A TWO-STAGE STIRLING CRYOCOOLER

A TWO-STAGE STIRLING CRYOCOOLER DRIVEN BY OPPOSED PISTON LINEAR COMPRESSOR FOR SPACE APPLICATIONS.

V.K. Bhojwani1, M.D. Atrey1, K.G. Narayankhedkar2, S.L. Bapat1

1Mechanical Engineering department, Indian Institute of Technology

Bombay, Powai, Mumbai – 400 076, India 2Veermata Jeejabai Technology Institute, Matunga, Mumbai, India

ABSTRACT

A two-stage, split Stirling cryocooler with a capacity of 2 W at 100 K and 0.5 W at 50 K is designed. A second order cyclic analysis is used to decide the final geometry of the unit. A compressor with an opposed piston configuration is developed. A flexure stack suspended piston and displacer are used. A moving coil linear motor is developed. Sensors are installed for measuring strokes of pistons and displacer; and pressure at the outlet of the compressor. A 10-channel oscilloscope provides the variations of parameters. The load tests indicated that the cooling capacity is close to the design values. The power input for the compressor is 110 W as compared to 69 W predicted by the analysis. The experimental no-load temperature for the first stage is observed to be close to the predicted temperature. Two identical expanders are developed to check the repeatability. For Expander I, the temperatures attained for a load of 3.33 W on Stage I and 0.95 W on Stage II are 106 K and 74 K, respectively, with a power input of 110 W. The corresponding values for Expander II are 128 K and 66.7 K, respectively, with a power input of 105.6 W. KEYWORDS: Two-stage Stirling cryocooler, Free piston free displacer, Stirling cycle INTRODUCTION

Stirling cryocoolers, with low capacity requirements, have found immense applications for infra-red imaging in space due to valveless operation and high COP. Free- piston, free-displacer arrangements with a motorized displacer (linear motor) make the Stirling cryocoolers reliable and an ideal choice for space applications. A two-stage

678

Downloaded 11 Jan 2011 to 115.184.40.50. Redistribution subject to AIP license or copyright; see http://proceedings.aip.org/about/rights_permissions

Stirling cryocooler with cooling capacities of 2 W at 100 K at Stage I and 0.5 W at 50 K at Stage II has been designed. Cyclic analysis by Atrey et al. [1] was used to predict the geometry of the cryocooler. Various losses viz. regenerator ineffectiveness, shuttle heat transfer, temperature swing loss, PV loss, loss due to heat conduction, pumping loss and loss due to the pressure drop in various components, and mechanical efficiency were calculated to determine the impact on refrigeration and power input. The respective losses are subtracted from the ideal refrigeration and added to the ideal input power to get the net refrigeration and net power input. Optimization of various geometric and operating parameters was carried out to achieve the desired objective. TABLE 1 gives the performance predicted by the cyclic analysis for the operating conditions and geometric parameters given below: Operating frequency 40 Hz Diameter of the piston 22.5 mm Stroke of the piston 10 mm Stroke of the expander displacer 3 mm Diameter of the displacer Stage I 17 mm Length of the regenerator at Stage I 50 mm Diameter of the displacer Stage II 9 mm Length of the regenerator at Stage I 25 mm

TABLE 1 shows the effect of pressure on the performance of the cryocooler. It can be observed that at pressures of 15 bar and higher, the desired performance can be obtained. Even though the requirement of the cryocooler is 2 W at Stage I and 0.5 W at Stage II, cooling capacity of the order of 3.151 W is desired at Stage I and 2.496 W at Stage II at 50 K. Due to variation of the heat capacity of the regenerator material at temperatures of the order of 50 K, larger margin has been kept for Stage II. FIGURE 1 and FIGURE 2 give the cross-section of the opposed piston linear compressor and the two stage expander driven by linear motor. EXPERIMENTAL INVESTIGATIONS

