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Doctoral Degree Defense. A MINIATURE REVERSE-BRAYTON CYCLE CRYOCOOLER AND ITS KEY COMPONENTS: HIGH EFFECTIVENESS HEAT RECUPERATOR AND MINIATURE CENTRIFUGAL COMPRESSOR. Defender: Lei Zhou Advisor: Dr. Louis C. Chow Dr. Jay Kapat

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Doctoral Degree Defense

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Doctoral Degree Defense

A MINIATURE REVERSE-BRAYTON CYCLE CRYOCOOLER AND ITS KEY COMPONENTS: HIGH EFFECTIVENESS HEAT RECUPERATOR AND MINIATURE CENTRIFUGAL COMPRESSOR

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 10th,2003


Research Background

  • Applications

    • Oxygen/Nitrogen liquefaction

    • Infrared image sensor array

    • Electronic device cooling

    • Out space exploration

    • HTS (High temperature superconductor) cooling


General refrigeration cycle


General COP


Technology

Pros

Cons

S

High efficiency, compact

Vibration, unreliable

P

Compact, reliable, no moving parts

Efficiency lower than Stirling

G

Simple, reliable

Bulky, gas purity sensitivity, inefficient

R

Compact, no vibration, efficient

Moving part

Major Cryogenic Technologies

  • Stirling machine

  • Pulse tube

  • Gifford-McMahon

  • RTBC (reverse Turbo-Brayton Cycle)


Cycle efficiency vs. compressor electrical power

Proposed miniature RTBC

Courtesy of Ray Radebaugh, NIST-Boulder


Cycle efficiency vs. operating temperature

Proposed miniature RTBC

Courtesy of Ray Radebaugh, NIST-Boulder


Cryocooler Applications and Operating Regions

Proposed miniature RTBC


Heat exchanger to Ambient

5

Heat regenerator

turbine

motor

3

6

4

generator

compressor

1

2

Heat Load

RTBC concept

COP=Heat removed from heat load end / Power input to system


RTBC Mollier Diagram

Heat out

Work in

Heat exchange

Work out

Heat in


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


Advantages of miniature RTBC cryocooler

  • Portability

  • Suitable for weight/size critical applications

  • Simplicity

  • Low maintenance

  • Low cost


Miniature cryocooler concept

m

mm

cm

m

Macro scale

Micro scale

Meso scale

0.1kW

W

10mW

Poor COP

Good COP

Best COP


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


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


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


Heat exhausted=261W

5

Heat regenerator

turbine

motor

3

6

4

generator

compressor

1

2

Cooling Load=20W

Result of thermal efficiency analysis:system parameters

COP=0.083

T5=300.2K

Mass flow rate=2.81g/s

Eff=0.993

T4=440K

T3=299.5K

Pmotor=262W

T6=76.0K

Pressure ratio=1.75

T1=64K

T2=74.4K


Insulated surface

d

L

w

s

d

Micro-channel heat recuperator

Tcold,in

Thot,in

Cold end

Hot end


Stacked multi-layer construction


Y

X

Z

d

Cold Neon

w

d

Hot Neon

Physical model


Hot gas node

Wall

Cold gas node

Interface

Insulation material

Hot fluid

Hj+1

Hj

Wj

Wt

Wj+1

Metal Material

Cj

Cj+1

Cold fluid

Numerical Model (1-D)


Fig.5 axial heat conduction in wall

Numerical simulation for single material


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


1-D Numerical for two material


Comparison of heat conductivity


Configuration

1+1 (1mm)

10+10 (10mm)

40+40 (40mm)

T(K)

3

30

120

SiO2

0.377

0.848

0.853

Metal

0.4005

0.5333

0.6198

Alternative Insulator/Metal

0.4003

0.937

0.991

Comparison of single material and two materials


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.


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


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


Compressor Design flow chart

Basic layout design

Basic thermodynamics and sizing

Geometry design

1-D flow calculation

3-D CFD verification

Manufacturing and testing


  • Radial IGV

  • Mixed flow impeller

  • Axial diffuser

Basic Layout

Flow direction


R-Z plane

X-Y plane


3-D geometry design ---- hub-shroud contour (R-Z plane)

Impeller hub curve

Impeller shroud curve

IGV shroud curve

IGV hub curve


0,0

3-D geometry design ---- X-Y plane blade angle

X-Y projection line of blade at shroud/hub surface

Trailing edge

Leading edge


Geometry implementation in Pro/Engineer(1)


Geometry implementation in Pro/Engineer(2)


Geometry implementation in Pro/Engineer(3)


Introduction of 2-zone model of impeller


3-D view of IGV


3-D view of diffuser


Compressor assembly (1)


Compressor assembly (2)


3-D CFD geometry#

#: 3-D simulation results is provided by Xiaoyi Li


3-D results


3-D results


CFD results

Flow Separation

inertia force and centrifugal force

Suggestion

reduce the length of IGV

add deswirl vane


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.


Pressure and Temperature at Inlet

Compressor Testing Run set up

Mass Flow Controller

Power Out of Motor

Motor Case Temperature

Motor Bearing Temperature

Pressure and Temperature after Mixer

Bearing Temperature

Pressure and Temperature at Diffuser Exit

Motor Bearing Temperature

Power In

Bearing Temperature


Coupler design speed:

~30,000 RPM

Coupler with steel sleeve in test run

~97,000 RPM

Testing assembly (coupler improvement)


Testing assembly

To mass flow rate meter


  • ‘Blank Shaft’ Test

  • Motor efficiency = 40% to 70%

    • 90,000 rpm = 65% with load

  • Loss per bearing = 105 Watts at 90,000 rpm


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

Compressor test


Compressor test


Compressor test

Theoretical points


Compressor efficiency

  • Actual output conditions:

    • 93,984 rpm

    • 1.29 pressure ratio

    • 61.2% isentropic efficiency

    • 5.1 grams per second mass flow rate


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


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


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