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Meshing very long ducts

Question asked by 1-DVFMOQ on Aug 15, 2008
Latest reply on Aug 15, 2008 by Bill McEachern

We started recently to use Floworks to analyse thermal profiles on bus ducts and we are running into problems and problems. The tech support is not really helping us a lot because they have never worked with such geometry. Maybe somebody can help us here... We took the 2 days training but I would really like to have a "Meshing for Dummies" book on hand right now....

Here is a brief description of the geometry and the setup:

- The bus consist of 2 concentric cylinders. The internal cylinder has 14.5" O.D. X 0.6" thk. and the external 31" x 0.25" thk.
- The total lenght is approximately 250 feet.
-Air is going in by one of the tube and returning by the 2 others.
- This is an external analysis and the computational domain has been reduced to just include the full geometry (it was way too large)
- For this run, we had around 2.5 millions elements and it took around 100 hours to converge.

Problems we are running into :

- Flow trajectories going out of the model. SW Tech support said that this is a bug in the post-processor and they are working on this. We first tought that it would be a coarse mesh issue but they said that this is probably not the root cause.

- Mass flow rates do not balance between the inlet and the outlets (30% lost approx). Tech support said that it might be a problem with the pst-processor, but we are measuring these rates with surface goals (refer to screenshot013), which are calculated by the solver, not the post-processor. These goals are measured on the internal surface of the inlet and outlet lids. Can it be again a meshing issue ? Since the mass flow rates do not balance, I do not trust my temperatures.... If really 30% of my air is lost somewhere, it will certainly affect the results...

Meshing is the complex part in this kind of project. Does it exist somewhere a good guide or manual that explains the meshing ?

FW help is not really easy to understand and the examples / tutorials are not similar to our application...

Any help or ideas are welcome....

Thanks in advance