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11.3.16 Setting Radiation Boundary Conditions

When you set up a problem that includes radiation, you will set additional boundary conditions at walls, inlets, and exits.

Define $\rightarrow$ Boundary Conditions...

Inlet and Exit Boundary Conditions

Emissivity

When radiation is active, you can define the emissivity at each inlet and exit boundary when you are defining boundary conditions in the associated inlet or exit boundary panel ( Pressure Inlet panel, Velocity Inlet panel, Pressure Outlet panel, etc.). Enter the appropriate value for Internal Emissivity. The default value for all boundary types is 1.

For non-gray DO models, the specified constant emissivity will be used for all wavelength bands.

!! The Internal Emissivity boundary condition is not available with the Rosseland model.

Black Body Temperature

FLUENT includes an option that allows you to take into account the influence of the temperature of the gas and the walls beyond the inlet/exit boundaries, and specify different temperatures for radiation and convection at inlets and exits. This is useful when the temperature outside the inlet or exit differs considerably from the temperature in the enclosure. For example, if the temperature of the walls beyond the inlet is 2000 K and the temperature at the inlet is 1000 K, you can specify the outside-wall temperature to be used for computing radiative heat flux, while the actual temperature at the inlet is used for calculating convective heat transfer. To do this, you would specify a radiation temperature of 2000 K as the black body temperature.

Although this option allows you to account for both cooler and hotter outside walls, you must use caution in the case of cooler walls, since the radiation from the immediate vicinity of the hotter inlet or outlet almost always dominates over the radiation from cooler outside walls. If, for example, the temperature of the outside walls is 250 K and the inlet temperature is 1500 K, it might be misleading to use 250 K for the radiation boundary temperature. This temperature might be expected to be somewhere between 250 K and 1500 K; in most cases it will be close to 1500 K. (Its value depends on the geometry of the outside walls and the optical thickness of the gas in the vicinity of the inlet.)

In the flow inlet or exit panel ( Pressure Inlet panel, Velocity Inlet panel, etc.), select Specified External Temperature in the External Black Body Temperature Method drop-down list, and then enter the value of the radiation boundary temperature as the Black Body Temperature.

!! If you want to use the same temperature for radiation and convection, retain the default selection of Boundary Temperature as the External Black Body Temperature Method.

!! The Black Body Temperature boundary condition is not available with the Rosseland model.

Wall Boundary Conditions for the DTRM, and the P-1, S2S and Rosseland Models

The DTRM and the P-1, S2S, and Rosseland models assume all walls to be gray and diffuse. The only radiation boundary condition required in the Wall panel is the emissivity . For the Rosseland model, the internal emissivity is 1. For the DTRM and the P-1 and S2S models, you can enter the appropriate value for Internal Emissivity in the Thermal section of the Wall panel. The default value is 1.

Partial Enclosure Wall Boundary Condition for the S2S Model

When the S2S model is used, you also have the option to define a ``partial enclosure"; i.e., you can disable view factor calculations for walls that are not participating in the radiative heat transfer calculation. This feature allows you to save time computing the view factors and also reduce the memory required to store the view factor file during the FLUENT calculation.

To make use of this feature, turn off the Participates in S2S Radiation option in the Thermal section of the Wall panel for each relevant wall. You can specify the Temperature of the partial enclosure under Partial Enclosure in the Radiation Model panel (Figure  11.3.12). The partial enclosure is treated like a black body with the specified temperature.

!! If you change the definition of the partial enclosure by including or excluding some of the boundary zones, you will need to recompute the view factors.

!! The Flux Reports panel will not show the exact balance of the Radiation Heat Transfer Rate because the radiative heat transfer to the partial enclosure is not included.

Wall Boundary Conditions for the DO Model

When the DO model is used, you can model opaque (diffuse or specular) and semi-transparent walls, as discussed in Section  11.3.6.

You can use a diffuse wall to model wall boundaries in many industrial applications since, for the most part, surface roughness makes the reflection of incident radiation diffuse. For highly polished surfaces, such as reflectors or mirrors, the specular boundary condition is appropriate. The semi-transparent boundary condition is appropriate for modeling glass panes in air, for example.

Opaque Wall Boundary Conditions for the DO Model

In the Radiation section of the Wall panel, select opaque in the BC Type drop-down list to specify an opaque wall. Opaque walls are treated as gray if gray radiation is being computed, or non-gray if the non-gray DO model is being used. Once you have selected opaque as the BC Type, you can specify the fraction of the irradiation that is to be treated as diffuse. By default, the Diffuse Fraction is set to 1, indicating that all of the irradiation is diffuse. If you specify a value less than 1, the diffuse fraction will be reflected diffusely (as described in Section  11.3.6) and the remainder will be reflected specularly.

