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Old   October 27, 2007, 12:07
Default problem with heat transfer
  #1
kostas
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Hi I am trying to model a stack (2.5X1.6X0.5) with all walls except one adiabatic, on wall at T=const, inlet as static pressure=0 ant Tair=25 C, and outlet as const mass flow rate. I am modeling as steady state. With air at 25C And with thermal model with buoyancy turbulence

While the pressure, the velocities and the k-e are ok, the thermal model at the solution is very unsteady and the residuals are vey high.

Any ideas for this?

Also any ideas about BC so that I can run this as physical model (not to imply an outlet BC with mass flow rate, but to calculate the flow due to buoyancy)

thnks

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Old   October 27, 2007, 13:31
Default Re: problem with heat transfer
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Wooster
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Have you set the initial temperature of the fluid?(Initial conditions dialog)? I had something similar occur to me and by Initializing the fluid temperature to the inlet condition (or suspected fluid temperature) I was able to get my thermal residuals to converge.

If we are talking about the same problem, then CFX basically sets the fluid temperature to the highest temperature set by the user (the hottest temperature in the model). This can cause the residuals to just "hang" at higher values. In my model they wouldn't converge until 1000+ iterations. Hope this helps. -W
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Old   October 28, 2007, 17:33
Default Re: problem with heat transfer
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Glenn Horrocks
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Hi,

I think you will find there is no steady state solution. Often in turbulent convection driven problems you get large scale transient structures in the flow which cannot be "averaged" out with a turbulence model. The only way to proceed is to do a transient simulation.

At a minimum you should check the steady state solution gives the same answer as a transient solution.

Glenn Horrocks
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Old   October 31, 2007, 04:26
Default Re: problem with heat transfer
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kostas
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Thank you. I finally manage to get a result, with transient solution, but only in hybrid ventilation.

I still can't find bountary conditions for naturally ventilation

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