Electromagnetic levitation: global instabilities and the flow inside a molten sample

Electromagnetic levitation is a well-known technique for containerless processing of metals and alloys both in the solid and in the molten state. In experiments the levitated bodies often times show different types of instabilities resulting in a rotating and oscillating motion of the sample. We present results on the reason of such spontaneous instabilities. The theoretical predictions are verified by model experiments using solid Al or Mg spheres. These instabilities can obviously be avoided by system parameters below the corresponding non-dimensional threshold. If this is not possible, an active damping method using DC magnetic fields has been developed. The DC field can either be produced by permanent magnets or by an electromagnetic superposition to the levitation coils. Experiments will be shown demonstrating the stabilization effect due to the DC magnetic fields.
Numerical simulations for the flow inside the molten levitated sample will be given as well as results for the linear stability of this internal flow. Flow and flow stability are also analyzed for the case of a molten droplet subject to a global rotation. It is shown that the global rotation may have a significant stabilizing influence on the internal flow field.