
The P1 radiation model is the simplest case of the more general PN model, which is based on the expansion of the radiation intensity into an orthogonal series of spherical harmonics [ 38, 234]. This section provides details about the equations used in the P1 model.
The P1 Model Equations
As mentioned above, the P1 radiation model is the simplest case of the PN model. If only four terms in the series are used, the following equation is obtained for the radiation flux :
where is the absorption coefficient, is the scattering coefficient, is the incident radiation, and is the linearanisotropic phase function coefficient, described below. After introducing the parameter
Equation 11.39 simplifies to
The transport equation for is
where is the StefanBoltzmann constant and is a userdefined radiation source. FLUENT solves this equation to determine the local radiation intensity when the P1 model is active.
Combining Equations 11.311 and 11.312, the following equation is obtained:
(11.313) 
The expression for can be directly substituted into the energy equation to account for heat sources (or sinks) due to radiation.
Anisotropic Scattering
Included in the P1 radiation model is the capability for modeling anisotropic scattering. FLUENT models anisotropic scattering by means of a linearanisotropic scattering phase function:
Here, is the unit vector in the direction of scattering, and is the unit vector in the direction of the incident radiation. is the linearanisotropic phase function coefficient, which is a property of the fluid. ranges from to 1. A positive value indicates that more radiant energy is scattered forward than backward, and a negative value means that more radiant energy is scattered backward than forward. A zero value defines isotropic scattering (i.e., scattering that is equally likely in all directions), which is the default in FLUENT. You should modify the default value only if you are certain of the anisotropic scattering behavior of the material in your problem.
Particulate Effects in the P1 Model
When your FLUENT model includes a dispersed second phase of particles, you can include the effect of particles in the P1 radiation model. Note that when particles are present, FLUENT ignores scattering in the gas phase. (That is, Equation 11.315 assumes that all scattering is due to particles.)
For a gray, absorbing, emitting, and scattering medium containing absorbing, emitting, and scattering particles, the transport equation for the incident radiation can be written as
where is the equivalent emission of the particles and is the equivalent absorption coefficient. These are defined as follows:
and
In Equations 11.316 and 11.317, , , and are the emissivity, projected area, and temperature of particle . The summation is over particles in volume . These quantities are computed during particle tracking in FLUENT.
The projected area of particle is defined as
(11.318) 
where is the diameter of the th particle.
The quantity in Equation 11.315 is defined as
(11.319) 
where the equivalent particle scattering factor is defined as
and is computed during particle tracking. In Equation 11.320, is the scattering factor associated with the th particle.
Heat sources (sinks) due to particle radiation are included in the energy equation as follows:
(11.321) 
Boundary Condition Treatment for the P1 Model at Walls
To get the boundary condition for the incident radiation equation, the dot product of the outward normal vector and Equation 11.311 is computed:
Thus the flux of the incident radiation, , at a wall is . The wall radiative heat flux is computed using the following boundary condition:
where is the wall reflectivity. The Marshak boundary condition is then used to eliminate the angular dependence [ 188]:
Substituting Equations 11.324 and 11.325 into Equation 11.326 and performing the integrations yields
If it is assumed that the walls are diffuse gray surfaces, then , and Equation 11.327 becomes
Equation 11.328 is used to compute for the energy equation and for the incident radiation equation boundary conditions.
Boundary Condition Treatment for the P1 Model at Flow Inlets and Exits
The net radiative heat flux at flow inlets and outlets is computed in the same manner as at walls, as described above. FLUENT assumes that the emissivity of all flow inlets and outlets is 1.0 (black body absorption) unless you choose to redefine this boundary treatment.
FLUENT includes an option that allows you to use different temperatures for radiation and convection at inlets and outlets. This can be useful when the temperature outside the inlet or outlet differs considerably from the temperature in the enclosure. See Section 11.3.16 for details.