CFD-ACE+ has a unique Boundary Condition feature called "Thin Walls". This feature can be used to model very thin entities such as baffles, plates, and thermal resistances. Since the entity is very thin, you can actually ignore the volume it displaces, but not from a momentum, heat transfer, and/or scalar transport standpoint. For the flow field, a thin-wall acts simply as a wall boundary, i.e it appears as a blockage to flow. In simulations where the users want to assign an internal BC as Wall, you can assign it as a Thin Wall. For the thermal field, it may be used to model:

1. Contact resistance, which is brought about by poor contact at a material interface (solid-solid, or fluid-solid interface)
2. Lead, which is a thin layer of metal with much higher values of conductivity than the materials on either side of it. For the scalar field, it is used to model only the contact resistance.
3. Thermal resistance, where two conductive metals are connected to each other by a layer of material with low thermal conductivity (e.g. a ceramic)

In addition to the above applications, the Thin Wall boundary condition also provides for high fidelity contact resistance calculation through the thermal gap model. The thermal gap model is used to calculate the heat transfer between solid bodies separated by a gap. The thermal gap model accounts for a) the gaseous conduction within the gap, b) the effects of direct-contact conduction (solid-solid conduction), and c) radiation heat transfer across the gap. Applications of the thin wall boundary condition are often found in the semi-conductor and the electronics / thermal packaging industries.

Activating Thin Wall's in CFD-ACE-GUI

1. Go to the BC tab and change the Boundary Condition Setting Mode to Thin-Wall.



Figure 1. Change the BC Setting Mode to Thin Wall
2. Pick a domain-domain interface boundary condition from either the Viewer Window or the BC Explorer.
3. Press the Set button to convert the Interface to a Thin-Wall boundary condition. (Press the Unset button to revert the boundary condition back to an Interface.)

Figure 2. Use 'Set' and 'Unset' to create the Thin Walls
4. Once you have hit the 'Set' button, you may enter the thickness of the material and the thermal conductivity to use for the material.

Thermal Resistance Modeling in CFD-ACE+

This user tip will demonstrate how thin walls can be used to study the effect of a thermally resistive material, which connects two thermally conductive metal solids, without having to grid the thermal resistance layer. Two Cases are studied, one with the thermal resistance layer as a separate volume and one with the Thin Wall BC, to compare the resulting temperature field.

Case 1: Heat Transfer Between Two Solids Using a Thermal Resistance Layer

Case 2: Heat Transfer Between Two Solids Using Thin Wall BC's to Model the Thermal Resistance Layer

The Thin Wall BC has two inputs, the thickness of the layer and its thermal conductivity. This is used to calculate the effective thermal conductivity with the assumption that the heat flux across the thin wall is continuous. To read more on how the effective thermal conductivity is calculated, please refer to the theory section of the Thin Walls Chapter.

For this case, the thickness of the thermal resistance layer is 0.00254m (0.1") The thermal conductivity is set to the value of the thermal resistance layer, which is 10W/m-K is this case. Metal Solid 1 and Solid2 are given the properties of 1020 Stainless Steel.

Results

Case 1 and Case 2 are solved using the Heat Transfer module in CFD-ACE+. Figure 6 (a) and (b) shows a plot of the temperature fields for Case 1 and Case 2 respectively. In Figure 6 (b), you can see that there is a temperature jump across the Thin Wall. Figure 7 (a) and (b) shows the temperature field in Metal Solid 2 for Case 1 and Case 2. It can be seen that the temperature values match well between the two cases, which indicates that Thin Wall BC's can be used effectively to model thermal resistance layers without the need to grid the layer.

Figure 6. (a) Temperature profile using the Thin Wall option (b) Temperature profile using the thermal resistance layer	Figure 7. (a) Temperature profile using the Thin Wall option (b) Temperature profile using the thermal resistance layer

If you would like to have the files used in this tip, please click here to download them. Once you download the files, you can look in the Heat Transfer Summary located in the output file and verify that the heat flux is the same for both Case 1 and Case 2, 1.7419E+04W/m, across the "Thermal Resistance" (whether it be a Thin Wall or separate volume).

If you have any questions about this feature or would like us to discuss some other topic in the future, please let us know.

Amit Saxena
Applications Engineer
ESI CFD Customer Support