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Turbulent Schmidt Number in Ansys CFX 12.1

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Old   May 17, 2011, 07:54
Default Turbulent Schmidt Number in Ansys CFX 12.1
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Hello Everybody,

I am modelling a swirler in which natural gas is being mixed with air. I use Ansys CFX 12.1 and the SST turbulence model. I want to help the mixing, therefore I want to set my turbulent Schmidt number to 0.2 instead of the larger default values.

I have read some forum posts and changed the turbulent Schmidt number at four locations. (Two regimes and two coefficients in the sst model):

Domain/Fluid Models/Advanced Turbulence Control/k-omega regime + k-epsilon regime/k coefficients + omega coefficients/Turbulent flux closure.


However, there is no significant difference in my results concerning mixing. I think I do something wrong. I actually want to set in the turbulent Schmidt number for the CH4 mass transport eqauation. That should give a better result.



So, Does someone know how to set the turbulent Schmidt number manually in Ansys CFX??? Can you enlighten me about this subject?

Thanks Already,

Greetings, Diddypablo
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Old   May 17, 2011, 19:10
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This does not sound like a good approach to me. Fiddling with turbulence model parameters usually does not work. Why don't you just adjust the diffusivity of the component?
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Old   May 18, 2011, 03:21
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Dear Ghorrocks,

Your right that fiddling with the turbulence model does not really has an impact. The results do not change at all.

So, thanks for proposing to adjust diffusivity. But, according to literature, I have to use a turbulent Schmidt number of 0.2.

Do you know how the turbulent Schmidt number and the general kinematic diffusivity (As in fluid models/component/Kinematic diffusivity) are related to eachother? In this way I hope to set the diffusivity value corresponding to Sct = 0.2.

Thanks and greetings,

Diddypablo
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Old   May 18, 2011, 04:51
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there is a difference between turbulent schmidt number for diffusion and mixing ( default value = 0.9) in CFX and turbulent schmidt number for turblence model in
Domain/Fluid Models/Advanced Turbulence Control/k-omega regime + k-epsilon regime/k coefficients + omega coefficients/Turbulent flux closure.
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Old   May 18, 2011, 05:09
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Dear Fek66,

Thanks for your reply. Do you know where to set this turbulent schmidt number for diffusion and mixing in CFX???

Gr. Diddy
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Old   May 18, 2011, 06:11
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This is all described in the CFX documentation.

If you want to use a schmidt number of 0.2 then do that. Do not try to approximate it by adjusting diffusivity. If you are trying to match models which used schmidt number 0.2 then you should also.
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Old   May 18, 2011, 06:44
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Hi, thnx for your quick respons Ghorrocks,

Yeah, Ideally I just want to set my turbulent Schmidt number. Changing it by finding relations and adjusting other parameters is not my prefered gameplan.

But the question remains: Where in Ansys CFX can the turbulent Schmidt number for species diffusion and mixing be set?? In the GUI there are no provisions made for this (as far as I know).

Gr. Diddy
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Old   May 23, 2011, 11:20
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Hello Guys,

I think I found an answer to my own question and many other threads here:

The turbulent schmidt number in the species transport equation can easily be adjusted in the Fluid Models tab in ANSYS CFX 13.0!

Thanks for Ansys 13.0!!

Greetings,

DiddyPablo
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