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 fcollonv October 7, 2011 12:17

Weller test case for XiFoam: results discrepancy

5 Attachment(s)
Dear foamers,

I'm trying to do the simulation used by Weller et al. to validate XiFoam
[H.G. Weller et al., 1998, Application of a Flame-Wrinkling LES Combustion Model to a Turbulent Mixing Layer, Twenty-Seventh Symposium (International) on Combustion/The Combustion Institute].

For that first I took simply the tutorial case, refine a bit the mesh at the shear layer and boundary layers and update the U and k file to use the experimental profiles.
Then the flame was effectively wrinkled (see XiFoamInit.jpg VS WellerResults.jpg - on the last the top picture is from the experiment). But the temperature and the pressure were crazy. In XiFoamInitTemperature.jpg I use a threshold filter to select the cells with a temperature between 260K and 290K. That range is totally unphysical as the temperature should not go below 293K.
So I changed the numerical schemes (cf. attachment). And now the pressure and the temperature are great: p is roughly constant and the minimal temperature is 292.6 K However the instability of the flame is gone.

Any suggestions will be appreciate.
Thanks,

Frederic

* max CFL = 0.5
* no gravitation
* the mesh is not has large in the homogeneous direction as in the Weller's paper. The dimensions are those of the tutorial case
* the combustion properties are those of the tutorial case
* it looks like the wrong smaller temperature was triggering the instabilities due to a bigger density difference between the fresh gas and the burnt one.

 hannes October 7, 2011 14:38

Hello Frederic,

I have noticed the same behaviour of the Weller model as you when I simulated this testcase with OF-1.5dev some years ago and I did not find the reason for that.
I have implemented two other combustion models and tested them on the same testcase (same grid and BC's like tutorial case) and they yielded much more physical results.
Take a look here:
http://www.openfoamworkshop.org/08/p...nesKroeger.pdf

Also, I did not succeed in applying the weller model to a partially premixed bunsen flame case. The newton solver for temperature did not converge quite often and also unphysical temperatures occurred.

Regards, Hannes

 alberto October 8, 2011 01:16

Since the author of the paper is also one of the developers of OpenFOAM, a report of this issue on mantis ( http://www.openfoam.com/mantisbt/main_page.php ) might give you the answer and eventually a solution.

Best,

 fcollonv October 8, 2011 03:03

Thanks Hannes and Alberto for the quick answer.

@Hannes: When using the tabulated chemistry, do you read the density from the table? If so, the pressure used in the ideal gas law is the one set in Cantera (presumably 1 atm.) to compute the flamelet, isn't it? You were consequently using a kind of incompressible ideal gas law (as named in Fluent) in which one makes use of the so-called operating pressure independent of the local pressure. Can you confirm that?
For another combustion model, I implemented such incompressible ideal gas law to get rid of numerical acoustic effects as OpenFOAM has no perfectly non-reflecting BC. The improvement was important...

@Alberto: Thank you for the suggestion. I actually thought of it. But I was unsure as it isn't really a bug in OpenFOAM... I will give it a try.

 alberto October 8, 2011 03:06

Quote:
 Originally Posted by fcollonv (Post 327133) @Alberto: Thank you for the suggestion. I actually thought of it. But I was unsure as it isn't really a bug in OpenFOAM... I will give it a try.
I don't want to encourage users to report setup problems on bugzilla, but if you can't reproduce a result in the literature using the same model, it might be a bug. In the end, it's better for everyone to have this clarified ;-)

 fcollonv October 8, 2011 03:17

Quote:
 In the end, it's better for everyone to have this clarified ;-)
I agree totally :)

 fcollonv December 30, 2011 06:58