13.3.6 Defining Boundary Conditions for Radiation

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

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 dialog box ( Pressure Inlet dialog box, Velocity Inlet dialog box, Pressure Outlet dialog box, etc.). Enter the appropriate value for Internal Emissivity. The default value for all boundary types is 1. Alternatively, you can specify a user-defined function for emissivity. For more information, see this section in the separate UDF Manual.

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

ANSYS 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 dialog box ( Pressure Inlet dialog box, Velocity Inlet dialog box, 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 dialog box 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 dialog box. The default value is 1. Alternatively, you can specify a user-defined function for emissivity. For more information, see this section in the separate UDF Manual.

Partial Enclosure Wall Boundary Condition for the S2S Model

When the S2S model is used, you can define a "partial enclosure" (i.e., you can disable view factor calculations for walls and inlet and exit boundaries 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 ANSYS FLUENT calculation.

To make use of this feature for walls, you can disable the Participates in S2S Radiation option in the Radiation tab of the Wall dialog box for each relevant wall. Similarly, you can disable the view factor calculations for any inlet or exit boundary by highlighting the boundary in the Boundary Conditions task page, clicking the Edit... button and disabling the Participates in S2S Radiation option (this can also be done through the define/boundary-conditions text command). You can specify the Temperature of the partial enclosure under Partial Enclosure in the Radiation Model dialog box (Figure  13.3.3). 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 dialog box 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 walls, as discussed in
this section in the separate Theory Guide , as well as semi-transparent walls ( this section in the separate Theory Guide).

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 can be appropriate, for example, when modeling for glass panes in air.

Opaque Walls

In the Radiation tab of the Wall dialog box (Figure  13.3.9), 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. If the non-gray DO model is being used, the Diffuse Fraction can be specified for each band.

Figure 13.3.9: The Wall Dialog Box Showing Radiation Conditions for an Opaque Wall

After you have selected opaque as the BC Type, you can specify the fraction of reflected radiation flux that is to be treated as diffuse. By default, the Diffuse Fraction is set to $1$, indicating that all of the radiation is diffuse. A diffuse fraction equal to $0$ indicates purely specular reflected radiation. A diffuse fraction between $0$ and $1$ will result in partially diffuse and partially specular reflected energy. See this section in the separate Theory Guide for more details.

You will also be required to specify the internal emissivity in the Thermal tab of the Wall dialog box (Figure  13.3.10). For gray-radiation DO models, enter the appropriate value for Internal Emissivity. (The default value is 1.) The value that you specify will be applied to the diffuse component only. 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.) Alternatively, you can specify a user-defined function (UDF) for internal emissivity. For more information, see this section in the separate UDF Manual.

Figure 13.3.10: The Wall Dialog Box Showing Internal Emissivity Thermal Conditions for an Opaque Wall

You can also specify the external emissivity and external radiation temperature for a semi-transparent wall when the thermal conditions are set to Radiation or Mixed in the Wall dialog box (Figure  13.3.11). Alternatively, you can specify a UDF for these parameters.

Figure 13.3.11: The Wall Dialog Box Showing External Emissivity and External Radiation Temperature Thermal Conditions

For more information on boundary condition treatment at opaque walls,
see this section in the separate Theory Guide.

Semi-Transparent Walls

To define radiation for an exterior semi-transparent wall, click the Radiation tab in the Wall dialog box and then select semi-transparent in the BC Type drop-down list(Figure  13.3.12). The dialog box will expand to display the semi-transparent wall inputs needed to define an external irradiation flux (Figure  13.3.12).

Figure 13.3.12: The Wall Dialog Box for a Semi-Transparent Wall Boundary

Then perform the following steps:

1.   Specify the value of the irradiation flux (in W/m $^2$) under Direct or Diffuse Irradiation. If the non-gray DO model is being used, a constant Direct or Diffuse Irradiation can be specified for each band.

