ANSYS Workbench Tutorial for
Transient Thermal Analysis
Introduction
Transient thermal analyses determine temperatures and other thermal quantities
that vary over time. The variation of temperature distribution over time is of interest
in many applications such as with cooling of electronic packages or a quenching
analysis for heat treatment. Also of interest are the temperature distribution results
in thermal stresses that can cause failure. In such cases the temperatures from a
transient thermal analysis are used as inputs to a structural analysis for thermal
stress evaluations. Transient thermal analyses can be performed using the ANSYS or
Samcef solver.
Many heat transfer applications such as heat treatment problems, electronic package
design, nozzles, engine blocks, pressure vessels, fluid-structure interaction problems,
and so on involve transient thermal analyses.
Point to Remember
A transient thermal analysis can be either linear or nonlinear. Temperature
dependent material properties (thermal conductivity, specific heat or density), or
temperature dependent convection coefficients or radiation effects can result in
nonlinear analyses that require an iterative procedure to achieve accurate solutions.
The thermal properties of most materials do vary with temperature, so the analysis
usually is nonlinear.
Preparing the Analysis
Typically, a steady-state thermal analysis include several steps.
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Create Analysis System
Define Engineering Data
Attach Geometry
Define Part Behaviour
Define Connections
Apply Mesh Controls/Preview Mesh
Establish Analysis Settings
Define Initial Conditions
Apply Loads and Supports
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Review Results
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Solve
Create Analysis System
From the Toolbox, drag the Transient Thermal or the Transient Thermal
(Samcef) template to the Project Schematic.
Define Engineering Data
Thermal Conductivity must be defined for a steady-state thermal analysis. Thermal
Conductivity can be isotropic or orthotropic, and constant or temperaturedependent.
There are several material in the Engineering Data Sources that we can use
directly. By clicking the Engineering Data Sources, then Thermal Material, then
clicking the plus near Aluminium.
Attach Geometry
A step file of a 1U CubeSat can be get from the following website.
https://confluence.cornell.edu/download/attachments/203031258/CubeSat.STEP?version=2&modif
icationDate=1358271342000&api=v2
1. Select the Geometry cell in an analysis system schematic.
2. Browse to the CAD file from the following access points:
 Right-click on the Geometry cell in the Project Schematic and
choose Import Geometry.
3. Double-click on the Model cell in the Project Schematic. The Mechanical
application opens and displays the geometry.
Define Part Behaviour
The properties of the geometry are usually defined as Structure Steel, we can
select several components and change them to Aluminium, which was added in
previous step.
Define Connections
In a thermal analysis only contact is valid. Any joints or springs are ignored.
With contact the initial status is maintained throughout the thermal analysis, that is,
any closed contact faces will remain closed and any open contact faces will remain
open for the duration of the thermal analysis. Heat conduction across a closed
contact face is set to a sufficiently high enough value (based on the thermal
conductivities and the model size) to model perfect contact with minimal thermal
resistance. If needed, you can model imperfect contact by manually inputting
a Thermal Conductance value.
In this case the contact type has already been set.
Apply Mesh Controls/Preview Mesh
There are no specific considerations for steady-state thermal analysis itself. However
if the temperatures from this analysis are to be used in a subsequent structural
analysis the mesh must be identical. Therefore in this case you may want to make
sure the mesh is fine enough for structural analysis.
It is recommended to use the multizone method, which may help generate a more
reasonable result.
Establish Analysis Settings
For a steady-state thermal analyses you typically do not need to change these
settings. The basic controls are:
Step Controls allow you to control the rate of loading which could be important in a
steady-state thermal analysis if the material properties vary rapidly with
temperature. When such nonlinearities are present it may be necessary to apply the
loads in small increments and perform solutions at these intermediate loads to
achieve convergence. You may wish to use multiple steps if you a) want to analyze
several different loading scenarios within the same analysis or b) if you want to
change the analysis settings such as the time step size or the solution output
frequency over specific time ranges.
