A modernized ANSYS-based finite element model for the thermal-electrical design of aluminum reduction cells

Abstract

Heat balance and magnetohydrodynamics are critical to the design of an aluminum reduction cell since they largely determine its operational window. Furthermore, an inadequate lining design generally leads to degraded cell performance and premature failures. The first task in lining design is to determine the position of the frozen ledge and the cell superheat for a range of operational parameters.

Although several different modeling approaches and computational domains have been proposed to solve the Stefan problem, a widely accepted methodology, first proposed by Dupuis [1], is based on the iterative repositioning of the ledge front in a thermoelectrical (TE) Finite Element (FE) model. The algorithm involves successive displacements of the solidification front nodes based on the calculated temperature field until the entire ledge-to-liquids interface reaches the bath solidification temperature. The superheat is adjusted to minimize the difference between the cell internal heat generation and the integrated heat losses over the control volume. Originally, this approach was limited to two layers of first order elements across the ledge thickness moving horizontally and did not include the liquids.

This paper presents a generalization as well as improvements to the original methodology, enabling the prediction of the ledge profile using an arbitrary number of first or second order elements through the ledge thickness while including the metal pad and the bath. The proposed modeling framework has been implemented in ANSYS using the ANSYS Parametric Design Language (APDL) scripting language and designed to minimize the computational cost of moving the ledge. Another benefit is that the generic core macros developed also efficiently handle the ledge front displacement in any orientation. Current technology ANSYS elements are used, in such a manner that high-performance computing solvers can be leveraged.

The robustness of this improved methodology is illustrated in this article by comparing the results obtained for a fictitious 300 kA cell technology against those computed by the standard approach.