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MINIATURE ENGINEERING SYSTEMS GROUP

FUNDAMENTAL TECHNICAL ISSUES

Zero boil-off of cryogenic propellants such as lh 2 in space Innovative requirement Cryocooler simultaneously optimized for Compactness, Light weight, Reliability and High Efficiency Appropriate result Usage of Reverse Turbo Brayton Cryocoolers Features Compact, Light-weight, Reliable and Highly Efficient No unbalanced or reciprocating components

Proposed Technical Solutions

Optimal mixture of in-house technical innovation and selection of off-the-shelf technology where available Partnered w/ Rini Tech. Inc,. for fast product development if not Frequent communication w/ cryogenic specialists at AFRL, NIST, ONR/DARPA, NASA KSC, NASA GRC and NASA Arc GRC We directly approached the corresponding best possible resource to have a solution in short span of time Gas foil bearings Dr. Nagaraj Arakere, UFL Mr. Jay vaidya, EAI Motor Rotordynamics

Other Sources of Financial Support

1. NASA KSC – Miniature Joule Thompson cryocooler, 2002-03 2. NASA KSC CCDF (through ASRC) – Miniature Joule Thompson cryocooler, 2003-04 3. RTI – Development of Miniature compressor and heat Exchanger – 2002-03 4. MDA SBIR (through AFRL) – Development of a compact recuperator, Nov 02 – May 03 5. MDA SBIR (through AFRL) – Design of a reliable turbo reverse Brayton cryocooler, Nov 03 - May 04 6. NASA SBIR (JSC) - Development of a 77K Reverse-Brayton Cryocooler with Multiple Coldheads, March 04 – Sep 04 Pending 1. MDA SBIR Phase II (through AFRL) – Design of a reliable turbo reverse Brayton cryocooler, summer 04 2. MDA SBIR (through AFRL) – Reliable and highly efficient multi-stage centrifugal compressor for low temperature cryocoolers, selected for Phase I award.

3. MDA SBIR (through AFRL) – MEMS fabricated, highly effective, compact recuperative heat exchanger for a miniature, reliable cryocooler, selected for Phase I award

PUBLICATIONS

1) “ 2) “ 3) “ 4) “ Two-Stage Cryocooler Development for Liquid Hydrogen Systems of the American Vacuum Society, Orlando, FL, March 17-20, 2003.

Two-Stage Cryocooler Development for Liquid Hydrogen Systems Design of a Super-High Speed PMSM for Cryocooler Application Mesoscopic Energy Systems ”, Joint Symposium of the Florida Society of Microscopy and the Florida Chapter ”, Space Cryogenics Workshop, Alyeska Resort, AK, September 18-19, 2003.

”, Space Cryogenics Workshop, Alyeska Resort, AK, September 18-19, 2003.

”, accepted to be published in Annual Review of Heat Transfer, 2004.

5) “ Development of a Super-High Speed PMSM Controller and Analysis of the Experimental Results Orlando, FL, July, 2004.

6) “ ”, to be presented at The Eighth World Multi Conference on Systemics, Cybernetics and Informatics (SCI 2004), Development of a New V/f Control for a Super-High Speed PMSM ”, to be presented at The Eighth World Multi-Conference on Systemics, Cybernetics and Informatics (SCI 2004), Orlando, FL, July, 2004.

7) “ Design and Simulation of a Cryogenic Electrical Motor ”, submitted to 2004 AP-S International Symposium and USNC/URSI National Radio Science Meeting, Monterey, California, June 20-26, 2004.

PUBLICATIONS

8) “ Design of a Super-High Speed Cryogenic Permanent Magnet Synchronous Motor ”, submitted to Eleventh International Power Electronics and Motion Control Conference (EPE-PEMC 2004), Riga, Latvia, September 2-4, 2004. (Invited paper: Special section about high and super-high speed motor).

9) “ Design of a Super-High Speed Axial Flux PMSM ”, submitted to IEE Proceedings on Electric Power Applications.

10) “ Design and Preliminary Testing of a Compact, High Speed Centrifugal Compressor 11) “ Compressor 12) “ 13) “ ”, submitted to ASME International Mechanical Engineering Congress and Exposition, 2004.

