Plasma Arc Cutting - Reversed Swirl Ring, Electrode Thread and Cut Direction Effects on Kerf Geometry

Plasma arc cutting is used to cut any conductive material. It consists in blowing pressurized gas and feed current to an arc, leading to a thin plasma dart able to melt down the material and blow it away, creating a kerf. Its quality depends on its shape. This paper shows, through experimental measurements, how the inner geometry of the torch can affect the cut quality. It appears that one side of the kerf is much more oblique and sensitive to factors variation than the other. A theory based on a computational fluid dynamics model is proposed to investigate the causes of these phenomena

THEORETICAL PLASMA CHARACTERIZATION DURING CURRENT PULSE

International audienceThe economic/ecological contexts and the CO2 regulation lead the automotive industry to improve the spark ignited engines. A way of improvement is the admission of a lean mixture or of a diluted mixture by recirculation of exhaust gases in the combustion chamber. The main difficulty in these conditions is to start the combustion. To overcome this problem, ignition systems are studied and more particularly the spark one. This discharge leads to the apparition of plasma and the understanding of the energy transfer mechanisms between this plasma and the reactive mixture is essential. This work is focus on the modeling of a spark during its electrical arc phase in order to predict the hydrodynamic behavior of the arc and the shock wave propagation. The difficulty on the choice of initial conditions for the model is highlighted. A two dimensional model based on ANSYS Fluent software is developed. This model allows us to show the role of each initial parameter as well as their impacts on the plasma flow. One calculation case presents the shock wave propagation and the plasma kernel. Finally a parametric study is presented. Without a complete model describing all the phases of the spark the choice of initial conditions is essential, nevertheless experimental measurements are difficult to perform. The interdependence of the initial parameters is demonstrated and care is needed in case of incomplete set of initial conditions which should be completed

0D KINETIC MODEL: APPLIED TO SF6

This work is related to the chemical kinetics modelling of plasma during extinction. A zero-dimensional model (0D) has been developed. Two hypotheses were used: (A) a constant pressure or (B) a constant mass density. Three initial data categories are generally required for the model: (1) the chemical reactions that govern the kinetic scheme, (2) the chemical composition at the local thermodynamic equilibrium (LTE) and (3) a law of temperature decay as a function of time representing the cooling rate. The developed model is presented and applied to SF6, gas commonly used in high voltage circuit breakers (HVCB), in order to be validated. We present the evolution of the species during the temperature decay for several cooling rates. The results give the evolution of species densities and the departures from equilibrium according to the cooling rate. Consideration of SFx molecules is essential in order to avoid erroneous interpretations

Modelling and Simulation of SF6 High-Voltage Circuit-Breakers - an Overview on Basics and Application of CFD Arc Simulation Tools

The paper gives an overview on the basics of CFD arc simulation tools with respect to the simulation of the fluid mechanical processes in the interrupter unit of SF6 high-voltage circuit-breakers at no-load and short-circuit switching-off processes. On the example of SF6 self-blast circuit-breakers the complete process from the analysis of the switching-off process to the creation of a modular simulation model consisting of several sub models is illustrated. Details to the modelling in the particular sub modules and to the implementation are given. The capability of a CFD arc simulation tool based on the program package ANSYS/FLUENT is demonstrated on the basis of selected simulation results. Furthermore case examples for the application of the presented CFD arc simulation tool in the development process of high-voltage circuit-breakers are given

ARC MOTION IN LOW VOLTAGE CIRCUITBREAKER (LVCB) EXPERIMENTAL ANDTHEORETICAL APPROACHES

International audienceAbstract This paper is related to the study of the arc motion in simple low voltage circuit breaker geometry.Experimental and theoretical approaches are investigated respectively by fast camera and by a magneto hydrodynamicmodel. Two theoretical methods have been developed to characterize the arc movement called MECM (Mean ElectricalConductivity Method) and GCRM (Global Current Resolution Method). The results obtained by the two models are ingood agreement with the experimental observations. The MECM allows obtaining faster results but the stagnation phasesare well represented with the GRCM and this last method is easier to implement in more complex geometry. The resultsshow also the importance of the exhaust description on the arc behavior

EXPERIMENTAL AND SIMULATION STUDIES ON THE VOLTAGE DROP OF ARC IN LOW-VOLTAGE CIRCUIT BREAKER

International audienceIn low voltage circuit breaker (LVCB) apparatus, a current limitation is performed by increasing the arc voltage. This increase is mainly realized in the splitters plates of the arc chamber by additional drop voltages due to anode and cathode sheaths regions. The consideration of the voltage drops near-electrodes regions is so one of the most important mechanism to improve the description of the arc behavior in LVCB. In this paper, the arc voltage evolution has been studied by experimental and simulation by considering a simple geometry constituted by two rails runner with one or two splitters plates. One magneto hydrodynamic model in three dimensions (3D) was developed to simulate the arc motion and the arc splitting process. In order to compare with the model results, experimental tests have been carried out

