The radiation model uses a true view factor calculation which provides as accurate energy balance because it enforces reciprocity between solids. Temperature and energy balance accuracy are ensured for geometries with widely varying feature sizes.
Radiative heat transfer through transparent media is supported, as well as geometric symmetry. The radiation model computes radiative heat transfer to moving solids and moving surfaces, and is the basis of the solar heating model. The radiation model has very rigorous “bookkeeping” to keep track of the radiative energy balance, and reports the amount of heat transfer due to radiation and the radiative energy balance for each part in a model. The result is that reciprocity is enforced, to ensure that the radiative heat transfer between parts with large size differences is computed accurately.
Radiation works with all of the supported geometry types: two and three dimensional Cartesian and axisymmetric about the X and Y axes.
To use radiation, specify an emissivity for every solid material type in the model. If there are no solids present, specify an emissivity for the surrounding walls by setting an emissivity on the fluid material. (You will have to create a new material, but it can be based on a database material.) Because the radiation algorithm does not allow the fluid medium to participate, emissivity specified on a fluid material is automatically applied to the walls touching the fluid.
Note that the default value of 1 as the emissivity is not generally recommended because it indicates a perfectly emittive surface. Such a case may not be physically realistic for most applications.
Enable Heat Transfer and Radiation on the Solve dialog.
Radiation can be run with or without flow.
An important consideration is that fluid parts that contact one another should not be extrusion meshed. The reason is that if either (or both) use extruded elements, the interface between the extrusion faces and the tetrahedral faces, also known as a non-conformal interface, is not supported by the radiation model. The radiation model must have a matching mesh at all fluid-solid interfaces. This guideline applies to solid-solid interfaces as well if one or both of the solids is a transparent medium.
When an assembly is enclosed by an air volume, it is very important that a non-zero value of emissivity be assigned to the air (which then gets applied to the walls). If a value of 0 is used, the wetted surfaces (that do not touch solids) will behave as perfect mirrors, and no energy will be lost to the environment--a non-physical situation. Apply a temperature boundary condition to the external air surface that represents the correct environmental temperature, and specify a realistic emissivity for the air.
The radiation model computes true view factors for every part. This is more accurate than the flux-based method used in the radiation model in previous versions. The view factors between every part are written to the “.sol” file, and should sum to 1 for each part. Tables of view factors are produced for opaque as well as transparent materials.
A sample view factor list for one part in an assembly is shown:
| Opaque Part-To-Part View Factors |
| Part 1 viewing Part 1, VF = 0 |
| Part 1 viewing Part 2, VF = 0.00870629 |
| Part 1 viewing Part 3, VF = 0.0575024 |
| Part 1 viewing Part 4, VF = 0.021062 |
| Part 1 viewing Part 5, VF = 0.338157 |
| Part 1 viewing Part 6, VF = 0.574572 |
| Part 1 sum of all view factors = 1 |
Because this model uses a true view factor calculation, it can more accurately solve the radiative heat transfer for models with parts that have large size differences. Surface to surface reciprocity is enforced to ensure a more accurate energy balance.
To improve the performance of the view factor calculation, element faces are clustered together into larger polygons to reduce the effective number of view factor faces. The result is faster view factor forming, better reciprocity enforcement, and faster solution of the radiosity matrix at each iteration.
Radiation face clustering occurs automatically, but can be controlled by a flag (ClusterFaces) in the Flag Manager.
The fact that the radiation model computes view factors and reciprocity between every face of every part leads to a high level of accuracy and a good energy balance for radiation calculations. The model is, however, resource intensive. During initial startup, a view factor is calculated between all element faces of every part with a line of sight. Additionally, the radiation matrix must be built that tracks all of this data.
The required amount of RAM increases with the square of the number of surface element faces. Depending on the number of surfaces in a geometry, the amount of RAM required to compute the view factors may be in excess of 1 Gigabyte. The amount of time required to compute the view factors at startup can be quite significant as well. A progress bar indicates the relative progress of this calculation during initial startup.
The default amount of memory currently available for radiation view factor calculation is 4 GB.
To increase this amount, modify the value for the rad_matrix_size flag in the Flag Manager. The argument is the amount of RAM in megabytes. For example, to set the limit to be 10 GB of RAM, specify a value of 10000.
The radiation model automatically adjusts the accuracy of the computation as a function of the available RAM. The algorithm probes the system to determine how much RAM is available, and then it will adjust the optical sampling rate so that the final radiosity matrix will fit into the available RAM. It will also determine whether it should use in-memory or out-of-core storage of view factors, radiosity matrix terms, and even the type of solver employed. So even with only 256 Mbytes of memory, it is possible to run radiation calculations. It will take longer and be less accurate than results generated on a machine with 4 Gbytes of RAM, however.
If, however, the analysis model simply cannot be run with the available RAM, an error will be given advising that the radiation model cannot be run due to the lack of system resources.
Fortunately, this calculation is only performed at the beginning of an analysis. It does not occur for subsequent restarts of the analysis if the mesh does not change. Because the radiation model employs a surface integral method, it has been shown to not require a high mesh density to provide accurate results. Please be sure to balance the meshing requirements of the other physical phenomena in an analysis model as appropriate.
Radiation is supported for moving solids. When radiation is enabled for a motion analysis, the view factors are automatically recomputed when the moving part has traveled 2% of the maximum diagonal of the domain bounding box. Enable this flag in the Flag Manager to change this behavior:
where the argument is the percent of the diagonal. To recompute view factors every 5%, for example, enable this flag:
ViewFactorUpdate 5
Notes:
While perfect vacuums are extremely rare in most industrial applications, there are some applications in which the solution of radiation within such an environment is useful. A fluid has to be included in the environment, but Flow can be turned off (on the Solve dialog) to remove any convection effects. A general procedure for radiation analysis in a quasi-vacuum is to:
Related Topics
Example modifying the emissivity property