Development of two-stage small Stirling cycle cooler for temperatures below 20 K

Development of two-stage small Stirling cycle cooler for temperatures below 20 K* M. Kyoya, K. Narasaki, K. Ito, K. Nomi, M. Murakami t, H. Okuda, H. Murakami ¢, T. M a t s u m o t o * * and Y. Matsubara tt Sumitomo Heavy Industries, Ltd, Toyo Works, Toyo, Ehime 799-13, Japan *Institute of Engineering Mechanics, University of Tsukuba, Tsukuba, Ibaraki 305, Japan tlnstitute of Space and Astronautical Science, Sagamihara 229, Japan ** Department of Astrophysics, Nagoya University, Nagoya 464, Japan ttAtomic Research Institute, Nihon University, Funabashi, Chiba 274, Japan A two-stage small Stirling cycle cooler for temperatures below 20K has been developed for space applications. The prototype cooler has a two-stage displacer driven by a linear motor in a cold head. The working gas, helium, is compressed by a compressor with dual opposed pistons. The typical cooling power is =169 mW at 20 K and the lowest temperature achieved is 12.6K. The input power to the compressor is 82W. This paper reports the results from preliminary tests of the prototype cooler.

Keywords: Stirling cycle cryocooler; two-stage regenerator; space cryogenics

Sumitomo Heavy Industries, Ltd (SHI) had developed single-stage Stirling cycle coolers, which provide refrigeration at 80K for several years, and are currently producing them commercially. SHI is now concentrating on the development of a two-stage small Stirling cycle cooler for temperatures below 20 K. Components for infrared detection systems require cooling at 4 K or below. Such systems presently require stored cryogens in space. However, the lifetime is limited by the mass of cryogen and by the parasitic heat flowing into the Dewar. The key to extending this lifetime is the use of mechanical coolers to reduce the parasitic heat through the shields. A cooler producing a few hundred milliwatts at temperatures between 20 and 40 K might be suitable for such an application. Infrared detection systems in space also require high efficiency, compactness, low weight and long life. Stirling cycle coolers have a high potential to satisfy these requirements 1 5. An example of the application of 20K Stirling coolers to space systems is the IRIS (Infra-red Imaging Surveyor) cryostat, which is currently undergoing feasibility studies and is planned to be launched early in the 21st century. A conceptual drawing of the IRIS cryostat is shown in Figure 1. A special feature of this cooling system is the combination of superfluid helium and a 20 K mechanical cooler which cools the innermost vapour-cooled shield (VCS) directly. By adopting such a hybrid system of liquid helium and a mechanical cooler, a one year lifetime may be achieved with 200dm 3 of helium. In this case, the IRIS cryostat will

have a 1.8 times greater lifetime expectancy with the application of a mechanical cooler.

Description of cooler The prototype cooler is a split Stirling cycle cooler and the schematic drawing is shown in Figure 2. The cooler

- OUTER',

3HEL.

C~CS -ZVCS

SUsPCRS-PAP

SLFE~FLUI© -EL !!.M T ~ N

*Paper presented at the 1993 Space Cryogenics Workshop, 20-21 July 1993, San Jose, CA, USA

0011-2275/94/050431-04 @ 1994 Butterworth-Heinemann Ltd

t<

Figure 1 Conceptual drawing of IRIS cryostat

Cryogenics 1994 Volume 34, Number 5 431

Stirling cycle cooler: M. Kyoya et al. HERMETIC CONNECTOR CONNECTING ]

PISTON PERHANENT MAGNET --

2

7//

PERMANENT MAGNET _ EXPANSION STAGE /-

GLASS

]

" 2nd

2nd EXPANSION STAGE~

MERMETIC CONNECTOR

1 s t STAGE DISPLACER ~'~./REGENERATOR

1st

SPRING .....

E

,/GLASS

PIPE

--

POSITION SENSOR

POSITION SENSOR

STAGE

....

