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 3-3 gives the overriding rules for these groups.
1
Temperature, Open Boundary, Open Boundary, Inflow Heat Flux
2
Thermal Insulation, Symmetry, Periodic Condition
3
Heat Flux
4
Boundary Heat Source, Radiation Group, Line Heat Source on Axis
5
Diffuse Mirror, Prescribed Radiosity, Diffuse Surface
6
Opaque Surface, Incident Intensity, Continuity on Interior Boundary
7
Thin Layer, Thermal Contact, Thin Film
8
Isothermal Domain Interface
Table 3-3: Overriding Rules for Heat Transfer Boundary conditions
A\B
1
2
3
4
5
6
7
8
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-Opaque Surface
 
 
 
 
 
X
 
 
7-Thin Layer
X
 
 
 
 
 
X
 
8-Isothermal Domain Interface
 
 
 
 
 
 
 
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 3-3 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 Diffuse Surface boundary condition is applied on the same boundary afterward.
•
Temperature belongs to group 1.
•
Diffuse Surface belongs to group 5.
•
The cell on the line of group 1 and the column of group 5 is empty so Temperature and Diffuse Surface contribute.
This mechanism can be checked on the COMSOL Desktop, in the Override and Contribution section of each feature, as shown in the following table:
Table 3-4: Override and Contribution sections
Temperature
Diffuse Surface
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 on the COMSOL Desktop, in the Override and Contribution section of each feature, as shown in the following table:
Table 3-5: Override and Contribution sections
Heat Flux
Symmetry
In Example 2 above, if Symmetry followed by Heat Flux is added, the boundary conditions contribute.