Diffuse Surface
Diffuse surfaces reflect radiative intensity uniformly in all directions. This node handles radiation with a view factor calculation. The feature adds one radiosity shape function per spectral interval to its selection and uses it as surface radiosity.
It adds a radiative heat source contribution
on the side of the boundary where the radiation is defined, where ε is the surface emissivity, G is the irradiation, and eb(T) is the blackbody hemispherical total emissive power. Where the radiation is defined on both sides, the radiative heat source is defined on both sides too.
Model Inputs
This section has fields and values that are inputs to expressions that define material properties. If such user-defined property groups have been added, the model inputs are included here.
There is one standard model input — the Temperature T. The default is the temperature variable in the Heat Transfer interface or 293.15 K in the Surface-to-Surface Radiation interface. This model input is used in the expression for the blackbody radiation intensity and, when multiple wavelength intervals are used, for the fractional emissive power. The temperature model input is also used to determine the variable that receives the radiative heat source. When the model input does not contain a dependent variable, the radiative heat source is ignored.
Radiation Settings
When Wavelength dependence of emissivity is set to Constant in the Radiation Settings section of the physics interface (available when the Surface-to-surface radiation check box is selected), select a Radiation direction based on the geometric normal (nx, ny, nz): Opacity controlled, Negative normal direction, Positive normal direction, or Both sides.
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Opacity controlled requires that each boundary is adjacent to exactly one opaque domain. Opacity is controlled by the Opacity domain subfeature. For external boundaries, the exterior side opacity is controlled by the Exterior radiation setting at the interface level. This is the default option when the node is added from any version of the Heat Transfer interface with the Surface-to-surface radiation check box is selected.
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Select Negative normal direction to specify that the surface radiates in the negative normal direction. An arrow indicates the negative normal direction that corresponds to the direction of the radiation emitted by the surface.
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Select Positive normal direction if the surface radiates in the positive normal direction. An arrow indicates the positive normal direction that corresponds to the direction of the radiation emitted by the surface.
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Select Both sides if the surface radiates on both sides. This is the default option when the node is added from the Heat Transfer in Thin Shells interface or the Surface-to-Surface Radiation interface.
When Wavelength dependence of emissivity is set to Solar and ambient or Multiple spectral bands in the Radiation Settings section of the physics interface (available when the Surface-to-surface radiation check box is selected), select a Radiation direction for each spectral band: Opacity controlled, Negative normal direction, Positive normal direction, Both sides, or None. The Radiation direction defines the radiation direction for each spectral band similarly as when Wavelength dependence of emissivity is Constant. Defining a radiation direction for each spectral band makes it possible to build models where the transparency or opacity properties defers between spectral bands.
This is useful for example to represent glass opaque to radiation outside of the 0.3–2.5 µm wavelength range.
None is used when adjacent domains are either both transparent or both opaque for a given spectral band.
When the Surface-to-surface radiation check box is not selected or not available, the Radiation Settings section can be displayed by clicking the Show button () and selecting Advanced Physics Options. Select a Radiation direction between Negative normal direction and Positive normal direction.
The Thin Layer boundary also defines the layer opacity that determines the side of the layer where the radiation occurs, depending on radiation direction. When the Surface-to-surface radiation check box is not selected or not available, the thin layer is set opaque.
Ambient
If the Surface-to-surface radiation check box is selected, select Define ambient temperature on each side when the ambient temperature differs between the sides of a boundary. This is needed to define ambient temperature for a surface that radiates on both sides and that is exposed to a hot temperature on one side (for example, fire) and to a cold temperature on the other side (for example, external temperature). By default, Define ambient temperature on each side is not selected when the node is added from any version of the Heat Transfer interface; but it is selected when the node is added from the Heat Transfer in Thin Shells interface or the Surface-to-Surface Radiation interface.
Set the Ambient temperature Tamb. For User defined, enter a value or expression. Else, select an Ambient temperature defined in the Ambient Settings section of a Heat Transfer or Heat Transfer in Shells interface. When Define ambient temperature on each side is selected, define the Ambient temperature Tamb, u and Tamb, d on the up and down side, respectively. The geometric normal points from the down side to the up side.
Set Tamb to the far-away temperature in directions where no other boundaries obstruct the view. Inside a closed cavity, the ambient view factor, Famb, is theoretically zero and the value of Tamb therefore should not matter. It is, however, good practice to set Tamb to T or to a typical temperature value for the cavity surfaces in such cases because that minimizes errors introduced by the finite resolution of the view factor evaluation.
By default, a diffuse irradiation contribution Idiff is included into the external irradiation. For User defined, enter a value or expression. When considering solar irradiation, it accounts for the irradiation from the sun, scattered by the atmosphere, and supposed to be isotropic. Else, select a Clear sky noon diffuse horizontal irradiance defined in the Ambient Settings section of a Heat Transfer or Heat Transfer in Shells interface.
To consider only the direct irradiation defined in the External Radiation Source feature, clear the Include diffuse irradiance check box.
