STABILITY OF ALUMINUM CELLS - A NEW APPROACH.

Abstract

Small periodic disturbances are superposed to the equilibrium shape of the interface between cryolite and aluminum, and a linear analysis of their properties is proposed. Perturbations of electric quantities are first calculated, and then perturbations of velocities and pressure in the two liquids follow from 3D Navier-Stokes equations with a linear drag. The result is a characteristic equation which relates the parameters of the disturbance (wave length, rate of amplification) to the parameters of the cell (current densities, thicknesses of liquid layers, friction coefficients, equilibrium velocities. . . ). It is found that two different kinds of instabilities may develop. One is the classical Kelvin-Helmholtz instability of sheared interfaces slightly modified by MHD effects; it may generate small scale disturbances (20 cm). The other is a new instability, essentially electromagnetic in nature, able to generate large scale waves (1. 5 m) propagating in the direction of horizontal current.