Coupled
Thermal-Electric Line
LINK68 Element Description
LINK68 is a uniaxial element in 3-D space with the ability to conduct heat and electrical current between its nodes. Joule heat generated by the current flow is also included in the heat balance. The element has two degrees of freedom, temperature and voltage, at each node. The thermal-electrical line element may be used in a steady-state or transient thermal analysis, although no transient electrical capacitance or inductance effects are included in the element.
The element is linear but requires an iterative solution to include the Joule heating effect in the thermal solution. If no electrical effects are present, the conducting bar element (LINK33) may be used. If the model containing the thermal-electrical element is also to be analyzed structurally, the element should be replaced by an equivalent structural element. See LINK68 in the Mechanical APDL Theory Reference for more details about this element.
LINK68 Input Data
The geometry, node locations, and the coordinate system for this thermal-electrical line element are shown in Figure 68.1: LINK68 Geometry. The element is defined by two nodes, the cross-sectional area, and the material properties. In an axisymmetric analysis the area should be input on a full 360° basis. The thermal conductivity and electrical resistivity are in the element longitudinal direction. The specific heat and density may be assigned any values for steady-state solutions.
The electrical material property, RSVX, is the resistivity of the material. The resistance of the element is calculated from RSVX*length/AREA. The resistivity, like any other material property, may be input as a function of temperature. Properties not input default as described in the Material Reference.
The word VOLT should be input for the Lab variable on the D command and the voltage input for the value. The word AMPS should be input for the Lab variable on the F command and the current into the node input for the value.
Element loads are described in Nodal Loading. Element body loads may be input as heat generation rates at the nodes. The node J heat generation rate HG(J) defaults to the node I heat generation rate HG(I). This rate is in addition to the Joule heat generated by the current flow.
The current being calculated via this element can be directly coupled into a 3-D magnetostatic analysis [BIOT].
A summary of the element input is given in "LINK68 Input Summary". A general description of element input is given in Element Input.
LINK68 Input Summary
- Nodes
I, J
- Degrees of Freedom
TEMP, VOLT
- Real Constants
AREA - Cross-sectional area
- Material Properties
MP command: KXX, DENS, C, ENTH, RSVX
- Surface Loads
None
- Body Loads
- Heat Generations --
HG(I), HG(J)
- Special Features
Birth and death - KEYOPTS
None
LINK68 Output Data
The solution output associated with the element is in two forms:
Nodal temperatures and voltages included in the overall nodal solution
Additional element output as shown in Table 68.1: LINK68 Element Output Definitions
The heat flow and the current flow into the nodes may be printed with the OUTPR command. The Joule heat generated this substep is used to determine the temperature distribution calculated for the next substep. A general description of solution output is given in Solution Output. See the Basic Analysis Guide for ways to view results.
The Element Output Definitions table uses the following notation:
A colon (:) in the Name column indicates that the item can be accessed by the Component Name method (ETABLE, ESOL). The O column indicates the availability of the items in the file Jobname.OUT. The R column indicates the availability of the items in the results file.
In either the O or R columns, “Y” indicates that the item is always available, a number refers to a table footnote that describes when the item is conditionally available, and “-” indicates that the item is not available.
Table 68.1: LINK68 Element Output Definitions
| Name | Definition | O | R |
|---|---|---|---|
| EL | Element Number | Y | Y |
| NODES | Nodes - I, J | Y | Y |
| MAT | Material number | Y | Y |
| VOLU: | Volume | Y | Y |
| XC, YC, ZC | Location where results are reported | Y | 1 |
| HGEN | Heat generations HG(I), HG(J) | Y | - |
| TG | Thermal gradient at centroid | Y | Y |
| TF | Thermal flux at centroid (heat flow/cross-sectional area) | Y | Y |
| EF | Electric field (voltage gradient) | Y | Y |
| JS | Current density (voltage flux) | Y | Y |
| CUR | Current | Y | Y |
| JHEAT: | Joule heat generation per unit volume | Y | Y |
Available only at centroid as a *GET item.
Table 68.2: LINK68 Item and Sequence Numbers lists output available through the ETABLE command using the Sequence Number method. See The General Postprocessor (POST1) in the Basic Analysis Guide and The Item and Sequence Number Table in this reference for more information. The following notation is used in Table 68.2: LINK68 Item and Sequence Numbers:
- Name
output quantity as defined in the Table 68.1: LINK68 Element Output Definitions
- Item
predetermined Item label for ETABLE command
- E
sequence number for single-valued or constant element data
LINK68 Assumptions and Restrictions
Heat and current are assumed to flow only in the element longitudinal direction.
The element must not have a zero length, that is, nodes I and J may not be coincident.
A free end of the element (that is, not adjacent to another element and not subjected to a boundary constraint) is assumed to adiabatic.
No conversion is included between electrical heat units and mechanical heat units.
The resistivity may be divided by a conversion factor, such as 3.415 Btu/Hr per watt, to get Joule heat in mechanical units. Current (input and output) should also be converted for consistent units.
If a current is specified at the same node that a voltage is specified, the current is ignored.
The electrical and thermal solutions are coupled via an iterative procedure.
There is no conversion required when consistent units are used.
This element may not be compatible with other elements with the VOLT degree of freedom. To be compatible, the elements must have the same reaction force (see Element Compatibility in the Low-Frequency Electromagnetic Analysis Guide).