LINK34
Convection
Link
LINK34 Element Description
LINK34 is a uniaxial element with the ability to convect heat between its nodes. The element has a single degree of freedom, temperature, at each node point. The convection element is applicable to a 2D (plane or axisymmetric) or 3D, steady-state or transient thermal analysis.
If the model containing the convection element is also to be analyzed structurally, the convection element should be replaced by an equivalent (or null) structural element. The element may have a nonlinear film coefficient which may also be a function of temperature or time. See LINK34 in the Mechanical APDL Theory Reference for more details about this element.
LINK34 Input Data
The geometry and node locations for this convection element are shown in Figure 34.1: LINK34 Geometry. The element is defined by two nodes, a convection surface area, two empirical terms, and a film coefficient. In an axisymmetric analysis the convection area must be expressed on a full 360° basis. The empirical terms n (input as EN) and CC determine the form of the convection equation in conjunction with KEYOPT(3).
The convection function is defined as follows:
q = hf*A*E*(T(I) - T(J))
where:
| q = heat flow rate (Heat/Time) |
| hf = film coefficient (Heat/Length2*Time*Deg) |
| A = area (Length2) |
| T = temperature (this substep) (Deg) |
E = empirical convection term = 
|
| Tp = temperature (previous substep) (Deg) |
| n = empirical coefficient (EN) |
| CC = input constant |
Note: E = F if n and CC = 0.0. F = 1.0 unless KEYOPT(3) = 2. If KEYOPT(3) = 3, E equals the larger of ITp(I) - Tp(J)In or CC/hf.
A special option obtained with KEYOPT(3) = 2 allows an alternate input for hf and an input scale factor (F). This option uses the VAL1 field of the SFE command with KVAL = 0 for the hf value and KVAL = 2 for the F value. If the hf value is zero (or blank), the HF material property is used for hf. If the F value is zero (or blank) or negative, a value of 1.0 is assumed for F. Note, the F value input in this field will ramp within a load step if KBC = 0. An SFE command must be included (even if the values are left blank) for all LINK34 elements having KEYOPT(3) = 2.
Heat generation rates may be input as element body loads at the nodes. The node J heat generation rate HG(J) defaults to the node I heat generation rate HG(I).
A summary of the element input is given in "LINK34 Input Summary". A general description of element input is given in Element Input.
LINK34 Input Summary
- Nodes
I, J
- Degrees of Freedom
TEMP
- Real Constants
AREA - Convection surface area EN - Empirical coefficient CC - Input constant - Material Properties
MP command: HF
- Surface Loads
- Convections --
1 - Alternate input of HF and F if KEYOPT(3) = 2 (see text above)
- Body Loads
- Heat Generation --
HG(I), HG(J)
- Special Features
Birth and death Nonlinearity if real constant EN is not equal to zero or if KEYOPT(3) = 3 - KEYOPT(2)
Evaluation of film coefficient:
- 0 --
Use average of T(I) and T(J) to evaluate HF
- 1 --
Use greater of T(I) or T(J) to evaluate HF
- 2 --
Use lesser of T(I) or T(J) to evaluate HF
- 3 --
Use differential |T(I) - T(J)| to evaluate HF
- KEYOPT(3)
Film coefficient and scale factor:
- 0 --
Standard element input and empirical term
- 2 --
Use alternate input for HF and F (input with SFE command)
- 3 --
Use discontinuous empirical term
LINK34 Output Data
The solution output associated with the element is in two forms:
Nodal temperatures included in the overall nodal solution
Additional element output as shown in Table 34.1: LINK34 Element Output Definitions
The heat flow rate is in units of Heat/Time and is positive from node I to node J. In an axisymmetric analysis, the heat flow is on a full 360° basis. 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 letter or number refers to a table footnote that describes when the item is conditionally available, and “-” indicates that the item is not available.
Table 34.1: LINK34 Element Output Definitions
| Name | Definition | O | R |
|---|---|---|---|
| EL | Element Number | Y | Y |
| NODES | Nodes - I, J | Y | Y |
| XC, YC | Location where results are reported | Y | 1 |
| H | Film coefficient (includes empirical term) | Y | Y |
| AREA | Input area | Y | Y |
| TEMP | Temperature at node I and node J | Y | Y |
| HEAT RATE | Heat flow rate from node I to node J | Y | Y |
Available only at centroid as a *GET item.
Table 34.2: LINK34 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 34.2: LINK34 Item and Sequence Numbers:
- Name
output quantity as defined in the Table 34.1: LINK34 Element Output Definitions
- Item
predetermined Item label for ETABLE command
- E
sequence number for single-valued or constant element data
- I,J
sequence number for data at nodes I and J
LINK34 Assumptions and Restrictions
If Tp(I) = Tp(J) and n are nonzero, the first term of E is defined to be zero.
Since all unspecified nodal temperatures are initially set to the uniform temperature, a nonzero value of n may result in no heat flowing through the element in the first substep of a thermal solution.
Nodes may or may not be coincident.
The element is nonlinear if n is nonzero or KEYOPT(3) = 3. However, the solver always assumes the element is nonlinear and, therefore, always performs an iterative solution. (Only 2 iterations are performed if the element is linear.)
LINK34 Product Restrictions
When used in the product(s) listed below, the stated product-specific restrictions apply to this element in addition to the general assumptions and restrictions given in the previous section.
Ansys Mechanical Pro —
Birth and death is not available.
Nonlinearity is not available. Real constant EN cannot equal 0 and KEYOPT(3) cannot equal 2 nor 3.