Surface Fractional Emissive Power
This section is only available when the Surface-to-surface radiation check box is selected.
This section is available when the Wavelength dependence of emissivity is defined as Solar and ambient or Multiple spectral bands for the physics interface (see Radiation Settings).
When the Fractional emissive power is Blackbody/Graybody, the fractional emissive power is automatically computed for each spectral band as a function of the band endpoints and surface temperature.
When the Fractional emissive power is User defined, define the Fractional emissive power, FEPBi for each spectral band. All fractional emissive powers are expected to be in [0,1] and their sum is expected to be equal to 1.
Surface Emissivity
In diffuse gray and diffuse spectral radiation models, the surface emissivity and the absorptivity must be equal. For this reason it is equivalent to define the surface emissivity or the absorptivity.
The surface emissivity settings are defined per spectral interval.
When the Radiation direction is Opacity controlled, Negative normal direction, or Positive normal direction for a spectral band, by default, the Surface emissivity ε (dimensionless) uses values From material. This is a property of the material surface that depends both on the material itself and the structure of the surface. Make sure that a material is defined at the boundary level (by default materials are defined at the domain level).
When the Radiation direction is set to Both sides for a spectral band, define the Material on upside and Material on downside:
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The defaults for both Material on upside and Material on downside use Boundary material. The list has options based on the materials defined in the model.
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Define the Surface emissivity on the upside and downside, respectively. The geometric normal points from the down side to the up side. Set the surface emissivity  to a number between 0 and 1, where 0 represents diffuse mirror and 1 is appropriate for a perfect blackbody. The proper value for a physical material lies somewhere in-between and can be found from tables or measurements.
When the Radiation direction is set to None for a spectral band, no information is needed for this spectral band in the Surface Emissivity section.
Initial Values
This section is only available when the Surface-to-surface radiation check box is selected.
The surface radiosity initial values are defined per spectral interval.
When the Radiation direction is Opacity controlled, Negative normal direction, or Positive normal direction for a spectral band Bi, the default Surface radiosity JBi, init is defined as
When Both sides is selected as the Radiation direction,
Enter initial values for the Surface radiosity JBi, init, u and JBi, init, d. The default Surface radiosity is ht.JBiinitU and ht.JBiinitD.
When None is selected as the Radiation direction, no surface radiosity is defined; hence no initial value is needed.
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In the notation used here, Bi stands for B1, B2,... up to the maximum number of spectral intervals.
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When the model contains one spectral interval, JBi, init, JBi, init, u and JBi, init, d are named, respectively, Jinit, Jinit, u and Jinit, d.
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If this feature is combined with heat transfer in 2D and 1D, the thickness is assumed to be infinite for the view factor computation. The user-defined value for d is still used in the heat transfer equation.
To define temperature dependencies for the user inputs (surface emissivity for example), use the temperature variable ht.T, that corresponds to the appropriate variable (upside, downside, or average temperature of a layer, wall temperature with turbulence modeling), depending on the model configurations. See Boundary Wall Temperature for a thorough description of the boundary temperature variables.
Several settings for this node depend on the Wavelength dependence of emissivity setting, which is defined for the physics interface when the Surface-to-surface radiation check box is selected.
In addition, the Transparent media refractive index is equal to 1 by default, and can be set when the Surface-to-surface radiation check box is selected.
See Radiation Settings.
Theory for Surface-to-Surface Radiation
Upside and downside settings can be visualized by plotting the global normal vector (nx, ny, nz), that always points from downside to upside. Note that the normal vector (ht.nx, ht.ny, ht.nz) may be oriented differently.
See Tangent and Normal Variables in the COMSOL Multiphysics Reference Manual.
Heat Generation in a Disc Brake: Application Library path Heat_Transfer_Module/Thermal_Contact_and_Friction/brake_disc
Location in User Interface
Context menus
Heat Transfer>Radiation>Diffuse Surface
Heat Transfer in Solids>Radiation>Diffuse Surface
Heat Transfer in Fluids>Radiation>Diffuse Surface
Heat Transfer in Porous Media>Radiation>Diffuse Surface
Bioheat Transfer>Radiation>Diffuse Surface
Heat Transfer with Surface-to-Surface Radiation>Radiation>Diffuse Surface
Surface-to-Surface Radiation>Diffuse Surface
Heat Transfer with Radiation in Participating Media>Radiation>Diffuse Surface
Heat Transfer in Thin Shells>Radiation>Diffuse Surface
Ribbon
Physics Tab with Surface-to-Surface Radiation selected:
Boundaries>Surface-to-Surface Radiation>Diffuse Surface
Physics Tab with Heat Transfer, Heat Transfer in Solids, Heat Transfer in Fluids, Heat Transfer in Porous Media, Heat Transfer in Building Materials, Bioheat Transfer, Heat Transfer with Surface-to-Surface Radiation, Heat Transfer with Radiation in Participating Media, or Heat Transfer in Thin Shells selected:
Boundaries>Radiation>Diffuse Surface