Surface-to-Ambient Radiation (Heat Transfer interface)
Use this node to account for radiation from boundaries to the ambient. The net inward heat flux from surface-to-ambient radiation is
where ε is the surface emissivity, σ is the Stefan-Boltzmann constant (a predefined physical constant), and Tamb is the ambient temperature.
Model Input
This section has fields and values that are inputs to expressions that define material properties. If such user-defined property groups are added, the model inputs appear here.
Temperature
This section is available when temperature-dependent material properties are used. By default the temperature of the parent interface is used and the section is not editable. To edit the Temperature field, click Make All Model Inputs Editable (). The available options are User defined (default), Common model input (the minput.T variable, set to 293.15 [K] by default) and all temperature variables from the physics interfaces included in the model. To edit the minput.T variable, click the Go to Source button (), and in the Common Model Inputs node under Global Definitions, set a value for the Temperature in the Expression for remaining selection section.
Radiation Settings
Define on which side of the boundary is the ambient domain that receives the radiation, by selecting a Radiation direction: Upside or Downside.
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.
Surface-to-Ambient Radiation
Ambient temperature
For User defined, enter an Ambient temperature Tamb. The default value is approximately room temperature, 293.15 K (20ºC). Else, select an Ambient temperature defined in an Ambient Thermal Properties node under Definitions.
Surface emissivity
The default Surface emissivity ε (a dimensionless number between 0 and 1) is taken From material. For User defined, it should be specified. An emissivity of 0 means that the surface emits no radiation at all while an emissivity of 1 means that it is a perfect blackbody.
Location in User Interface
Context menus
 Heat Transfer in Solids>Surface-to-Ambient Radiation
Ribbon
Physics Tab with interface as Heat Transfer in Solids selected:
Boundaries>Surface-to-Ambient Radiation