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What’s the Deal with Electronics Cooling CFD?

To many professional analysts with a background in Computational Fluid Dynamics, most electronics cooling applications look pretty straightforward. There aren’t any complex physical processes to model like combustion (well hopefully not), high pressures or high temperatures (again, hopefully not) or high speed flow. Typically there are no free surfaces or multiphase flows, and no moving geometry. So, nothing really challenging from a CFD perspective then?

Um… well not exactly. All of the above are true. Well, OK some are only partly true – fans rotate and heat pipes involve phase change, but these can be handled without modeling the fan’s rotation or the phase change directly. So why not use a general-purpose CFD tool? Should be a piece of cake, right?

In theory general-purpose software can be used, but as the saying goes:
“In theory there’s no difference between theory and practice. But in practice there is”

The short answer is ‘productivity’. Electronics products are characterized by extremely short design cycles, requiring exceptional productivity across the CFD process. The challenge is to compress the most time consuming steps in the traditional CFD process – typically geometry preparation and meshing.

Traditional work flow for CFD analysis

Traditional work flow for CFD analysis

General-purpose CFD tools are often used late in the design process. The starting point is to prepare manufacturing CAD geometry for the analysis. As an aside, such CAD geometry is sometimes wrongly referred to as ‘dirty’ and so needs to be ‘cleaned’. Actually it was just created for a different purpose and so needs to be re-worked to make it suitable for analysis.

Once prepared, the geometry has to be meshed. Almost all general-purpose CFD tools use a body-fitted mesh. This is created by first generating a 2D mesh on the surfaces in the model, from which a fully unstructured volume mesh is generated. This is easiest when dealing with relatively few non-abutting objects. Meshing is often the most time-consuming part as both mesh quality and density have to be controlled. Meshing is a BIG subject in CFD. There many techniques and even more software solutions out there, which (rightly) suggests it’s not trivial.

The process above often uses different tools for geometry preparation and meshing. Once created, the mesh is imported into the CFD pre-processor. The pre-processor applies boundary conditions to cell faces and volumes within the mesh to define walls, inlets, outlets, heat sources etc. The process works well provided the geometry is not expected to change, or not change significantly, as small changes can often be accommodated by distorting the mesh. Otherwise, every time the geometry changes the mesh has to be reconstructed and the boundary conditions reapplied. Ouch! – geometry change is generally what product design is all about.

From the outset, electronics cooling CFD tools took a radically different approach to the way geometry is handled and the way the model is meshed – more on that in later posts…

Design Process, CFD, Mesh Generation, Meshing

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About John Parry

John ParryI started my career in the consultancy group at CHAM Ltd., using PHOENICS for a variety of CFD applications. From the consultancy group I moved into support, helping customers debug models, and figuring out how to model new applications. That broadened into delivering training courses and creating training material. I was invited to join Flomerics when it started in 1989 to head up Customer Services, and I jumped at the chance to work for a startup. After a few years supporting customers using FloTHERM I moved across into research, developing thermofluid models of common electronic parts, like fans and IC packages, later managing the DELPHI and SEED projects. More recently I worked with Flomerics’ Finance Director on the acquisition of MicReD, helping to integrate MicReD’s business into Flomerics Group which was great fun. Since Flomerics acquired Nika, I’ve been responsible for promoting the FloEFD suite in education, and moved into marketing. I now work as part of the Mechanical Analysis Division’s Corporate Marketing group, responsible for ElectronicsCooling Magazine and the division’s Higher Education Program. Expertise: I’m a chemical engineer by training and did a PhD in reactor design before getting involved with CFD more than 25 years ago. Visit John Parry’s Blog

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