The Rosseland approximation assumes that the media is optically thick and that radiant energy emitted from other locations in the domain are quickly absorbed and have no influence in the local transport. This implies that the approximation is not valid near walls. In CFX, special treatment is applied to wall boundaries to over come this limitation. Other boundaries are not given any special treatment.

The Rosseland approximation is extremely convenient to use because it does not solve for an additional transport equation. It is usually valid for an optical thickness/depth greater than 5. It should not be used whenever the optical thickness is below 1 because it will affect robustness of the flow solver.

For details, see General Radiation Considerations.