Output Controls allow you to specify the time points at which results should be
available for postprocessing. In a nonlinear analysis it may be necessary to perform
many solutions at intermediate load values. However i) you may not be interested in
all the intermediate results and ii) writing all the results can make the results file size
unwieldy. In this case you can restrict the amount of output by requesting results
only at certain time points.
Nonlinear Controls allow you to modify convergence criteria and other specialized
solution controls. Typically you will not need to change the default values for this
control.
Nonlinear Controls are exposed for the ANSYS solver only.
Analysis Data Management settings enable you to save specific solution files from
the steady-state thermal analysis for use in other analyses.
Select Analysis Setting, in the Details, change the Number of Steps to 5.
Input the End time for each steps, the intervals between steps are not necessarily
be the same.
By doing this, we can see the thermal effect various with time.
Define Initial Conditions
For a steady-state thermal analysis you can specify an initial temperature value. This
uniform temperature is used during the first iteration of a solution as follows:
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To evaluate temperature-dependent material properties.
As the starting temperature value for constant temperature loads.
It is better to run a steady thermal analysis, but in this case, for simplify the
procedure, we just set the Initial Temperature Values in the Details of Initial
Temperature as 20 as default.
Apply Loads and Supports
The following loads are supported in a steady-state thermal analysis:
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Temperature
Convection
Radiation
Heat Flow
Perfectly Insulated
Heat Flux
Internal Heat Generation
Imported Temperature
Imported Convection Coefficient
Fluid Solid Interface
Loads and supports vary as a function of time even in a static analysis as explained
in the Role of Time in Role of Time in Tracking. In a static analysis, the load’s
magnitude could be a constant value or could vary with time as defined in a table or
via a function. Details of how to apply a tabular or function load are described
in Defining Boundary Condition Magnitude. In addition, see the Apply Loads and
Supports section for more information about time stepping and ramped loads.
By right click Transient Thermal, we can add different thermal load on the object.
For a CubeSat, Convection, Radiation and Heat Flux are the most important
part.
After add a Convection load, two highlight cells can be seen in the Details, which
are Geometry and Film Coefficient.
Click Geometry and select all component, then select Apply.
Convection data can be get by click the triangle at the right side of the Film
Coefficient, and select Import Temperature Dependent.
For simplify the analysis, selecting Stagnant Air – Simplified Case.
Other time-independent loads such as radiation can also be applied in a similar way.
Since Heat Flux is a time-dependent variable that might disappear in eclipse period,
therefore we need to click the triangle at the right side of the Magnitude cell,
selecting Tabular (Time). By doing this, we can add the Heat Flux values we
calculated before in the column at the right side.
Please note that it is better to change the Time[s] of step 5 first, the values of the
upper lines shall always smaller than the lower lines.
Solve
The Solution Information object provides some tools to monitor solution
progress.
Solution Output continuously updates any listing output from the solver and
provides valuable information on the behavior of the structure during the analysis.
Any convergence data output in this printout can be graphically displayed as
explained in the Solution Information section.
You can also insert a Result Tracker object under Solution Information. This
tool allows you to monitor temperature at a vertex as the solution progresses.
By right click the Solution button, we can ask the solver to generate several kinds
of result. In this case, we select Temperature.
Review Results
Applicable results are all thermal result types.
Once a solution is available you can contour the results or animate the results to
review the response of the structure.
As a result of a nonlinear analysis you may have a solution at several time points.
You can useprobes to display the variation of a result item over the load history. Also
of interest is the ability to plot one result quantity (for example, maximum
temperature on a face) against another results item (for example, applied heat
generation rate). You can use the Charts feature to develop such charts.
Note that Charts are also useful to compare results between two analyses of the
same model.
By click the Graph tap at the bottom line, we can see a graph shows the maximum
and minimum temperature various with time. More specific data shows in the right
columns.
The temperature distribution shows in the contour figure in the main workspace,
which can help us analyse the thermal effect on the Cubesat.