17.6.2.19. Radiation

Applies thermal radiation to a surface of a model (an edge in a 2D model). You can define the exchange of radiation between a body and the ambient temperature, or between two surfaces.

For thermal related analyses that use the Mechanical APDL solver, the actual calculation of the radiation exchange between two surfaces is performed using the Radiosity Solver method. The Radiosity Solver method accounts for the heat exchange between radiating bodies by solving for the outgoing radiative flux for each surface, when the surface temperatures for all surfaces are known. The surface fluxes provide boundary conditions to the finite element model for the conduction process analysis in Workbench. When new surface temperatures are computed, due to either a new time step or iteration cycle, new surface flux conditions are found by repeating the process. The surface temperatures used in the computation must be uniform over each surface facet to satisfy the conditions of the radiation model.

For models that are entirely symmetrical, you can account for symmetry using Symmetry Regions or Cyclic Regions. The Radiosity Solver method respects plane or cyclic symmetries. Using a model's symmetry can significantly reduce the size of the model. The Radiosity Solver method will take symmetry into account and the Radiation Probe solution results will be valid for the full model. Settings for the Radiosity Solver method are available under the Analysis Settings object in the Radiosity Controls category.

Related References

This discussion of the Radiation loading condition assumes that you have an understanding of the related terminology. For a listing of terminology, see the Definitions section of the Mechanical APDL Thermal Analysis Guide. In addition, for more references, see the Mechanical APDL References and Notes topic at the end of the section.

This page includes the following sections:

Analysis Types

Radiation is available for the following analysis types:

Dimensional Types

The supported dimensional types for the Radiation boundary condition include:

  • 3D Simulation

  • 2D Simulation

Geometry Types

The supported geometry types for the Radiation boundary condition include:

  • Solid

  • Surface/Shell

Topology Selection Options

The supported topology selection options for Radiation include:

  • Body

  • Face: Supported for 3D only.

  • Edge: Supported for 2D only.

  • Element Face

Magnitude Options

The supported Magnitude options for Radiation include the following:

  • Constant

  • Tabular (Time Varying) - for Ambient Temperature

  • Tabular (Temperature Varying): Supported for Emissivity if Correlation = Surface To Surface.

  • Function (Time Varying)

Loading Types and Loading Definition

The unique loading characteristics and definitions for the Radiation boundary condition are described below.

Ambient Temperature Radiation

When the Correlation property is specified as To Ambient in the Details pane of a Radiation object, all of the radiation energy is assumed to be exchanged with the surroundings at ambient temperature, that is, the Form Factor is assumed to be 1.0.

You can set the following additional radiation properties in the Details pane:

  • Emissivity: The ratio of the radiation emitted by a surface to the radiation emitted by a black body at the same temperature.

  • Ambient Temperature: The temperature of the surrounding space.

Note:  Radiation exchange between surfaces is restricted to gray-diffuse surfaces. Gray implies that emissivity and absorptivity of the surface do not depend on wavelength (either can depend on temperature). Diffuse signifies that emissivity and absorptivity do not depend on direction. For a gray-diffuse surface, emissivity = absorptivity and emissivity + reflectivity = 1. Note that a black body surface has a unit emissivity.

Surface to Surface Radiation

When the Correlation property is specified as Surface to Surface in the Details pane of a Radiation object, the radiation energy is exchanged between surfaces. In this context, "surface" refers to a face of a shell or solid body in a 3D model, or an edge in a 2D model. You can then specify Emissivity, Ambient Temperature (defined above), Enclosure, and the Enclosure Type. Emissivity must be a positive value that is not greater than 1. Emissivity can also be defined by Tabular Data.

You should assign the same Enclosure number to surfaces radiating to each other. Specify the Enclosure Type as either Open (default) or Perfect as suited for a simulation of the closed radiation problems. Furthermore, closed radiation problems have no dependence on Ambient Temperature so that property is removed from the Details pane during closed problems.

Caution:  You cannot apply a Surface to Surface Radiation load to a geometric entity that is already attached to another Radiation load.

Recommendation:  For surfaces emitting radiation, make sure that the mesh does not penetrate into other bodies.

