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;

Doctoral Degree Defense

Defender: Lei ZhouAdvisor: Dr. Louis C. Chow

Dr. Jay KapatCommittee members: Dr. Louis C. Chow; Dr. Jay Kapat; Dr. Q.

Chen; Dr. R. Chen; Dr. Larry AndrewDepartment of MMAE

University of Central FloridaNOV 10th,2003

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

AND MINIATURE CENTRIFUGAL COMPRESSOR

Research Background

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

General refrigeration cycle

H eat re jection Q hto am bient at T h

H eat absorption

Q c watts at T c

Fluid expansionto reduce

tem perature

W ork done onprocess flu id

Power in v iacom pressoror drive unit

Q H X

H TS load at ~T c

ch

cCarnot TT

T

Carnotideal

1COP

)3.01.0(

COPCOP ideal

real

General COPOPERATING

TEMPERATURE CARNOT

COP (Watt Input per Watt

Lifted)

"TYPICAL" COP FOR >100 WATT HEAT LOADS

(Watt Input at 300 K per

Watt Lifted at Top) 273 K 0.11 ~ 0.4

200 K 0.52 ~ 2 150 K 1.01 ~ 4 100 K 2.03 ~ 8-10 77 K 2.94 ~ 12-20 50 K 5.06 ~ 25-35 40 K 6.58 ~ 35-50 30 K 9.10 ~ 50-75

Treject = 303 K

Major Cryogenic Technologies

• Stirling machine

• Pulse tube

• Gifford-McMahon

• RTBC (reverse Turbo-Brayton Cycle)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

Cycle efficiency vs. compressor electrical power

0

5

10

15

20

25E

FF

ICIE

NC

Y

(% o

f C

arno

t)

10 10 10 10 100 1 2 3 4

COM PRESSO R INPUT POW ER (W )

TRW

TRW

LM

LM

TRW

NIS T

4-valve

Turbo-B ray ton

M ixed -gas JT

M ixed -gas JT80 K

90 K

G iffo rd -M cM ahon

0.5 W@ 80 K

LM

NIS T

TRW

TRW

LM

TRW

P u lse Tube (Stirling -type )

Stirling


P u lse Tube(G M -type )

C ryocoo lerE ffic iencyT = 80 K

1 W

10 W

50 W

100 W

5 W Activebuffer

Turbo-B rayton

85%

85%

85%

85%

Stirling

JPLLM

Courtesy of Ray Radebaugh, NIST-Boulder

Proposed miniature

RTBC

Cycle efficiency vs. operating temperature

1 5 10 50 100 300

EF

FIC

IEN

CY

(%

Car

not)

TEM PE R ATU R E (K )Radebaugh 2001

C ryocoo ler E ffic iency

0

5

10

15

20

25

30

Turbo-B ray ton

M ixed -gas JT

P u lse Tube (Stirling -type )

Stirling


P u lse Tube(G M -type )

(<10 kW com pressor input)

Courtesy of Ray Radebaugh, NIST-Boulder

Proposed miniature

RTBC

Cryocooler Applications and Operating Regions

Proposed miniature

RTBC

RTBC concept

2

turbine

6

generator compressor

motor

Heat exchanger to Ambient

Heat Load

1

34

5Heat regenerator

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

system

RTBC Mollier Diagram

Work in

Work out

Heat exchange

Heat in

Heat out

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 cm mmm

Micro scale Meso scale Macro scale

10mW W 0.1kW

Poor COP Good COP Best COP

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

),,,,,,( 411 prdTEETPTfCOP tc

1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0 3.20.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

DT1.5 DT1.7 DT1.9 DT2.1 DT2.3 DT2.5 DT3.0 DT4.0

CO

P

Pressure Ratio


1.5 2.0 2.5 3.0 3.5 4.00.235

0.240

0.245

0.250

0.255

0.260

0.265

0.270

0.275

0.280

COP

CO

P

Delta T (K)


