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

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The development and successful validation of that model was considered as a major achievement at the
time, but the model potential application in an anode retrofit study was not considered practical then. Later
on, several anode prototypes were successfully designed with that type of half anode model.

1986: 3D thermo-electric cathode side slice and cathode corner
model

The next step was the development of a 3D cathode side slice thermo-electric model that included the
calculation of the thickness of the solid electrolyte phase on the cell side wall (References 1, 2 and 3, see
Figure 2). This involved the addition of an extra convergence loop on the geometry of the model in order to
satisfy both the temperature and the heat flux boundary conditions on the solidified surface.

This loop was programmed in a .COM file in the VAX VMS environment. In that loop, ANSYS was called
as a subroutine. The calculation of the new nodes position was computed in an external FORTRAN
program also called as a subroutine in the .COM file. The mesh had obviously to be very crude as the extra
geometry convergence loop added around an order of magnitude to the CPU time required by ANSYS to
solve a fixed geometry thermo-electric problem. In this first side ledge thickness convergence loop
implementation, the solution from the previous loop was not used as an initial guess to solve the next loop.

Despite the very serious limitations on the size of the mesh, a full cathode corner (Reference 3) was built
next, more to demonstrate the capability to do so than to actually use the model as a design tool as the
model turn around time was not at all convenient!

1989: 3D cathode potshell plastic deformation mechanical model

After the migration of the computer platform from a VAX VMS environment to a SGI UNIX environment
and the recoding of the cathode ledge thickness convergence loop directly in ANSYS using APDL and the
successful application of the above models to design some cell prototypes, it was decided to expand the
modeling capability into a new direction.

The new model type addresses a different aspect of the physics of an aluminum reduction cell, namely the
mechanical deformation of the cathode steel potshell under its thermal load and more importantly its
internal pressure load. The internal pressure buildup is coming from the gradual swelling of the cell lining
as it absorbs sodium during the cell operation.

The requirements for that type of model were quite different from those of the thermo-electric models. The
full quarter of the shell structure had to be meshed and that mesh had to be fine enough to be able to
accurately compute the level of stress in the potshell structure. Obviously, 2D shell plastic mechanical
elements were used, initially only the triangular shape was available! Providing an accurate thermal loading
on the potshell structure was a difficulty as no quarter thermo-electric models were available.

Finally, running in plastic mode was quite a challenge for three reasons:

1) Even on the brand new P-IRIS 4D/20, the CPU time required to solve such a model was excessive, the

initial debugging runs had to be done on a Convex C1 computer running in England;

2) The ANSYS non-linear solver was not very robust at the time;

3) Since potshell designs were very weak, most of them were actually failing under their maximum load

in real life (Reference 4, see Figure 3).

Yet, over the years, this type of model turned out to be quite an asset, as most if not all of the potshell were
eventually redesigned in order to prevent excessive plastic deformation. This drastically reduced potshell
repair costs and contributed to increase cell life.

1992: 3D thermo-electric quarter cathode model

With the upgrade of the P-IRIS to 4D/35 processor, and the option to run on a CRAY XMP supercomputer,
the severe limitations on the CPU usage were finally partially lifted. This opened the door to the possibility