2. Doctoral Degree Defense Defender: Lei Zhou Advisor: Dr. Louis C. Chow Dr. Jay Kapat Committee members: Dr. Louis C. Chow; Dr. Jay Kapat; Dr. Q. Chen; Dr. R. Chen; Dr. Larry Andrew Department of MMAE University of Central Florida NOV 10 th,2003 A MINIATURE REVERSE-BRAYTON CYCLE CRYOCOOLER AND ITS KEY COMPONENTS: HIGH EFFECTIVENESS HEAT RECUPERATOR AND MINIATURE CENTRIFUGAL COMPRESSOR

3. Research Background Applications –Oxygen/Nitrogen liquefaction –Infrared image sensor array –Electronic device cooling –Out space exploration –HTS (High temperature superconductor) cooling

4. General refrigeration cycle

5. General COP

6. Major Cryogenic Technologies Stirling machine Pulse tube Gifford-McMahon RTBC (reverse Turbo-Brayton Cycle) Technology ProsCons S High efficiency, compactVibration, unreliable P Compact, reliable, no moving partsEfficiency lower than Stirling G Simple, reliableBulky, gas purity sensitivity, inefficient R Compact, no vibration, efficientMoving part

7. Cycle efficiency vs. compressor electrical power Courtesy of Ray Radebaugh, NIST-Boulder Proposed miniature RTBC

8. Cycle efficiency vs. operating temperature Courtesy of Ray Radebaugh, NIST-Boulder Proposed miniature RTBC

9. Cryocooler Applications and Operating Regions Proposed miniature RTBC

10. RTBC concept 2 turbine 6 generator compressor motor Heat exchanger to Ambient Heat Load 1 3 4 5 Heat regenerator COP=Heat removed from heat load end / Power input to system

11. RTBC Mollier Diagram Work in Work out Heat exchange Heat in Heat out

12. Miniature RTBC cryocooler Proposed cooling power: 20Watt at 77K Proposed COP: 0.08~0.1 Miniature size –Miniature single stage mixed flow centrifugal compressor –Micro channel heat recuperator –Integrated high efficiency motor/alternator –Advanced air-foil bearings

13. Advantages of miniature RTBC cryocooler Portability Suitable for weight/size critical applications Simplicity Low maintenance Low cost

14. Miniature cryocooler concept mm cmmmm Micro scale  Meso scale  Macro scale 10mW   W 0.1kW Poor COP  Good COPBest COP

15. Thermal efficiency analysis of miniature reverse- Brayton cycle(1)

16. Thermal efficiency analysis of miniature reverse-Brayton cycle(2)

17. Thermal efficiency analysis of miniature reverse- Brayton cycle(3)

18. Result of thermal efficiency analysis: system parameters 2 turbine 6 generator compressor motor Heat exhausted=261W Cooling Load=20W 1 3 4 5 Heat regenerator COP=0.083 Eff=0.993 T1=64K T5=300.2K T6=76.0K T2=74.4K T3=299.5K T4=440K P motor =262W Pressure ratio=1.75 Mass flow rate=2.81g/s

19. Micro-channel heat recuperator s d d w L Insulated surface T hot,in T cold,in Hot end Cold end

20. Stacked multi-layer construction

21. Physical model Cold Neon Hot Neon d d w Y X Z

22. Numerical Model (1-D) Hot gas node Cold gas node Wall Interface HjHj H j+1 CjCj C j+1 WjWj W j+1 WtWt Metal Material Insulation material Hot fluid Cold fluid

23. Numerical simulation for single material Fig.5 axial heat conduction in wall

24. dt VS. Length (total temperature different =220K)

25. 1-D Numerical for two material

26. Comparison of heat conductivity

27. Comparison of single material and two materials Configuration1+1 (1mm)10+10 (10mm)40+40 (40mm)  T(K) 330120 SiO2 0.3770.8480.853 Metal 0.40050.53330.6198 Alternative Insulator/Metal 0.40030.9370.991

28. Conclusion of micro-channel heat recuperator design 1-D numerical simulation is suitable for the performance estimation of the micro-channel heat recuperator With proper parameter selection, the micro-channel heat recuperator can achieve 0.99 effectiveness at an acceptable pressure loss For the reason of manufacturing, this heat recuperator may be constructed as many thin layers stacked together. It provides the possibility of two materials (one have high heat conductivity and another have very low heat conductivity) stacked alternatively to provide 0.99 effectiveness. This simulation provides the guidance to select the material to manufacture the heat recuperator. LTCC may be a good candidate due to its low heat conductivity and high solidity after cured.