-

CONPRESSOR

COI_D HEAD

Figure 2 Schematic drawing of prototype cooler consists of a cold head unit with a two-stage displacer, a compressor and a gas feed connecting pipe. The total weight of the prototype cooler is 24 kg. It would not be difficult to reduce the weight to under 15 kg, because the prototype cooler has various auxiliary accessories such as the dummy pressure cans, with glass windows for visual observation of the motion in the compressor, and a pressure transducer to measure the pressure waveform. The compressor has dual opposed pistons. This configuration reduces the vibration levels. Each piston in the compressor is directly coupled to the moving coil of the linear motor in a permanent magnet system. The geometry and dimensions of the permanent magnet system are optimized with the aid of a finite element magnetic field program. The program permits the inclusion of analytically intractable effects, such as saturation and leakage. The permanent magnet system provides a constant field of = 0 . 5 2 T in the gap around the iron. In this gap, the coil connected to the piston moves up and down driven by an a.c. current through the coil. The operating frequency is around 15 Hz and the swept volume is ~9.5 cm 3. The compression heat is dissipated through the water cooling tube. The cold head unit has a two-stage displacer moving up and down in the cylinder, driven by a linear motor. The mechanism for the linear motor in the cold head is the same as in the compressor. The dimensions of the first and second displacers are 12.6 mm in diameter and 50 mm long, and 6.2 mm in diameter and 60 mm long, respectively. The first and second regenerators consist

Figure 4 Schematic diagram of experimental apparatus of two different mesh sizes of stainless steel. The cylinder is constructed from copper and stainless steel with a thin wall. The dimensions of the connecting pipe are 3.2mm in diameter and 250mm long. A photograph of the cooler is shown in Figure 3.

Test method A schematic diagram of the experimental apparatus is shown in Figure 4. The two-stage Stirling cycle cooler is tested in a vacuum chamber. The second stage is surrounded by a thin wall aluminium radiation shield, which is attached to the first expansion stage. The temperatures are measured with a Cu-constantan thermocouple on the first stage and an A u + F e - C r thermocouple on the second stage. Electric resistance heaters are mounted to the first and second expansion stages to measure the cooling power. The pressure waveform generated by the compressor is measured with a pressure transducer located at the flange of the cold head. The motions of the compressor piston and the displacer are measured with laser position sensors on the side of the compressor and the displacer. The phase angle between the compressor piston and the displacer is analysed by the fast Fourier transform (FFT) analyser. Two power supplies with electric frequency inverters are used to drive the compressor motors and the cold head motor, independently. The phase angle between the motion of the compressor and the displacer is also electrically controlled.

Test results and discussion

No-load temperature Figure 5 shows the experimental result of the cool-

Figure 3 Photograph of cooler

432

Cryogenics 1994 Volume 34, Number 5

down performance of the two-stage Stirling cycle cooler. The temperature of the second stage reached ~13 K in 25 min and the temperature of the first stage reached =86 K in 45 min with a total input power of 82 W. The lowest temperature achieved so far is 12.6 K. Figure 6 shows the effects of operating frequency on no-load temperatures. The charge pressure and the stroke of the compressor piston were fixed. When the

Stirling cycle cooler: M. Kyoya et al. I

I

I

I

I

I

I

I

E

I

100

I

I

I

]

L

I

250

300 200

8o 1st STAGE TEMPERATURE LU cr < re LM B..

LU

100

....

.--" -

20 2rid STAGE IEM IERATURE . . . . ...... I

I

I

20

X

I

I

40

........

610

~

- m---.----.-~

I

I

m

t

•

I

/

I

I

80

I

100

I

I

]

I

I

I

I

Figure

7

I

I

1.2

I

I

1.4

16

1.

(MPa)

I

I

I

t

I

•

8O

150

~-- --'4

LU n" 60

n ¢tLU n

In 100 Z

INPUT POWER - _ _. ........

L[J p-

40 2nd STAGE TEMPERATURE

•

169mW

2nd STAGE TEMPERATURE

50

/ •

1st STAGE TEMPERATURE

v

re UJ

w re I - 60 < re LU n

/

[

Effects o f c h a r g e p r e s s u r e (with 15 Hz f r e q u e n c y )

100

250

2oo

•

I

1

CHARGE PRESSURE

1st STAGE TEMPERATURE

•

I

0.8

120

~ - 8O

20

50

12.6K j

I

Figure 5 C o o l - d o w n p e r f o r m a n c e (with 0.98 MPa c h a r g e press u r e and 8 2 W i n p u t p o w e r )

I

2nd STAGE TEMPERATURE

1

Time (min)

100

Q._ 100 Z

INPUT POWER

b-

_85~8_~

~__ . . . . . .

