INTERFACIAL INSTABILITY DUE TO MHD MODE-COUPLING IN ALUMINUM REDUCTION CELLS

This paper analyses instabilities on the cryolite/aluminium interface in an aluminium reduction cell. The simplified cell model is a finite rectangular tank containing the two fluid layers, and carrying a unifo rm normal current. The magnetic field is assumed to be a linear functi on of position. Several previous studies have considered waves consist ing of a single Fourier component but here we consider perturbations w hich are a general combination of the normal gravity-wave modes. We de rive a system of coupled ordinary differential equations for the time- development of the mode amplitudes, and show that instability can occu r via mode interactions, the electromagnetic perturbation force due to one mode feeding energy into the other. Growth rates are determined b y computing the eigenvalues of an interaction matrix, and an approxima te method using only the three leading diagonals is developed. If two modes have similar frequencies they may resonate and become unstable a t a very low threshold current. We consider the influence of various c ell parameters and draw some general conclusions about cell design.