Electrode evaporation and its effects on thermal arc behavior

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 (SF₆ in the case of circuit breakers) and electrode vapor, whose properties are substantially different from those of pure SF₆. 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.