2.    Apply Direct Irradiation Parallel to the Beam is the default means of specifying the scale of irradiation flux. When enabled, ANSYS FLUENT assumes that the value of Direct Irradiation that you specify is the irradiation flux parallel to the Beam Direction. When deselected, ANSYS FLUENT instead assumes that the value specified is the flux parallel to the face normals and will calculate the resulting beam parallel flux for every face. See this figure in this section in the separate Theory Guide for details.

3.   Define the Beam Width by specifying the beam Theta and Phi extents. Beam width is specified as the solid angle over which the irradiation is distributed. The default value for beam width is $1e^-6$ which is suitable for collimated beam radiation. A beam width less than this is likely to result in zero irradiation flux.

4.   Specify the ( X,Y,Z) vector that defines the Beam Direction. The beam direction is defined as the vector of the centroid of the solid angle (beam width). You can specify the Beam Direction as a constant, a profile or a UDF. This is especially useful in applications where the shape of the radiative source is circular or cylindrical (or non-linear). For information about boundary profiles, see Section  4.6.

Note that the actual direction of the beam of radiation that enters the domain will be further influenced by the solid angles available from the number of divisions set up; the effective direction will be the direction vector of the solid angle that the incoming beam falls into. Finally, any non-zero diffuse fraction will act to spread out (hemispherically, proportional to the diffuse fraction) the irradiation that enters the domain.

For a UDF example that specifies the beam direction, see this section in the separate UDF Manual.

5.   Specify the fraction of the irradiation that is to be treated as diffuse as a real number between $0$ and $1$. By default, the Diffuse Fraction is set to 1, indicating that all of the irradiation is diffuse. A diffuse fraction of $0$ treats the radiation as purely specular. If you specify a value between $0$ and $1$, the radiation is treated as partially diffuse and partially specular. If the non-gray DO model is being used, the Diffuse Fraction can be specified for each band. See this section in the separate Theory Guide for details.

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

If Heat Flux conditions are specified in the Thermal tab of the Wall dialog box, 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 ANSYS FLUENT. Internal emissivity is ignored for semi-transparent surfaces.

Note that when a boundary wall is made semi-transparent ANSYS FLUENT calculates the amount of radiation leaving as well as entering the domain. If you do not provide a source of irradiation or a radiating thermal condition (e.g. Mixed or Radiation) then you are effectively radiating to a temperature of $0$ K and it is highly likely you may observe temperatures in your model that are lower than expected. Ensure that the external (incoming) radiant conditions give good account of the surroundings.

You can also specify the external emissivity and external radiation temperature for a semi-transparent wall when the thermal conditions are set to Radiation or Mixed in the Wall dialog box (Figure  13.3.11). Alternatively, you can specify a user-defined function (UDF) for these parameters. For more information, see this section in the separate UDF Manual.

For a detailed description of boundary condition treatment at semi-transparent walls, see this section in the separate Theory Guide.

To define radiation for an interior (two-sided) semi-transparent wall, in the Wall dialog box click the Radiation tab and then select semi-transparent in the BC Type drop-down list (Figure  13.3.13). Then specify the Diffuse Fraction as described for the previous case.

Note that for semi-transparent walls, the internal emissivity defined under thermal conditions is ignored. Emissivity and absorptivity on a semi-transparent surface can only be effected volumetrically in the wall thickness as a consequence of the wall material absorption coefficient. See notes near the end of this section in the separate Theory Guide discussing limitations around working with the wall thickness.

Figure 13.3.13: The Wall Dialog Box for an Interior Semi-Transparent Wall

Solid Cell Zones Conditions for the DO Model

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, enable the Participates In Radiation option in the Solid dialog box (Figure  13.3.14).

Figure 13.3.14: The Solid Dialog Box

In general, you should not disable the Participates In Radiation option for any fluid zones.

See this section in the separate Theory Guide for more information on solid semi-transparent media.

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  7.3.14). 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, ANSYS FLUENT allows you to specify the convective and radiative portions of the heat flux separately.