Numerical Modelling of a Free-Burning Arc in Argon. A Tool for Understanding the Optical Mirage Effect in a TIG Welding Device

Transcription

1 Presented at the COMSOL Conference 2009 Milan Numerical Modelling of a Free-Burning Arc in Argon A Tool for Understanding the Optical Mirage Effect in a TIG Welding Device J.M. Bauchire, E. Langlois-Bertrand, C. de Izarra GREMI, UMR 6606 CNRS/Université d Orléans, France

2 INTRODUCTION Electric Arc & TIG Welding Device Electric arcs at atmospheric pressure are thermal plasmas with: High energy density High temperature High light emissivity High electric current intensity Point-to-plane discharge configuration, freeburning arc, nearly TIG welding device configuration Standard diagnostic of electric arcs: emission spectroscopy

3 INTRODUCTION Experimental Observation As lens is shifted upward, cathode tip is still visible, whereas the lens optical axis is above the nozzle exit...! Rays of light, emitted from the cathode tip, are bent when passing through the plasma. Optical mirage effect...? Numerical modelling of the electric arc + ray-tracing...

4 FREE-BURNING ARC SIMULATION General Assumptions Axisymmetry Flow Inlet and surrounding gases Temperature Radiative losses Gravity effect Electrode erosion, electrode sheath Electric current 2D (r,z) simulation Laminar and steady-state Argon at atmospheric pressure Local thermodynamic equilibrium Net emission coefficient method Not taken into account Not taken into account DC

5 FREE-BURNING ARC SIMULATION Model & COMSOL Application Modes Laminar Non-Isothermal Flow Weakly Compressible Navier-Stokes J B Explicit coupling General Heat Transfer J E U rad Meridional Induction and Electric Currents, Potentials σ V = 0 A = µ J 0 E = V J = σ V B = A

6 FREE-BURNING ARC SIMULATION Input data Mass density (kg.m -3 ) Viscosity (kg.m -1. s -1 ) Temperature (K) Temperature (K) Thermodynamic properties & transport coefficients depend on temperature

7 FREE-BURNING ARC SIMULATION Input data Thermal conductivity (W.m -1.K -1 ) Electric conductivity (A.V -1. m -1 ) Temperature (K) Temperature (K) Implicit coupling

8 FREE-BURNING ARC SIMULATION Calculation Domain & Boundary Conditions Flow inlet (0.5 m/s, 300 K) Cathode tip Current density I = 100 A Axis (6 mm) Pointed electrode 1000 K T profile Nozzle 2.5 mm T profile Computation domain Open Boundary (300 K) (15 mm) Mesh points = Triangular elements = Degrees of freedom = Solver = PARDISO stationary Flat electrode (20 mm) Grounded electrode Insulation or T profile

9 FREE-BURNING ARC SIMULATION Results Electric potential (V) Velocity (m/s) Good agreement with: Experimental results Previous simulations based on finite volume method

10 FREE-BURNING ARC SIMULATION Results Temperature (K) Temperature (K) Anode boundary condition: Insulation Anode boundary condition: T profile Only weak influence on temperature field in cathode and nozzle exit regions

11 FREE-BURNING ARC SIMULATION Results Refractive index & index gradients (streamlines) λ = nm Anode boundary condition: Insulation Validation of refractive index gradients Mainly for low temperatures Nozzle region Ray-tracing Anode boundary condition: T profile

12 RAY-TRACING Theory The ray path in a non homogeneous zone can be calculated with vectorial formulation of Snell- Descartes laws: d ds ds is the curvilinear abscissa u ( nu) ( n) = the unit vector tangent at any point in the trajectory of the light n the refractive index If the ray of light comes from a point M 0 (r 0,z 0 ) with a θ angle between the cathode axis and the ray propagation direction at M 0 point, we obtained: dr0 dl dz dl 0 = n 0 = n 0 sinθ cosθ This equation system is solved with Euler method

13 RAY-TRACING Results

14 CONCLUSION Demonstration of COMSOL Multiphysics capability to simulate arc discharges Still remain difficulties to reach convergence according to boundary condition type (Dirichlet s) Success in exporting and post-processing results (for ray-tracing) Further works on this subject to improve model and to take into account the electrodes