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Using ANSYS and CFX to Model Aluminum Reduction Cell since 1984 and Beyond
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2008: Weakly Coupled Thermo-Electro-Mechanical and MHD
Mathematical Models
To start with, the vertical potshell deformation problem is a fully coupled thermo-electro-mechanical
problem. The potshell deforms due to its thermal load and this thermal load is generated by the cell heat
dissipation characteristics.
Then, as demonstrated in Reference 35, the metal pad horizontal current density has a strong influence on
the MHD cell stability and the intensity of that horizontal current density is directly proportional to the
metal pad thickness. With a vertically deformed potshell, there is a strong longitudinal variation of that
horizontal current density even for the "static" bath/metal interface configuration.
A new version of MHD-Valdis cell stability model has been developed to take into account the longitudinal
deformation of the cathode block surface as computed by the mechanical model (Reference 36) and hence
take into account the strongly varying metal pad thickness and corresponding horizontal current density on
the non-linear MHD cell stability analysis.
2010: New Generation of 3D cathode potshell plastic deformation
and lining swelling mechanical model
The "empty shell" thermo-mechanical potshell model, like the other two types of models for that matter, is
based on the usage of the quadrilateral Finite Strain shell element (SHELL181) in the commercial code
ANSYSŪ. The temperature distribution obtained from the full cell quarter thermo-electric model [2] is
applied as a body load to the entire potshell structure. Also for all three types of model, it is possible to
solve the mechanical problem only by considering the elastic properties of the potshell steel structure or to
consider in addition the temperature dependent isotropic hardening von Mises plasticity behavior of the
potshell steel structure using the MISO non-linear hardening option in ANSYSŪ.
All this is relatively straightforward to setup, but there is still the internal forces generated from the thermal
and chemical lining expansion to be specified as boundary conditions in the model. Obviously, model
boundary conditions are model inputs, while those internal forces are a priori unknown and depend on the
potshell structural rigidity. Yet, it is now possible to apply as boundary conditions to the "empty shell"
model the contact interface pressure distribution extracted from the "almost empty shell" model solution
(References 37, 38). As can be seen in Figure 23, the resulting improved "empty shell" model displacement
solution is now quite similar to the one from the "almost empty shell" model" presented in Figure 24.
In the "almost empty shell" model type, more physic is incorporated by adding the lining geometry
between the potshell walls and the cathode blocks all around the potshell and by applying a pressure
loading as boundary condition at the carbon block/side lining interface that is lying on the Dewing strain-
stress relationship (i.e. that represents the carbon block equilibrium condition). In order to be able to do so,
a new convergence numerical scheme external to the ANSYS solver must be setup. Starting from an
assumed initial internal load, the task of that external convergence loop is to converge toward that cathode
block equilibrium condition pressure loading for each element face of the carbon block/side lining
interface.
The "almost empty shell" potshell model have been improved by decoupling the 2D potshell mesh from the
3D side lining mesh and by reconnecting the two parts using ANSYSŪ CONTA174 and TARGE170
contact pair elements. After the decoupling, it is possible to completely refine the 2D potshell mesh. This
was not possible before the decoupling. Figure 24 presents the resulting displacement solution. Apart for
the possibility to further refine the 2D potshell mesh, a second significant improvement is the added
possibility to extract from the solution the pressure that the side lining is applying on the potshell through
the contact interface.
In addition to the lining material located between the potshell walls and the cathode carbon blocks already
present in the "almost empty shell" potshell model type, the "half empty shell" potshell model type also