
The Rosseland or diffusion approximation for radiation is valid when the medium is optically thick ( ), and is recommended for use in problems where the optical thickness is greater than 3. It can be derived from the P1 model equations, with some approximations. This section provides details about the equations used in the Rosseland model.
The Rosseland Model Equations
As with the P1 model, the radiative heat flux vector in a gray medium can be approximated by Equation 11.311:
where is given by Equation 11.310.
The Rosseland radiation model differs from the P1 model in that the Rosseland model assumes that the intensity is the blackbody intensity at the gas temperature. (The P1 model actually calculates a transport equation for .) Thus . Substituting this value for into Equation 11.329 yields
(11.330) 
Since the radiative heat flux has the same form as the Fourier conduction law, it is possible to write
where is the thermal conductivity and is the radiative conductivity. Equation 11.331 is used in the energy equation to compute the temperature field.
Anisotropic Scattering
The Rosseland model allows for anisotropic scattering, using the same phase function (Equation 11.314) described for the P1 model in Section 11.3.4.
Boundary Condition Treatment for the Rosseland Model at Walls
Since the diffusion approximation is not valid near walls, it is necessary to use a temperature slip boundary condition. The radiative heat flux at the wall boundary, , is defined using the slip coefficient :
where is the wall temperature, is the temperature of the gas at the wall, and the slip coefficient is approximated by a curve fit to the plot given in [ 234]:
(11.335) 
where is the conduction to radiation parameter at the wall:
(11.336) 
and .
Boundary Condition Treatment for the Rosseland Model at Flow Inlets and Exits
No special treatment is required at flow inlets and outlets for the Rosseland model. The radiative heat flux at these boundaries can be determined using Equation 11.331.
FLUENT includes an option that allows you to use different temperatures for radiation and convection at inlets and outlets. This can be useful when the temperature outside the inlet or outlet differs considerably from the temperature in the enclosure. See Section 11.3.16 for details.