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

Define the .

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Keep the default value, to define a diffuse gray radiation model. In this case, the surface properties (emissivity, radiosity, reflectivity, transmissivity, critical angle) have the same definition for all wavelengths. The surface properties 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, λsol/amb], (solar radiation) and one for large wavelengths, [λsol/amb, +∞[, (ambient radiation). It is then possible to define the (SI unit: m), λsol/amb, 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 of the for each spectral band in the table, to define a diffuse spectral radiation model. These values should be set in an ascending order. The value of the for the next spectral band is updated in consequence. It is then possible to provide a definition of the surface properties for each spectral band.


	The first Left endpoint and the last Right endpoint 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.

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.

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For Ray Shooting 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.

Ray Shooting

The use of a ray shooting algorithm allows to generate the view factor data for the modeling of transmission and specular reflection. Select this method to enable the Opaque Surface (Surface-to-Surface Radiation Interface) node (for specular reflection) and Semi-Transparent Surface (Surface-to-Surface Radiation Interface) node (for transmission).

To compute the radiation intensity on surfaces, the ray shooting algorithm emits n rays in 2D and n² rays in 3D where n is the value selected for . The trajectories of these rays are computed as they are absorbed, reflected or transmitted on the model surfaces until their intensity becomes too small or if the rays go far away from the geometry. The threshold where the ray trajectory is no longer computed is controlled by the . During the rays trajectory computation the tiling is adapted up to a numbers of time defined by the .

To improve the accuracy of the radiation computation the user may increase the (default value is 8), decrease the (default value is 1e-3) or increase the (default value is 3). Conversely changing these values in the opposite direction should decrease computational time. Also, higher values of the under node may improve the results.

View Factors Update

This section is available when the is , by clicking the button (

) and selecting in the dialog box.

It allows controlling the frequency of view factors computation according to a given criterion in time-dependent simulation. The view factors are updated each time the changes by more than the , instead of being updated every time step.

For example, set to t and to 1e-3[s] to enforce the update every 1e-3s of simulation time.

Discretization

Surface Radiosity

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