If the non-gray DO model is being used, the Diffuse Fraction can be specified for each band.

Additionally, you will be required to specify the emissivity in the Thermal section of the Wall panel. For gray-radiation DO models, enter the appropriate value for Internal Emissivity. (The default value is 1.) For non-gray DO models, specify a constant Internal Emissivity for each wavelength band in the Radiation tab. (The default value in each band is 1.)

Semi-Transparent Wall Boundary Conditions for the DO Model

In the Radiation section of the Wall panel, select semi-transparent in the BC Type drop-down list to specify a semi-transparent wall.

For an external semi-transparent wall, you can define an external irradiation flux in the Wall panel (see Figure  11.3.15). For an internal semi-transparent wall, see the discussion on multiple-zone domains, below.

Figure 11.3.15: The Wall Panel for a Semi-Transparent Wall
\begin{figure} \psfig{file=figures/panel-semitrans-wall.ps} \end{figure}

The inputs for an external semi-transparent wall are as follows:

1.
Specify the value of the irradiation flux under Irradiation. If the non-gray DO model is being used, a constant Irradiation can be specified for each band.

2.
Define the Beam Width by specifying the beam Theta and Phi extents.

3.
Specify the ( X,Y,Z) vector that defines the Beam Direction.

4.
Specify the fraction of the irradiation that is to be treated as diffuse. By default, the Diffuse Fraction is set to 1, indicating that all of the irradiation is diffuse. If you specify a value less than 1, the diffuse fraction will be reflected diffusely (as described in Section  11.3.6), the transmitted portion will also be reflected diffusely, and the remainder will be reflected specularly.

If the non-gray DO model is being used, the Diffuse Fraction can be specified for each band.

!! Note that the refractive index of the external medium is assumed to be 1.

!! If Heat Flux conditions are specified in the Thermal section of the Wall panel, the specified heat flux is considered to be only the conduction and convection portion of the boundary flux. The given irradiation specifies the incoming exterior radiative flux; the radiative flux transmitted from the domain interior to the outside is computed as a part of the calculation by FLUENT.

Enabling Radiation in Specific Cell Zones (DO Model Only)

With the DO model, you can specify whether or not you want to solve for radiation in each cell zone in the domain. By default, the DO equations are solved in all fluid zones, but not in any solid zones. If you want to model semi-transparent media, for example, you can enable radiation in the solid zone(s). To do so, turn on the Participates In Radiation option in the Solid panel (Figure  11.3.16).

Figure 11.3.16: The Solid Panel
\begin{figure} \psfig{file=figures/panel-solid-rad-on.ps} \end{figure}

!! In general, you should not turn off the Participates In Radiation option for any fluid zones.

Two-Sided-Wall Boundary Conditions for the DO Model in Multiple-Zone Domains

For the DO model, you can specify the boundary condition on each side of a two-sided wall independently to be either diffuse or specular. Note that the two fluid zones bordering the wall will not be radiatively coupled (although you can choose them to be thermally coupled.)

You can also choose to couple the contiguous fluid or solid zones radiatively by making the two-sided wall between them semi-transparent. In this case, radiation will pass through the wall. You can specify the two-sided wall to be semi-transparent only if both the neighboring cell zones participate in radiation; both sides of the wall will be semi-transparent if you define one side to be semi-transparent. You can, however, specify a different diffuse fraction for each side.

It is also possible to associate a thickness with the two-sided wall. In this case, the refraction due to the wall thickness is accounted for as radiation travels through the boundary. You can specify a Wall Thickness and a wall Material Name in the Wall panel (as described in Section  6.13.1). The refractive index and the absorption coefficient are those of the specified wall material. Only a constant absorption coefficient is allowed for a solid material. The effective reflectivity and transmissivity of the wall are computed assuming a planar layer of the given thickness with absorption but no emission. The refractive indices of the surrounding media correspond to those of the surrounding fluid materials. (When an external wall is specified to be semi-transparent, the refractive index of the external medium is assumed to be 1.)

Thermal Boundary Conditions

In general, any well-posed combination of thermal boundary conditions can be used when any of the radiation models is active. The radiation model will be well-posed in combination with fixed temperature walls, conducting walls, and/or walls with set external heat transfer boundary conditions (Section  6.13.1). You can also use any of the radiation models with heat flux boundary conditions defined at walls, in which case the heat flux you define will be treated as the sum of the convective and radiative heat fluxes. The exception to this is the case of semi-transparent walls for the DO model. Here, FLUENT allows you to specify the convective and radiative portions of the heat flux separately, as explained above.


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Up: 11.3 Radiative Heat Transfer
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© Fluent Inc. 2003-01-25