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Optically Thin Assumption

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Old   April 22, 2015, 05:59
Default Optically Thin Assumption
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Hi,

I want to use optically thin assumption for modelling radiation in my case.

What is exactly meant by optically thin assumption? As far, I know it adds radiative source term to the energy equation. Please add more info.

Is there any tutorial in OpenFOAM using optical thin assumption for radiation modelling? What parameters should I define in my case setup to enable this assumption?

any idea?

Thanks
Cummins

Last edited by firefoam; April 23, 2015 at 13:42.
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Old   April 23, 2015, 03:55
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Optically thin means that the product of a characteristic length and the absorption coefficient is low, typically l * kappa < 0.25. This means that the radiation passes through this length (mostly) undisturbed.

Suitable radiation models for this can be fvDOM and possibly viewFactors. P1 model on the other hand is valid only for optically thick cases.
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Old   April 23, 2015, 06:22
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Quote:
Originally Posted by chriss85 View Post
Optically thin means that the product of a characteristic length and the absorption coefficient is low, typically l * kappa < 0.25. This means that the radiation passes through this length (mostly) undisturbed.

Suitable radiation models for this can be fvDOM and possibly viewFactors. P1 model on the other hand is valid only for optically thick cases.
Hi Chriss,

Thanks for your response. I understand that fvDOM is suitable for this case.

I have gone through one paper using a radiation model as describe here: http://www.sandia.gov/TNF/radiation.html

I am not sure whether this has been implemented in OpenFOAM or not. As per this model, RTE is not being solved and radiative source term is added to the energy equation. Can you please suggest me if this has been implemented in OpenFOAM if yes then how can I use this?

Thanks

Last edited by firefoam; April 23, 2015 at 07:37.
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Old   April 23, 2015, 07:04
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They are using a very simple T^4 model, which simply assumes that the radiation is not reabsorbed inside the domain. This is not directly available, but you can easily add such a term to the energy equation of the solver you want to use and recompile it.
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Old   April 23, 2015, 08:18
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Hi Chris,

Thank you very much for your response.

Actually I have started using fireFOAM soon and not good in programming. Is it possible for you to show me some path how can I add a source term in the energy equation? I am suing fireFoam solver. I think this is the part of code solving energy equation.

volScalarField& he = thermo.he();

fvScalarMatrix EEqn
(
fvm::ddt(rho, he) + mvConvection->fvmDiv(phi, he)
+ fvc::ddt(rho, K) + fvc::div(phi, K)
+ (
he.name() == "e"
? fvc::div
(
fvc::absolute(phi/fvc::interpolate(rho), U),
p,
"div(phiv,p)"
)
: -dpdt
)
- fvm::laplacian(turbulence->alphaEff(), he)
==
combustion->Sh()
+ radiation->Sh(thermo)
+ parcels.Sh(he)
+ surfaceFilm.Sh()
+ fvOptions(rho, he)
);

EEqn.relax();

fvOptions.constrain(EEqn);

EEqn.solve();

fvOptions.correct(he);

thermo.correct();


Thanks in advance.
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Old   April 23, 2015, 09:04
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You're right, this is the correct part to add it. I think it would probably be something like this:

Code:
fvScalarMatrix EEqn
    (
        fvm::ddt(rho, he) + mvConvection->fvmDiv(phi, he)
      + fvc::ddt(rho, K) + fvc::div(phi, K)
      + (
            he.name() == "e"
          ? fvc::div
            (
                fvc::absolute(phi/fvc::interpolate(rho), U),
                p,
                "div(phiv,p)"
            )
          : -dpdt
        )
      - fvm::laplacian(turbulence->alphaEff(), he)
     ==
        combustion->Sh()
      + radiation->Sh(thermo)
      + parcels.Sh(he)
      + surfaceFilm.Sh()
      + fvOptions(rho, he)
- 4 * thermo->p()*kappa*(pow(thermo->T(), 4) - pow(300, 4))
    );
However, if you want to consider different species, you will have to adjust this. I can't tell you much about that since I'm only working with single-species code myself. Same if you need temperature-dependent absorption coefficients.
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Old   April 23, 2015, 10:07
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Thanks Chriss,

I will try to do this.
Just one more thing is there any tutorial or forum discussion aiming to add energy source term?

Many Regards
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