Computational Analysis of a Compact, High speed Centrifugal ”, submitted to ASME International Mechanical Engineering Congress and Exposition, 2004.

Design and Fabrication of a Compact Recuperative Heat Exchanger with High Effectiveness ”, submitted to ASME International Mechanical Engineering Congress and Exposition, 2004.

Miniature Joule-Thomson (JT) Cryocoolers for Propellant Management ”, submitted to ASME International Mechanical Engineering Congress and Exposition, 2004.

PATENTS UNDER PREPARATION

• Compact, High Speed Centrifugal Compressor with High Efficiency.

• Compact, Recuperative Heat Exchanger with High Effectiveness.

• Compact, High Speed Permanent Magnet Synchronous Motor with High Energy Density and High Efficiency.

• Cryogenic High Speed Motor.

1. Statement of Work 2. Overall View 3. Single Stage Centrifugal Compressor Testing 4. CFD Simulation 5. Bearing Design 6. Design of Permanent Magnet Synchronous Motor and Control Electronics

Statement of Work

Phase I (15 months) July 1, 02 to September 30, 03 Focused on the overall cryocooler design, and design and demonstration of the key enabling component (compressor), and development and characterization (at room temperature) of TiN, MoS 2 , TiN/MoS 2 coatings.

Phase II (12 months) October 1, 03 to September 30, 04 Considers testing and optimization of the compressor and motor, and design of other components (recuperator and gas foil bearing). The tribological coating developed in Phase I will be characterized at cryogenic temperature. Also, an ultra low coefficient-of-friction MoS 2 coating will be developed.

SIGNIFICANCE OF MOTOR cycle optimization BASIS COOLING LOAD REQUIREMENT OF 50 W AT LH 2 TEMPERATURE CENTRIFUGAL COMPRESSOR MUST AT LEAST HAVE A DESIGN SPEED OF 200,000 RPM TO COMPENSATE FOR ITS SMALL SIZE (U tip ∞ D*N) AND TO COMPRESS A LIGHT GAS LIKE HELIUM (EVEN IF THE COMPRESSION PROCESS IS ACHIEVED IN FOUR STAGES) Solution To inevitably Speed Motor 200,000 rpm Design a High Efficient, Super High at lh 2 temperature that is capable of rotating the four-stage compressor at

Year 2 Tasks & Schedule 1.

Months – > Phase II, Task 1 Phase II, Task 2 Phase II, Task 3 Phase II, Task 4 Phase II, Task 5 Phase II, Task 6 Phase II, Task 7 Quarterly Reports Final Report 1 x x x x 2 x x x x 3 x x x x x 4 x x x x x 5 x x x x x x 6 x x x x x x x 7 x x x x x x x 8 x x x x x x 9 x x x x x 10 11 12 x x x x x x x x x x Feb 23 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 bearing for the overall system.

To develop and measure tribological properties for TiN/MoS 2 77K.

and DLC/MoS 2 bilayers at To achieve ultra low coefficient-of-friction MoS 2 coating.

Overall View

MINIATURE SINGLE STAGE CENTRIFUGAL COMPRESSOR testing

Compressor

Test Setup

Electric Motor Coupler

Gage Pressure Versus Speed

12.0

Cast Impeller 10.0

8.0

6.0

4.0

2.0

0.0

0 20000 40000 60000 80000 100000 120000 140000 160000

Speed (rpm )

Straight Blade Impeller Test 1 Straight Blade Impeller Test 2 Straight Blade Impeller w/ Data Acquisition Design Point

Testing of the single stage centrifugal compressor

Required speed  150,000 rpm. Achieved Speed  97,000 rpm Available Drivers Air-Turbine Turbocharger Speeds Electric Motor

158,000 rpm 250,000 rpm 200,000 rpm

Accessories

Controller TC-Control System Mist Lubricator Speed Controlling Unit

Coupling Cost

Flexible Coupler $ 2500 Spline Shaft $ 30,000 Spline Shaft $35,000

Coupling Issue

Flexible Couplers operating 150K rpm  not easily available in the market. A Flexible Coupler to be CUSTOM designed. Design of Flexible Coupler involves 1) Misalignments (angular and parallel) due to oscillating stresses 2) System natural frequency  Compressor and Motor 3) Pulsating torque values need to be quantified for max torque and number of pulses per revolution.

