Thermal coupling types

The type of thermal coupling you define is determined by the geometry of the source and target regions you specify, as well as the type of heat transfer such as conduction, convection, and radiation. This article explains the different types of thermal coupling and provides guidance on where each type should be used.

Modeling conductive thermal coupling

Conductive thermal couplings model conductance between unconnected objects, without explicitly modeling the heat transfer medium between them.

The conduction heat transfer is defined by

where G is the thermal conductance between two bodies at the contact interface.


Simulation Object	Type	Description	When to use
Thermal Coupling	Thermal Coupling	Models conductance between the surfaces or edges using the specified magnitude without explicitly modeling the heat transfer medium between them.	Use when the thermal conductance coefficient, G, is known.
	Perfect Contact	Creates an infinite conductance between each of the primary elements and the nearest secondary element.	Use only when the meshes are nearly aligned, or where the in-plane temperature gradients are insignificant. It may lead to convergence issues; however, you can substitute it with a thermal coupling that has a high conductance value relative to solid conductances. Generally, a value of 10,000 W/°C is considered effective.
	Additional Conductance	Creates explicitly conductances between selected elements. The thermal solver connects each primary element to all secondary elements without performing proximity or overlapping checks. It also applies the conductance values to each element, without considering area or any other factors.	Use in very simplified models where the geometry may not accurately represent the actual physical setup. For example, it is used in some system-level spacecraft models to link subsystems.
Interface Resistance Note: Not supported with the finite element method.	Surface Resistance	Applies the interface resistance to a free or shared face of a polygon body meshed with 3D elements.	Use for inserting additional conductances in series with existing conductances for two adjacent polygon bodies that are meshed with 3D elements that share nodes. The conductances of the coupling are added in series to the conductances of the 3D elements on either side of the shared surface.
	Edge Interface	Applies the interface resistance to a free or shared edge of a polygon body meshed with 2D elements.	Use for inserting additional conductances in series with existing conductances for two polygon faces sharing an edge, which are both meshed with 2D elements that share nodes along that edge. The conductance of the coupling is added in series to the conductances of the 2D elements on either side of the shared polygon edge. Use when you wish to introduce an additional conductance or resistance in series on a polygon face or an edge that has merged nodes. The interface conductance is placed in series with the conductance spanning the 2D elements and the polygon edge itself.
	Interface Between 2 Sets	Applies the interface resistance between faces, or edges, in any combination.	This type is equivalent to Thermal Coupling when the coupling method is element subdivision.
Thermal Coupling - Advanced	1 Way	Creates a chain of one-way conductances from the primary to the secondary elements.	Use this type to model advection/fluid flow.
Thermal Coupling - Advanced	User Function	Creates a linear conductance proportional to the overlap area of the primary region elements and the secondary region elements. You can specify the proportionality factor using subroutine USERF.	Use when you want to write a complex correlation in your USERF subroutine for the proportionality factor VDEP. VDEP represents the heat transfer coefficient. For example, your VDEP variable can be function of time or temperature of other elements.
Surface-to-Surface Contact	Automatic PairingManual	Models thermal conductances between two potentially connected 3D structural surfaces when you activate the thermal coupling between them. The conductance may be infinite, specified in terms of an effective conduction heat transfer coefficient, or in terms of the two different heat transfer coefficients: one when the two surfaces are in contact and the other one when they have a gap between them.	Use in a coupled thermal-structural analysis of a 3D model to apply a thermal coupling on a structural contact without the need to create a separate simulation object. In thermal only or couple thermal-flow models, they operate similarly to standard thermal couplings, with the distinction of having an automatic geometry-based selection.
Surface-to-Surface Gluing	Automatic PairingManual	Models thermal conductances between two firmly connected 3D structural surfaces when you activate the thermal coupling between them. The conductance may be infinite or specified in terms of a conduction heat transfer coefficient.	Use this type in a coupled thermal-structural analysis of a 3D model to apply a thermal coupling between two structural surfaces glued together without the need to create a separate simulation object.
Surface-to-Surface Contact/Gluing	Automatic PairingManual	Models thermal conductances between two structurally connected 3D surfaces when you activate the thermal coupling between them.	Use this type in a coupled thermal-structural analysis of a 3D model to apply a thermal coupling between two structural surfaces that can be defined as contact or glue and can be switched between them without the need to create a separate simulation object.
Edge-to-Edge Contact	Automatic PairingManual	Models thermal conductances between two potentially connected structural edges meshed with 2D axisymmetric, plane stress, or plane strain elements when you activate the thermal coupling between them. The conductance may be infinite, specified in terms of an effective conduction heat transfer coefficient, or in terms of the two different heat transfer coefficients: one when the two surfaces are in contact and the other one when they have a gap between them.	Use this type in a coupled thermal-structural analysis of a 2D-3D hybrid model to apply a thermal coupling on a structural contact without the need to create a separate simulation object
Edge-to-Edge Gluing	Automatic PairingManual	Models thermal conductances between two firmly connected structural edges meshed with 2D axisymmetric, plane stress, or plane strain elements when you activate the thermal coupling between them. The conductance may be infinite or specified in terms of a conduction heat transfer coefficient.	Use this type in a coupled thermal-structural analysis of a 2D-3D hybrid model to apply a thermal coupling between two structural edges glued together without the need to create a separate simulation object.
Edge-to-Edge Contact/Gluing	Automatic PairingManual	Models thermal conductances between two structurally connected edges meshed with 2D axisymmetric, plane stress, or plane strain elements when you activate the thermal coupling between them without the need to create a separate thermal coupling simulation object.	Use this type in a coupled thermal-structural analysis of a 2D-3D hybrid model to apply a thermal coupling between two structural surfaces that can be defined as contact or glue and can be switched between them without the need to create a separate simulation object.

