Ansys Fluent provides six radiation models that allow you to include radiation, with or without a participating medium, in your heat transfer simulations.
Heating or cooling of surfaces due to radiation and/or heat sources or sinks due to radiation within the fluid phase can be included in your model using one of the following radiation models.
In addition to these radiation models, Ansys Fluent also provides a solar load model that allows you to include the effects of solar radiation in your simulation.
Typical applications well suited for simulation using radiative heat transfer include the following:
radiative heat transfer from flames
surface-to-surface radiant heating or cooling
coupled radiation, convection, and/or conduction heat transfer
radiation through windows in HVAC applications, and cabin heat transfer analysis in automotive applications
radiation in glass processing, glass fiber drawing, and ceramic processing
You should include radiative heat transfer in your simulation
when the radiant heat flux, 
, is large compared to
the heat transfer rate due to convection or conduction. Typically
this will occur at high temperatures where the fourth-order dependence
of the radiative heat flux on temperature implies that radiation will
dominate.
The primary advantages of the DTRM are threefold: it is a relatively simple model, you can increase the accuracy by increasing the number of rays, and it applies to a wide range of optical thicknesses.
You should be aware of the following limitations when using the DTRM in Ansys Fluent:
DTRM assumes that all surfaces are diffuse. This means that the reflection of incident radiation at the surface is isotropic with respect to the solid angle.
The effect of scattering is not included.
The implementation assumes gray radiation.
Solving a problem with a large number of rays is CPU-intensive.
DTRM is not compatible with non-conformal interfaces or sliding meshes.
DTRM is not compatible with parallel processing.
The P-1 model has several advantages over the DTRM. For the P-1 model, the RTE (Equation 5–22) is a diffusion equation, which is easy to solve with little CPU demand. The model includes the effect of scattering. For combustion applications where the optical thickness is large, the P-1 model works reasonably well. In addition, the P-1 model can easily be applied to complicated geometries with curvilinear coordinates.
You should be aware of the following limitations when using the P-1 radiation model:
The P-1 model assumes that all surfaces are diffuse. This means that the reflection of incident radiation at the surface is isotropic with respect to the solid angle.
The implementation is restricted to either gray radiation or non-gray radiation using a gray-band model. The non-gray implementation assumes a constant absorption coefficient within each wavelength band. The weighted-sum-of-gray-gases model (WSGGM) cannot be used to specify the absorption coefficient in each band. The non-gray implementation also assumes the spectral emissivity at walls to be constant within each band. Further information about the gray-band model is provided in Advantages and Limitations of the DO Model.
There may be a loss of accuracy, depending on the complexity of the geometry, if the optical thickness is small.
The P-1 model tends to over-predict radiative fluxes from localized heat sources or sinks.
The Rosseland model has two advantages over the P-1 model. Since it does not solve an extra transport equation for the incident radiation (as the P-1 model does), the Rosseland model is faster than the P-1 model and requires less memory.
The Rosseland model can be used only for optically thick media. It is recommended for use when the optical thickness exceeds 3. Note also that the Rosseland model is not available when the density-based solver is being used; it is available with the pressure-based solver, only.
The DO model spans the entire range of optical thicknesses, and allows you to solve problems ranging from surface-to-surface radiation to participating radiation in combustion problems. It also allows the solution of radiation at semi-transparent walls. Computational cost is moderate for typical angular discretizations, and memory requirements are modest.
The current implementation is restricted to either gray radiation or non-gray radiation using a gray-band model. Solving a problem with a fine angular discretization may be CPU-intensive.
The non-gray implementation in Ansys Fluent is intended for
use with participating media with a spectral absorption coefficient 
that
varies in a stepwise fashion across spectral bands, but varies smoothly
within the band. Glass, for example, displays banded behavior of this
type. The current implementation does not model the behavior of gases
such as carbon dioxide or water vapor, which absorb and emit energy
at distinct wave numbers [447]. The modeling
of non-gray gas radiation is still an evolving field. However, some
researchers [181] have used gray-band
models to model gas behavior by approximating the absorption coefficients
within each band as a constant. The implementation in Ansys Fluent can
be used in this fashion if desired.
