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Using ANSYS and CFX to Model Aluminum Reduction Cell since 1984 and Beyond
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1998: 2D+ thermo-electric full cell slice model
At the same time, a 2D+ version of the same full cell slice model was developed. Solving a truly three
dimensional cell slice geometry using a 2D model may sound like a step in the wrong direction, but
depending on the objective of the simulation, sometimes it is not so. Having both models available is in
that context an advantage.
The 2D+ model uses beam elements to represent geometric features lying in the third dimension (the + in
the 2D+ model). Even that way, some geometric representation accuracy is of course lost compared to a
true 3D model, but on the other hand, the payback is almost an order of magnitude gain on the model usage
turn around time (Reference 1, see Figure 10). That type of speedup becomes very important in the context
of the utilization of that kind of model in transient mode.
1999: 2D+ transient thermo-electric full cell slice model
So not much later, a transient version of the 2D+ full cell slice model was developed. An interesting feature
of that model is the extensive APDL coding that computes other aspects of the process related to the
different mass balances like the alumina dissolution, the metal production and the cell controller actions
like the alumina feeding and the anode cathode adjustment (References 14 and 15, see Figure 11).
As that type of model has to compute the dynamic evolution of the ledge thickness, there is a lot more
involved than simply activating the ANSYS transient mode option. Unfortunately, as each transient load
step has to alternate with a geometry change and an initial condition initialization load step, the current
numerical scheme is far from being efficient. Clearly, there is a need for further development on that type
of model.
2000: 3D thermo-electric cathode slice erosion model
As the needs of the industry evolve, new types of models are required. In the past, cathode swelling was a
big problem. Now, with the new types of cathode block, it is the cathode fast erosion rate that creates a
problem.
As this erosion rate is proportional to the cathode surface current density and since the initial surface
current density is not uniform, the erosion profile will not be uniform. Furthermore, that initial erosion
profile will promote further local concentration of the surface current density that in turn will promote a
further intensification of the non-uniformity of the erosion rate.
That extra physics can be incorporated in a standard thermo-electric cathode model. In turn, that new type
of model can be used to investigate potential design improvements that are dealing with the erosion
problem (Reference 17, see Figure 12).
2000: 3D thermo-electro-mechanic half anode model
In the anode manufacturing process, cast iron is poured between the steel stud and the carbon block to joint
the two pieces. As the contact between the cast iron and the carbon remains imperfect, a significant voltage
drop occurs at that interface while the anode is in operation.
In the standard thermo-electric half anode model, that extra contact resistance is added to the model as an
extra radial electrical resistivity in the cast iron. So it is up to the model user to adjust the intensity of the
extra cast iron radial electrical resistivity in order for the model to accurately reproduce the measured
contact drop during model calibration (Reference 3).
Yet, that contact resistance per unit surface depends on the stud hole design, as it is inversely proportional
to the average contact pressure between the cast iron and the carbon. The average contact pressure itself
depends on the steel stud diameter, the average cast iron thickness, the details interface geometry and the
relative thermal expansion of the three materials involved. Finally, the relative thermal expansion itself
depends on the different materials operating temperature that in turn depends on the contact resistance as it
is responsible of an extra very localized production of Joule heat.