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 14-3 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	Diffuse Surface

6	Thin Layer

Table 14-3: 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 14-3 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.

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 14-4: Override and Contribution sections

Heat Flux	Symmetry

In Example 2 above, if Symmetry followed by Heat Flux is added, the boundary conditions contribute.