Thermal Analysis Module 6. Training Manual January 30, 2001 Inventory #001441 6-2 Thermal Analysis...
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Transcript of Thermal Analysis Module 6. Training Manual January 30, 2001 Inventory #001441 6-2 Thermal Analysis...
Thermal Analysis
Module 6
January 30, 2001
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Training ManualThermal Analysis
• In this chapter, we will briefly describe the procedure to do a steady-state thermal analysis.
• The purpose is two-fold:
– To reiterate the typical analysis steps that were introduced in Chapter 4.
– To introduce you to thermal loads and boundary conditions.
• Topics covered:
A. Overview
B. Procedure
C. Workshop
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Thermal Analysis
A. Overview
• Thermal analyses are used to determine the temperature distribution, thermal gradient, heat flow, and other such thermal quantities in a structure.
• A thermal analysis can be steady-state or transient.
– Steady-state implies that the loading conditions have “settled down” to a steady level, with little or no time dependency. Example: An iron that has already reached the desired temperature setting.
– Transient* implies conditions that are changing with time. Example: A casting in the process of cooling down from molten metal to solid.
* Not covered in this course
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Thermal Analysis
...Overview
• Thermal loading conditions can be:
Temperatures Regions of the model where temperatures are known.
Convections Surfaces where heat is transferred to (or from) surroundings by means of convection. Input consists of film coefficient h and bulk temperature of the surrounding fluid Tb.
Heat flux* Surfaces where the heat flow rate per unit area is known.
Heat flow* Points where the heat flow rate is known.
Heat generation* Regions where the volumetric heat generation rate is known.
Radiation* Surfaces where heat transfer occurs by means of radiation. Input consists of emissivity, Stefan-Boltzmann constant, and optionally, temperature at a “space node.”
Adiabatic surfaces “Perfectly insulated” surfaces where no heat transfer takes place.
* Not covered in this course
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Thermal Analysis
B. Procedure
• The procedure to do a steady-state thermal analysis is similar to that for a static stress analysis:
– Preprocessing
• Geometry
• Meshing
– Solution
• Loading
• Solve
– Postprocessing
• Review results
• Validate the solution
• Setting GUI preferences (Main Menu > Preferences) to Thermal may be helpful.
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Thermal Analysis - Procedure
Preprocessing
Geometry
• Can either be created within ANSYS or imported.
• Details of both methods will be covered later.
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Thermal Analysis - Procedure
...Preprocessing
Meshing
• First define element attributes: element type, real constants, and material properties.
Element type
• The table below shows commonly used thermal element types.
• There is only one DOF per node: TEMP
2-D Solid 3-D Solid 3-D Shell Line Elements
Linear PLANE55 SOLID70 SHELL57 LINK31,32,33,34
QuadraticPLANE77 PLANE35
SOLID90 SOLID87
Commonly used thermal element types
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Thermal Analysis - Procedure
...Preprocessing
Material properties
– Minimum requirement is thermal conductivity, KXX.
– Specific heat (C) will be needed if internal heat generation is to be applied.
– ANSYS-supplied material library (/ansys57/matlib) contains both structural and thermal properties for a few common materials, but we recommend that you create and use your own material library.
– Setting preferences to “Thermal” limits the Material Model GUI to display only thermal properties.
Real constants
– Primarily needed for shell and line elements.
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Thermal Analysis - Procedure
...Preprocessing
• Then mesh the geometry.
– Save the database.
– Use the MeshTool to create the mesh. The default smart-size level of 6 produces a good initial mesh.
• This completes the preprocessing step. Solution is next.
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Thermal Analysis - Procedure
Solution
Loading
• Prescribed Temperatures
– DOF constraints for a thermal analysis
– Solution > -Loads-Apply > Temperature
– Or the D family of commands (DA, DL, D)
• Convections
– These are surface loads
– Solution > -Loads-Apply > Convection
– Or the SF family of commands (SFA, SFL, SF, SFE)
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Thermal Analysis - Procedure
...Solution
• Adiabatic Surfaces
– “Perfectly insulated” surfaces where no heat transfer takes place.
– This is the default condition, i.e, any surface with no boundary conditions specified is automatically treated as an adiabatic surface.
• Other possible thermal loads:– heat flux (BTU / (hr-in2)
– heat flow (BTU / hr)
– heat generation (BTU / (hr-in3)
– radiation (BTU / hr)
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Thermal Analysis - Procedure
...Solution
Solve
• First save the database.
• Then issue SOLVE or click on Solution > -Solve- Current LS.
– Results are written to the results file, jobname.rth, as well as to the in-memory database.
• This completes the solution step. Postprocessing is next.
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Thermal Analysis - Procedure
Postprocessing
Review Results
• Typically consists of contour plots of temperature, thermal gradient, and thermal flux.
– General Postproc > Plot Results > Nodal Solu… (or Element Solu…)
– Or use PLNSOL (or PLESOL)
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Thermal Analysis - Procedure
...Postprocessing
• A useful option for contour plots in 3-D solid models is isosurfaces — surfaces of constant value. Use the /CTYPE command or Utility Menu > PlotCtrls > Style > Contours > Contour Style.
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Thermal Analysis - Procedure
...Postprocessing
• Validate the Solution
• Are temperatures within the expected range?
– You can generally guess the expected range based on prescribed temperatures and convection boundaries.
• Is the mesh adequate?
– Just as in the case of stresses, you can plot the unaveraged thermal gradients (element solution) and look for elements with high gradients. These regions are candidates for mesh refinement.
– If there is a significant difference between the nodal (averaged) and element (unaveraged) thermal gradients, the mesh may be too coarse.
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Thermal Analysis - Procedure
C. Workshop
• Refer to your Workshop Supplement for instructions on:
W3. Axisymmetric Pipe with Fins
