The main goal of a cell stability MHD model like MHD-Valdis is to help locate the busbars around the cell in a way that leads to the generation of a magnetic field inside the cell that itself leads to a stable cell operation.
Yet, as far as the cell stability is concerned, the uniformity of the current density in the metal pad is also extremely important and can only be achieved with a correct busbar network sizing.
This work compares the usage of a detailed ANSYS® based 3D
thermo-electric model with the one of the versatile 1D model part of MHD-Valdis to help design a well balanced busbar network.
The problem of choosing the busbar sizing in order to obtain a uniform current pick up in all the collector bars of a modern side by side high amperage aluminum electrolysis cell, while known to be critical to the cell MHD stability, is not often discussed in the literature.
References [1 and 2] are two exceptions, each presents an in-house computer code called respectively NEWBUS and BUSCAL designed specifically to do such a task. Both use a simple 1D line busbar network representation, a temperature dependent electrical resistivity and solve for the resulting non-linear problem by computing the voltage and temperature equations iteratively and alternately until convergence is reached. These days, such an in- house solver can be setup fairly rapidly in an Excel spreadsheet (see figure 1).
Typically, the calculated collector bars current pick up distribution and the different currents in the busbar network are then transferred to the metal pad current density solver and the metal pad magnetic field solver in preparation to run the MHD wave stability solver.
Much more recently, [3, 4, 5 and 6] ANSYS® based 3D full cell
and external busbar thermo-electric models have been developed in order to very accurately compute the metal pad current density field considering both the converged steady-state ledge profile and the busbar design. Of course, once developed, the 3D busbar model can also be solved stand-alone.
Figure 1. Simple 1D line network model of an anode studs, yoke and rod implemented in an Excel spreadsheet.
So, on one hand, it is possible to develop an in-house code to solve a simplified 1D line network busbar representation and use that tool to perform busbar sizing optimization and, on the other hand, it is possible to develop an ANSYS® based parametric 3D busbar
model to do the same.
Yet, there is now also a third option, using MHD-Valdis [7, 8, 9, 4, 5 and 10] which is a commercially available, fully non-linear MHD cell stability solver. The fact that it is fully non-linear, means that it is solving among other variables the busbar network current distribution at each time step. It is doing so using a versatile 1D line network busbar generator and solver called BUSNET also available to carry out busbar sizing optimization studies.
The three above options will be compared to try to identify the most efficient tool to carry out busbar sizing optimization studies, but before proceeding with the comparison exercise, it is important to take a step back and first review the background theory of the equations that need to be solved.