P Freton

Thermal plasma modelling

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.

Energy equation formulations for two-temperature modelling of ?thermal? plasmas

Based on an equilibrium fluid model, built from the commercial Fluent software previously validated for thermal plasma characterizations on several geometries, a non-equilibrium two-temperature model was developed. This kind of model needs the use of two energy equations: one for the electrons, and the other for heavy particles.Nevertheless, depending on the authors, divergences exist in the expressions of equations. The main differences are related to the attribution of the ionization term and to the components of thermal conductivity in the energy equations.The two-temperature model developed is applied in a transferred arc configuration where the medium is described using the different formulations for the energy equations.The right formulation, based on the Boltzmann equation, is then applied in a transferred arc configuration for two values of current intensity of 100 and 600xa0A. We show that in order to obtain coherent and physical results in all the cases, special attention needs to be given: the ionization term, the reactive thermal conductivity and the radiation line contribution need to be considered in electron energy equations, whereas the reactive thermal conductivity due to dissociations and the continuum radiation contributions need to be associated with the heavy energy equations.

Plasma–weld pool interaction in tungsten inert-gas configuration

A three-dimensional (3D) transient model of a transferred argon arc in interaction with an anode material is presented and the results discussed. The model based on a finite volume method is developed using the open software @Saturne distributed by Electricite de France. The 3D model includes the characterization of the plasma gas and of the work piece with a current continuity resolution in the whole domain. Transport and thermodynamic properties are dependent on the local temperature and on the vapours emitted by the eroded material due to the heat flux transferred by the plasma. Drag force, Marangoni force, Laplace and gravity forces are taken into account on the weld pool description. The plasma and the weld pool characteristics are presented and compared with experimental and theoretical results from the literature. For a distance between the two electrodes of d = 5 mm and an applied current intensity of I = 200 A, the vapour concentration is weak. The influence of the parameters used in the Marangoni formulation is highlighted. Finally, in agreement with some authors, we show with this global transient 3D model that it is not necessary to include the voltage drop in the energy balance.

Two-dimensional self-consistent modelling of the arc/cathode interaction

A theory based on the literature works is developed to define a self-consistent model of the arc?cathode interaction in thermal plasma domain. The proposed model, applied in this paper in a two-dimensional coordinate system includes a physical description of the arc?cathode interaction, the cathode bulk, the sheath, the pre-sheath and the local thermodynamic equilibrium plasma column.This model allows assuming the electric current conservation through a two-temperature treatment of the sheath and pre-sheath regions. This model has been applied in a free burning arc configuration in an argon gas with a cylindrical tungsten cathode and current intensity equal to 200?A. The orders of magnitude of the current density and of the heat flux are, respectively, 106?A?m?2 and 107?W?m?2.Three parametric studies are performed: on the ionization layer length, on the value of the secondary emission coefficient and on the formulation of the electrical conductivity in the pre-sheath. The size of the ionization layer and the chosen value of the secondary emission coefficient have a significant influence on the properties of the discharge.

Argon and Arcal.37 plasma characteristics in a TIG configuration

In a previous paper (Mougenot 2013 J. Phys. D: Appl. Phys. 46 135206), a 3D model based on the @Saturne software was presented in argon gas and the results compared with the literature results. This paper extends the analysis from pure argon and shows the influence of added helium on plasma and weld pool properties. The influence of vapours coming from the plasma interaction with the anode material is shown in Arcal.37 gas (composed of 30% argon and 70% helium), showing the necessity to consider their presence. The importance of the forces acting on the weld pool is illustrated and analysed. The Marangoni effect is the major force acting on the weld pool leading to plasma cooling, an increase in the width of the weld pool and a reduction in its depth.

Prediction of the cathodic arc root behaviour in a hollow cathode thermal plasma torch

The upper part of a well type cathode (WTC) plasma torch is modelled for several conditions in an air medium in the presence of an electric arc. The plasma flow created by the electric arc is described and the results compared with the data from the literature. Special attention is paid to the description of arc root attachment and to its movement due to the balance of forces. A fine description of the magnetic field produced by the external solenoid is reported. The model is based on the @Fluent software implemented with specific developments to be adapted to the thermal plasma domain. The paper shows the necessity to provide an accurate description of the external magnetic field due to the strong influence of the radial magnetic field component. Overall, we propose an original approach for arc root movement description which contributes to the understanding of the flow behaviour in the WTC torch.

Flow Behavior in High-Voltage Circuit Breaker

Since several years, high-voltage circuit breakers (voltages of up to 800 kV) have been studied by the thermal plasma community to attain high level of reliability, good operating economy, and long intervals without maintenance. The studies are also related to gas substitution in order to replace SF6. This feature consists to study, by a numerical tool (@Fluent) in a specific geometry, the main physical mechanisms occurring during the current intensity variation. Images of the SF6 plasma in the reactor are presented: geometry, temperature, and pressure fields.

3-D visualization of a 3-D free-burning arc model deflected by external magnetic or convective forces

We present in this paper, the influence of magnetic and convective external forces, applied on a transferred arc configuration.

Arc Movements in a Hollow Cathode of a High-Power Plasma Torch

Hollow-cathode arcs are used in industrial thermal-plasma applications like waste treatments or gas heater. The description of such kind of plasma torch is not easy to model due to three facts: vortex gas injection, the transient change of cathode root-attachment position, and the presence of an external magnetic coil to diminish the cathode erosion. This paper proposes a numerical 3-D modeling for representing the arc movement and studying the convection effects within the cathode.

Experimental characterisation of the weld pool expansion in a tungsten inert gas configuration

ABSTRACT In this study, we focus on the expansion of the weld pool in a welding process by tungsten inert gas (TIG). Use of high-speed imaging and metallographic investigations combined with electrical measurements allow us to determine the weld pool shape and dimensions evolution and the energy involved in the process for six durations of interaction (from 1 to 20u2005s) in a spot TIG configuration. Results show a fast increase of the weld dimensions on the first second. Otherwise, time does not seem to be an influent parameter on the weld shape. The comparison between experimental work and numerical model available in our team highlights differences between the predicted shapes and the observed ones.