Etude expérimentale et numérique d’un procédé de coupage plasma intensifié

International audienceLe procédé de coupage plasma, mettant en œuvre un arc électrique transféré entre une torche et une pièce métallique à découper, fait l’objet de développements continus depuis les années 1970 et d’une utilisation industrielle aujourd’hui essentielle à la construction métallique. Des études de caractérisation expérimentale et de modélisation détaillée, n’ont en revanche débuté que plus récemment, en raison de la mobilisation nécessaire d’approches [1] alliant mesures spectrométriques et simulation CFD.Le corpus de connaissance, et les méthodes de simulation, constitués et enrichis depuis, permettent désormais d’aborder l’amélioration des technologies industrielles de coupage plasma avec une approche d’innovation par la connaissance, i.e. s’appuyant sur une compréhension plus fine de la phénoménologie de mise en forme du jet plasma de coupage.La caractérisation d’une approche innovante proposée par la société AKRYVIA [2], est présentée ici. Cette approche s’appuie sur une démarche d’intensification du jet de plasma augmentant l’effet de constriction par la pression d’alimentation de la tuyère primaire associé à un contrôle actif de la cohérence spatiale du jet ainsi formé dans la tuyère secondaire, et dans l’espace entre la torche et la tôle.Cette technologie a pu être caractérisée expérimentalement sur les installations d’AKRYVIA à travers des largeurs de saignée de découpe ayant des niveaux de précision deux fois meilleures que les références industrielles de même puissance, des mesures de champ électrique moyen dans le plasma atteignant 26 kV/m et des images de l’arc en sortie de tuyère.Dans ce papier, une modélisation CFD réalisée au laboratoire LAPLACE est confrontée à certains de ces résultats. En particulier, ce modèle a permis de prédire des températures au cœur de l’arc inédites, supérieures à 30 000 K (figure 1)

Experimental Studies of Arc Motion Between Two Parallel Runners with Splitter Plates

In this paper, we present an experimental study in a simplified arc chamber geometry of Low-Voltage Circuit Breaker (LVCB). The influence of vent aperture on arc motion and the influence of splitter plates on arc voltage drop and arc motion are studied. The arc chamber is composed by two parallel arc runners and following the configuration chosen by one or two splitter plates. The experimental setup is completed by a generator (capacitor bench with triggered switch), a high-speed camera and electrical measurements. The arc ignition, arc displacement, arc splitting and arc re-strike have been observed. The results will be described and discussed for different experimental configurations

Influence of Copper Vapors in SF6 Plasma

In this study a theoretical approach allows estimating the ablation mass flux of copper from a corrected Hertz-Knudsen flux. The influence of the copper vapours coming from the anode electrode to an SF6 plasma is studied in a simplified 2D configuration. Depending on the plasma pressure an ablation or a diffusion state is considered. The amount of copper versus time is presented. An RMS current I=10 kA is applied leading at t=10 ms to an amount of copper equal to 0.55 mg. The vapours change the plasma properties mainly the electrical conductivity and radiation and so the plasma behaviour. At time t=5 ms the electrode erosion leads to a copper plasma. This simple case shows the necessity to well consider the copper erosion in plasma modelling as in High Voltage Circuit Breaker (HVCB) where higher current are considered

Compositions Chimiques et Propriétés Thermodynamiques à l’ETL des Plasmas à Haute Pression

International audienceL’étude des phénomènes physico-chimiques se produisant dans un moteur, lors de l’allumage d’un mélange stœchiométrique d’air et de méthane, nécessitent la connaissance préalable de la densité des espèces. A défaut certains auteurs utilisent les propriétés de l’air pur en supposant que la faible proportion du méthane n’a que peu d’influence [1-2]. Afin de compenser ce manque de données, notamment pour la mise en place des modèles et d’établir un état initial lors de l’étude de cinétique chimique [3-4], un code de calcul de composition et de propriétés thermodynamiques pour des gaz purs et mélanges de gaz, basé sur la loi d’action de masse a été développé. Le calcul est réalisé à l’équilibre thermodynamique pour des pressions de 1 bars <P< 300 bars et une gamme de température de 300 K<T<50 kK. La correction du Viriel est introduite pour assimiler le gaz au comportement d’un fluide réel. Pour différents gaz à P=1 bar, nous présenterons la confrontation des résultats du code avec des travaux de la littérature [3-4-5]. Une comparaison des propriétés thermodynamiques entre l’air et le méthane sera réalisée notamment sur la chaleur spécifique. A haute pression, les résultats montrent qu’il est indispensable de prendre en compte la correction du Viriel. Nous illustrerons la nécessité d’incorporer l’effet du Viriel dans le cas de l’hexafluorure de soufre [6] utilisé dans les disjoncteurs à haute tension. L’effet se fait ressentir notamment sur la densité de masse ou une différence d’environ 80% par rapport aux mesures expérimentales peut être trouvée s’il n’est pas considéré. La prédiction des effets convectifs par les modèles s’en trouve alors erronée