1. Thermal Analysis Module 6

2. Training Manual January 30, 2001 Inventory #001441 6-2 Thermal 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

3. Training Manual January 30, 2001 Inventory #001441 6-3 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

4. Training Manual January 30, 2001 Inventory #001441 6-4 Thermal Analysis...Overview Thermal loading conditions can be: TemperaturesRegions of the model where temperatures are known. ConvectionsSurfaces 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 T b. 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

5. Training Manual January 30, 2001 Inventory #001441 6-5 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.

6. Training Manual January 30, 2001 Inventory #001441 6-6 Thermal Analysis - Procedure Preprocessing Geometry Can either be created within ANSYS or imported. Details of both methods will be covered later.

7. Training Manual January 30, 2001 Inventory #001441 6-7 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 Commonly used thermal element types

8. Training Manual January 30, 2001 Inventory #001441 6-8 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.

9. Training Manual January 30, 2001 Inventory #001441 6-9 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.

10. Training Manual January 30, 2001 Inventory #001441 6-10 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)

11. Training Manual January 30, 2001 Inventory #001441 6-11 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)

12. Training Manual January 30, 2001 Inventory #001441 6-12 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.

13. Training Manual January 30, 2001 Inventory #001441 6-13 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)

14. Training Manual January 30, 2001 Inventory #001441 6-14 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.

15. Training Manual January 30, 2001 Inventory #001441 6-15 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.

16. Training Manual January 30, 2001 Inventory #001441 6-16 Thermal Analysis - Procedure C. Workshop Refer to your Workshop Supplement for instructions on: W3. Axisymmetric Pipe with Fins