0.60 0.65 0.70 0.75 0.80 0.85 0.90

0.10

0.15

0.20

0.25

0.30

COP

CO

P

Ec,Et

Result of thermal efficiency analysis:system parameters

2

turbine

6

generator compressor

motor

Heat exhausted=261W

Cooling Load=20W

1

34

5Heat regenerator

COP=0.083

Eff=0.993

T1=64K

T5=300.2K

T6=76.0K

T2=74.4K

T3=299.5K

T4=440K

Pmotor=262W

Pressure ratio=1.75

Mass flow rate=2.81g/s

Micro-channel heat recuperator

s

d

dw

L

Insulated surface

Thot,in

Tcold,in

Hot end Cold end

Stacked multi-layer construction

Physical model

Cold Neon

Hot Neon

d

d

wY

X

Z

0)]()([4

0)]}()([)]()([{

0)]()([4

2

2

xTxTdx

Tcm

xTxTxTxTdx

TkA

xTxTdx

Tcm

whhh

p

hhccw

sw

wccc

p

30PrRe

nk

dUCpPe

Numerical Model (1-D)

Hot gas node

Cold gas node

Wall

Interface

HjHj+1

Cj Cj+1

Wj Wj+1WtMetal

Material

Insulation material

Hot fluid

Cold fluid

Numerical simulation for single material

Fig.5 axial heat conduction in wall

0.000 0.005 0.010 0.015 0.02080

100

120

140

160

180

200

220

240

260

hot duct wall cold duct

Tem

pera

ture

(K)

Length (m)

20 40 60 80 100 1202

3

4

5

6

7

8

9

10

dT

cold

end t

em

pera

ture

diffe

rence

dT

(K)

Length L (mm)

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

L

dU

L

m

kNuL

TCpmdT

n

2

4

1-D Numerical for two material

Comparison of heat conductivity

Comparison of single material and two materials

Configuration 1+1 (1mm) 10+10 (10mm) 40+40 (40mm)

T(K)3 30 120

SiO20.377 0.848 0.853

Metal0.4005 0.5333 0.6198

Alternative Insulator/Metal

0.4003 0.937 0.991

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

Basic Layout

Flow direction

•Radial IGV

•Mixed flow impeller

•Axial diffuser

R-Z plane X-Y plane

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

IGV shroud curve

IGV hub curve

Impeller shroud curve

Impeller hub curve

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

Geometry implementation in Pro/Engineer(1)



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.

Compressor Testing Run set up

Pressure and Temperature at Diffuser

Exit

Motor Case Temperature

Mass Flow Controller

Power InBearing Temperature

Power Out of Motor

Motor Bearing Temperature

Pressure and Temperature at Inlet

Motor Bearing Temperature

Bearing Temperature

Pressure and Temperature after Mixer

Testing assembly (coupler improvement)Coupler design speed:

~30,000 RPM

Coupler with steel sleeve in test run

~97,000 RPM

Testing assembly

EyeP, T sensor here

Impeller outT sensor here

Diffuser outP, T sensor here

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

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

Compressor testPower versus Speed

0.0

200.0

400.0

600.0

800.0

1000.0

1200.0

1400.0

0 20000 40000 60000 80000 100000 120000

Speed (rpm)

Po

wer

(W

atts

)

Cast Impeller

StraightBladeImpeller Test1

StraightBladeImpeller Test2

StraightBladeImpeller w/DataAcquisition

Compressor testGage Pressure Versus Speed

0.0

2.0

4.0

6.0

8.0

10.0

12.0

0 20000 40000 60000 80000 100000 120000 140000 160000

Speed (rpm)

Gag

e P

ress

ure

(p

sig

)

Cast Impeller

Straight BladeImpeller Test 1

Straight BladeImpeller Test 2

Straight BladeImpeller w/ DataAcquisition

Design Point

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

Speed Impeller Work Mass Flow Raterpm Watts g/s50000 60 3.849 0.57160000 90 3.159 0.37670000 150 3.389 0.35580000 200 3.952 0.38190000 225 5.139 0.612

Compressor Efficiency

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

Documents

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;