Specifying thermal conductance magnitude

Use the following thermal conductances:

Total Conductance, G, in power per temperature units, when the thermal conductance is known.
Total Resistance, 1/G, to specify the inverse of a thermal conductance value in temperature per power units.
Heat Transfer Coefficient, h=G/A, to specify HTC value which is equal to thermal conductance per unit area.
Edge Contact to define the thermal conductance between the edge primary elements and the secondary elements. You must specify conductance per length coefficient.
Conductive Gap to define conductance in a gap between the primary elements and secondary elements. You must specify the gap thermal conductivity of the interstitial material which equal to the thermal conductance multiplied by the distance to the secondary element along the primary element's surface normal and divided by the interface area.

Modeling convective thermal coupling

Convective thermal couplings model the convective heat transfer between a solid surface and a fluid that is in contact with it.

The convective heat transfer is defined as

where G is the thermal conductance between surface and 1D duct elements.


Simulation object	Type	Description	When to use
Thermal Coupling —Convection	Convection Coupling	Models convective couplings between the fluid ducts and the solid convecting region, using the specified heat transfer coefficient.	Use when the heat transfer coefficient is known.
	Forced Convection Coupling	Models forced convection heat transfer computed using standard correlations for forced convection in a 1D flow system with a developing boundary layer, or a fully developed boundary layer.	Use when forced convection occurs between a fluid in motion and a solid surface in different geometrical configurations.
	Free Convection Coupling	Models free convection heat transfer computed using standard correlations for free convection from plates, spheres, cylinders, and channels.	Use when free or natural convection occurs between convective surfaces and a moving fluid, in which the fluid motion is not induced by an external force.
	Across Gap Convection Coupling	Models heat transfer between two parallel surfaces where a convecting fluid is the medium of heat exchange using correlations for modeling convection across a gap.	Use when free convection occurs between closely spaced parallel walls.
	Duct Node Convection Coupling	Models convection heat transfer between fluid duct nodes and the duct wall using the specified heat transfer coefficient.	Use when multiple 1D fluid ducts connect at a single point (often referred to as a mixing node) and when establishing convection coupling between this mixing node and the wall is required.

Modeling radiative thermal coupling

Radiative thermal couplings model heat transfer between two bodies through the exchange of electromagnetic waves.

The radiation heat transfer is defined by

where G is the radiation conductance between two surfaces.


Simulation object	Type	Description	When to use
Thermal Coupling – Radiation	Gap Radiation	Models radiative heat paths between close parallel surfaces with known emissivity.	Use this multi-layer insulating panels, honeycomb panels, or any pair of broad thin surfaces separated by a narrow gap, across which radiative conductances are significant.
Thermal Coupling – Radiation	Object to Object Radiation	Models radiative heat paths between objects with known emissivity and known view factors to each other. It uses average temperature of the groups for the coupling.	Use this type to model radiation heat transfer between objects that are not close to one another and where the assumption of view factor = 1.0 is not valid.