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MINIATURE ENGINEERING SYSTEMS GROUP

(http://www.mmae.ucf.edu/~kmkv/mini)

Two-Stage CryoCooler Development for Liquid Hydrogen

Systems

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Miniature Engineering Systems GroupCore 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

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Motivation and Objective Storage of cryogenic propellants (LH2 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.

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Approach and Innovation

Two Stage Reverse Turbo Brayton Cycle (RTBC) CryoCoolerreliable, efficient, compact and light weightRTBC 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 (MoS2), diamond-like-carbon (DLC) coatings, bilayer coatings of DLC/MoS2 for low values of coefficient of friction at cryogenic temperatures.

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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, coatingFY03-04 (12 months): Design: 4-stage compressor1, Rotor system, GFB2

Fab/testing: Motor, Recuperative heat exchanger3, coatingFY04-05 (12 months): Fab/testing: 4-stage comp., other HXs, turbo-expander4, GFB w/coatingsFY05-06 (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.

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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 MoS2 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.

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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 tribological coatings – coefficient of

friction, hardness, wear resistance and durability at cryogenic temperatures.

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Performance Comparison – Quantifiable Research

Results

Component Performance characteristic

Commercially available

Short term goal

Long term goal

Motor Speed(rpm)

Efficiency

150,000(Koford Motors)

30%

200,000

60%

200,000

96%

Compressor

(mesoscale)

Efficiency 35%

(Creare)

45% 75%

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Performance Comparison – Quantifiable Research Results

Component Performance characteristic

Commercially available

Short term goal Long term goal

Heat

Exchanger

Effectiveness

Size

(Length)

98%

67 cm

(Creare Inc,.)

95%

8 cm

99%

< 8 cm

Controller Efficiency 80% 60% 95 %

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Component Performance 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 770K LN2 and

finally satisfactory operation in

the Cryocooler

TiN (20-25 GPa)

Coefficient of Friction (COF) DLC

0.1-0.15 COF

At Room Temp TiN=0.143

TiN/MoS2 on Glass = 0.05-0.1

TiN/ MoS2 on Al

= 0.12-0.18

TiN/ MoS2 on Si wafer = 0.045

At 770K LN2 0.24-0.48

Nitrides

At Room Temp TiN (< 0.1)

At 770K LN2 ZrN (0.4-0.8)

Performance Comparison – Quantifiable Research

Results

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Overall System

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Thermodynamic Schematic

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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.

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Design of a Single Stage Compressor

inlet guide vanes

impeller blades

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flow inlet

Inlet guide vanes Contoured

endwall

Full blades and splitter blades

Vaned, axial diffuser with multiple vane segments

flow exit

List of Innovations

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Compressor Assembly Animation

This animation represents the assembly of the compressor housing containing the internal parts, coupled with the motor.

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Fabrication

Rapid PrototypingStereo lithography models used for visualization and fit.

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Fabrication

Manufacturing Impeller cast from A356

Aluminum.4-axis CNC machining of

diffuser and inlet guide vane.

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Rotor Balancing

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Testing

Measurement InstrumentationThermocouples for

temperature measurements.

Pressure transducers for pressure measurements.

Mass flow meter for flow rate measurements.

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Testing Performance Instrumentation

Digital Multimeter for power measurements.Oscilloscope for motor speed measurements.

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Sliding Mesh - Grid

1. Most accurate method – unsteady fluid field gives interaction between igv-rotor-stator.

2. More computationally demanding.

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Mixing Plane-Grid

1. Steady state solution – losing interaction between stator and rotor.

2. Cost effective.

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Solution for IGV

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Preliminary Results from Impeller Flow Simulation

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Preliminary Mechanical Design of 4-stage Helium

Compressor

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Four Stage Compressor – Assembly Structure

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Four Stage Compressor – Rotating Part

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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.

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Permanent Magnet Synchronous Motor

Specifications

Output Shaft Power 2000 W

Shaft Speed 200,000 rpm

Shaft Diameter 16 mm

Max. Length 100 mm

Max. Outer Diameter 44 mm

Supply DC Voltage 28 V

Efficiency > 90 %

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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.

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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

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Shaft Profile

Shaft will not fail under:

Bending stress

Shear stress

Shaft whirl

Operational speed is above the 1st critical speed.

Dynamic deflection at first critical speed :

0.361mm < 0.5mm (gap)

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Shaft Centrifugal Stress

Maximum Stress developed:

1210 MPa < 1700 MPa (Yield Strength of the Titanium based alloy)

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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).

