Electrical arcs and, more generally thermal plasmas, are widely used in many applications and the understanding or the improvement of the corresponding processes or systems, often requires precise modelling of the plasma. We present, here, a double approach to thermal plasma modelling, which combines the scientific procedure with an engineering point of view. First, we present the fundamental properties of thermal plasmas that are required in the models, followed by the basic equations and structures of the models. The third part is devoted to test cases, and its objectives are the study of some basic phenomena to show their influence on arc behaviour in simple configurations, and the validation of the models: we point out the roles of radiation, thermal conductivity and electrical conductivity for a stationary or transient wall-stabilized arc and we validate a three-dimensional model for a free-burning arc.Sections 4–6 deal with several industrial configurations and the model is useful in each case for studying important phenomena or processes in greater detail. For transferred arcs, such as those used in metallurgy, the energy transfer from the arc to the anode, and the presence of metallic vapour and pumping gas are essential. For a non-transferred plasma torch used for plasma spraying, we illustrate the relevance of a three-dimensional model and we present the interaction of the plasma with powders. Problems related to high- and low-voltage circuit-breakers are then presented, and various typical mechanisms are modelled. Finally, several non-equilibrium models useful for quasi-thermal conditions are presented in detail, showing how they take into account the chemical kinetics and two-temperature plasmas occurring under particular conditions, such as decaying arcs or inductively coupled plasmas.

https://iopscience.iop.org/article/10.1088/0022-3727/38/9/R01/pdf

Thermal plasma modelling

Metal vapour is formed in arc welding processes by the evaporation of molten metal in the weld pool, and in the case of gas–metal arc welding, in the wire electrode and droplets. The presence of metal vapour can have a major influence on the properties of the arc and the size and shape of the weld pool. Previous experimental and computational works on the production and transport of metal vapour in welding arcs, in particular those relevant to gas–metal arc welding and gas–tungsten arc welding, are reviewed. The influence of metal vapour on the thermodynamic, transport and radiative properties of plasmas is discussed. The effect of metal vapour on the distributions of temperature, current density and heat flux in arcs is examined in terms of these thermophysical properties. Different approaches to treating diffusion of metal vapour in plasmas, and the production of vapour from molten metal, are compared. The production of welding fume by the nucleation and subsequent condensation of metal vapour is considered. Recommendations are presented about subjects requiring further investigation, and the requirements for accurate computational modelling of welding arcs.

https://iopscience.iop.org/article/10.1088/0022-3727/43/43/434001/pdf

The effects of metal vapour in arc welding

In circuit breakers, arc heaters and arc welding apparatus, metal vapor resulting from electrode erosion is inevitably injected into the arc plasma. The arc then burns in a mixture of the working gas (SF/sub 6/ in the case of circuit breakers) and electrode vapor, whose properties are substantially different from those of pure SF/sub 6/. The present work is a computational investigation into the effects of electrode vapor on the behavior of a supersonic nozzle arc under dc conditions. The arc and electrodes are treated as a coupled system. The arc plasma is assumed to be in local thermodynamic equilibrium (LTE). The thin non-LTE layer between the electrode and the arc is not included in the computation but its effects are taken into account in the energy balance at the electrode surface. A conservation equation for the electrode vapor is solved together with the governing equations for mass, momentum, and energy of the gas mixture. Comparisons were made between the results with and without electrode vaporization. Results show that vaporization of the electrode material (copper in the present investigation) has a cooling effect at the arc center and broadens the arc column. The arc voltage is, however, not sensitive to the presence of electrode vapor for the gas discharge conditions investigated.

Electrode evaporation and its effects on thermal arc behavior

In this paper, the complicated arc phenomena in air dc circuit breakers that are widely used in rail transit and vessels are investigated with both experimental and numerical approaches. Initially, the basic arc motion characteristics in the switch are presented by the recorded waveforms and images in the carefully designed breaking experiments. Phenomena of arc stagnation and back commutation are revealed and the influence of arc chamber width on the arc stagnation is preliminarily discussed according to the experimental results. Then, to make the nature of arc motion clear and to determine the influence details of the arc chamber width on arc motion characteristics, a simplified model based on magnetohydrodynamics theory is built and four cases with the arc chambers of different widths are calculated. The simulated arc behavior is presented by the temperature distribution sequences. In addition, the formation process of back commutation is described in detail by the variation of arc column. The influence mechanism of arc chamber width on arc motion is analyzed according to the differences of pressure propagation and temperature distribution in the arc chamber. Furthermore, the calculated arc displacement curves show that the arc stagnation time could be shortened and the risk of back commutation could be reduced by decreasing the arc chamber width within the reasonable extent.

Investigation on Arc Behavior During Arc Motion in Air DC Circuit Breaker

Calculated values of the viscosity, thermal conductivity and electrical conductivity of nitrogen and mixtures of nitrogen and hydrogen, including ammonia and hydrazine, at high temperatures are presented. Combined ordinary, pressure, temperature and electric field diffusion coefficients are also given for the mixtures. The calculations, which assume local thermodynamic equilibrium, are performed for plasmas in the temperature range from 300 to 30,000 K and the pressure range from 0.1 to 5 atm. The results are compared with those of previously published studies. Reasonable and in some cases very good agreement is found; those discrepancies that exist are attributed to the improved values of the some of the collision integrals used here. Different mixing rules are tested, with two found to give very good results: that of Yun et al. [K.-S. Yun, S. Weissman, E.A. Mason, Phys. Fluids 5 (1962) 672] for the viscosity and thermal conductivity, and a simple interpolation weighted by mole fraction for the electrical conductivity.

Transport coefficients of plasmas in mixtures of nitrogen and hydrogen

PC-based arc modeling taking account of the effects of nozzle ablation and the mixing process of PTFE vapor with SF/sub 6/ has been carried out for a full-scale auto-expansion circuit breaker with a moving contact. An equation to calculate the PTFE concentration in the mixture is introduced together with overall mass, momentum, and energy conservation equations for the mixture. The thermodynamic and transport properties of SF/sub 6/-PTFE vapor mixture calculated by the transport theory are given. An empirical relationship between radiation reabsorption factor and the relevant arcing parameters (nozzle size and instantaneous arcing current) has been employed to make predictions. The features of temperature, pressure, PTFE mass concentration, and flow fields during the whole arcing period are discussed. It is shown that the energy brought into the expansion volume by the hot PTFE vapor is mainly responsible for the pressure buildup. The propagation of the pressure wave in the expansion volume is responsible for the pressure peaks in the second loop of arcing. The predicted arc voltage and pressure variation are compared with the test results and with those predicted using the thermodynamic and transport properties of pure SF/sub 6/. Results show that the use of the properties of SF/sub 6/-PTFE vapor mixture substantially improves the agreement between the predicted and measured pressure rise in the expansion volume.

Computational investigation of arc behavior in an auto-expansion circuit breaker contaminated by ablated nozzle vapor

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Computational investigation of arc behavior in an auto-expansion circuit breaker contaminated by ablated nozzle vapor