
The radiative transfer equation (RTE) for an absorbing, emitting, and scattering medium at position in the direction is
where  =  position vector  
=  direction vector  
=  scattering direction vector  
=  path length  
=  absorption coefficient  
=  refractive index  
=  scattering coefficient  
=  StefanBoltzmann constant (5.672 10 W/m K )  
=  radiation intensity, which depends on position ( and direction  
=  local temperature  
=  phase function  
=  solid angle 
is the optical thickness or opacity of the medium. The refractive index is important when considering radiation in semitransparent media. Figure 13.3.1 illustrates the process of radiative heat transfer.
The DTRM and the P1, Rosseland, and DO radiation models require the absorption coefficient as input. and the scattering coefficient can be constants, and can also be a function of local concentrations of H O and CO , path length, and total pressure. FLUENT provides the weightedsumofgraygases model (WSGGM) for computation of a variable absorption coefficient. See Section 13.3.8 for details. The discrete ordinates implementation can model radiation in semitransparent media. The refractive index of the medium must be provided as a part of the calculation for this type of problem. The Rosseland model also requires you to enter a refractive index, or use the default value of .