1 MINIATURE ENGINEERING SYSTEMS GROUP ( Two-Stage CryoCooler Development for Liquid Hydrogen Systems

2 Miniature Engineering Systems Group Core Group of Faculty Dr. Louis Chow Director System design, spray cooling, thermal management, thermal fluids design/ experiment, thermodynamics Dr. Jay Kapat Co-Director System design, design of turbo machinery, heat transfer and fluidic components, component and system testing Dr. Quinn Chen Associate Director for Educational Programs Micro-fabrication and tribology, actuators Dr. Linan An Polymer-derived ceramic micro-fabrication Dr. Joe Cho Bio-MEMS, Magnetic MEMS, MOEMS, micro/nano fabrication, micro fluidics Dr. Neelkanth Dhere Tribological coatings, multilayer thin films, sensors Dr. Chan Ham Control, micro-satellites Dr. K.B. Sundaram Micro-fabrication, thin film, sensors, micro- and meso-scale motors and generators Dr. Abraham Wang Vibration and control, health monitoring, piezoelectric materials, shape memory alloy Dr. Tom Wu RF MEMS, miniature electromagnetic devices

3 Motivation and Objective  Storage of cryogenic propellants (LH 2 and LOX) for extended periods have become increasingly important within NASA. There would be loss of propellants in storage tanks as well as in transfer lines both in space and ground applications due to heat leak.  The objective of this project is to design and build a cryocooler, which is capable of removing 50W of heat at liquid hydrogen temperature and thus contribute to NASA efforts on ZBO storage of cryogenic propellants and to attain extremely high hardness, extremely low coefficient of friction, and high durability at temperatures lower than 60 K for the tribological coatings to be used for this cryocooler.

4 Approach and Innovation  Two Stage Reverse Turbo Brayton Cycle (RTBC) CryoCooler reliable, efficient, compact and light weight RTBC bottom stage with He as the working fluid (immediate goal) RTBC top stage with Ne as the working fluid Key Enabling / Innovative Features for the bottom stage: Compressor – Four stage centrifugal compressor with very high efficiency in its class. Design incorporates intercooling, inlet guide vanes, deswirler vanes, end wall contouring, axial diffuser at the exit integrated with after- or inter-cooler. Motor – The motor would be a high speed three phase PMSM with a magnet integrated rotor and high frequency soft switching control system. Recuperative heat exchanger for regeneration –Non-conventional design for reduction of axial or parasitic heat conduction, massively parallel design with micro-channels and special manifolds for ultra-high effectiveness, low pressure drop and uniform flow distribution. Gas foil bearings – Completely hydrodynamic gas foil bearings for both radial and axial support - key in minimizing losses associated with the compressor and the motor. Tribological coatings – Extremely hard coatings of titanium nitride (TiN), bilayer coatings of TiN and molybdenum disulphide (MoS 2 ), diamond-like-carbon (DLC) coatings, bilayer coatings of DLC/MoS 2 for low values of coefficient of friction at cryogenic temperatures.

5 Overall Project Outline At A Glance FY01-03 (15 months): Design: system/cycle, motor, 4-stage compressor, gas foil bearings (GFB) Fab/testing: 1-stage compressor, coating FY03-04 (12 months): Design: 4-stage compressor 1, Rotor system, GFB 2 Fab/testing: Motor, Recuperative heat exchanger 3, coating FY04-05 (12 months): Fab/testing: 4-stage comp., other HXs, turbo-expander 4, GFB w/coatings FY05-06 (12 months): Integration and system testing Notes: 1.This will be based on continued testing and optimization of the 1-stage compressor, which will be funded through an MDA/AFRL SBIR project awarded to Rini Technologies (RTI, our partner). This effort will be further helped if RTI wins another SBIR contract from NASA MFC in the Sep03 cycle. 2.We expect our partner Electrodynamics Associates to win an NASA GRC SBIR funds on LH2-cooled hyper-conducting motor with gas foil bearings. These funds will help in our efforts on GFB. 3.This effort is helped by (a) existing NASA KSC funds for a compact JT-system for in-line ZBO/pre-chilling/densification, (b) MDA/AFRL SBIR funds received by RTI on MEMS recuperative heat exchanger. This effort would be further helped if (c) RTI wins another SBIR from NASA GSFC on this topic in the Sep03 cycle. 4.This effort would be possible (a) through collaboration with Elliott Energy Systems (Stuart, FL) through scaling of their micro-turbines, and (b) potentially also through an SBIR from NASA KSC that RTI is competing on in the Sep03 cycle.

