Electromagnetics & CFD: A mutual attraction
Fred Mendonça, CD-adapco London

Design Engineers are all too often faced with an inexorable link between traditional engineering disciplines - fluid dynamics, structural mechanics, thermodynamics, electromagnetics. The overall performance of an engineering component, or system, requires knowledge of how these separate functions interact. Combining the analyses from separate physical disciplines, for a required repeatable product development workflow, through process integration is commonly referred to as ‘Vertical Applications’.

Take the example of switch-gear in electrical power plants. A component in a high voltage (HV) circuit breaker assembly contains a copper rod and sleeve which, when connected, completes an electrical circuit. The electrical current, as it flows through the metal matrix, experiences a resistance which generates heat usually very close to the metal surface. For safety reasons, this heat needs to be dissipated, and it is most effectively done through conduction and convective air-cooling.

CD-adapco and ABB Corporate Research in Baden, Switzerland, have recently validated a modeling of the interaction between electromagnetic heat generation, heat transfer and air-cooling. According to Prof. Zoran Andjelic, Principal Senior Scientist at ABB, “Safety regulations and component operational efficiencies require that the apparatus stays within reasonable temperature limits. While our electromagnetic codes can tell us how much heat is generated from the current flow in such components, CD-adapco has shown us how to combine this with fluidic heat transfer, and we are quite satisfied that the phenomena are well captured”.

Over an operating time of twenty minutes, CD-adapco’s worldleading CFD software STAR-CD takes information about the system heat-losses from the EMC code POLOPT, and computes the cooling air-flow and thermodynamics around the starting switch to predict the metal surface temperatures exposed to the atmosphere. Fred Mendonça, Manager of Vertical Applications at CD-adapco adds, “There are important enablers necessary for this form of Process Integration to work successfully; analysts increasing believe in PLM, and realise the need for a strong link between their CAD definition and the design analysis processes. Our products, embedded in the most widely used CAD software, are linked to fast computational analysis by robust automatic meshing using polyhedral cells, a technology pioneered here at CD-adapco”.

STAR-CD is embedded into the native CAD systems thanks to the STAR-CAD Gateways (STAR-Pro/E Gateway is shown here in the Pro-ENGINEER environment) this means that the starting-point for this multi-disciplinary analysis is common. It’s a key ingredient for design analysis. Surface and volume meshes for both the EMC and CFD can be generated automatically while still in the CAD environment. The CFD boundary conditions and solver settings can also be defaulted in the CAD environment. Heat losses computed in POLOPT are then passed to STAR-CD, and reinterpreted as electromagnetic heating, raising the metal surface temperatures. This creates a buoyant plume, which has the useful property of convecting the heat away from the surface and bringing cooler air from below into contact with the metal.

How well does the integrated modeling represent reality? Validation of the virtual modeling must always be considered as an essential part of CAE analysis. As the simulation advances in time, one can monitor the temperature of the surface, and compare these predicted temperatures with measurements on the actual apparatus. In fact, to give confidence that the EMC-CFD coupled modeling works over a wide range of operating conditions, different current charges ranging between 2 and 18 kilo-Amperes have been run through the starting switch, then both measured and simulated temperatures compared – they agree to within a few degrees. This gives Design Engineers the confidence to test different design variants in a virtual environment, early in the design process and long before the design space has been fixed. Prof. Andjelic concludes, “Once a model is validated, the time and resource savings added to the design benefits which are inherent to this form of integrated virtual simulation are very clear.”