COMPARISON OF MHD MODELS FOR ALUMINIUM REDUCTION CELLS

The self sustained waves at the aluminium-electrolyte interface, known as ‘MHD noise’, are observed in the most of commercial cells under certain conditions. The instructive analysis is presented how a step by step inclusion of different physical coupling factors is affecting the wave development in the electrolysis cells. The early theoretical models for wave development do not account for the current distribution at the cathode, instead assuming a uniform current density J z at the bottom. When the electric current is computed according to the actual electrical circuit, the growth rate is significantly lower, and if a sufficient dissipation is included, does not lead to instability. The inclusion of the horizontal circulation-generated turbulence is essential in order to explain the small amplitude self-sustained oscillations. The horizontal circulation vortices create a pressure gradient contributing to the deformation of the interface. The full time dependent model couples the nonlinear fluid dynamics and the extended electromagnetic field that covers the whole bus bar circuit and the ferromagnetic effects.