150 O ~

~ 40

lst STAGE TEMPERATURE

~ 4o

re LU

w re 7 I - 6o < nLIJ n

200

•

20

e-- -----------e

2~~~,6mW .L 1

I

I 13

I

I 15

I

I 17

FREQUENCY Figure

6

I

I 19

I

t 21

i

0

(Hz)

Effects o f o p e r a t i n g f r e q u e n c y (with 0.98 MPa c h a r g e

pressure)

I

i

I

1 oo

~

200

2nd S T A G E

I

i

I

300

i

400

H E A T LOAD

500 (mW)

C o o l i n g p o w e r versus t e m p e r a t u r e o f s e c o n d stage, Figure 8 with no heat load to first s t a g e (with 0.98 MPa c h a r g e p r e s s u r e ; O, D, 8 2 W i n p u t p o w e r ; O, B, 1 2 8 W i n p u t p o w e r )

operating frequency was 15 Hz, the temperature of the second stage was the lowest. Figure 7 shows the effects of charge pressure on noload temperatures with the operating frequency and the stroke of the compressor piston fixed. The lowest temperature of the second stage was obtained at a charge pressure of 0 . 9 8 M P a and in this case the operating pressure swing was from 0.76 to 1.20MPa. The temperature of the second stage was obtained when the phase angle between the compressor and the displacer was = 8 5 °. Under these conditions the cooling power of the cooler was measured.

25

i

I

QI=0.6W Q2=0.2W LU

cr

20

/

//

p< cr w B_

LU p-

15

LU < F-

co

Q1=0.6~

Q1 =0.3W

Q1 =0W Q2=0W

(.9

10

"O ¢--

Net cooling power

OJ

Figure 8 shows the cooling power versus t e m p e r a t u r e of

the second stage with no heat load to the first stage, for two input powers, 82 and 128W. The relationship between the cooling power and the t e m p e r a t u r e of the second stage was found to be linear. The typical cooling power was 169roW at 2 0 K , and 5 0 0 m W at 33.5 K with

5 80

I

I

90

100

1st S T A G E

TEMPERATURE

110 (K)

C o o l i n g p o w e r o f first a n d s e c o n d s t a g e w i t h i n p u t 9 p o w e r o f 8 2 W (and c h a r g e p r e s s u r e o f 0.98 MPa)

Figure

Cryogenics 1994 Volume 34, N u m b e r 5

433

Stirling cycle cooler: M. Kyoya et al. 25

i

Conclusions

I

Q1 = 0 . 6 W Q2=0.2W

LJJ

n- 2O rr W n

Q2=

.

"

=

LLI I.--

The two-stage small Stirling cycle cooler has been designed, fabricated and tested. The optimum operating conditions were found by the tests. The test results showed the prototype cooler has the capability, with respect to cooling power at 20 K, required for use in the IRIS. The test results provide additional information for the detailed design of the cooler for the next phase.

W

(5

QI=0W Q2=0W

r~ 10

QI=0.3W

QI=0.6W

Acknowledgements

"o c cxl

The authors wish to thank K. Kanazawa, N. Watanabe and K. Kanao of SHI Hiratsuka Research Laboratories for their technical assistance in this work. 5

80

,

I

,

I

,

90 100 110 1st STAGE TEMPERATURE (K)

Figure 10 Cooling power of first and second stage with input power of 128W (and charge pressure of 0.98 MPa)

a total input power of 82 W. When the input power was increased to 128 W the cooler reached a base temperature of l l . 9 K , and the cooling power was 2 2 6 m W at 20K and 5 0 0 m W at 28.9 K. Figures 9 and 10 show the cooling power of the first and second stages as a function of their temperatures. These figures indicate that the cooling power of the first stage has little effect on the second stage temperature. During these tests, heat was rejected at ~320 K.

434

C r y o g e n i c s 1994 V o l u m e 34, N u m b e r 5

References

1 Walker, G Cycle analysis for Stirling refrigerator with multiple expansion stages, perfect regeneration and isothermal processes IntJ Refrig (1990) 13 13-19 2 Walker, G. Cryocoolers: Part 1 and 2 Plenum Press, New York, USA (1983) 3 Bradshaw, T.W. First results on a prototype two stage miniature Stirling cycle cooler for spaceflight applications Proc Fourth lnt Cryocooler Conf Easton, MD, USA (Sept 1986) 303-309 4 Orlowska, A.H., Bradshaw, T.W. and Hieatt, J. Closed cycle coolers for temperatures below 30K Cryogenics (1990) 30 246248 5 Kenug, C.S. and Lindale, E. Effects of leakage through clearance seals on the performance of a 10 K Stirling cycleregrigerator Proc Third Cryocooler Conf Boulder, CO, USA (Sept 1984) 127-134