# Interfaces¶

In a CHT analysis, an interface defines the physical behaviour between the common boundaries of two solids (CHT analysis requires a multi-region mesh). For instance, it is possible to model how heat is exchanged between a wall and the fluid in a room. This is done defining an interface between the adjacent faces of the wall and the fluid. For these overlapping boundaries the user can define the Velocity and Thermal conditions as described in the following sections.

## Preparation¶

The interface is defined by two adjacent surfaces. These must have the same area and overlap completely. There might be cases in which the latter conditions are not satisfied straightaway. For instance, only a part of a face needs to be set as interface. To overcome this difficulty, the face must be split, so that only the relevant area is separated from the rest of the surface (this operation has to be done on the CAD file before importing it in the SimScale project). Figure 1 shows a heat sink which will be coupled with a chip. The chip shape is imprinted on the heat sink (red face in the figure), so the heat sink face is actually split in two. Only this part will be involved in the interface definition.

Fig. 1: Surface splitting on a heat sink. The area in red will be set as interface.

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 must be less than 1.5 times the size of the others. Figure 2 shows an example of this issue. In (a) it is possible to see the chip in grey. In (b) the adjacent heat sink is shown. The cells at the interface (red) are too big with respect to those on the chip (a). In (c) the cells in the red face are approximately the same size of that on the chip.

Fig. 2: Chip mesh (top); Bad mesh sizing at the heat sink interface (bottom left); Good mesh sizing at the interfaces (bottom right);

## Creation¶

Important

If an interface is not defined by the user, it will be automatically detected. A No-slip velocity condition and a Coupled thermal type will be assigned to it (see the description below). Consequently, the settings applied to manually-selected interfaces, override the default options.

In the simulation tree, navigate to the Domain menu, select Interfaces and add a new one. Simscale supports only the Region interface. The user is asked to specify two options for the interface, Velocity and Thermal. Figure 3 shows the interface definition between a chip and a heat sink.

Fig. 3 Interface definition

## Velocity¶

The Velocity options define the fluid velocity conditions at the interface. The two options available for the interfaces are the No-slip and Slip. In case the interface is between two solids, this option is irrelevant.

• No-Slip

The ‘no-slip’ option imposes a friction wall (or real wall) condition by setting the velocity components (tangential and normal) to Zero value at the interface i.e. $$V_t=V_n=0$$. This is the default option assigned to interfaces not user-defined.

• Slip

The ‘slip’ option imposes a frictionless wall condition. In this case the tangential velocities at the interface are adjusted according to the flow conditions, while the normal component is zero.

## Thermal¶

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

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

• Coupled

This is the simplest choice when the heat exchange between two regions needs to be taken into account. It models a perfect heat transfer across the interface. This is the default setting, in case an interface is not directly defined by the user.

• Contact interface material

This type of interface is used when the user wants to simulate a layer with thickness t and thermal conductivity $$\kappa$$ between the two regions with interface. For example, it is possible to model the thermal paste between a chip and a heat sink without actually drawing and meshing it. The latter operation is usually a problem, considering that the thickness of these layers is two or three orders of magnitude smaller than the other dimensions.

• Specific conductance

This interface type is very similar to the Contact Interface material but now, as the name suggests, it is only necessary to set the specific conductance which is defined as $$K = \frac{\kappa}{t}$$. For instance, this option may be used for an interface with a layer which thickness is unknown. One meaningful application is a household radiator for which the paint coat specific conductance may be given instead of its thickness and $$\kappa$$.

• Total resistance

The Total Resistance interface is the setting that allows the user 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 $$R = \frac{1}{K A} = \frac{1}{\frac{\kappa}{t} A}$$. It is worth noticing that the area of the interface appears in the latter 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 tubes wall is only known as a total resistance. A first simulation proves that performances are insufficient. Consequently the tubes length is increased. The new simulation will only be correct if the total resistance is changed accordingly to the new area of the tubes.

Interfaces reference project