Thermoelastic Analysis in Design

1. Thermoelastic Analysis in Design William Bell & Paul-W. Young Topsfield Engineering Service, Inc. John Stewart, Saber Design and Analysis Services, LLC.

2. Purpose This study explores the capability of Thermal Desktop to map temperatures from a thermal model to a Nastran model to evaluate thermal stress and distortion

3. Applications • Rapid cool-down to cryogenic temperatures • Differential thermal expansion causing leakage, failure, galling, or seizing • Electronics components • Misalignment due to thermal distortion • Time dependent and steady state conditions • Space optics - optical alignment • Gasket/seal seating - pressure containing • Thermal contact joint design

4. Tools Used • Thermal Desktop from C & R Technologies – Version 4.7 patch 16 • FEMAP V8.3 and NX NASTRAN V2.0

5. Study Assembly • ½” thick heated plate with a serpentine pipe 1/8” sch 40 pipe attached to the plate for temperature control • 20 watts/in2 • 15 watts/in2 • Heat Loads

6. Thermal Model Development • Evolved from a early version of a Thermal Desktop model • Rebuilt using latest modeling objects without simplifying dimensions • Picked off dimensions from the Autocad drawing for creation of the Nastran model • Result - there were some discrepancies

7. Thermal Study Conditions • Mass Flow cooling - Coolant – 100 lb/hr of Nitrogen gas at -200 F and 40 psig – built-in properties for Nitrogen • No Radiation Heat Transfer • Plate is heated with 1150 watts • Conduction within plate and pipe walls • Built in convection equations for heat transfer from pipe to Nitrogen • Steady State Conditions (although Thermal Desktop can solve time dependent cases and search for worst case conditions)

8. Material Properties • The structural and thermal properties used in the analysis models are values commonly used for Stainless Steel, Aluminum, and the attachment techniques employed • The property data used can be found in the Nastran and Thermal Desktop model files • In a “real world” problem, the material data would be detailed out and agreed to prior to beginning any analysis. Due to the large temperature differences, temperature dependent properties would also be used

9. Thermal Desktop Model Construction • Pipe with wall (1/8” nps - sch 40) built on a polyline • Lumps and paths within pipe • Ties representing the convective heat transfer from the pipe wall to the fluid lumps • Three brick objects with edge nodes merged for the plate except for Case D where the plate was created from the Nastran grooved plate. Plate is ½” thick • Heat flux applied to the bottom surface of two of the bricks • Contactor object to represent the pipe to plate bond. In the groove the bond thickness is 0.003”. The weld to the flat plate is an 1/8” fillet

10. Cases evaluated in Nastran • A - Pipe bonded to grooved plate – Nastran pipe and plate from chexa elements • B - Pipe bonded to grooved plate – Nastran pipe from cquad4 elements and plate from chexa elements • C - Pipe welded to flat plate – Nastran pipe from cquad4 elements and plate from chexa elements • D - Pipe bonded to grooved plate – Nastran pipe and plate from chexa elements – TD plate from the Nastran plate

11. Case Material Combinations • Case At1, Bt1, Dt1 - SS plate; SS pipe; easyflo braze • Case At2, Bt2 -  Al plate; Al pipe; Al braze • Case At3, Bt3 - Al plate; SS pipe; epoxy bond • Case Ct1 - SS plate; SS pipe; SS weld • Case Ct2 -  Al plate; Al pipe; Al weld

12. Case A – pipe and plate from chexa elements Cases A and B • Pipe bonded to a groove in the plate. • Case B – pipe from cquad4 elements and plate from chexa elements

13. Case C • Pipe with cquad4 elements attached with chexa solid elements to the top surface of the solid plate of chexa solid elements.

14. Pipe and Plate from chexa elements Case D • Pipe bonded to a groove in the plate. • TD plate from Nastran plate above, with groove.

