This node defines a boundary that is opaque to radiation. It prescribes the incident intensities on a boundary and takes into account the net radiative heat flux,
qr,net, which is absorbed by the surface.
See Opaque Surface for more details on the definition of the radiative intensities on the boundary.
If this node is selected from the Pairs menu, choose the pair to apply this condition to. A pair must to be created first. See
Identity and Contact Pairs in the
COMSOL Multiphysics Reference Manual for more details.
There is one standard model input — the Temperature T, which is used in the expression of the blackbody radiative intensity.
This section is available when the Wavelength dependence of radiative properties is defined as
Solar and ambient or
Multiple spectral bands in the Radiation in Participating Media interface (see
Participating Media Settings).
When the Fractional emissive power is
Blackbody/Graybody, the fractional emissive power
FEPk is automatically calculated for each spectral band as a function of the band endpoints and temperature.
When the Fractional emissive power is
User defined for each band, define the
Fractional emissive power,
FEPk for each spectral band in the table displayed underneath. All fractional emissive powers are expected to be in [0,1] and their sum must be equal to 1. Select the
Define fractional emissive power on each side check box to set specific
Upside and
Downwside values in the table.
Select a Surface type to define the behavior of the surface:
Gray surface or
Black surface. The definition of the incident intensities used to define the incoming and outgoing radiative heat fluxes depends on the option selected in this list.
If Gray surface is selected, the surface emissivity should be set. This is a property of the material surface that depends both on the material itself and the structure of the surface.
An emissivity of 0 means that the surface does not emit any radiation and that all outgoing radiation is diffusely reflected by this boundary. An emissivity of
1 means that the surface is a perfect blackbody: all outgoing radiation is fully absorbed on this boundary. When the node is added in the Radiation in Participating Media interface with the Discrete Ordinates Method, the radiative intensity along the discrete incoming directions on this boundary is defined by
The default value of the Emissivity ε is taken
From material. Make sure a material is defined at the boundary level (by default materials are defined at the domain level).
For User defined enter another value or expression. You can define a temperature-dependent emissivity using the variable
rpm.T or
rasm.T.
If the Wavelength dependence of radiative properties is
Solar and ambient or
Multiple spectral bands, the wavelength can be accessed by the
rpm.lambda or
rasm.lambda variable. Any expression defined for the emissivity is then averaged over each spectral band to obtain a piecewise constant emissivity. If the average value of the emissivity on each band is known, you may use instead the option
User defined for each band to avoid the evaluation of the average.
If the Wavelength dependence of radiative properties is
Solar and ambient or
Multiple spectral bands, alternatively set the
Emissivity to
User defined for each band to enter a value for each spectral band in the table shown below. Within a spectral band, each value is assumed to be independent of wavelength.
Select the Define properties on each side check box to set specific values on each side of the boundary. The
Emissivity, upside and
Emissivity, downside should be set, with the same options as those described above.
If Black surface is selected, no user input is required, and the radiative intensity along the discrete incoming directions on this boundary is defined by
