All of the thermal properties discussed in this section—gap conductance, gap radiation, and gap heat generation—can be included in a contact property definition for both surface-based contact and element-based contact. All three types of thermal properties can be included in the same contact property definition.

The thermal contact property model between two surfaces can also be completely defined through user subroutine UINTER or VUINTER (see “User-defined interfacial constitutive behavior,” Section 22.1.5; “UINTER,” Section 25.2.28; and “VUINTER,” Section 25.3.3).

*SURFACE INTERACTION, NAME=name
*GAP CONDUCTANCE
*GAP RADIATION
*GAP HEAT GENERATION

*INTERFACE or *GAP, ELSET=name
*GAP CONDUCTANCE
*GAP RADIATION
*GAP HEAT GENERATION

*SURFACE INTERACTION, USER

Gap conductance and gap radiation are enforced in ABAQUS/Explicit with an explicit algorithm analogous to the penalty method for mechanical contact interaction. Therefore, gap conductance and gap radiation can influence the stability condition; although in a fully coupled temperature-displacement analysis the mechanical portion of the system usually governs the overall stability condition (see “Fully coupled thermal-stress analysis,” Section 6.5.4). Extremely large values of gap conductance or gap radiation can result in a decrease in the stable time increment, which will be accounted for by the automatic time incrementation algorithm in ABAQUS/Explicit.

Gap heat generation is applied within whichever algorithm (kinematic or penalty) is used to enforce the mechanical contact constraints. Gap heat generation has no effect on the stable time increment.

Thermal contact fluxes may be inaccurate during increments in which mesh adaptivity occurs if the mechanical contact constraints are enforced kinematically, because mesh adjustments occur in ABAQUS/Explicit between the determination of the mechanical contact state for kinematic contact and the calculation of thermal contact fluxes. For example, mesh adjustments for adaptivity may cause discontinuity in the contact pressure: for pressure-dependent gap conductance, the gap conduction coefficient will be set based on the pressure determined by the kinematic contact algorithm prior to the mesh adjustment, even though the thermal contact flux is applied after the mesh adjustment. The significance of this inaccuracy on the solution will depend on the size and frequency of the mesh adjustments and the degree of variation in the conduction coefficient. This inaccuracy can be avoided by enforcing the mechanical contact constraints with the penalty method.

The conductive heat transfer between the contact surfaces is assumed to be defined by

You can define directly or, in ABAQUS/Standard, in user subroutine GAPCON (“GAPCON,” Section 25.2.9).

Defining the gap conductance directly

When defining directly, define it as

where

is the average of the surface temperatures at and ,

is the clearance between and ,

is the contact pressure transmitted across the interface between and ,

is the average of any predefined field variables at and , and

is the average of the magnitudes of the mass flow rates per unit area of the contact surfaces at and (this variable is not considered in an ABAQUS/Explicit analysis).

Defining gap conductance as a function of clearance

You can create a table of data defining the dependence of on the variables listed above. The default in ABAQUS is to make a function of the clearance . When is a function of gap clearance, , the tabular data must start at zero clearance (closed gap) and define as increases. At least two pairs of points must be given to define as a function of the clearance. The value of drops to zero immediately after the last data point, so there is no heat conductance when the clearance is greater than the value corresponding to the last data point. If gap conductance is not also defined as a function of contact pressure, will remain constant at the zero clearance value for all pressures, as shown in Figure 22.2.1–1(a).

Input File Usage:

*GAP CONDUCTANCE , ,

ABAQUS/CAE Usage:

Interaction module: contact property editor: ThermalThermal Conductance: Definition: Tabular, Use only clearance-dependency data

Figure 22.2.1–1 Examples of input data to define the gap conductance as a function of clearance or contact pressure.

Defining gap conductance as a function of contact pressure

You can define as a function of the contact pressure, . When is a function of contact pressure at the interface, the tabular data must start at zero contact pressure (or, in the case of contact that can support a tensile force, the data point with the most negative pressure) and define as increases. The value of remains constant for contact pressures outside of the interval defined by the data points. If gap conductance is not also defined as a function of clearance, is zero for all positive values of clearance and discontinuous at zero clearance, as shown in Figure 22.2.1–1(b).