15. Thermal Desktop Geometry Cases A and B Thermal Model Geometry Case C Thermal Model Geometry Case D Thermal Model Geometry

16. Thermal Desktop ties Ties from the fluid lumps to the pipe wall

17. Thermal Desktop contactors Contactor connections – shown in yellow

18. Case A & D Nastran Model Geometry chexa elements thru pipe Bond shown in yellow

19. Case B Nastran Model Geometry Pipe with cquad4 elements

20. Case C Nastran Model Geometry Weld bead shown in yellow

21. Thermal model elements – Cases A & B • Pipe • 2448 TD/RC Nodes • 1 pipe • 1 contactor • Plate • 1880 TD/RC Nodes • 3 fdsolids • 2 heat loads • 1 contactor • 12,038 conductors connecting plate and pipe • Fluid • 103 lumps • 2 plenums • 101 junctions • 102 paths • 1 tie

22. Thermal model elements – Case D • Pipe • 2448 TD/RC Nodes • 1 pipe • 1 contactor • Plate • 78,213 TD/RC Nodes • 25,482 plates • 65,240 solids • 2 heat loads • 1 contactor • 909,152 conductors connecting plate and pipe • Fluid • 103 lumps • 2 plenums • 101 junctions • 102 paths • 1 tie

23. Nastran Model Construction • Plate and bond built with 95,480 chexa elements for Cases A, B, and D • Plate and weld built with 112,216 cquad4 elements for Case C • Pipe built with 70,908 chexa elements for Case A & D • Pipe built with 23,636 cquad4 elements for Case B & C

24. Temperature Mapping Procedure • Step 1 – Temperatures from TD plate to Nastran plate • Step 2 - Temperatures from TD plate to Nastran bond, if required • Step 3 - Temperatures from TD pipe to Nastran pipe • This avoids mixing pipe and plate temperatures when mapping

25. Mapping tolerances • Thermal Desktop plate to the Nastran plate and bond, if required – 1e-5” • Thermal Desktop pipe to Nastran pipe – 0.00025”

26. Nastran Temperature TD Temperature Deflection Stress Results – Case At1

27. Nastran Temperature TD Temperature Deflection Stress Results – Case Bt1

28. Nastran Temperature TD Temperature Deflection Stress Results – Case Ct1

29. Nastran Temperature TD Temperature Stress Deflection Results – Case Dt1

30. Dt1 Stress At1 Stress At1 Stress Dt1 Stress Results – Case At1 versus Dt1

31. Nastran Temperature TD Temperature Deflection Stress Results – Case At2

32. Nastran Temperature TD Temperature Stress Deflection Results – Case Bt2

33. Nastran Temperature TD Temperature Deflection Stress Results – Case Ct2

34. Nastran Temperature TD Temperature Deflection Stress Results – Case At3

35. Nastran Temperature TD Temperature Deflection Stress Results – Case Bt3

36. Case At1 Thermal Results Cross section for temperature and Nastran Results Thermal model node numbers

37. Case At1 Thermal Results Temperatures in TD plate Temperatures in TD pipe

38. Case Dt1 Thermal Results Temperatures in TD plate from Nastran model Temperatures in TD pipe

39. Temperatures in Nastran plate from TD model Case At1 Thermal Results

40. Nastran Results Summary

41. Lessons Learned - thermal • Spend some time reviewing thermal results: • Determining if nodalization is sufficient – distortion or stress • Choosing materials and material thermal properties • Assuring convergence • Getting separate files for each component of the model and putting each component on a separate layer • Plan out the combinations with the design team • Carefully check to see if the temperature mapping is accurate • Let go of the fear of finite elements

42. Lessons learned - structural • Spend some time working with the thermal analyst: • Getting dimensions consistent • Sorting out materials and structural properties up front • Determining the mounting constraint • Getting separate files for each component of the model • Plan out the combinations with the design team • Carefully check to see if the temperature mapping is accurate • Do hand calculations as a check on stresses and deflections

43. Models • The Nastran and Thermal Desktop models are available as a down load by going to the following URL: • http://www.topeng.com/downloads