13.3.5 Defining Material Properties for Radiation

When you are using the P-1, DO, or Rosseland radiation model in ANSYS FLUENT, you should be sure to define both the absorption and scattering coefficients of the fluid in the Create/Edit Materials dialog box. Note that you can either enter a constant value for these parameters, or you can specify them using a user-defined function (UDF). For more information, see this section in the separate UDF Manual.

Materials

If you are modeling semi-transparent media using the DO model, you should also define the refractive index for the semi-transparent fluid or solid material. When using the Rosseland model, you can specify the refractive index only for the fluid material. When using the P1 model, you should define the refractive index for the fluid material only. For the DTRM, you need to define only the absorption coefficient.

If your model includes gas phase species such as combustion products, absorption and/or scattering in the gas may be significant. The scattering coefficient should be increased from the default of zero if the fluid contains dispersed particles or droplets which contribute to scattering. Alternatively, you can specify the scattering coefficient as a user-defined function (UDF). For more information, see this section in the separate UDF Manual.

ANSYS FLUENT allows you to input a composition-dependent absorption coefficient for CO $_2$ and H $_2$O mixtures, using the WSGGM. The method for computing a variable absorption coefficient is described in this section in the separate Theory Guide. Section  8.8 provides a detailed description of the procedures used for input of radiation properties.

Absorption Coefficient for a Non-Gray DO Model

If you are using the non-gray DO model, you can specify a different constant absorption coefficient for each of the bands used by the gray-band model, as described in Section  8.8. You cannot, however, compute a composition-dependent absorption coefficient in each band. If you use the WSGGM to compute a variable absorption coefficient, the value will be the same for all bands. Alternatively, you can specify a user-defined function (UDF) for the absorption coefficient. For more information, see this section in the separate UDF Manual.

Refractive Index for a Non-Gray DO Model

If you are using the non-gray DO model, you can specify a different constant refractive index for each of the bands used by the gray-band model, as described in Section  8.8. You cannot, however, compute a composition-dependent refractive index in each band.