6 Efforts in Alternative Funding Sources  NASA KSC – Miniature Joule Thomson (JT) CryoCoolers for Propellant Management (funded). KSC Partners: Bill Notardonato and George Haddad.  Defense Advance Research Projects Agency (DARPA) – We have been invited to the pre- solicitation workshop on micro cryocoolers. Communicating with potential team members and planning a proposal. Also, inviting Dr. Ray Radebaugh and Dr. Marty Nisenoff (leading, world-renowned experts in cryocoolers) to UCF campus for this DARPA proposal and ongoing projects.  Harris Corporation – We have reciprocal visits for possible joint proposals to DoD.  Technology Associates, Inc. (based in Boulder, CO) – They have recently opened a branch in UCF research park in order to collaborate with us, and have provided UCF subcontracts on multiple of their DoD/NASA contracts on MEMS cryocoolers.  Rini Technologies, Inc. – partner on this project and several DoD SBIR projects on miniature cryocoolers.  Lockheed Martin Missiles and Fire Control (LMMFC) – They have provided initial funding for miniature coolers. We are currently exploring opportunities of mutual interests. Their engineers are on this project time as part-time graduate students.  Proposal for collaboration in materials research in the areas of ultra-low friction (COF<0.01) of MoS 2 and carbon-based coatings with European Research Institutes from National Science Foundation is being solicited. Preparing to submit.  Proposal on Threat Control submitted to DTRA in January 2003, which included miniature cryocoolers for sensors for nuclear treaty verification. Communicating with DTRA.

7 Important Parameters to Measure Performance  Performance of the two stage cryocooler (with emphasis on performance of the bottom stage) – COP, weight and size.  Compressor performance – weight, size, efficiency.  Heat exchanger performance – effectiveness, size, pressure drop, and weight.  Motor performance – speed, weight, size, efficiency.  Motor control system performance – switching frequency, efficiency.  Gas foil bearings – load bearing capacity, wear during start and stop, dynamic stability.  Performance of t ribological coatings – coefficient of friction, hardness, wear resistance and durability at cryogenic temperatures.

8 Performance Comparison – Quantifiable Research Results ComponentPerformance characteristic Commercially available Short term goal Long term goal MotorSpeed (rpm) Efficiency 150,000 (Koford Motors) 30% 200,000 60% 200,000 96% Compressor (mesoscale) Efficiency35% (Creare) 45%75%

9 Performance Comparison – Quantifiable Research Results ComponentPerformance characteristic Commercially available Short term goalLong term goal Heat Exchanger Effectiveness Size (Length) 98% 67 cm (Creare Inc,.) 95% 8 cm 99% < 8 cm ControllerEfficiency80%60%95 %

10 ComponentPerformance characteristics Commercially Available Achieved to date all at RT Long Term Goal Tribological Coatings Hardness DLC (40 GPa)TiN (25 GPa) COF < 0.15 at 77 0 K LN 2 and finally satisfactory operation in the Cryocooler TiN (20-25 GPa) Coefficient of Friction (COF) DLC COF At Room Temp TiN=0.143 TiN/MoS2 on Glass = TiN/ MoS2 on Al = TiN/ MoS2 on Si wafer = At 77 0 K LN Nitrides At Room Temp TiN (< 0.1) At 77 0 K LN 2 ZrN ( ) Performance Comparison – Quantifiable Research Results

11 Overall System

12 Thermodynamic Schematic

13 Single Stage Centrifugal Compressor  Purpose: As the four stage compressor is very difficult to be designed, fabricated and tested, and since a number of innovations are planned for size where no data is available in literature, we design, fabricate and test a single stage compressor representing one of the four stages of the proposed four stage compressor in order to obtain initial results and to generate a database for future design optimization of the four stage compressor.

14 Design of a Single Stage Compressor inlet guide vanes impeller blades

15 flow inlet Inlet guide vanes Contoured endwall Full blades and splitter blades Vaned, axial diffuser with multiple vane segments flow exit List of Innovations

16 Compressor Assembly Animation  This animation represents the assembly of the compressor housing containing the internal parts, coupled with the motor.

17 Fabrication  Rapid Prototyping  Stereo lithography models used for visualization and fit.

18 Fabrication  Manufacturing  Impeller cast from A356 Aluminum.  4-axis CNC machining of diffuser and inlet guide vane.

19 Rotor Balancing

20 Testing  Measurement Instrumentation  Thermocouples for temperature measurements.  Pressure transducers for pressure measurements.  Mass flow meter for flow rate measurements.

