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Using ANSYS and CFX to Model Aluminum Reduction Cell since 1984 and Beyond

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1993: 3D electro-magnetic full cell model

The development of a finite element based aluminum reduction cell magnetic model clearly represented a
third front of model development. The ability to solve the magnetic field inside an aluminum reduction cell
is a key requirement in order to be able to design a stable high amperage cell (i.e. a cell that does not
amplify small bath-metal interface perturbation into long MHD driven traveling waves).

For a finite element solver, it is not a straightforward application as the requirements are multiple, the
model must be able to:

1) deal with locally distributed current sources;

2) deal with far field current sources;

3) deal with a multiply connected ferro-magnetic thin wall structure (the potshell) that is shielding the

inside of the cell from the far field current sources.

Because of the presence of the ferro-magnetic shielding structure, the solution of the magnetic problem
cannot be reduced to a simple Biot-Savard integration scheme. Instead, solving the problem using the finite
element method requires meshing the empty space around the cell up to the point where semi-infinite
special boundaries elements can be used. Obviously, the model solution is non-linear because of the non-
linear magnetic properties of the ferro-magnetic shielding structure.

Starting with version 5.0A, ANSYS could be used to solve that problem. Obviously, because the full cell
and its surrounding empty space had to be meshed, this type of model required a tremendous amount of
computing resources. An experimental version of that type of model was developed and run on a CRAY
C90 supercomputer (Reference 8, see Figure 7).

That model required so much computer resources, that the next planned development phase, the extension
of the full cell thermo-electric model presented above into a full cell thermo-electro-magnetic model was
cancelled. To this day, the aluminum industry still relies on in-house developed boundary element codes to
compute the cell magnetic field (Reference 10), often using a very approximate representation of the ledge
geometry leading to a magnetic field calculation based on very approximate locally distributed current
sources (Reference 7).

1993: 3D transient thermo-electric full quarter cell preheat model

Driven by an urgent plant request, the cathode quarter thermo-electric model was extended into a full
quarter cell geometry in preheat configuration and ran in transient mode in order to analyze the cell preheat
process (References 11, 12 and 13, see Figure 8). The need was urgent, but again due to its huge computing
resources requirements, the model was not ready in time to be used to solve the plant problem at the time.

Fortunately, since then, the model results were finally put to contribution to solve the plant problem when it
resurfaced. So again, we have here an example of delayed model success story.

1998: 3D thermo-electric full cell slice model

In 1994, the aluminum industry was in the middle of a smelting overcapacity crisis and reacted among
other things by a wave of R&D budgets cut. Cutting the funding for the development of expensive and
"unproductive" aluminum reduction cell models seemed the right thing to do in that context.

So at that time, the author stopped working for the R&D organization of a major aluminum company and
started working as a consultant. After a few years of consolidation, he was able to finance his own R&D
activities in the field of aluminum reduction cell model development. The 3D thermo-electric full cell slice
model is the first new model developed that way.

As described previously, the 3D half anode model and the 3D cathode side slice model have been
developed in sequence, and each separately required a fair amount of computer resources. Merging them
together was clearly not an option at the time, yet it would have been a natural thing to do. Many years
later, the hardware limitation no longer existed so they were finally merged (Reference 1, see Figure 9).