PHOENICS is supplied with five models of radiation, namely:-
Models a, b, c and d use the "radiative-conductivity" concept, whereas model e allows fully for angular effects.
Of these, only d can also handle conjugate heat transfer (i.e. heat conduction within large immersed solids) and two-phase flow (i.e. additional suspended solids within the flowing medium).
Model a is restricted to Cartesian and cylindrical-polar grids, whereas models b, c and d are applicable to BFC grids also.
The PHOENICS implementation of all models is restricted to "gray" radiation, i.e. to that in which the influence of wave-length can be neglected.
Models d and e can handle radiation between solids separated by non-absorbing media, whereas the others cannot. Model e is, in principle, the more accurate; model d is the more economical.
Because of its novelty and wide applicability, model d (IMMERSOL) is presented first, in section 3.
Sections 4, 5 and 6 are devoted to the older models a, b and c. Model d is the only one to combine universal applicability with economic practicability for complex geometries.
Model e is not described in this article; but information about it can be found in the lecture entitled "Surface to Surface Radiation" in the Lectures on PHOENICS section of POLIS.
Other models of radiation are known, for example:-
PHOENICS implementations of the discrete-transfer and discrete- ordinates methods have been reported by Kjaldman  and Muller et al , respectively.