The Differential Approximation or P1 adds an additional transport equation to the simulation, consequently it is more costly. The P1 model is valid for an optical thickness greater than 1. For example, the model has proved adequate for the study of pulverized fuel (PF) flames, in regions away from the immediate vicinity of the flame. However, it has been used for lower values with varying success.

The P1 model implementation in CFX only allows for opaque diffuse walls. That is, the diffuse fraction setting would then be ignored.

Open boundaries: Inlets, outlets and openings are treated as fully transparent boundaries. That is, they absorb all outgoing energy, and the incoming energy is computed as a blackbody at either the local temperature or at a user specified external blackbody temperature. For details, see General Radiation Considerations.