Input File Usage:

*GAP CONDUCTANCE, PRESSURE , ,

ABAQUS/CAE Usage:

Interaction module: contact property editor: ThermalThermal Conductance: Definition: Tabular, Use only pressure-dependency data

Gap conductance as a function of both clearance and contact pressure

can depend on both clearance and pressure. A discontinuity in is allowed at and . At the state of zero clearance and zero pressure the value of corresponding to the zero pressure data point is used, as shown in Figure 22.2.1–2(a).

Figure 22.2.1–2 Examples of input data to define the gap conductance as a function of both clearance and contact pressure.

In the case of no-separation contact, once contact occurs the conductance is always evaluated based on the portion of the curve that defines the pressure dependence. The gap conductance, , remains constant for contact pressures outside of the interval defined by the data points, as shown in Figure 22.2.1–2(b). The pressure dependence of is extended into the negative pressure region even if no data points with negative pressure are included.

Input File Usage:	*GAP CONDUCTANCE , , *GAP CONDUCTANCE, PRESSURE , ,
	For example, the following input defines for the zero clearance data point and for the zero pressure data point: *SURFACE INTERACTION, NAME=name *GAP CONDUCTANCE 20.0, 0.0 10.0, 0.1 … *GAP CONDUCTANCE, PRESSURE 50.0, 0.0 65.0, 100.0 70.0, 250.0 …

ABAQUS/CAE Usage:

Interaction module: contact property editor: ThermalThermal Conductance: Definition: Tabular, Use both clearance- and pressure-dependency data

Using gap conductance to model convective heat transfer from a surface in ABAQUS/Standard

Generally, mass flow rates are defined in ABAQUS/Standard (see “Forced convection through the mesh” in “Uncoupled heat transfer analysis,” Section 6.5.2) only for nodes associated with forced convection elements. However, they can be defined for any node in a model. By using the dependence of on the average mass flow rate at the interface, it is possible for the contact property definition to simulate convective heat transfer to the boundary layer between a solid and a moving fluid. If mass flow rates are given only for nodes on one side of the interface, which is typically the case when simulating convective heat transfer, the average mass flow rate used to define will be half the magnitude specified.

Defining gap conductance to be a function of predefined field variables

The gap conductance can be dependent on any number of predefined field variables, . To make the gap conductance depend on field variables, at least two data points are required for each field variable value.

Input File Usage:

*GAP CONDUCTANCE, DEPENDENCIES=n , , ,

ABAQUS/CAE Usage:

Interaction module: contact property editor: ThermalThermal Conductance: Definition: Tabular, Clearance Dependency and/or Pressure Dependency, Number of field variables: n

*GAP CONDUCTANCE, USER

ABAQUS assumes that radiative heat transfer between closely spaced contact surfaces occurs in the direction of the normal between the surfaces. In models using surface-based contact this normal corresponds to the master surface normal (see “Defining contact pairs in ABAQUS/Standard,” Section 21.2.1; “Defining contact pairs in ABAQUS/Explicit,” Section 21.4.1; and “Surfaces: overview,” Section 2.3.1). In models using the contact elements available in ABAQUS/Standard the element's connectivity defines the normal direction.

The gap radiation functionality in ABAQUS is intended for modeling radiation between surfaces across a narrow gap. A more general capability for modeling radiation is available in ABAQUS/Standard (see “Cavity radiation,” Section 24.1.1).

Radiative heat transfer is defined as a function of clearance between the surfaces through the effective viewfactor. ABAQUS maintains the radiative heat flux even when the surfaces are in contact. This causes only a minor inaccuracy since normally the heat flux from conduction is much larger than the radiative heat flux.

ABAQUS defines the heat flow per unit surface area between corresponding points as

The viewfactor must be defined as a function of the clearance, , and should have a value between 0.0 and 1.0. At least two pairs of points are required to define the viewfactor, and the tabular data must start at zero clearance (closed gap) and define the viewfactor as the clearance increases. The value of drops to zero immediately after the last data point, so there is no radiative heat transfer when the clearance is greater than the value corresponding to the last data point (see Figure 22.2.1–3).

