Opaque Surface (Radiation in Participating Media and Radiation in Absorbing-Scattering Media Interfaces)
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.
The net radiative heat flux is defined as the difference between the incoming and outgoing radiative heat fluxes:
The incoming and outgoing radiative heat fluxes are defined from the weighted sums of the incident intensities.
See Opaque Surface for more details on the definition of the radiative intensities on the boundary.
If the boundary is semitransparent, consider using the Semitransparent Surface (Radiation in Participating Media and Radiation in Absorbing-Scattering Media Interfaces) node instead.
Pair Selection
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.
Models Input
This section has fields and values that are inputs to expressions that define material properties. If such user-defined materials are added, the model inputs appear here.
There is one standard model input — the Temperature T, which is used in the expression of the blackbody radiative intensity.
Fractional Emissive Power
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.
Surface Radiative Properties
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.
Gray Surface
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
When it is added in the Radiation in Absorbing-Scattering Media interface with the Discrete Ordinates Method, no emission is taken into account, and the radiative intensity along the incoming discrete 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.
Black Surface
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
if the node is added in the Radiation in Participating Media interface. When the node is added in the Radiation in Absorbing-Scattering Media interface,
Values of radiative intensity along discrete outgoing directions are not prescribed.
When P1 approximation is selected as the Radiation discretization method for the physics interface, there is additional theory, equations, and variables described in P1 Approximation Theory.
Radiative Heat Transfer in a Utility Boiler: Application Library path Heat_Transfer_Module/Thermal_Radiation/boiler
Location in User Interface
Context Menus
Ribbon
Physics tab with Radiation in Participating Media or Radiation in Absorbing-Scattering Media selected: