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Valued Contributor
Posts: 58
Registered: ‎01-08-2009
Accepted Solution

Convective Heat Transfer - Buoyancy Flow Vectors Are Moving In Wrong Direction

199 Views, 1 Replies
07-30-2013 06:09 AM

I've sent the model in to tech support but wanted to ask the question here too because it might help someone else.  Picture attached.




  • Heat sink with total power of about 130 watts applied to a surface.
  • Ambient temp about 55C
  • No forced flow.  Cube shaped air volume with 0 psig on inlet and outlets to allow for buoyancy flow.
  • 50C temp applied at bottom of cube.
  • Using symmetry (half model)


  • The analysis gets very close to converging after 400 iterations but never stops.  Temperature deltas are extremely small so I stop analysis.
  • The flow vectors are correctly rising from the heat source but only locally near the surface of the heat sink.  About 10 to 20 in/sec.
  • Elsewhere in the air volume, the vectors are pointing in the opposite direction (towards gravity) and moving fast - 200 in/sec

I can't figure out what is causing the air to flow in the opposite direction.  It really is acting as though I have placed a velocity on the top of the volume

but I haven't.  Only zero pressure.  Has anyone else seen this in their models?  If so, how did you resolve it?



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Valued Contributor
Posts: 58
Registered: ‎01-08-2009

Re: Convective Heat Transfer - Buoyancy Flow Vectors Are Moving In Wrong Directi

07-30-2013 10:58 AM in reply to: mrm01

Seems I had forgotten to set the air to "Variable" instead of "Fixed" though I swear I made that selection before running the analysis. Running correctly now.

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