*GAP RADIATION 
,  
,  
,  
…

*PHYSICAL CONSTANTS, ABSOLUTE ZERO=

*PHYSICAL CONSTANTS, STEFAN BOLTZMANN=

In fully coupled temperature-displacement or coupled thermal-electrical simulations, ABAQUS allows for heat generation due to the dissipation of energy created by the mechanical or electrical interaction of contacting surfaces. The source of the heat in a fully coupled temperature-displacement analysis is frictional sliding; the source in a coupled thermal-electrical simulation is the flow of electrical current across the interface surfaces. By default, ABAQUS releases all of the dissipated energy as heat between the surfaces and distributes it equally between the two interacting surfaces.

You can specify the fraction of dissipated energy converted into heat, (default is 1.0), and the weighting factor, (default is 0.5), for distribution of the heat between the interacting surfaces. often includes a factor to convert mechanical energy into thermal energy.

= 1.0 indicates that all of the generated heat flows into the first (slave) surface of the contact pair. = 0.0 indicates that all of the generated heat flows into the opposite (master) surface. Unless valid experimental data suggest otherwise, it is best to assume the default value of = 0.5 because this value evenly distributes the generated heat between the surfaces.

If user subroutine UINTER or VUINTER is used to define the interfacial constitutive behavior, all gap heat generation effects will be turned off; you must supply an additional heat flux in the user subroutine to model these effects.

*GAP HEAT GENERATION 
, 

Heat generated due to frictional sliding

In coupled thermal-mechanical surface interactions, the rate of frictional energy dissipation is given by

where is the frictional stress and is the slip rate. The amount of this energy released as heat on each surface is assumed to be

where and are defined above. The heat flux into the slave surface is , and the heat into the master surface is .

Heat generated due to flow of electrical current in ABAQUS/Standard

In a coupled thermal-electrical analysis (see “Coupled thermal-electrical analysis,” Section 6.6.2), the rate of electrical energy dissipated by electric current flowing across the interface is

where is the electrical current density and and are the electrical potentials on the two surfaces. The amount of this energy released as heat on each of the interface surfaces is assumed to be

where and are defined in the same way as for frictional dissipation. Again, the heat flux into the slave surface is , and the heat into the master surface is .

ABAQUS provides many output variables related to the thermal interaction of surfaces. In ABAQUS/Standard the values of these variables are always given at the nodes of the slave surface. In ABAQUS/Explicit these variables can be output for master and slave surfaces, although they are not available for analytical surfaces. The variables are available only for simulations that use surface-based contact definitions. They can be requested as surface output to the data, results, or output database files (see “Surface output from ABAQUS/Standard” in “Output to the data and results files,” Section 4.1.2, and “Surface output” in “Output to the output database,” Section 4.1.3, for details).

ABAQUS/Standard provides the heat flux per unit area across the thermal gap elements as output. Request element output of the variable identifier HFL to the data, results, or output database file (see “Element output” in “Output to the data and results files,” Section 4.1.2, and “Element output” in “Output to the output database,” Section 4.1.3, for details). The only nonzero component will be HFL1: there is no heat flux tangential to the interface defined by the gap element. A positive value of HFL1 indicates heat flowing in the direction of the normal to the master surface side of the element (see “Gap contact elements,” Section 23.2.1, for the definition of this normal for DGAP elements).

Contours of the heat flux across the thermal contact elements can be plotted using ABAQUS/CAE.

Use node-based surfaces with caution when there is thermal interaction between the two bodies. ABAQUS calculates the thermal interaction between the bodies in terms of nodal heat fluxes that must consider the actual contact surface area associated with each node. In ABAQUS/Standard this area must be specified precisely for each node in the node-based surface to calculate the correct heat fluxes; in ABAQUS/Explicit a unit area is assigned to each node of a node-based surface (see “Defining node-based surfaces,” Section 2.3.3).

When the surfaces involved in a thermal interaction are defined on shell elements that have multiple temperature degrees of freedom at each node, the choice of the temperature degree of freedom at a given node for the thermal interaction depends on how the surface is defined. For an element-based surface the temperature degree of freedom closest to the surface is chosen; i.e., the first temperature degree of freedom at the node for the bottom surface and the last temperature degree of freedom at the node for the top surface. For a node-based surface the first temperature degree of freedom at the node is always chosen for a thermal interaction.