13.3.8 Postprocessing Radiation Quantities

Available Variables for Postprocessing

ANSYS FLUENT provides radiation quantities that you can use in postprocessing when your model includes the solution of radiative heat transfer. You can generate graphical plots or alphanumeric reports of the following variables/functions:

In the Radiation... category:

• Incident Radiation (P-1 and DO models)

• Absorption Coefficient (DTRM, P-1, DO, and Rosseland models)

• Scattering Coefficient (P-1, DO, and Rosseland models)

• Refractive Index (P1, DO, and Rosseland models)

• Radiation Temperature (P-1 and DO models)

• Surface Cluster ID (S2S model)

In the Wall Fluxes... category:

• Radiation Heat Flux (all radiation models)

• Surface Incident Radiation (S2S, DTRM, and DO models)

• Absorbed Radiation Flux (DO model, semi-transparent wall)

• Reflected Radiation Flux (DO model, semi-transparent wall)

• Transmitted Radiation Flux (DO model, semi-transparent wall)

• Beam Irradiation Flux (DO model, semi-transparent wall)

See Chapter  31 for definitions of these postprocessing variables. Note that in addition, incident radiation, transmitted, reflected and absorbed radiation flux are also available on a per-band basis for the non-gray DO model.

The sign convention on the radiative heat flux is such that the heat flux from the wall surface is a positive quantity.

It is possible to export heat flux data on wall zones (including radiation) to a generic file that you can examine or use in an external program. See Section  13.2.3 for details.

Take care not to confuse Incident Radiation and Surface Incident Radiation. Incident Radiation is a volumetric quantity giving the total radiant load passing through the cell (in all directions), whereas Surface Incident Radiation is the total radiant load hitting the surface (which will subsequently be absorbed, transmitted and reflected). There is no direct means to report how much radiation has been absorbed/emitted/scattered in cells.

Reporting Radiative Heat Transfer Through Boundaries

You can use the Flux Reports dialog box to compute the radiative heat transfer through each boundary of the domain, or to sum the radiative heat transfer through all boundaries.

Reports Fluxes Set Up...

See Section  30.3 for details about generating flux reports.

Overall Heat Balances When Using the DTRM

The DTRM yields a global heat balance and a balance of radiant heat fluxes only in the limit of a sufficient number of rays. In any given calculation, therefore, if the number of rays is insufficient you may find that the radiant fluxes do not obey a strict balance. Such imbalances are the inevitable consequence of the discrete ray tracing procedure and can be minimized by selecting a larger number of rays from each wall boundary.

Displaying Rays and Clusters for the DTRM

When you use the DTRM, ANSYS FLUENT allows you to display surface or volume clusters, as well as the rays that emanate from a particular surface cluster. You can use the DTRM Graphics dialog box (Figure  13.3.16) for all of these displays.

Display $\rightarrow$ DTRM Graphics...

Figure 13.3.16: The DTRM Graphics Dialog Box

Displaying Clusters

To view clusters, select Cluster under Display Type and then select either Surface or Volume under Cluster Type.

To display all of the surface or volume clusters, select the Display All Clusters option under Cluster Selection and click the Display button.

To display only the cluster (surface or volume) nearest to a specified point, deselect the Display All Clusters option and specify the coordinates under Nearest Point. You may also use the mouse to choose the nearest point. Click on the Select Point With Mouse button and then right-click on a point in the graphics window.

Displaying Rays

To display the rays emanating from the surface cluster nearest to the specified point, select Ray under Display Type. Set the appropriate values for Theta and Phi Divisions under Ray Parameters (see Section  13.3.2 for details), and then click the Display button. Figure  13.3.17 shows a ray plot for a simple 2D geometry.

Figure 13.3.17: Ray Display

Including the Mesh in the Display

For some problems, especially complex 3D geometries, you may want to include portions of the mesh in your ray or cluster display as spatial reference points. For example, you may want to show the location of an inlet and an outlet along with displaying the rays. This is accomplished by enabling the Draw Mesh option in the DTRM Graphics dialog box. The Mesh Display dialog box will appear automatically when you enable the Draw Mesh option, and you can set the mesh display parameters there. When you click Display in the DTRM Graphics dialog box, the mesh display, as defined in the Mesh Display dialog box, will be included in the ray or cluster display.

Reporting Radiation in the S2S Model

When you use the S2S model, ANSYS FLUENT allows you to view the values of the view factor and radiation emitted from one zone to any other zone. You can use the S2S Information dialog box (Figure  13.3.18) to generate a report of these values in the console or as a separate file.

Report $\rightarrow$ S2S Information...

Figure 13.3.18: The S2S Information Dialog Box

The steps for generating the report are as follows:

1.   Specify the values in which you are interested by selecting View Factors and/or Incident Radiation.

2.   Choose the zones for which you would like data by selecting them in the lists under From and To (at least one zone must be selected under each list). To select all of the zones of a particular type, click on that category in the list under Boundary Types.

3.   Specify how you would like to present the data. To report the values in the console, click the Compute button. To write the data as an S2S Info File ( .sif format), click the Write... button and enter a file name in the Select File dialog box (see Section  2.1.6).

The following is an example of how the data is presented:

S2S Information
From wall1 to:
                         Viewfactor       Incident Radiation
               wall1         0.0000                   0.0000
               wall2         0.2929              171387.7813
               wall3         0.2929              155305.7969
               wall4         0.4142               29055.9023

From wall2 to:
                         Viewfactor       Incident Radiation
               wall1         0.2929              306451.9688
               wall2         0.0000                   0.0000
               wall3         0.4142              214195.0938
               wall4         0.2929               19153.2715

Note that the header listed above ( S2S Information) is not displayed in the console.