Two almost identical expander units have been fabricated. However, the performance of only one unit is discussed in this paper. A split Stirling cryocooler has been developed. Linear motors with moving coil arrangement have been used for the opposed piston compressor as well as the displacer. The pistons and the displacer are suspended on two stacks of flexure bearings each. The cryocooler was instrumented for the measurements of piston and displacer strokes, pressure variation, and temperature measurements at both stages. The output of all the sensors was fed to a high resolution 10-channel oscilloscope. The results are discussed below. TABLE 1. Result of cyclic simulation for various charge pressures

Pressure, bar 12 13 14 15 16 Ideal RE (Stage– I), W 6.66 7.058 7.78 8.143 8.88 Ideal RE (Stage– II), W 2.59 2.748 3.02 3.171 3.46 Ideal Power, W 34.44 36.54 40.18 42.16 45.92 Net Refrigerating effect (Stage-I), W 1.97 2.264 2.89 3.151 3.8 Net Refrigerating effect (Stage-II), W 2.0 2.098 2.41 2.496 2.83 Total power input, W 57.2 60.74 66.01 69.42 74.82

679


delivery

Pistons

Flexure stack 1

Linear motorFlexure stack 2

LVDT

delivery

Pistons

Flexure stack 1

Linear motorFlexure stack 2

LVDT

FIGURE 1. Cross-section of the opposed piston linear compressor

Two-stage displacer

High vacuum pump

Flexure stack 2

Linear motor

High pressure gas from compressor

FIGURE 2. Cross-section of the two-stage expander driven by a linear motor

Flexure stack 1

680


Experimental results

At a 16 bar charge pressure, a minimum temperature of 81.5 K at Stage I and 43 K at Stage II were observed. However, these temperatures stabilized only at 86 K at Stage I and 45 K at Stage II. The power input to the compressor and expander was 118.8 W and 3.28 W respectively. The stroke of the compressor was measured as 7.15 mm and 2.42 mm for the expander. Load Test – Stage I

Stable temperatures of 94 K at Stage I and 45 K at Stage II were achieved when a heat load of 1.33 W was applied on Stage I. It can be observed from FIGURE 3 that at a heat load of 2 W, a stable temperature of 98 K can be achieved at Stage I. When the heat load was increased to 3.33 W at Stage I, the temperature reached 106 K at Stage I. The load characteristic of Stage I is almost a straight line.

Load Test – Stage II

Next, heat load was applied to Stage II only. The results of this test are shown in FIGURE 4. It can be seen that the temperature that can be achieved at a design heat load of 0.5 W at Stage II is 56.1 K. When an electrical heat load of 0.95 W was applied to the heater on Stage II, the Stage II temperature increased to 66 K.

FIGURE 3. Load characteristics of Stage I

3.33, 106

0, 86

1.33, 94

1.83, 972, 98

2.33, 100

2.83, 103

80

85

90

95

100

105

110

0 0.5 1 1.5 2 2.5 3 3.5 4Heat load applied on stage I, W

Tem

pera

ture

of s

tage

I, K

Heat load applied on stage I, W

681


FIGURE 4 Load characteristics of Stage II Load Test - Heat load applied to both the stages simultaneously

Initially the heat loads of 1.33 W and 0.95 W were applied on Stage I and Stage II simultaneously. After this, the heat load on Stage I was increased as shown in the TABLE 2. The temperature for Stage II increased from 66 K to 74 K as a result of the change in heat load on Stage I. The same is marked in the FIGURE 5. Optimized performance

Experiments were conducted at 15, 16 and 17 bar and up to the limiting power of 120 W to the compressor and 3 W of power input to the expander with various mechanical phase shifts and frequencies. The best results are given in TABLE 3 and FIGURE 6. No load temperatures achieved, with both the heaters active, were 80 K at Stage I and 46.2 K at Stage II, at a frequency of 39 Hz, charge pressure of 15 bar, mechanical phase difference of 76 degrees, displacer stroke of 2.98 mm and piston stroke of 10.37 mm. When the Stage II heat load was 0.95 W and the Stage I heat load was 1.33 W, the temperatures attained at the first and the second stage were 91 K and 61.4 K, respectively. At this point, the heat load was increased in steps of 0.5 W at Stage I and the temperature variation is shown in FIGURE 6. TABLE 2. Load characteristics of the two-stage cryocooler (loads applied to both stages)

