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13.3.1 Overview and Limitations

FLUENT provides five radiation models which 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, 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:

You should include radiative heat transfer in your simulation when the radiant heat flux, $Q_{\rm rad} = \sigma(T^{4}_{\rm max} - T^{4}_{\rm min})$, 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.

Advantages and Limitations of the DTRM

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 FLUENT:

Advantages and Limitations of the P-1 Model

The P-1 model has several advantages over the DTRM. For the P-1 model, the RTE (Equation  13.3-1) 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:

Advantages and Limitations of the Rosseland Model

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.

Advantages and Limitations of the DO Model

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 FLUENT is intended for use with participating media with a spectral absorption coefficient $a_{\lambda}$ 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 [ 248]. The modeling of non-gray gas radiation is still an evolving field. However, some researchers [ 108] have used gray-band models to model gas behavior by approximating the absorption coefficients within each band as a constant. The implementation in FLUENT can be used in this fashion if desired.

The non-gray implementation in 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.

Advantages and Limitations of the S2S Model

The surface-to-surface (S2S) radiation model is good for modeling the enclosure radiative transfer without participating media (e.g., 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:

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