## 13.3.14 Defining Material Properties for Radiation

When you are using the P-1, DO, or Rosseland radiation model in FLUENT, you should be sure to define both the absorption and scattering coefficients of the fluid in the Materials panel. Note that you can either enter a constant value for these parameters, or you can specify them using a user-defined function (UDF). Click here to go to the UDF Manual for details.

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 should define the refractive index for the fluid or solid material. For the DTRM, you need to define only the absorption coefficient.

Define Materials...

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). Click here to go to the UDF Manual for details.

FLUENT allows you to input a composition-dependent absorption coefficient for CO and H O mixtures, using the WSGGM. The method for computing a variable absorption coefficient is described in Section  13.3.8. 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.