Stage I load, W 0 1.33 1.83 2.33 2.83 3.33 Stage II load, W 0 0.95 0.95 0.95 0.95 0.95 Stage I temp. K 86 94 97 100 103 106 Stage II temp. K 45 66 68 70 72 74 Compressor power, W 118.8 118.8 118.3 117.8 117.4 116.9

0, 45

0.45, 55

0.95, 66

0.5, 56.1

40

45

50

55

60

65

70

0 0.2 0.4 0.6 0.8 1 1.2

Heat load applied on stage II, W

Tem

pera

ture

at s

tage

II, K

Heat load on stage II, W

682


0.95, 740.95, 720.95, 700.95, 680.95, 66

0, 45

3.33, 1062.83, 1032.33, 1001.83, 971.33, 940, 86

0

20

40

60

80

100

120

0 0.5 1 1.5 2 2.5 3 3.5

Heat Load, W

Tem

pera

ture

, K

Heat load applied on stage I, W

Heat load aplied on stage II, W

Effect of heat load at stage I on stage II temperature with fixed load of 0.95 W at stage II.

Stage I

Stage II

FIGURE 5. Load characteristics for the two stages TABLE 3. Load characteristics of the two-stage cryocooler (loads applied to both stages)

COMPARISON OF THEORETICAL ESTIMATES WITH EXPERIMENTAL DATA

The parameters corresponding to the experimentally optimized conditions were input as data in the cyclic analysis program and the cooling capacities were determined. The program estimated a cooling capacity of 3.02 W at 96 K at Stage I as compared to 2 W at 96 K observed experimentally at 15 bar. Similarly, the estimated cooling capacity at Stage II from the analysis program is 2.526 W at 58 K as compared to 0.5 W at 58 K observed experimentally.

This indicates that the safety factor for estimation of cooling capacity at Stage I turns out to be 3.02/2 = 1.5 times whereas for Stage II a factor of 2.526 W/ 0.5 W = 5.05 results. This is an important conclusion for designing cryocoolers of 50 K temperature range using cyclic analysis.

Stage I load, W 0 1.33 1.83 2.33 2.83 3.33 Stage II load, W 0 0.95 0.95 0.95 0.95 0.95 Stage I temp. K 80 91 95 98.4 103 108 Stage II temp. K 46.2 61.4 63.7 66.2 68.5 70.8 Compressor power, W 120.0 119.4 118.9 118.4 117.9 117..4

683


0, 80

3.33, 1082.83, 1032.33, 98.4

1.83, 951.33, 91

0.95, 70.80.95, 68.50.95, 66.20.95, 63.70.95, 61.4

0, 46.2

0

20

40

60

80

100

120

0 0.5 1 1.5 2 2.5 3 3.5

Heat load, W

Tem

pera

ture

, K

Heat load on stage I, WHeat load on stage II, W

Effect of heat load at stage I on stage II temperature with fixed load of 0.95 W at stage II.

FIGURE 6. Load characteristics for the two stages under experimentally optimized conditions

CONCLUSIONS

The cryocooler provided a cooling capacity of 3.33 W at 106 K for Stage I. However, when the cryocooler was fully loaded (i.e. 2 W at Stage I and 0.5 W at Stage II) the temperatures achieved were 96 K and 58 K at Stage I and Stage II, respectively (interpolated values). Thus, it can be said that even though the cryocooler satisfies the full heat load requirement of Stage I, Stage II still needs some modification by way of an increase in the second stage expansion volume. ACKNOWLEDGEMENTS

The authors acknowledge the support received from ISAC, Indian Space Research Organization, Bangalore towards this work. REFRENCES

1. Atrey, M. D., Bapat, S. L. and Narayankhedkar K. G., “Cyclic simulation of Stirling cryocoolers”, Cryogenics, 29, pp. 341-347, (1990).

684


A TWO-STAGE STIRLING CRYOCOOLER

Documents

Transcript of A TWO-STAGE STIRLING CRYOCOOLER