29. Centrifugal compressor design Advantages of single stage centrifugal compressor –Simplicity: only 1 moving part –Reliability (better than reciprocating compressor) –Possible high efficiency –No vibration: high revolution speed (>>100 kRPM) –Compact Disadvantages: –Difficult design: complicated flow field –Relatively expensive: manufacturing rows of blades in small size –Low compression ratio

30. Testing Compressor specifications Working fluid: Nair Operating pressure: 1 bar Operating temperature: 300K Mass flow rate: 4.5 g/s Compression ratio: 1.7 Bearing: conventional ball bearing Driver type: direct Motor

31. Compressor Design flow chart Basic layout design Basic thermodynamics and sizing Geometry design 1-D flow calculation 3-D CFD verification Manufacturing and testing

32. Basic Layout Flow direction Radial IGV Mixed flow impeller Axial diffuser

33. R-Z planeX-Y plane

34. 3-D geometry design ---- hub-shroud contour (R-Z plane) IGV shroud curve IGV hub curve Impeller shroud curve Impeller hub curve

35. 3-D geometry design ---- X-Y plane blade angle   0,0 Leading edge Trailing edge X-Y projection line of blade at shroud/hub surface

36. Geometry implementation in Pro/Engineer(1)

37. Geometry implementation in Pro/Engineer(2)

38. Geometry implementation in Pro/Engineer(3)

39. Introduction of 2-zone model of impeller

40. 3-D view of IGV

41. 3-D view of diffuser

42. Compressor assembly (1)

43. Compressor assembly (2)

44. 3-D CFD geometry # #: 3-D simulation results is provided by Xiaoyi Li

45. 3-D results

47. CFD results Flow Separation inertia force and centrifugal force Suggestion reduce the length of IGV add deswirl vane

48. Conclusion of compressor design 2-zone model is the most powerful 1-D design tool in centrifugal compressor design. With proper mathematics and interactive program codes, 3-D geometry can be designed and then implemented with pro/engineering software 3-D CFD simulation show the improvements should be done in next design. Mixed flow impeller with axial diffuser may have severe flow separation problem at the bending section. A deswirl vane is needed before this section Impeller may need to be refined with inducer to reduce entrance separation.

49. Compressor Testing Run set up Pressure and Temperature at Diffuser Exit Motor Case Temperature Mass Flow Controller Power In Bearing Temperature Power Out of Motor Motor Bearing Temperature Pressure and Temperature at Inlet Motor Bearing Temperature Bearing Temperature Pressure and Temperature after Mixer

50. Testing assembly (coupler improvement) Coupler design speed: ~30,000 RPM Coupler with steel sleeve in test run ~97,000 RPM

51. Testing assembly To mass flow rate meter

52. ‘Blank Shaft’ Test Motor efficiency = 40% to 70% –90,000 rpm = 65% with load Loss per bearing = 105 Watts at 90,000 rpm

53. Compressor test Curved Blade Impeller –89,485 rpm, 3.13 g/sec, 2.70 psig Straight Blade Impeller –93,984 rpm, 5.14 g/sec, 5.05 psig

54. Compressor test

55. Theoretical points

56. Compressor efficiency Actual output conditions: –93,984 rpm –1.29 pressure ratio –61.2% isentropic efficiency –5.1 grams per second mass flow rate

57. Testing conclusion Straight Blade Impeller more effective than Curved Blade Impeller In order to run at full speed, an integrated Motor/compressor design is needed Compressor was on way to design conditions –Pressure ratio of 1.7 at Operating speed of 150,000 rpm –Mass flow rate of 4-8 grams per second Reduce losses –Improve alignment Implement laser aligning procedures Introduce rigid coupler Incorporate one shaft throughout the assembly –Incorporate air foil bearing / air journal bearing Only if power consumption remains high

58. CONCLUSIONS Miniature RTBC which can provide middle cooling power (1-20 Watt at 77K) may have high efficiency and small footprint.Its unique features including reliability, vibration free and low maintenance may have promising applications Its key components, including 0.99 effectiveness micro heat recuperator and meso-scale centrifugal compressor and related bearing technologies are key enabling technologies which can make it have good COP comparing to other competing cryogenic systems. The design of micro-scale heat recuperator and compressor is on the way to successful which provide solid evidence to the success of miniature RTBC technology