Settings for the Surface-to-Surface Radiation Interface

The is the default physics interface name.

The is used primarily as a scope prefix for variables defined by the physics interface. Refer to such physics interface variables in expressions using the pattern <name>.<variable_name>. In order to distinguish between variables belonging to different physics interfaces, the name string must be unique. Only letters, numbers, and underscores (_) are permitted in the field. The first character must be a letter.

The default (for the first physics interface in the model) is rad.

Radiation Settings

This section is always available for The Surface-To-Surface Radiation Interface. To display this section for any version of the Heat Transfer interface, select the check box in the section.

Define the .

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Keep the default value, to define a diffuse gray radiation model. In this case, the surface emissivity has the same definition for all wavelengths. The surface emissivity can still depend on other quantities, in particular on the temperature.

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Select to define a diffuse spectral radiation model with two spectral bands, one for short wavelengths, [0, λ1], (solar radiation) and one for large wavelengths, [λ1, +∞[, (ambient radiation). It is then possible to define the (SI unit: m), λ1, to adjust the wavelength intervals corresponding to the solar and ambient radiation. The surface properties can then be defined for each spectral band. In particular it is possible to define the solar absorptivity for short wavelengths and the surface emissivity for large wavelengths.

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Select and set the value (to ), to define a diffuse spectral radiation model. It is then possible to provide a definition of the surface emissivity for each spectral band. Update (SI unit: m), λ1, λ2, ..., to define the wavelength intervals [λi − 1, λi[ for i from 1 to the .


	The first and the last endpoints, λ0 and λN (with N equal to the value selected to define the Number of wavelength bands), are predefined and equal to 0 and +∞, respectively.

Modify the if it is different from 1 and corresponds to vacuum refractive index, which is usually a good approximation for air refractive index.

In the menu, select or to define if the exterior of the heat transfer interface selection should be considered as transparent or opaque, respectively, when option is used to define in surface-to-surface radiation boundary conditions. It has no effect on a boundary where option is not selected.

Also select the check box to enable the ability to define radiation groups, which can, in many cases, speed up the radiation calculations.

Select the : (the default) or .

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For Direct Area Integration select a — is the default.

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For Hemicube select a — is the default.

Hemicube

Hemicube is the default method for the heat transfer interfaces. The more sophisticated and general hemicube method uses a z-buffered projection on the sides of a hemicube (with generalizations to 2D and 1D) to account for shadowing effects. Think of it as rendering digital images of the geometry in five different directions (in 3D; in 2D only three directions are needed), and counting the pixels in each mesh element to evaluate its view factor.

Its accuracy can be influenced by setting the of the virtual snapshots. The number of z-buffer pixels on each side of the 3D hemicube equals the specified resolution squared. Thus the time required to evaluate the irradiation increases quadratically with resolution. In 2D, the number of z-buffer pixels is proportional to the resolution property, and thus the time is, as well.

For an axisymmetric geometry, Gm and Famb must be evaluated in a corresponding 3D geometry obtained by revolving the 2D boundaries about the axis. COMSOL Multiphysics creates this virtual 3D geometry by revolving the 2D boundary mesh into a 3D mesh. The resolution can be controlled in the azimuthal direction by setting the number of azimuthal sectors, which is the same as the number of elements to a full revolution. Try to balance this number against the mesh resolution in the rz-plane.

Direct Area Integration

COMSOL Multiphysics evaluates the mutual irradiation between surface directly, without considering which face elements are obstructed by others. This means that shadowing effects (that is, surface elements being obstructed in nonconvex cases) are not taken into account. Elements facing away from each other are, however, excluded from the integrals.

Direct area integration is fast and accurate for simple geometries with no shadowing, or where the shadowing can be handled by manually assigning boundaries to different groups.


	If shadowing is ignored, global energy is not conserved. Control the accuracy by specifying a Radiation integration order. Sharp angles and small gaps between surfaces may require a higher integration order for accuracy but also more time to evaluate the irradiation.

Discretization

To display this section, click the button (

) and select .

Select(the default), ,, or to define the discretization level used for the shape function.

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About the Heat Transfer Interfaces

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Feature Nodes for the Surface-to-Surface Radiation Interface

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The Surface-To-Surface Radiation Interface

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Theory for Surface-to-Surface Radiation