ANODE-BOUNDARY-LAYER BEHAVIOR IN A TRANSFERRED, HIGH-INTENSITY ARC

The effects of the plasma's gas-flow rate on phenomena such as the neg ative anode fall and the heat flux to the anode in high-intensity arcs are discussed on the basis of a numerical model of the anode's bounda ry layer. The modelling system consists of a rotationally symmetrical argon plasma formed between the outlet of a constrictor tube and a wat er-cooled flat copper anode perpendicular to the axis of the plasma fl ow which is directed towards the anode. The are is operated at atmosph eric pressure and at a current level of 200 A. The boundary conditions for the electron temperature and the electron number density at the a node's surface are obtained by solving the electron-energy equation an d the diffusion equation at the anode's surface simultaneously with al l conservation equations for the calculation domain. A diffuse anode a ttachment is obtained for a mass flow rate exceeding 0.2 g s(-1) and a constricted anode attachment is found for rates below 0.02 g s(-1). T here is a (probably unstable) transition region between these flow rat es. The anode boundary layer of the diffuse attachment is strongly aff ected by the mass-flow rate. Increasing the mass-flow rate shifts the location of the peak of the electrical potential towards the anode whi le its magnitude decreases and the total heat flux to the anode increa ses. This is a consequence of the effects of the mass-flow rate on the axial profiles of electron and heavy-particle temperatures and of the electron number density. In contrast, the anode boundary layer of the constricted attachment seems not to be affected by the mass-flow rate .