21 Testing  Performance Instrumentation  Digital Multimeter for power measurements.  Oscilloscope for motor speed measurements.

22 Sliding Mesh - Grid 1.Most accurate method – unsteady fluid field gives interaction between igv-rotor- stator. 2.More computationally demanding.

23 Mixing Plane-Grid 1.Steady state solution – losing interaction between stator and rotor. 2.Cost effective.

24 Solution for IGV

25 Preliminary Results from Impeller Flow Simulation

26 Preliminary Mechanical Design of 4-stage Helium Compressor

27 Four Stage Compressor – Assembly Structure

28 Four Stage Compressor – Rotating Part

29 Compressor Future Work  To continue with the single stage compressor simulation and testing and to verify its design.  To integrate the single stage compressor into a four stage centrifugal helium compressor for the bottom cycle.  To design and check the fabrication feasibility of the four stage compressor.

30 Permanent Magnet Synchronous Motor Specifications Output Shaft Power2000 W Shaft Speed200,000 rpm Shaft Diameter16 mm Max. Length100 mm Max. Outer Diameter44 mm Supply DC Voltage28 V Efficiency> 90 %

31 Challenge  In the motor design, high speed will  Increase core loss in the rotor and stator.  Increase copper loss in the winding due to increased eddy current loss.  Increase bearing loss.  Increase mechanical stress in the shaft.  In the motor control, high speed will  Increase switching loss due to increased sampling frequency.  Need sensorless control due to the unavailability of the commercial position sensor at such a high speed.

32 PMSM Configuration  3-phase, 2-pole  Slotless design  Rectangular Litz-wire  Laminated low loss stator core  Active length: 25.4 mm  Stator outer diameter: 38 mm  Winding pitch: 10/15

33 Shaft Profile  Shaft will not fail under:  Bending stress  Shear stress  Shaft whirl  Operational speed is above the 1 st critical speed.  Dynamic deflection at first critical speed : 0.361mm < 0.5mm (gap)

34 Shaft Centrifugal Stress Maximum Stress developed: 1210 MPa < 1700 MPa (Yield Strength of the Titanium based alloy)

35 Air Gap Flux Density and Torque  Low harmonics of the normal flux density.  Tangent flux density is due to large airgap.  Torque is simulated with 60.8 A phase current (back EMF: 12 V). 1.5% ripple

36 Major Task & Achievements in Control Design  Task:  Design of reliable and efficient 3 phase, 2 pole 200krpm motor controller with 10% speed margin.  95% efficiency of the control electronics (without LPF).  Achievements:  Currently achieved 164 krpm.  82 krpm for a 100 krpm sample motor, which is equivalent to 164 krpm for the 200 krpm motor.  This speed limitation is mainly resulted from the insufficient capability of the sample motor.  80% efficiency of the control electronics including a LPF (89% without the LPF).  Tests on the efficiency, power factor, harmonics and the LPF.

37 Controller Hardware and Software

38 PMSM Loss Copper loss16.9 W Shaft eddy loss20 W Iron loss10.4 W Bearing loss10 W Filters loss11 W Windage loss12.8 W Total loss81.1W Motor Efficiency: Control Efficiency: Total Efficiency:

39 Permanent Magnet Synchronous Motor Ongoing Research & Future Work  Performing dynamic simulation of the shaft.  Fabricating and testing of a test-motor with ball bearings.  Designing of a controller for the new motor.  Enhancing the efficiency by improving the PWM and the LPF designs.  Realizing the ‘close loop control’.

40 Gas Foil Bearings Configuration

41 Gas Foil Bearings Dimensions and Present Analysis Journal bearing outer diameter = 43 mm Journal bearing inner diameter = 33 mm Inner hollow cylinder thickness = 1 mm Inner hollow cylinder axial length = 7 mm Foil axial length = 6 mm Gap between inner hollow cylinder and shaft = 5 mm Present Analysis: To determine the minimum shaft rotational speed at which the foils lift-off and the shaft is completely air borne.

