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Thermal Analysis

Module 6. Thermal Analysis. 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.

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Thermal Analysis

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  1. Module 6 Thermal Analysis

  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 January 30, 2001 Inventory #001441 6-2

  3. 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 Thermal AnalysisA. Overview January 30, 2001 Inventory #001441 6-3

  4. 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 Thermal Analysis...Overview January 30, 2001 Inventory #001441 6-4

  5. Thermal AnalysisB. 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. January 30, 2001 Inventory #001441 6-5

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

  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 January 30, 2001 Inventory #001441 6-7

  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. January 30, 2001 Inventory #001441 6-8

  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. January 30, 2001 Inventory #001441 6-9

  10. Thermal Analysis - ProcedureSolution 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) January 30, 2001 Inventory #001441 6-10

  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) January 30, 2001 Inventory #001441 6-11

  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. January 30, 2001 Inventory #001441 6-12

  13. Thermal Analysis - ProcedurePostprocessing 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) January 30, 2001 Inventory #001441 6-13

  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. January 30, 2001 Inventory #001441 6-14

  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. January 30, 2001 Inventory #001441 6-15

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

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