## Chapter 10: Radiation Modeling

This chapter describes:

- Comparison of the Radiation Models
- Terminology
- Material Properties for Radiation
- Rosseland Model
- The P1 Model
- The Discrete Transfer Model
- The Monte Carlo Model
- General Radiation Considerations

CFX includes several radiation modeling options: The Rosseland model (or Diffusion Approximation model), the P-1 model (also known as the Gibb’s model or Spherical Harmonics model), the Discrete Transfer model and the Monte Carlo model.

Many fluid flows of practical interest occur in situations where the fluid and/or the enclosing boundaries are hot. In such situations, the effect of radiant heat transfer may become important. A typical environment where radiation plays a significant role is a furnace or other such combustion chamber.

Two limits can be identified in the way that radiation interacts with a fluid or solid medium. One extreme is the situation where the medium is transparent to radiation at wavelengths in which the majority of the heat transfer occurs. In this case, the radiation only affects the medium by heating or cooling the surfaces of the domain, with no radiant energy transfer directly to the medium. Only the Monte Carlo model should be used for this limiting case. The Discrete Transfer model has also been used in this limit, but with limited success.

The opposite extreme is the situation in which the medium is optically dense, and radiation interacts with the medium throughout the interior of the domain, as well as at surfaces. If the medium is optically dense, radiant energy is either scattered, or absorbed and re-emitted in all directions with a small length scale compared to the size of the domain. This situation is known as the “diffusion limit,” because radiant intensity is independent of direction. (Note that there is no assumption that the radiation is “diffuse,” in the sense of “rarefied.”) In this limit, the Rosseland and P1 models are an attractive alternative to the Discrete Transfer and Monte Carlo models because of their simplicity.

For general cases, ranging from optically thin (transparent) to optically thick (diffusion) regions, like combustion, the Discrete Transfer and the Monte Carlo models more accurately represent the solution of the radiative transfer equation.

The Thermal Radiation Model can be selected from the Fluids/Solids Models form on the Domains tab whenever a heat transfer model has been set to Thermal Energy or Total Energy. The default for the Thermal Radiation Model is `None`.

Once a thermal radiation model has been selected, two additional submodels must also be chosen: the spectral model and the scattering model. For details, see Spectral Model and Scattering Model.

The radiation modeling options in CFX enable you to:

- Set up a gray/non-gray media enclosed by opaque diffuse surfaces, except at openings (inlets, outlets and openings), which are considered fully transparent. Symmetry planes and periodic boundaries are treated as specular surfaces when using the Discrete Transfer or Monte Carlo models.

- Select any of the thermal radiation models in any fluid or porous domains[1]. For solid domains, only the Monte Carlo option is available.

- Set up spectral dependent radiation quantities (material properties, radiation sources, directions, surface properties) via CEL expressions by using any of the available spectral variables: frequency, wavelength in vacuum, or wavenumber in vacuum.

- Setting of boundary conditions at walls, domain interfaces, and open boundaries: inlets, outlets and openings.

[1] As long as radiation does not travel through the solid separating two fluid domains, you may have different radiation models on each side of the solid.

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