Thermal Mass

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MASS71 Element Description

MASS71 is a point element having one degree of freedom, temperature, at the node. The element may be used in a transient thermal analysis to represent a body having thermal capacitance capability but negligible internal thermal resistance, that is, no significant temperature gradients within the body. The element also has a temperature-dependent heat generation capability. The lumped thermal mass element is applicable to a 1-D, 2-D, or 3-D steady-state or transient thermal analysis. See MASS71 in the Mechanical APDL Theory Reference for more details about this element.

In a steady-state solution the element acts only as a temperature-dependent heat source or sink. Other elements having special thermal applications are the COMBIN14 and COMBIN40 elements. These elements, which are normally used in structural models, may be used for thermally analogous situations.

If the model containing the thermal mass element is also to be analyzed structurally, the thermal element should be replaced by an equivalent structural element (such as MASS21)

Figure 71.1:  MASS71 Geometry

MASS71 Geometry

MASS71 Input Data

The lumped thermal mass element is defined by one node (as shown in Figure 71.1: MASS71 Geometry) and a thermal capacitance (Heat/Degree). When used with axisymmetric elements, the thermal capacitance should be input on a full 360° basis. The thermal capacitance (CON1) may be input as a real constant or calculated (KEYOPT(3)) from the real constant volume (CON1) and either the DENS and C or ENTH material properties. KEYOPT(3) determines whether CON1 is interpreted as volume or thermal capacitance.

The heat generation is applied directly as a nodal load and is not first multiplied by the volume. Thus, if KEYOPT(3) = 0 (that is, when using the specific heat matrix), the heat generation must be adjusted to account for the volume. For an axisymmetric analysis the heat generation rate should be input on a full 360° basis. A temperature-dependent heat generation rate of the following polynomial form may be input:

where T is the absolute temperature from the previous substep. The constants, A1 through A6, should be entered as real constants. If any of the constants A2 through A6 are nonzero, KEYOPT(4) must be set to 1. Also, if temperatures are not absolute, the offset conversion [TOFFST] must be specified.

Alternately, the heat generation expression may be defined as a temperature-dependent material property (QRATE) with the MP commands. QRATE can be input as numerical values or as tabular inputs evaluated as a function of temperature, time, and location.

A summary of the element input is given in "MASS71 Input Summary". A general description of element input is given in Element Input. See Harmonic Axisymmetric Elements for more details.

MASS71 Input Summary



Degrees of Freedom


Real Constants
CON1, A1, A2, A3, A4, A5
See Table 71.1: MASS71 Real Constants for a description of the real constants
Material Properties

MP command: QRATE, DENS, C, ENTH if KEYOPT(3) = 0 or QRATE if KEYOPT(3) = 1

Surface Loads


Body Loads

None (heat generation may be defined as a function of temperature by using real constants A1, A2, ... or by the QRATE material property definition.)

Special Features
Birth and death
Nonlinearity if heat generation is defined as a function of temperature

Interpretation of real constant CON1:

0 -- 

Interpret CON1 as volume (with either DENS and C or ENTH supplied as material properties)

1 -- 

Interpret CON1 as thermal capacitance (DENS*C*volume)


Temperature dependent heat generation:

0 -- 

No temperature-dependent heat generation (required if all real constants A2-A6 are zero)

1 -- 

Include temperature-dependent heat generation (required if any real constants A2-A6 are nonzero)

Table 71.1:  MASS71 Real Constants

1CON1Volume or thermal capacitance (see KEYOPT(3))
2A1Constant for temperature function
3A2Constant for temperature function
4A3Constant for temperature function
5A4Constant for temperature function
6A5Constant for temperature function
7A6Constant for temperature function

MASS71 Output Data

The solution output associated with the element is in two forms:

The heat generation is in units of Heat/Time and is positive into the node. 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 71.2:  MASS71 Element Output Definitions

ELElement NumberYY
XC, YC, ZCLocation where results are reportedY 1
TEMPElement (node) temperatureYY
HEATHeat generation rate into nodeYY

  1. Available only at centroid as a *GET item.

Table 71.3: MASS71 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 71.3: MASS71 Item and Sequence Numbers:


output quantity as defined in the Table 71.2: MASS71 Element Output Definitions


predetermined Item label for ETABLE command


sequence number for single-valued or constant element data

Table 71.3:  MASS71 Item and Sequence Numbers

Output Quantity Name ETABLE and ESOL Command Input

MASS71 Assumptions and Restrictions

  • When using the element with a temperature-dependent heat generation in a steady-state solution, an iterative solution is required.

  • The heat generation is calculated at the uniform temperature for the first substep.

MASS71 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.

ANSYS Mechanical Premium 

  • Birth and death is not available.

Release 18.2 - © ANSYS, Inc. All rights reserved.