Intermittent air breakdown between an electrode and liquid metal free surface

The present paper presents the experimental and numerical results of an electrically induced flow in a cylindrical container (188 mm diameter) filled with a In-Ga-Sn alloy. The electric current is applied from copper electrodes of various diameters (4-8mm) and varied from 100 to 700 Amps. The deformation of the free surface just under the electrode is reported in term of depth. Experimentally an arc develops around the electrode when the applied current exceeds a critical value. Just before the arc develops the electrode was still in good contact with the liquid metal. It is not yet clear on whether the arc develops because of contact lost or because of the occurrence of an electric gas breakdown near the electrode surface. In the later mechanism, the electric current would flow in parallel in the liquid metal as well as through the plasma surrounding the electrode. In a previously the numerical model was not able to be applied for cases of high electric current where the electric contact is lost. In present work, we present a numerical model able to simulate the breakdown and the arc phenomena across the air gaps between the electrode and the liquid metal surface. The evolution of the air/liquid metal surface is modelled with a VOF tracking method coupled with an MHD solver. The mechanism of breakdown is based on the development of a large numbers of thin, weakly ionized channels called streamers. The model has been first validated through comparison to Townsend's discharge theory and experiments for different electric field. In the present case it is found that the streamer starts to be emitted laterally by the electrode before even losing contact with the liquid metal. Self-sustaining discharge and ionization are predicted to occur very quickly under sufficient voltage difference. Once the level of ionization is high enough, the simulation assumes equilibrium; an Ohm's law is used to relate the electric current with the local plasma temperature dependant electric conductivity. The plasma temperature reaches temperature in the level of 104 K. Due to the electromagnetic forces and to the plasma gas jet, the surface of the liquid metal is highly instable. The simulation shows that the frequency at which the liquid metal comes in contact with the electrode is chaotic. The simulations suggest that although streamers can be emitted even when the electrode is still in contact with the liquid metal, breakdown occurs only when the electric contact is totally lost. (author)