[mehtab]

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

[chriss85]

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.

[mehtab]

Originally Posted by chriss85

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

[chriss85]

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.

[mehtab]

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.

[chriss85]

You're right, this is the correct part to add it. I think it would probably be something like this:

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.

[mehtab]

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

[sahmed]

Hi Mehtab
Its a little late, but could you please tell me if you could successfully add the radiation model in the energy equation with optically thin approximation from the TNF workshop. It would be of great help if you share your thoughts with me.
Thanks