5.3.9 Choosing a Radiation Model

For certain problems one radiation model may be more appropriate than the others. When deciding which radiation model to use, consider the following:

• Optical thickness: The optical thickness $aL$ is a good indicator of which model to use in your problem. Here, $L$ is an appropriate length scale for your domain. For flow in a combustor, for example, $L$ is the diameter of the combustion chamber. If $aL \gg 1$, your best alternatives are the P-1 and Rosseland models. The P-1 model should typically be used for optical thicknesses $>1$. For optical thickness $>3$, the Rosseland model is cheaper and more efficient. For high optical thickness cases, a second-order discretization scheme for the DO model is recommended. The DTRM and the DO model work across the full range of optical thicknesses, but are substantially more expensive to use. Consequently, you should use the "thick-limit'' models, P-1 and Rosseland, if the problem allows it. For optically thin problems ( $aL<1$), the DTRM and the DO model, only, are appropriate.

• Scattering and emissivity: The P-1, Rosseland, and DO models account for scattering, while the DTRM neglects it. Since the Rosseland model uses a temperature slip condition at walls, it is insensitive to wall emissivity.

• Particulate effects: Only the P-1 and DO models account for exchange of radiation between gas and particulates (see Equation  5.3-8).

• Semi-transparent walls (interior and exterior): Only the DO model allows you to model semi-transparent walls of various types (e.g., glass).

• Specular walls : Only the DO model allows specular reflection (e.g., for dust-free mirror).

• Partially-specular walls : Only the DO model allows specular reflection (e.g., dusty mirror).

• Non-gray radiation: Only the DO model allows you to compute non-gray radiation using a gray band model.

• Localized heat sources: In problems with localized sources of heat, the P-1 model may over-predict the radiative fluxes. The DO model is probably the best suited for computing radiation for this case, although the DTRM, with a sufficiently large number of rays, is also acceptable.

• Enclosure radiative transfer with non-participating media: The surface-to-surface (S2S) model is suitable for this type of problem. The radiation models used with participating media may, in principle, be used to compute the surface-to-surface radiation, but they are not always efficient.

External Radiation

If you need to include radiative heat transfer from the exterior of your physical model, you can include an external radiation boundary condition in your model (for details, see this section in the separate User's Guide). If you are not concerned with radiation within the domain, this boundary condition can be used without activating one of the radiation models.