When using the Surface to Surface correlation with shell bodies, the Details also includes a Shell Face property that enables you apply the load to Both faces, to the Top face only, or to the Bottom face only.

Applying a Radiation Boundary Condition

To apply a Radiation:

  1. Select the Radiation option from the Environment Context tab. Alternatively, right-click the Environment tree object or in the Geometry window and select Insert>Radiation.

  2. Define the Scoping Method as either Geometry Selection or Named Selection and then specify the geometry.

  3. Modify the Ambient Temperature as needed.

Details Pane Properties

The selections available in the Details pane are described below.

CategoryProperty/Options/Description
Scope

Scoping Method: This property specifies how you perform geometric entity selection. Options include Geometry Selection (default) and Named Selection.

Geometry: Visible when the Scoping Method is set to Geometry Selection. Displays the type of geometry (Body, Face, etc.) and the number of geometric entities (for example: 1 Body, 2 Edges) to which the boundary has been applied using the selection tools.

Named Selection: Visible when the Scoping Method is set to Named Selection. This field provides a drop-down list of available user–defined Named Selections.

Recommendation:  Ansys recommends that you do not create multiple Radiation objects with the same scoping.

Shared Reference Body: This property is a scoping feature used to apply a Radiation to the shared faces of two bodies, one solid and one shell. When you have properly scoped the geometric entities, using either Geometry Selection or an appropriate Named Selection, the property provides a drop-down list of the names of the bodies that share the scoped features. Select the desired body from the list. Once selected, the Geometry or Named Selection property displays a parenthetical of the shared face, “(1 Shared Face),” to indicate the condition.

Note:  The Shared Reference Body property is not supported when the Correlation property is set to Surface to Surface.

Shell Face: For this loading condition, this property is supported for 3D shell bodies only and requires that you activate beta options[a] as well as a beta feature selection for thermal variation.

  • When you set the Correlation property to Surface to Surface setting. The Shell Face setting enables you the choice of applying the load to Both faces, to the Top face only, or to the Bottom face only.

    Or...

  • When you 1) set the Correlation property to To Ambient and 2) set the beta feature/property Thermal Variation (Beta) property to an option other than No Variation Along Thickness. The Shell Face setting enables you to apply the load to the temperature degree of freedom associated with the selected face.

Important:  If you specify a shell body as a Shared Reference Body, the solver does not support the Linear Variation and Quadratic Variation options of the Thermal Variation (Beta) property. In addition, you need to the Offset Type property to Bottom.

Definition

Type: Read-only field that displays boundary condition type - Radiation.

Correlation

Emissivity

Ambient Temperature (not visible if Correlation property = Surface to Surface and the Enclosure Type property = Perfect)

Enclosure (only visible if the Correlation property = Surface to Surface)

Enclosure Type (only visible if the Correlation property = Surface to Surface)

Note:  If a solver error occurs when the Enclosure Type is set to Perfect, Ansys recommends that you change the setting to Open.

Suppressed: Include (No - default) or exclude (Yes) the boundary condition.

[a] Activate beta options and beta feature selection:

  1. On the Workbench Project page, select the Tools drop-down menu and then Options.

  2. In the dialog, select the Appearance group and then the Beta Options check box.

  3. Click OK.

  4. Again, select the Tools drop-down menu and then Options.

  5. In the dialog, select the Mechanical group.

  6. Under the heading "MAPDL Solver," select the Allow thermal variation along shell thickness check box.

  7. Click OK.

  8. The application prompts you to restart. Do so, and then open your shell model in Mechanical for a supported analysis type.

Mechanical APDL References and Notes

For additional information for this boundary condition, see the following Mechanical APDL sections.

  • Using the Radiosity Solver Method in the Thermal Analysis Guide.

  • Radiation and Radiosity Solution Method sections in the Mechanical APDL Theory Reference:

  • For a perfectly closed system, the application uses the VFSM command. Specifically, Mechanical performs the VFSM,,N,1 command with the perfect enclosure number N. However, note that the reciprocity relationship is not satisfied using the command. Review the Notes topic in the VFSM section of the Mechanical APDL Command Reference for additional information.

API Reference

For specific scripting information, see the Radiation section of the ACT API Reference Guide.