Working with multi-layer shell elements

In thermal meshing, you can use thin shell (2D) elements with a multi-layer physical property table to model any broad, thin geometry (such as panels, layers, etc.) in which you want to obtain a detailed picture of the conduction through the geometry without using thin solid (3D) elements. Multi-layer shell elements present a smaller conductance matrix than solid elements.

You can create two different types of multi-layer shell elements:

• Multi-Layer Shell UniformWith uniform multi-layer shells, each layer has the same material and the same thickness. They are not typically used to model physical layers. Instead, they are more commonly used to model detailed through-plane conduction. For each element in the mesh, each layer provides a calculation point for the thermal solver, which allows it to calculate a different temperature for each layer.

• Multi-Layer Shell Non-UniformWith non-uniform multi-layer shell elements, each layer can have a different material and a different thickness. For each element in the mesh, each layer provides a calculation point for the thermal solver, which allows it to calculate a different temperature for each layer.

Creating multi-layer shell elements

Depending on the type of multi-layer shell element you are creating, you first define the multi-layer properties by creating either a Multi-Layer Shell Uniform or a Multi-Layer Shell Non-Uniform physical property table. See Multi-Layer Shell Uniform dialog box (Thermal/SST/ESC) and Multi-Layer Shell Non-Uniform dialog box (Thermal/SST/ESC).

Note:

For the Multi-Layer Shell Non-Uniform physical property table, you must create each individual layer using the Layer type modeling object. Once you have those defined, you can use the Stack Layers option on the Multi-Layer Shell Non-Uniform physical property table dialog box to specify which layers to stack to form the shell element.

With the physical property tables created, you then assign those physical property tables by creating a 2D shell element mesh collector where the collector Type is either Multi-Layer Shell Uniform or Multi-Layer Shell Non-Uniform. See 2D Mesh Collector dialog box (Thermal/Flow/SST/ESC).

Applications of multi-layer shell elements

The following table details different applications of multi-layer shell elements

Application	Description
Thermal Boundary Conditions	Thermal loads and temperature constraints can be applied to any or all layers.
Radiation	Since elements can only radiate from their top or bottom sides, for radiation only the top and bottom layers of a multi-layer shell are used to calculate radiative fluxes. Intermediate layers can however model conduction between the top and bottom sides.
Thermal Couplings	When connecting a multi-layer mesh to another mesh with a thermal coupling, the coupling is established either from the top or the bottom layer, depending on which layer is closer to the connected mesh.
Convection	Flow surfaces convect from the appropriate layer of multi-layer shells.
Post Processing	You can view layer results under the nodes labeled Ply 1, Ply 2, and so on, depending on the number of layers, going from bottom (Ply 1) to top (Ply N) where N is the number of layers.

Understanding the Multi-Layer Shell-Uniform type

For isotropic materials, the thermal solver uses the thermal conductivity material property to calculate a conductive heat transfer coefficient between each pair of adjacent layers. For orthotropic materials, the thermal solver uses the Z-axis thermal conductivity material property to create the conductive thermal couplings.

In-plane conduction is calculated for the middle layer only. For this reason, the solver requires an odd number of layers to ensure that there is a unique middle layer. In-plane conduction is most accurately modeled with few layers (one layer being the most accurate). When calculating in-plane conduction for a multilayer shell, the solver assigns the whole thickness of the shell to the middle layer.

By calculating in-plane conduction in one layer, the numerical model is greatly simplified. This also implies that, when using the Multi-Layer Shell Uniform type, the through-plane heat flow should predominate over the in-plane heat flow.

The accuracy of through-plane conduction modeling is enhanced by dividing the shell into a greater number of layers. However, more layers result in additional computational time and reduced accuracy of in-plane conduction modeling.

For materials with a constant thermal conductivity, all conductances are equal between the layers. For materials with a constant specific heat, each layer has the same capacitance. For nonlinear materials, the layers may have different values.

Material properties within the thin geometry that are highly temperature-dependant can be accurately represented using the multi-layer physical property, provided that heat flow through the plane predominates over the in-plane heat flow.

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