A Study Of Mathematical Modeling And Its Application For The Melts Motion In Aluminium Reduction Cells

The integrated technical level of industrial aluminium electrolysis largely depends on working performance of alumiunium reduction cell, the core equipment for this production process. The melts motion in operating cells directly affects working performance of the cells, and it's significant for both design of large-scale cells and optimization of field process to resolve the melts behaviour accurately by building up reliable flow models, which is also an important basis of further promoting efficiency and saving energy.In view of the limitations in flow modeling for aluminium electrolysis home and abroad, such as separate computations of metal and bath flow, analysis of current efficiency (CE) without allowing for the melts motion, disregard of the bubble effect, etc, a set of more faultless flow models, well coincided with the industrial system, have been developed and successfully applied to analysis of the melts motion and CE in a real industrial cell, supported by a national "863" program, a national "973"program and a national natural science foundation.The main innovations and achievements are as follows:(1) Taking account of the relationship among motion of each phase and the integrity of flow in whole cell, an inhomogeneous steady flow model of three phases (metal-bath-bubble) has been built up, so as to numerically resolve the metal flow field, the bath flow field and the metal-bath interface deformation simultaneously in one model. Using this model, the global steady flow in a real industrial cell was analyzed, and the results show:electromagnetic forces, one of the two types of main driving forces, tends to form the flow pattern with large-scale circulation, while, gas bubble force, as the other type, tend to form small-scale loops surrounding each anode; due to the difference in driving forces, the metal and bath flow fields represent different characteristics; furthermore, the profile of anode projection can be observed on the metal-bath interface as a result of the motion of gas bubble.(2) Based on an analysis of current loss mechanism and combined with the theory of mass transfer across the fluid-fluid interface, a model for resolving local and global CE, connecting with the melts flow, has been developed, and the theoretic calculation of CE distribution in large-scale cells has been realized. The CE of a real industrial cell was analyzed with this model, and the results indicate that CE is higher on the regions under anodes than on the other regions because of the difference in local cathode current density and turbulence eddy dissipation. (3) According to the shallow water future of fluid system in aluminium reduction cells, a nonlinear shallow water model, suitable for transient analysis of the melts flow, has been developed, so as to realize numerical transient computation of the metal and bath flow fields and the wave of the metal-bath interface. Based on this, the effect of anode gas bubbles has been introduced into the nonlinear model, and the reliability of transient computation has been further enhanced. A transient study with application of the nonlinear model shows:the interface wave tends to be unstable from a stable state with decreasing anode-cathode distance; MHD stability can be improved markedly by reducing the vertical magnetic flux density in the metal layer; with considering the effect of anode gas bubbles, the transient interface wave is weakened, however, the steady interface deformation is enlarged.