0.0000 0.0001 0.0002 0.0003 0.0004-0.12

-0.10

-0.08

-0.06

-0.04

-0.02

0.00

Torq

ue (N

.m)

Time (s)

1.5% ripple

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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.

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Controller Hardware and Software

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PMSM Loss

Copper loss 16.9 W

Shaft eddy loss 20 W

Iron loss 10.4 W

Bearing loss 10 W

Filters loss 11 W

Windage loss 12.8 W

Total loss 81.1W

%96)1.812000(2000 m

%95c%91 mc

Motor Efficiency:

Control Efficiency:

Total Efficiency:

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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’.

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Gas Foil BearingsConfiguration

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Gas Foil BearingsDimensions 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.

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Gas Foil BearingsSection under consideration for present analysis

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Section meshed in Gambit

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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 CoatingsObjective

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SampleID

N2 : ArRatio

Atomic PercentN : Ti

AverageHardness

AverageElastic

Modulus(GPa)

GPa HV(Kgf/mm2)

1 0.5: 6 N :Ti = 50.3:49.7 9.32 878.47 144.20

2 0.5 : 4 N :Ti = 53.05:46.95 ----- ----- -----

3 1: 4 N :Ti = 52:48 16.62 1567.02 200.21

Good adhesion, thickness uniformity and stoichiometry of TiN and MoS2 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 MoS2 bilayers on Si wafer, glass and aluminum are prepared.

Expected excellent coefficient of friction (COF) and wear resistance.

TiN by DC and MoS2 by RF Magnetron Sputtering

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TiN Coating – Aluminum and Steel Substrates

Trial Run

Coefficient of Friction

Load (g) Average

Friction

1 0.15 27.4

0.1432 0.14 27.4

3 0.14 27.4

Fully reacted characteristic golden color.All trials run at 42 rpm at RT.Average COF for TiN/Al coating substrate =

0.143 TiN/MoS2 for friction measurement at USF.TiN for friction measurement at USF.

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TiN and MoS2 broad peaks indicate nanocrystalline nature of sample.

XRD Plot - MoS2 Coating on Glass

0

20

40

60

80

100

120

0 10 20 30 40 50 60 70

2 Theta (Degrees)

Inte

ns

ity

(C

ou

nts

) (006)

X-ray Diffraction – MoS2 and TiN

XRD Plot - TiN Coating on Glass

0

20

40

60

80

100

120

140

30 35 40 45 50 55 60 65 70 75 80

2 Theta (Degrees)

Inte

ns

ity

(C

ou

nts

)

(111)

(200)

(311)

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Scanning Electron Microscopy

- TiN and MoS2

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 = 52.91 : 47.09).

TiN MoS2

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TiN / MoS2 - Si Wafer

Coefficient of friction tests at UCF with the assistance of Dr. Chen and his colleagues requires deposition of TiN/MoS2 coating on bumps prepared by photolithography technique and also on plain Si wafer (1 cm2) to minimize the contact area between two rubbing samples and thereby providing more accurate coefficient of friction measurements.

TiN/MoS2 Sample

Si Sample with TiN /MoS2 film on bump

Bumps

SlidingForce

SlidingForce

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Friction Test [1N Normal Load ]

0

0.05

0.1

0.15

0.2

1 21 41 61 81 101 121 141 161 181

Time (Sec)

Co

effi

cien

t o

f F

rict

ion

Friction Test – Si Wafer

Average coefficient of friction for the TiN/MoS2 Bilayer

coating on Si wafer with 1 N normal load was = 0.045.

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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

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Tribological Coatings Research Progress

Literature values for coefficient of friction (COF)

Hard Coating COF COF - 770K LN2 Hardness Wear Resistance

DLC 0.1-0.15 0.24-0.48 40 GPa Good

Nitrides Less than 0.1 (TiN) 0.4-0.8 (ZrN) 20-25 GPa Good

Current results obtained for this project better than state of art

Substrate roughness is an important criteria for COF

Tribological Coating Coating Pair Substrate COF Hardness

TiN Steel Aluminum 0.143 25 GPa

TiN / MoS2 TiN / MoS2 Si Wafer 0.045 -

TiN / MoS2 TiN Glass 0.05-0.1 -

TiN / MoS2 TiN Aluminum 0.12-0.18 -

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Deposition parameters for TiN, MoS2 and TiN+MoS2 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

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Cryogenic temperatures degrade tribological properties.

However, hydrogen improves lubrication.

TiN and DLC provides good hardness and low friction.

Improved tribological properties expected for TiN/MoS2 and DLC/MoS2 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) MoS2 coating study in collaboration with Dr. Martin, France.

Tribological CoatingsFuture Research

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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.