The non-gray implementation in Ansys Fluent is compatible with all the models with which the gray implementation of the DO model can be used. Thus, it is possible to include scattering, anisotropy, semi-transparent media, and particulate effects. However, the non-gray implementation assumes a constant absorption coefficient within each wavelength band. The weighted-sum-of-gray-gases model (WSGGM) cannot be used to specify the absorption coefficient in each band. The implementation allows the specification of spectral emissivity at walls. The emissivity is assumed to be constant within each band.
You should be aware of the following limitations when using the DO radiation model:
The DO model is not supported for use with granular (fluid-solid) Eulerian multiphase flows.
The DO/Energy Coupling method is not supported with multiphase models.
Non-conformal interfaces are not supported with the accelerated DO model with GPU. For more information on the accelerated DO model, see Accelerating Discrete Ordinates (DO) Radiation Calculations in the Fluent User's Guide.
The surface-to-surface (S2S) radiation model is good for modeling the enclosure radiative transfer without participating media (for example, spacecraft heat rejection systems, solar collector systems, radiative space heaters, and automotive underhood cooling systems). In such cases, the methods for participating radiation may not always be efficient. As compared to the DTRM and the DO radiation models, the S2S model has a much faster time per iteration, although the view factor calculation itself is CPU-intensive. This increased time for view factor calculation will be especially pronounced when the emitting/absorbing surfaces are the polygonal faces of polyhedral cells.
You should be aware of the following limitations when using the S2S radiation model:
The S2S model assumes that all surfaces are diffuse.
The implementation assumes gray radiation.
The storage and memory requirements increase very rapidly as the number of surface faces increases. This can be minimized by using a cluster of surface faces, although the CPU time will be independent of the number of clusters that are used if the face to face basis is used to calculate the view factors.
The S2S model cannot be used to model participating radiation problems.
The S2S model with the Hemicube view factor method cannot be used if your model contains symmetry or periodic boundary conditions.
The S2S model with Cluster to Cluster and Ray Tracing view factor calculation does not support periodic non-conformal interfaces.
The S2S model does not support hanging nodes or hanging node adaption on radiating boundary zones.
The S2S model does not support semi-transparent boundaries.
View factor files cannot be written on mixed windows-linux platforms when clustering is enabled.
The MC model can solve problems ranging from optically thin (transparent) regions to optically thick (diffusion) regions, like combustion. It allows you to calculate quasi-exact solutions. While it is more accurate compared to other available models, it has a higher computational cost.
You should be aware of the following limitations when using the MC radiation model:
For the Monte Carlo model, all the physical quantities of interest are calculated as surface or volume averages.
The following notable features/models are supported with the Monte Carlo model:
Supports meshes with non-conformal interfaces based on the one-to-one auto-pairing method, such as meshes generated using the Fault Tolerant Meshing (FTM) workflow.
Shell conduction is not at all supported with semi-transparent walls and is not supported with opaque walls with boundary source specified, see Opaque Walls for the MC Model.
Symmetry planes and periodic boundaries are treated as specular surfaces when using the Monte Carlo model.
Transient.
Heat exchanger model.
Euler-Euler Multiphase models (VOF, Mixture, Eulerian models).
Note: The radiation field for multiphase is assumed to be homogeneous, and only solves for a single radiation field (not per phase). Radiation bulk material properties are assumed to be a volume fraction weighted average from the phase values. This makes the solution rather sensitive to the mesh, if more than one phase is transparent. Opaque phases should be modeled with high absorption coefficient.
For dispersed phases (as distinct from continuous morphology) that have small droplet sizes, scattering effects are not included from such small droplets. In general, the accuracy of the approach will reduce as the dispersed phase size gets smaller in relation to the mesh size.
The following are currently not supported with the Monte Carlo model:
2D cases.
Rotating / moving / overset meshes.
CutCell meshes.
Hanging nodes in the mesh.
Thin walls (for example, as baffles).
DPM.
Euler-Lagrangian Multiphase models.
Porous medium (or NETM model).
Combustion PDF transport.
Boundary sources of radiation for flow boundaries/openings.
Two zones with an interior boundary between them must both participate in radiation.
Isotropic radiation flux option is not available for the boundary source of radiation on opaque walls.
Opaque walls with boundary source and shell conduction (shell conduction is disabled if you enable both).
Semi-transparent walls with shell conduction, see Opaque Walls for the MC Model.
Mesh adaption is not supported with Monte Carlo model.