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# Contacts in Conjugate Heat Transfer

In a Conjugate Heat Transfer (CHT) analysis, an interface defines the physical behavior between the common boundaries of two regions that are in contact, e.g. solid-solid, or solid-fluid.

Important

In CHT simulations, all the fields at the interfaces are fully constrained by the Interface type. Therefore, defining boundary conditions to faces assigned to interfaces is not allowed. This would result in an overconstrained model.
If an interface is assigned to a boundary condition, the following error message is displayed when the user tries to start a simulation:

To solve this error, the user needs to unassign the interfaces from the boundary conditions.
Additionally, interfaces between two flow regions are not possible and will result in an error when running simulations.

### Automatic Interface Detection

When creating a new CHT simulation,  all possible interfaces will automatically be detected and populated in the simulation tree. Interfaces will be grouped together and defined as Coupled thermal interface.

#### How To Modify Specific Interfaces?

Individual interfaces or a group of interfaces can be filtered via entity selection. Select the entities (faces or volumes) for which you want to select all interfaces that exist between them.

Once you filter the interfaces of interest, a window opens with additional options for the Interface type for the selected contacts.

Interfaces which differ in settings from the standard bulk interfaces group will stay exposed individually in the simulation setup tree.

#### Partial Contacts

An interface is required to always be defined between two congruent surfaces, meaning that these surfaces must have the same area and overlap completely. After contact detection, the platform will also perform a check for partial contacts. If partial contacts are detected, the platform will show a warning and recommend an imprinting operation.

Imprinting is a single-click operation built into SimScale, which splits existing faces into smaller ones in order to guarantee perfect overlap between contacting faces. It is recommended to perform an imprint operation in order to guarantee accurate heat transition modeling for the simulation.

By default, any detected partial contact will be defined as an adiabatic interface, and not participate in heat conduction unless specified otherwise.

#### Contact Detection Errors

As all possible interfaces are detected automatically, it is no longer possible to manually add an interface or to change the entity assignment for a specific interface. In case no interfaces can be detected automatically, SimScale will show an error message.

In this case, it is not possible to create a mesh or start a simulation run for this simulation. Instead, the CAD model needs to be investigated for potential errors that prevent successful contact detection. Some common causes are:

• Duplicate parts: Sometimes the CAD modeling history makes it so that parts can be duplicated upon export. Please check that such a condition is not present in your model.
• Interfering parts: If the face pair is intersecting into each other by a margin higher than the CAD tolerance, the contact will not be detected. Please move the parts away so that the faces are in perfect contact, or you can also perform a boolean operation to create coincident faces.
• Small gaps between the faces: Opposite to the previous condition, if the face pair is separated by a gap higher than the CAD tolerance, the interface will not be detected. Sometimes the gap is so small that it is difficult to find it by visual inspection. Please move the parts together or extrude the faces to create the proper contact.

Some of the fix operations described above can be performed in the CAD mode. You can also reach out to support via email or chat in case you encounter this issue and can not solve it using the above suggestions.

### Interface Settings

#### Interface Type

The Interface type options define the heat exchange conditions at the interface. The five types available for the interfaces are reported below:

Coupled

The coupled thermal interface models a perfect heat transfer across the interface. This is the default setting, in case an interface is not defined by the user.

In this case, thermal energy cannot be exchanged between the domains across the interface.

Total Resistance

The Total Resistance interface allows users to model an imperfectly matching interface (e.g. due to the surface roughness) which reduces the heat exchange across it. The total resistance is defined as:

$$R = \frac{1}{K A} = \frac{1}{\frac{\kappa}{t} A} \tag{1}$$

It is worth noticing that the area of the interface appears in the definition. So this option must be assigned only to the relevant face. Let’s suppose that a heat exchanger is being simulated. The effect of solid sediment on the tube’s wall is only known as a total resistance. A first simulation proves that heat exchange performance is insufficient. Consequently, the length of the tubes is increased. The new simulation will only be correct if the total resistance is changed according to the new area of the tubes.

Specific Conductance

This interface type is very similar to the Thin layer resistance (below). It only requires users to set the specific conductance of the interface $$K$$, which is defined as:

$$K = \frac{\kappa}{t} \tag{2}$$

with thickness $$t$$ $$(m)$$ and thermal conductivity $$\kappa$$ $$(\frac {W}{mK})$$ between the two interface regions.

For instance, this option may be used for an interface where the layer thickness is negligible or unknown, i.e., a radiator for which the paint coating’s specific conductance may be given instead of its thickness and $$\kappa$$.

Thin Layer Resistance

The Thin layer resistance allows modeling a layer with thickness $$t$$ and thermal conductivity $$\kappa$$ between the two interface regions.

For example, it is possible to model the thermal paste between a chip and a heat sink without needing to resolve it in the geometry. Adding a thin layer to the geometry and meshing it is a problem, considering that the thickness of these layers is two or three orders of magnitude smaller than other components in the assembly.

### CAD and Mesh Requirements

A CHT simulation always requires a multi-region mesh. As far as the mesh is concerned, it is fundamental that the cell size at the interface is similar between the two faces. As a rule of thumb, the cells on one face should be less than 1.5 times the size of the others. The figure below shows an example of this issue. In the left case, the cells at the interface on the inner region are too small with respect to those on the outer body. In the case on the right side, the cells on the interface are approximately the same size.

Last updated: June 21st, 2022