperature plasmas (AP-LTPs). It combines a thermal-chemical non-equilibrium fluid model for the quasi-neutral plasma region and a simplified sheath model for the electrode sheath region. The free-burning argon arc is selected as a model system because both the electrical-thermal-chemical equilibrium and non-equilibrium regions are involved simultaneously in this arc plasma system. The modeling results indicate for the first time that it is the strong and synergistic interactions among the mass, momentum and energy transfer processes that determine the self-consistent non-equilibrium characteristics of the AP-LTPs. An energy transfer process related to the non-uniform spatial distributions of the electron-to-heavy-particle temperature ratio has also been discovered for the first time. It has a significant influence for self-consistently predicting the transition region between the “hot” and “cold” equilibrium regions of an AP-LTP system. The modeling results would provide an instructive guidance for predicting and possibly controlling the non-equilibrium particle-energy transportation process in various AP-LTPs in future.

Three-Dimensional Computations of Free-Burning Arcs and Their Surroundings

This paper is concerned with developing a capability to model free-burning high-intensity argon arcs and enhancing the accuracy of numerical results according to three-dimensional calculations. It was found that the computed temperatures along the axis between the cathode tip and the anode surface show good agreement with two different measured data. Although the LTE model can reasonably predict the overall arc voltage for free-burning arcs, it fails to account accurately what happens at the regions near electrodes.

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.

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.

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.

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.

Comparison of turbulence models for a free-burning high-intensity argon arc

Free-burning arcs where the work piece acts as an anode are frequently used for a number of industrial applications. Our investigation was concerned with developing a capability to model free-burning high-intensity argon arcs and enhancing the accuracy of numerical results according to a comparative study of turbulence models. For the arc modeling involved complicated interactions between the flow and electromagnetic fields, we modified the Navier-Stokes equations to take into account the radiation transport, the electrical power input and the electromagnetic driving forces with the relevant Maxwell equations. For the turbulence modeling, which is critical to the arc edges due to a steep temperature gradient between the arc column and the surrounding gas, zero- and two-equation eddy-viscosity models were used. The major arc parameters of temperature, axial velocity, electric potential difference and pressure-rise from ambient atmospheric pressure were summarized under laminar and turbulent conditions. It was found that the standard k-ɛ model modified to take into account the effect of a steep temperature gradient at the edge of free-burning arcs could predict reasonable temperature profiles better than the cases with the assumption of laminar flow and Prandtl’s mixing length model used for switching arcs.

Two-temperature chemically non-equilibrium modelling of transferred arcs

A two-temperature chemically non-equilibrium model describing in a self-consistent manner the heat transfer, the plasma chemistry, the electric and magnetic field in a high-current free-burning arc in argon has been developed. The model is aimed at unifying the description of a thermionic tungsten cathode, a flat copper anode, and the arc plasma including the electrode sheath regions. The heat transfer in the electrodes is coupled to the plasma heat transfer considering the energy fluxes onto the electrode boundaries with the plasma. The results of the non-equilibrium model for an arc current of 200 A and an argon flow rate of 12 slpm are presented along with results obtained from a model based on the assumption of local thermodynamic equilibrium (LTE) and from optical emission spectroscopy. The plasma shows a near-LTE behaviour along the arc axis and in a region surrounding the axis which becomes wider towards the anode. In the near-electrode regions, a large deviation from LTE is observed. The results are in good agreement with experimental findings from optical emission spectroscopy.

Sheath and arc-column voltages in high-pressure arc discharges

Electrical characteristics of a 1 cm-long free-burning atmospheric-pressure argon arc are calculated by means of a model taking into account the existence of a near-cathode space-charge sheath and the discrepancy between the electron and heavy-particle temperatures in the arc column. The computed arc voltage exhibits a variation with the arc current I similar to the one revealed by the experiment and exceeds experimental values by no more than approximately 2 V in the current range 20–175 A. The sheath contributes about two-thirds or more of the arc voltage. The LTE model predicts a different variation of the arc voltage with I and underestimates the experimental values appreciably for low currents but by no more than approximately 2 V for I ≳ 120 A. However, the latter can hardly be considered as a proof of unimportance of the space-charge sheath at high currents: the LTE model overestimates both the resistance of the bulk of the arc column and the resistance of the part of the column that is adjacent to the cathode, and this overestimation to a certain extent compensates for the neglect of the voltage drop in the sheath. Furthermore, if the latter resistance were evaluated in the framework of the LTE model in an accurate way, then the overestimation would be still much stronger and the obtained voltage would significantly exceed those observed in the experiment.

Cathode erosion in a high-pressure high-current arc: calculations for tungsten cathode in a free-burning argon arc

The motion of an evaporated atom of the cathode material in a near-cathode plasma is considered. It is shown that the evaporated atom is ionized almost instantly. The created ion, under the influence of a strong electric field existing in the cathode proximity, has a high probability of returning to the cathode. A small fraction of evaporated atoms are able to diffuse away from the cathode to the region where they are involved in plasma flow and lose their chance to return to the cathode. The fraction of the total evaporated atoms, which do not return to the cathode, the escape factor, determines the net erosion rate. In order to calculate this factor, the distributions of the plasma parameters in the near-cathode plasma were considered. Calculations showed that the escape factor is on the order of a few per cent. Using experimental data on the plasma and cathode temperatures, we calculated the net erosion rate for a free-burning 200 A argon arc with a thoriated tungsten cathode. The calculated erosion rate is close to 1 µg s−1, which is in agreement with available experimental data.

Time resolved characterization of a free-burning argon arc after ignition by optical emission spectroscopy

Time resolved properties of a free-burning argon arc after ignition have been characterized using optical spectroscopic method. After ignition, when the arc current keeps constant, the plasma temperature decreases with time at any position of the arc. The decrease of the plasma temperature is associated with the increase of the arc cathode surface temperature. It is suggested that the variation of the cathode surface temperature, which changes the current density distribution over the cathode surface, leads to the decrease of the plasma temperature in the free-burning arc after ignition.