Special considerations for thermal meshing

This article provides details on the considerations for defining a thermal mesh.

Appropriately defining the mesh size

Usually many fewer elements are needed to accurately model thermal conduction than are needed for FE-based stress analysis. A common mistake is to create meshes which are much too fine. Although this does not adversely affect accuracy, it does affect solution computational time. There are no hard and fast rules for meshing. It is best to try different mesh spacings for a particular application and choose a mesh which produces good results at reasonable solve times.

In general, meshes should be finer in areas where temperature variations are largest. Meshes must also be finer in areas of specific interest. A single component may be modeled with only a few elements in a large system model. However, a component may also be modeled with hundreds of elements for a detailed component analysis. It is easier to create a coarse mesh and refine the mesh if and where necessary, rather than to start off with fine meshes and their correspondingly longer solve times.

Conduction and boundary conditions with 3D elements

Conduction between 3D elements and 2D elements share basic rules with the application of boundary conditions. It is thus important to understand how conduction works.

The software creates 3D elements within a polygon body. Within the body, all elements are 3D and share nodes with their neighbors (barring meshing errors). Among elements of the same type, in this case 3D elements, conduction is modeled by default if the elements share nodes.

	Conduction via shared nodes
	Boundary condition or 2D element on the face of a meshed polygon body

Boundary conditions are most often applied to surfaces (2D elements) rather than volumes (3D elements). Typically, the surface has the same material properties as the volume.

• If you wish to apply a boundary condition to a face of a meshed polygon body, simply select the face. There is no need to mesh the surface unless its properties are different from the underlying volume. During the analysis, the solver will resolve the free faces of the 3D elements and create temporary elements that share nodes with the 3D elements.

• If you wish to model a coating or layer of a different material on the face of the body, you must mesh the face with 2D elements. These 2D elements must share all their nodes with the 3D elements they are coating. When 2D elements and 3D elements share nodes in this way, the calculation points for the elements can be connected. Conduction can then occur between the volume and the surface.Note: After creating 2D elements of faces of bodies meshed with 3D elements (or vice versa) always perform a model check and merge coincident nodes.

Depending on the type of 3D elements in the volume, use the appropriate type of 2D element to ensure node sharing:

• Use a triangular 2D mesh for the free faces of tetrahedral elements or the triangular free faces of wedge elements.

• Use a quadrilateral 2D mesh for the free faces of hexahedral elements or the quadrilateral free faces of wedge elements.

Conduction and boundary conditions with 2D elements

2D elements are created on and associated with a face. On a face, all elements are 2D and share two nodes with their neighbors. Heat is conducted in-plane only; temperature is assumed to be constant through the thickness of the 2D element. When the thermal solver encounters a boundary condition on an un-meshed edge of a meshed face, the solver automatically creates temporary 1D elements that share nodes with the 2D elements. Therefore there is no need to mesh the edge, unless you want to apply different geometric or material characteristics to the edge. In this case, mesh the edge with 1D elements that share nodes with the 2D elements. In either case, conduction takes place between the surface and the edge.

	Boundary condition or 1D element on the edge of a meshed face

Conduction and boundary conditions with 1D elements

1D element conduction is similar to 2D and 3D conduction. One node on the end of a 1D element must connect to its neighbor. The neighbor could be another beam element or a mass element. Heat is conducted along the length of the 1D element. However, in the case of a boundary condition at the end of a 1D element, you must create a 0D element.

	Boundary condition on 0D element at the end of a meshed 1D element

Example of improperly connected elements

The mesh show below shows several examples of elements that have been improperly connected. This mesh will not conduct heat between the elements.

• 1D element connected to the edge of a 3D solid element. The 1D element must be connected to an edge of a 2D element or to another 1D element.

• 2D element connected along an edge to a 3D solid element. All nodes on a 2D element must be connected to all nodes on the face of a solid or connected along their edge to another 2D element.

• 1D element connected at one end to a 2D element. The 1D element must be connected to an edge of a 2D element or to another 1D element.

• Mass element connected to the corner of a 2D or 3D element. Mass elements must be placed on the end of 1D elements.

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