A thermal resistance network can be used to approximate the effect of heat sources and heat transfer from that source to the surrounding domain without explicitly having to resolve the source geometry itself. An example application is a PCB with multiple small resistors, LEDs, and/or processor chips mounted on top, which, compared to the overall domain, are small enough for their individual geometry to play only a minor role on the result.
Thermal resistance networks are only available in the Conjugate heat transfer and Conjugate heat transfer v2.0 analysis types.
Only perfectly rectangular objects (solids) can be approximated via a thermal resistance network. In case the original geometry resolves more detailed features of the object you would like to model via a thermal resistance network, simply replace them with rectangular boxes.
In the image below, the left-hand side model represents a complex geometry containing several leads and a casing, whereas the right-hand side geometry shows a simplified version of the geometry, which is good for a simulation with thermal resistance network:
Did you know?
It is possible to perform the CAD clean up from Figure 1 in the CAD mode environment. It would take 2 simple operations:
1. Deleting the leads with a Delete body operation
2. Running a Simplify operation in Box mode. As the name suggests, this operation simplifies a complex geometry with a simpler entity.
The figure below shows how the geometry looks like, after each step:
Creation of a Thermal Resistance Network
In your Conjugate heat transfer analysis, navigate to Advanced concepts and add a ‘Thermal resistance network’.
Assign the top face of the part you want to approximate as thermal resistance network. Top face in this case means the one facing in opposite direction of where the object is fixed on. See the approximation diagram below for context:
The thermal resistance network assumes a simplified thermal model, in which the resistance between the top face and bounding region, side faces and bounding region, as well as bottom face and bounding region can be specified. This is done in the next step.
Define thermal resistance in all directions. The network will model each face of your rectangular body with an average temperature, which then transfers heat via convection to the surrounding medium based on the thermal resistance specified.
Note that, in the image above, the assignment contains a single face (the top face). For clarification of each resistance term, the diagram below depicts the thermal resistance model:
For all four side faces, a single thermal resistance value needs to be specified. For the top and bottom faces, one additional resistance component can be specified. In case there is a thermal conduction paste between the board and the chip, it’s possible to define it as a Board interface resistance.
As important notes, the topology that is assigned to a thermal resistance network is not going to be resolved in the mesh, since it is treated as a cell zone. Additionally, all contacts defined between topology assigned to thermal resistance networks and surrounding regions will be ignored, meaning thermal resistance networks are always taking priority over contacts.
When creating a simulation run, a warning will be shown that some faces have been assigned to both a thermal resistance network and an interface. This warning can be ignored. The simulation will prioritize thermal resistance networks over contacts.
If you are interested in a comparison between a simulation using the thermal resistance network model and a simulation with detailed geometry, make sure to check this example project.
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