4) System excitation as well as Internal Flexure response must be analyzed.

Bearing Issue

Bearing: The misalignment and failure of the coupler leads to the failure of presently used ceramic ball bearings.

Laser alignment of the system may reduce this problem considerably.

Gas foil bearings are being designed to replace conventional bearings at a later stage.

CFD-Simulation

Sliding Mesh Model (SMM) Geometry

Compressor Parts Pro/E Drawing of the Compressor

Sliding Mesh Model (SMM) Geometry (continued)

1.7M cells, 6 hours/time step on 1 sun Sparc station (30 time steps = 1 blade passing)

Pro/E drawing of Simulation Geometry

Sliding Mesh Model (SMM) (90,000rpm)

The geometry was simulated under the condition that impeller rotating at 90,000rpm. The mass flow rate was found to be 3 g/s at Pout= 1.3 bar. This value agrees well to the experimental measurement.

Sliding Mesh Model (SMM) (150,000 rpm)

1. A strong reversed flow and separation occur inside diffuser.

2. Losses at the diffuser inlet is caused by high impeller outlet flow angle and swirl at the bend.

Geometry

Summary of CFD work

• It is necessary to redesign the diffuser.

• We need deswirler vanes to reduce the swirl flow losses and correct the flow inlet angle at the diffuser inlet.

• Geometry should be simplified without affecting performance so that CFD runs can be faster and the design cycle can be shortened.

Future work of CFD

a. Modify diffuser to have 15 or 20 vanes.

b. Simulate the compressor with cyclic boundary condition in order to shorten the computation time c. Calculate correct flow/vane angles for the new design d. Simulate the redesigned compressor Since the common factor between rows is 5, for the case with 20 diffuser vanes, only 2 IGVs, 1 impeller blade, and 2 diffuser vanes are needed in the computational domain. Comparing to the previous SMM simulation, the current implementation of cyclic boundary condition has reduced the calculation time from 6 hr per time step to 1 hr per time step (for 15 diffuser vanes), and further to half an hour per time step (for 20 diffuser vanes).

Design with 15 diffuser vanes IGV (4), Impeller (2), Diffuser (3)

Bearing Design

Gas Foil Bearing

course of action To acquire a commercially available gas foil bearing suitable for the intended application of testing the motor and study it design.

’ s To continue the design of gas foil bearings with the acquired knowledge base.

Design of Permanent magnet synchronous motor and control electronics

Proposed Motor Test Setup

Sectional View

Properties

Shaft :: Material : Titanium ( 6Al – 4V) Yield Strength (77 K) : 1.4e+8 N/m 2 Density : 4600 kg/m 3 Magnet :: Samarium Cobalt (SmCo) (2-17) Compressive Strength : 833e+6 N/m 2 Density : 8500 kg/m 3

Analysis

Max stress due to centrifugal force is 419 MPa Shaft Tolerance 5 –0.01

0 mm Magnet Tolerance 5 0 +0.01

Assembling Issues of Shaft and Magnet The Titanium Shaft has CTE = 8.8 to 9e-6 /°C The Magnet(SmCo) has CTE = 8.0 to 11e-6 /°C Heat the shaft to 248°C and also cool the magnet to –196°C and assemble it at room temperature.

Max Thermal Stress developed during this process is 700 MPa. Resultant Thermal Stress Induced in the body while cooling it down to 77K is 914 MPa.

