The Rosseland approximation is not valid near walls. Therefore, a special boundary condition must be specified when heat conduction is comparable to radiation heat transfer. It has been proposed  [24] that a temperature slip boundary condition should be in this region. From [24] the heat flux at the wall, is given by:

(8–28)

where is the slip co-efficient, is the wall temperature and is the gas temperature at the wall. is computed as the solution of:

(8–29)

where is given by:

(8–30)