Modeling additional heat paths

A related group of simulation object commands let you create additional heat paths between model objects in contact or proximity, but without shared nodes. These commands are collectively referred to as “Thermal Coupling families.” Since the thermal solver models conductive heat flow only when nodes are shared, these commands are needed to define conductances when the mesh is dissimilar or disconnected. This can be extremely effective in reducing model size and complexity.

Thermal coupling commands

Thermal Coupling

Creates heat paths between (usually parallel) model objects including perfect contact. It can also take into account varying thickness of a conductive material that separates two model objects. For more information, see Thermal Coupling.

Thermal Coupling — Radiation

Creates radiative conductances between two model objects. For more information, see Thermal Coupling — Radiation.

Thermal Coupling — Advanced

Creates heat paths in unusual ways: one way conductances between two model objects, or conductances defined by the expert user. For more information, see Thermal Coupling — Advanced.

Interface Resistance

Defines additional conductances in series with other conductances that exist between two objects, or between one object and a radiative or convective environment. For more information, see Interface Resistance.

Surface to Surface Contact

Defines conductances between two objects where they contact each other. The conductance may be infinite, or may be specified in terms of a conduction heat transfer coefficient and/or a radiation option. Surfaces in contact may be paired automatically or manually. For more information, see Surface-to-Surface Contact.

Thermal Coupling — Convection

Creates convective conductances between two model objects, between model objects and fluid in a duct network, or between model objects and an environmental fluid. For more information, see Thermal Coupling — Convection.

How the thermal coupling commands work

Note:

The following explanation discusses only polygon faces meshed with 2D elements. The explanation applies equally to faces of polygon bodies that are meshed with 3D elements. Analogous explanations apply to the other selection options as well.

A typical thermal coupling creates a heat path simulating contact between a smaller primary region and a larger secondary region. The elements on the primary region are called primary elements; those on the secondary region, secondary elements. Each primary element is connected to the secondary element(s) that it overlaps. This creates a heat path that emulates conduction between the surfaces in contact, but avoids the necessity of creating a lattice of elements with shared nodes. It also allows you to introduce terms to model the specific conductance more accurately or easily.

Note that primary or secondary surface designation is not related to the direction of heat flow; either surface can be hotter or cooler than the other. If you select a small surface as a primary surface, only the nearest secondary elements are coupled. This is the typical way to model heat transfer due to close proximity or contact between surfaces of differing sizes.

However, if you are modeling convective or radiative heat paths between surfaces that are further apart, selecting the larger surface as the primary surface may be more appropriate. In this case, each of the elements on the larger surface will be coupled to the nearest element on the smaller surface. For the Thermal Coupling — Convection and Thermal Coupling — Radiation types, your judgment in selecting primary and secondary elements is essential for accurate modeling. The important fact to remember is that each primary element is coupled to the nearest secondary element(s).

Primary elements are subdivided according to the coupling resolution option you select, increasing accuracy when the primary elements are larger than, or poorly aligned with, the secondary elements. Each primary sub-element is coupled to the nearest secondary element.

Avoiding false in-plane conductances with thermal couplings

Thermal couplings are most accurate when the conductances between primary and secondary elements are relatively low compared to the in-plane conductances. If the conductances modeled by the Thermal Coupling are relatively high, the Thermal Coupling can create false in-plane conductive paths via heat flows which cross and re-cross the gap bridged by the coupling.

Avoiding false in-plane conductances

This problem is avoided if the magnitude of the coupling conductances are small compared to the conductance of the primary and secondary elements. For greatest accuracy, try to make mesh size and alignment between primary and secondary elements as close as possible. For the Thermal Coupling and Thermal Coupling — Radiation commands, if the value of the coupling conductance is large compared to the in-plane conductances you should use One to One resolution option.

Excluding thermal coupling elements from the convection to environment

When you define a thermal coupling and a convection to environment boundary condition on the same selection, the solver excludes the thermal coupling elements from the convection to environment.

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Modeling additional heat paths, Simcenter 3D 2021.1 Series

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