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Light Metals 2010 Edited by TMS (The Minerals, Metals & Materials Society, 2010)
DEVELOPMENT AND APPLICATION OF AN ANSYS® BASED THERMO-ELECTRO-MECHANICAL
ANODE STUB HOLE DESIGN TOOL
Marc Dupuis
GéniSim Inc.
3111 Alger St., Jonquière, Québec, Canada, G7S 2M9

Abstract
At the 2009 TMS conference, we could observe a renewed interest
for the optimization of anode stub hole design. This is not
surprising considering that 50 mV is costing about 1 MM$ per year
to a typical modern prebaked anode smelter producing around
220,000 T per year according to Richard [1] and that we can
estimate that the contact resistance voltage drop at the cast
iron/anode carbon interface is about 60 mV (assuming 0.1 m2 of
contact surface per stub hole, 3000 A of current per stub and 2
micro-ohm m2 of the average contact resistance) which translate to
1.2 MM$ per year of operational cost for a typical modern smelter
for that cast iron/anode carbon contact resistance alone.
Considering the above, it is easy to understand that there is a good
incentive to optimize the stub hole design in order to minimize the
cast iron/anode carbon contact resistance voltage drop. For that
reason, the author took advantage of the recent development of
ANSYS® contact elements library to develop an ANSYS® version
12.0 based fully coupled TEM anode stub hole design tool that is
now available to the whole aluminium industry through GeniSim
Inc.
Historical background
The typical approach for the last 20 years has been to optimize the
stub hole using 3D thermo-electric (TE) mathematical modeling
tools [2,3]. The weakness of this approach is that the contact
resistance has to be considered as constant and the value of that
constant as to be defined as a model input. As a result, the model is
only sensitive to the cast iron/anode carbon contact interface
surface area leading designers to increase that interface surface area
(see per example figure 2 of [3]) disregarding completely the
mechanical impact of those stub hole design changes.
This approach can be very misleading because the value of the
contact resistance that has to be assumed constant in TE models, is
in reality strongly dependant of the applied pressure at the contact
interface as initially reported in [4,5], and again at the 2009 TMS
conference in [6]. Furthermore Richard [1], conveniently fitted the
raw data into a 12 parameters equation that is function of both
pressure and temperature.
The geometry of the stub hole is such that the cast iron/anode
carbon interface contact pressure is function of the local
temperature in that region that is itself function of the local Joule
heating that is itself function of the contact resistance that is itself
function of the contact pressure.
Because of that cycle dependency, only a fully coupled thermo-
electro-mechanical (TEM) modeling tool can be reliably used as a
stub hole design tool because only this type of model is able to
fully reproduce the full complexity of the contact resistance
physical behavior.
Richard [1] was the first to develop an ANSYS® based TEM anode
stub hole model and to use such a model to do some stub hole
design optimization work. Unfortunately, the ANSYS® version
available at the time was not supporting thermo-electro-mechanical
contact elements preventing the development of a fully coupled
model. For that reason, the model he developed was only weakly
coupled. Furthermore, the ANSYS® version available at the time
was not even supporting thermo-electrical contact elements forcing
the usage of "clumsy" link elements to represent the thermo-
electrical contact behavior which added a lot of complexity into the
model development work.
Following Richard's initial effort, Goulet developed a fully coupled
TEM model based on Laval University proprietary finite element
code FESh++ [7,8,9]. FESh++ is a fantastic academic finite
element code in advance of ANSYS® for the implementation of
complex material behavior law elements so it is extremely useful to
carry-up fundamental research work. Unfortunately, it is not the
most practical tool to carry-up design optimization modeling work
in the aluminium industry. For that, ANSYS® have been the code
of choice of the industry for over 25 years now (see per example
[10,11]).
ANSYS® version 12.0 based thermo-electro-mechanical
anode stub hole model development

For that reason, the author took advantage of the recent
development of ANSYS® contact elements library to develop an
ANSYS® version 12.0 based fully coupled TEM anode stub hole
design tool that is now available to the whole aluminium industry
through GeniSim Inc. That model is based on the usage of
ANSYS® SOLID226 3D thermo-electro-mechanical second order
element together with CONTA174 and TARGE170 thermo-electro-
mechanical contact pair elements. Furthermore, CONTA174
element supports the setup of a pressure and temperature TCC
(thermal contact conductance) and ECC (electrical contact
conductance) values through the %table% option.