Overriding Mechanism for Heat Transfer Boundary Conditions
Many boundary conditions are available in heat transfer. Some of these can coexist (for example, Heat Flux and Thin Layer); others cannot (for example, Heat Flux and Thermal Insulation).
Several categories of boundary condition exist in heat transfer. Table 14-4 gives the overriding rules for these groups.
1
Temperature, Outflow
2
Thermal Insulation, Symmetry, Periodic Condition
3
Heat Flux
4
Boundary Heat Source, Line Heat Source on Axis
5
Surface-to-Ambient Radiation
6
Thin Layer
Table 14-4: Overriding Rules for Heat Transfer Boundary conditions.
A\B
1
2
3
4
5
6
1-Temperature
X
X
 
 
 
X
2-Thermal Insulation
X
X
 
 
X
 
3-Heat Flux
X
X
 
 
 
 
4-Boundary heat source
 
 
 
 
 
 
5-Radiation
 
X
 
 
X
 
6-Thin Layer
X
 
 
 
 
X
When there is a boundary condition A above a boundary condition B in the model tree and both conditions apply to the same boundary, use Table 14-4 to determine if A is overridden by B or not:
•
Locate the line that corresponds to the A group (see above the definition of the groups). In the table above only the first member of the group is displayed.
•
Locate the column that corresponds to the group of B.
•
If the corresponding cell is empty, A and B contribute. If it contains an X, B overrides A.
Group 3 and group 4 boundary conditions are always contributing. That means that they never override any other boundary condition. But they might be overridden.
Example 1
Consider a boundary where Temperature is applied. Then a Surface-to-Ambient Radiation boundary condition is applied on the same boundary afterward.
•
Temperature belongs to group 1.
•
Surface-to-Ambient Radiation belongs to group 5.
The cell on the line of group 1 and the column of group 5 is empty so Temperature and Surface-to-Ambient Radiation contribute.
Example 2
Consider a boundary where Heat Flux is applied. Then a Symmetry boundary condition is applied on the same boundary afterward.
•
Heat Flux belongs to group 3.
•
Symmetry belongs to group 2.
The cell on the line of group 3 and the column of group 2 contains an X so Heat Flux is overridden by Symmetry.
This mechanism can be checked in the COMSOL Desktop, in the Override and Contribution section of each feature, as shown in the following table:
Table 14-5: Override and Contribution sections.
Heat Flux
Symmetry
In Example 2 above, if Symmetry followed by Heat Flux is added, the boundary conditions contribute.