This node defines a boundary opaque to radiation. It prescribes incident intensities on a boundary and accounts for the net radiative heat flux,
qr,net, that is absorbed by the surface.
If this node is selected from the Pairs menu, choose the pair on which to apply this condition. A pair has 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 blackbody radiative intensity expression.
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 computed for each spectral band as a function of the band endpoints and temperature.
When the Fractional emissive power is
User defined, 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 is expected to 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 Wall type to define the behavior of the wall:
Gray wall or
Black wall. The definition of the incident intensities used to define the incoming and outcoming radiative heat fluxes depends on the option selected in this list.
If Gray wall 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 emits no radiation at all and that all outgoing radiation is diffusely reflected by this boundary. An emissivity of
1 means that the surface is a perfect blackbody: 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 incoming discrete directions on this boundary is defined by
The default Surface emissivity ε value is taken
From material. Make sure that 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 may set a temperature-dependent emissivity through the use of the variable
rpm.T or
rasm.T.
If Wavelength dependence of radiative properties is
Solar and ambient or
Multiple spectral bands, the wavelength may be accessed through the
rpm.lambda or
rasm.lambda variable. Any expression set for the emissivity is then averaged on 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
User defined for each band option to avoid the evaluation of the average.
If Wavelength dependence of radiative properties is
Solar and ambient or
Multiple spectral bands, set alternatively the
Surface emissivity to
User defined for each band to enter a value for each spectral band in the table displayed underneath. Within a spectral band, each value is supposed to be wavelength-independent.
Select the Define surface emissivity on each side check box to set specific values on each side of the boundary. The
Surface emissivity, upside and
Surface emissivity, downside should be set, with the same options as those described above.
If Black wall is selected, no user input is required, and the radiative intensity along the incoming discrete directions on this boundary is defined by