42 Gas Foil Bearings Section under consideration for present analysis

43 Section meshed in Gambit

44 To develop tribological coatings:  Attain extremely high hardness, extremely low coefficient of friction, and high durability at temperatures lower than 60 K. Hard coatings at cryogenic temperature:  Coatings such as diamond-like-carbon (DLC) and nitrides of high-melting metals (e.g. TiN, ZrN) have coefficient of friction < 0.1 at room temperature but much higher values at cryogenic temperatures  A special cryogenic tribometer is required for study of friction and wear at cryogenic temperatures. Tribological Coatings Objective

45 Sample ID N 2 : Ar Ratio Atomic Percent N : Ti Average Hardness Average Elastic Modulus (GPa) GPaHV (Kgf/mm 2 ) 10.5: 6N :Ti = 50.3: : 4N :Ti = 53.05: : 4N :Ti = 52:  Good adhesion, thickness uniformity and stoichiometry of TiN and MoS 2 coatings on glass and aluminum substrates verified by peel test, Dektak Profilometry and energy dispersive spectroscopy (EDS).  TiN micro hardness measurements results: HV –Vicker’s Hardness  TiN and MoS 2 bilayers on Si wafer, glass and aluminum are prepared.  Expected excellent coefficient of friction (COF) and wear resistance. TiN by DC and MoS 2 by RF Magnetron Sputtering

46 TiN Coating – Aluminum and Steel Substrates Trial Run Coefficient of Friction Load (g)Average Friction  Fully reacted characteristic golden color.  All trials run at 42 rpm at RT.  Average COF for TiN/Al coating substrate =  TiN/MoS 2 for friction measurement at USF.  TiN for friction measurement at USF.

47  TiN and MoS 2 broad peaks indicate nanocrystalline nature of sample. (006) X-ray Diffraction – MoS 2 and TiN (111) (200) (311)

48 Scanning Electron Microscopy - TiN and MoS 2  Nanocrystalline Grains (average grain size less than 100 nm) of TiN is observed.  EDS analysis have shown good stoichiometric ratio of Ti and N (Atomic Percent N : Ti = : 47.09).  TiN  MoS 2

49 TiN / MoS 2 - Si Wafer Coefficient of friction tests at UCF with the assistance of Dr. Chen and his colleagues requires deposition of TiN/MoS 2 coating on bumps prepared by photolithography technique and also on plain Si wafer (1 cm 2 ) to minimize the contact area between two rubbing samples and thereby providing more accurate coefficient of friction measurements. TiN/MoS 2 Sample Si Sample with TiN /MoS 2 film on bump Bumps Sliding Force Sliding Force

50 Friction Test – Si Wafer Average coefficient of friction for the TiN/MoS 2 Bilayer coating on Si wafer with 1 N normal load was =

51  Microwave assisted chemical vapor deposition (MWCVD) system has been installed.  Initial deposition and characterization of diamond-like- carbon (DLC) coatings being carried out. Microwave CVD System

52 Tribological Coatings Research Progress Literature values for coefficient of friction (COF) Hard CoatingCOFCOF K LN 2 HardnessWear Resistance DLC GPaGood NitridesLess than 0.1 (TiN) (ZrN)20-25 GPaGood Current results obtained for this project better than state of art Substrate roughness is an important criteria for COF Tribological CoatingCoating PairSubstrateCOFHardness TiNSteelAluminum GPa TiN / MoS 2 Si Wafer TiN / MoS 2 TiNGlass TiN / MoS 2 TiNAluminum

53  Deposition parameters for TiN, MoS 2 and TiN+MoS 2 Bilayer coatings on glass, Si wafer and aluminum substrates have been optimized.  Micro hardness and coefficient of friction for TiN on aluminum substrate comparable or better than state-of-art have been obtained.  Bilayer coatings expected to provide values comparable to RT at cryogenic temperatures.  Microwave assisted chemical vapor deposition (MWCVD) system has been installed.  Initial deposition and characterization of diamond-like- carbon (DLC) coatings is being carried out. Tribological Coatings Conclusion

54  Cryogenic temperatures degrade tribological properties.  However, hydrogen improves lubrication.  TiN and DLC provides good hardness and low friction.  Improved tribological properties expected for TiN/MoS 2 and DLC/MoS 2 bilayers at cryogenic temperatures.  Basic understanding of the role of hydrogen and effect of cryogenic temperatures on tribological properties.  Ultra-low COF (< 0.01 at RT) MoS 2 coating study in collaboration with Dr. Martin, France. Tribological Coatings Future Research

55 Next Year Work for the Project  To continue with the single stage compressor simulation and testing and to verify its design.  To design and check the fabrication feasibility of the four stage compressor.  To fabricate and test the permanent magnet synchronous motor.  To design and check the fabrication feasibility of high effectiveness micro channel heat exchanger.  To design and develop gas foil bearings for the overall system.  To achieve values of COF for the tribological coatings comparable to RT at cryogenic temperatures and finally satisfactory operation in the cryocooler.