As the Thermal Stress is developed due to Compression and Centrifugal Stress due to expansion , so while operating at 77K  Resultant Stress < 1400 MPa (Yield Strength of Ti)

Rotordynamic Analysis Results* for a Solid Shaft Gas Bearing Stiffness : 50,000 lb/in with 2 mm wall thickness • 1 st Bending Critical Speed : 150,708 rpm Gas Bearing Stiffness : 50,000 lb/in with 3 mm wall thickness • 1 st Bending Critical Speed : 152,410 rpm * Courtesy : Dr. Nagaraj Arakere (UFL)

Rotordynamic Analysis Results* for a Hollow Shaft Uniform wall thickness : 2 mm Gas Bearing Stiffness : 10,000 lb/in • 1 st Bending Critical Speed : 170,097 rpm Gas Bearing Stiffness : 50,000 lb/in • 1 st Bending Critical Speed : 227,790 rpm Gas Bearing Stiffness : 10,000 lb/in with a flexible coupling • 1 st Bending Critical Speed : 350,149 rpm Gas Bearing Stiffness : 50,000 lb/in with a flexible coupling • 1 st Bending Critical Speed : > 350,149 rpm * Courtesy : Dr. Nagaraj Arakere (UFL)

Manufacturing

The rotor is being machined by Wire EDM at present.

The rotor shaft is 106.68 mm long, 16 mm in diameter.

Specifications

Output Shaft Power Shaft Speed Shaft Diameter Max. Length Max. Outer Diameter Supply DC Voltage Efficiency Operating Temperature 2000 W 200,000 rpm 16 mm 100 mm 44 mm 28 V > 90 % 77 K (-196 ° C)

PMSM Cross Section

• Permanent magnet: SmCo (Samarium Cobalt) • Winding structure: 6 turns/phase/pole • Gap length: 1.0 mm • Shaft thickness: 1.5 mm • Stator length: 21 mm • Pitch: 1 to 15

Permanent Magnet

PM Structure Cross Section • Magnet material: EEC 2:17-27 MGOe (SmCo).

• Magnetized thru the 13 mm dimension

Motor Stator

• 0.005” silicon steel lamination

ID=0.970” (24.6 mm) OD= 1.380” (35.05 mm) Thickness: 21+/- 0.13 mm Total 165 laminations per stack

Winding

• Litz-wire is used – 3 x 25 strands @ AWG 36 – Diameter of AWG 36: 0.125 mm – Wire insulation: Heavy 200°C Polyesterimide overcoated with polyamideimide to MW 35 – 1x half-lap 0.025 mm sofimide (like kapton) wrap – Overall diameter: 1.60+/- 0.15 mm • This litz-wire can work at 77K.

Eddy Current of Solid Wire

I = 678 A Eddy current is very large when using solid wire, so multi strands Litz-wire is used to reduce eddy current loss.

Loss of 1/36 winding (Litz-wire) P ave =0.36 W Litz-wire: 75 strands @ AWG 36 Total copper AC loss = 36 x 0.36= 13.0 W

Back EMF and Current @ 77K

Simulated Torque=0.0975 N. m Very low harmonics in the back EMF waveforms.

Copper Loss DC Copper Loss AC Stator Iron Loss Rotor Loss Windage & gas bearing Loss Filter Loss Total Loss Motor Efficiency: Control Efficiency: Total Efficiency:

Loss Estimation

Unit R. T 77 K W W W W W 64.9

1.6

4.2

0 18.4 7.6

13.0

10.4

1.0 W 11 W 101.1

61.4 



    2000 ( 2000 95 % 



  61 .

4 )  92 .

2 % 97 % (@ 77 K)

Current Progress • We have ordered the designed permanent magnets and it will be shipped in 4 weeks.

• We have ordered the designed Litz-wire which can work at 77 K. The Litz-wire will also be shipped in 4 weeks.

• We are currently preparing the purchase order for the motor stators.

The Control Scheme

Requirements of key components • MOSFET – Stand up to 28 V voltage and 91 A current.

– Provide 2100 W output power.

– Low on-resistance.

– High switching frequency.

• Driver Chip – Provide enough drive current to new higher power MOSFETs to correctly turn on and off.

Accomplishments till date • Optimal thermodynamic, electrical and mechanical design of a 50 W cryocooler for pre-chilling, densification and ZBO of liquid hydrogen, • Complete validation of the single-stage compressor, • Validation of the design of motor and control system, • Design and fabrication of a compact recuperative heat exchanger for heat regeneration (done as a part of another project being funded by NASA KSC intended for a similar application).