Numerical and experimental study of transferred arcs in argon

The bidimensional model of the electric arc is enhanced with the plasma-electrodes interaction to predict the properties and the energy distribution of an argon arc operating with current intensities between 100 and 200A and electrode gaps of 10 and 20 mm. An adaptive numerical insulation is applied to the cathode, to properly simulate its thermionic emission mechanism and overcome the dependence on empirical distributions of the current density at its tip. The numerical results are quantitatively compared with the data obtained from calorimetric and spectroscopical measurements, performed on a device which generates a transferred arc between a water cooled copper anode and a thoriated tungsten cathode enclosed in a stainless steel chamber. The calculation of the heat fluxes towards the electrodes permits to determine the amount of power delivered to each component of the arc system (the anode, the cathode assembly and the chamber) and to evaluate the overall efficiency of the process for different configurations. The agreement between theory and data, over the range of parameters investigated, is sensible both in the temperature profiles and in the energy distributions. In such configurations, the conduction from the hot gas is the most relevant term in the overall heat transferred to the anode, but it is the electron transfer which rules the heat transfer in the arc attachment zone. The arc attachment radius is also dependent on the process parameters and increases with the arc current (from approximately 5mm at 100A to 7mm at 200 A) and the arc length. However the maximum heat flux reached on the axis decreases increasing the gap between the electrodes, although more power is delivered to the anode due to the radial spreading of the plasma. A 10mm 200A argon arc releases to the anode about 2.6kW, which corresponds to 75% of the total arc power available. If the arc is extended to 20mm the power transferred rises by nearly 350 W, but the overall efficiency drops to 65% due to the increased losses towards the chamber. The power delivered to the anode increases almost linearly with the arc current, presenting a slope of about 15WA(-1), independent of the arc length.