Experimental study of a pulse tube cold head driven by a low frequency thermal compressor

Experimental study of a pulse tube cold head driven by a low frequency thermal compressor

Y. Zhao1,2_{, W. Dai}1_{, S. Vanapalli}3_{, X. Wang}1_{, Y. Chen}1_{, E. Luo}1

1_{Key laboratory of Cryogenics, Technical Institute of Physics and Chemistry, Chinese Academy of} Sciences, 100190, Beijing, China

2_{University of Chinese Academy of Sciences, 100049, Beijing, China} 3

University of Twente, P.O.B. 217, 7500 AE, Enschede, The Netherlands ABSTRACT

Cryocoolers operating at liquid helium temperature span a number of application domains, such as cooling of superconducting magnets, SQUID devices etc. GM type cryocoolers are widely used at liquid helium temperature but with shortcomings of using an oil-lubricated compressor that require regular maintenance and rotary valves that reduces the efficiency of the cryocooler. We are developing an alternative system that makes use of a Vuilleumier type thermal compressor. The system consists of a Stirling type pulse tube cryocooler that provides a cold heat sink to a thermal compressor. The thermal compressor generates pressure wave to drive a second pulse tube cold head. We experimentally studied the influence of pre-cooling temperature and frequency on the performance of the pulse tube cold head. The lowest recorded temperature is 24.3 K with a pressure ratio of 1.18 and a frequency of 3 Hz. In this paper, the design of the cooling system and preliminary experimental results are presented.

Keywords: Pulse tube cryocooler, Thermal compressor, Liquid helium temperature, Displacer

1. INTRODUCTION

Cryocoolers operating at liquid helium temperatures are used in the several application areas including space exploration, military, medical and low-temperature physics. For many of these applications, a pulse tube cryocooler is a preferred candidate because of the absence of cryogenic moving parts. Due to the advances in cooler design and the development of magnetic regenerative materials (Mikulin et al. (1984), Zhu et al. (1990), Hashimoto et al. (1992)), a cooling temperature below 4 K was first obtained with a three-stage GM type pulse tube refrigerator (Matsubara and Gao (1994)). A lowest temperature of 2.13 K near the lambda line of 4He has recently been obtained (Thummes et al. (1996)). A two-stage pulse tube cooler reaching a no load cooling temperature of 2.3 K and a cooling power of 1.0 W at 3.82 K is reported (Wang et al. (1997), Xu et al. (2011)). These experimental achievements clearly demonstrate the capability of pulse tube coolers for cooling at or below 4.2 K.

Nowadays, GM-type or GM-type pulse tube cryocoolers are the main technologies to cater to the 4 K (sub-) temperature range (Radebaugh et al. (2008), Zhi and Han (2013)). The use of oil-lubricated compressor and the rotary valve in these cryocoolers lead to low efficiency, low power density and regularly require maintenance. The emergence of application of 4 K cryocoolers in critical areas such as superconducting magnets and in aerospace, place a stringent requirement in the specific power, efficiency and maintenance. As an alternative to GM-type cryocoolers, the Stirling type cryocooler has been one of the focus areas of research (Sun et al. (2009), Wang (1997), Wang (1997)). Lockheed Martin corporation and Zhejiang University of China have developed Stirling type cryocoolers, which can attain the liquid helium temperature (Qiu and Cao (2011), Qiu et al. (2012)), but the refrigeration capacity is very low and the thermal efficiency is poor compared to the GM type cryocoolers. The fundamental reason is that the operating frequency of the Stirling type cryocoolers is too high, which leads to the increase of losses especially in the low temperature regenerator packed with spheres. This paper presents the ongoing research efforts of an alternative type of pulse tube cryocooler driven by a Vuilleumier (VM) thermal compressor. The temperature difference between the ambient and a cryogenic temperature is utilized to produce a pressure wave to drive a pulse tube cold head with the aim to obtain liquid helium temperature. In 2002, Dai et al introduced a similar concept with liquid nitrogen as the cold sink. Using a work transfer tube in the system and a GM type cold head to precool the lower stage regenerator, a minimum temperature

of 3.5 K was obtained with operating frequency of 1 Hz and an average pressure of 2.1 MPa (Dai et al. (2003)). In this paper, a Stirling type pulse tube cryocooler is used as a cold sink for the thermal compressor, which allows flexibility of adjusting the sink temperature. The work transfer tube is also replaced with a more classical displacer.

2. SYSTEM CONFIGURATION

As shown in Fig.1, the configuration studied here includes the three sub-systems: VM thermal compressor, Stirling pulse tube cryocooler that provides the required cooling capacity for the thermal compressor and the pulse tube cryocooler with the aim of providing cooling at liquid helium temperature. The novelty of this concept is the use of VM type thermal compressor instead of a mechanical compressor. The thermal compressor generates pressure wave by making use of the temperature difference between the cryogenic sink temperature and the ambient. The cryogenic sink temperature is provided by a Stirling type pulse tube cryocooler through a copper thermal bridge.

Table 1 and 2 lists main component dimensions of the thermal compressor and the lower stage pulse tube cryocooler, i.e. subsystem A and C, respectively. Inside the thermal compressor, indigenously developed linear motor is used to drive the displacer at off-resonance mode. Since the required driving power is small for the displacer, the linear motor has a large power margin. In the Stirling type pulse tube cryocooler, i.e. subsystem B, two cold heads driven by a common pressure wave generator are used to provide sufficient cooling for the thermal compressor.

The thermal compressor without load, i.e., subsystem A plus B, has been systematically studied with the help of simulations and experiments (Dai et al. (2015), Zhao et al. (2015) ), and typically generates a pressure ratio of about 1.3 at 3 Hz with the cold end temperature being around 80 K. In this paper, sub-system C, i.e. the low temperature stage pulse tube cryocooler is added. It has a U-type pulse tube configuration and uses orifice-reservoir and double inlet as phase shifters to improve the phase relationship between mass flow and pressure.

1 2 3 4 5 6 7 8 13 12 10 11 14 15 16 17 18 19 20 21

Fig.1. Illustration of the VM type pulse tube cooler for liquid helium temperature

Subsystem A: 10. Ambient HX, 12.Regenerator, 13.Cold HX, 11.Displacer, 9. rod;

Subsystem C: low temperature pulse tube cryocooler

②

①

③

Subsystem B: Stirling type pulse tube cryocooler as pre-cooler

Subsystem B: 1. linear compressor, 2.Ambient HX, 3. Regenerator, 4.Cold HX, 5.Pulse tube, 6. Secondary ambient HX, 7. Inertance tube, 8. Reservoir;

Subsystem C: 14. Regenerator, 15. Low temperature stage cold head, 16. Low temperature stage pulse tube, 17. Low temperature stage secondary ambient HX, 18, 19 and 20. Low temperature stage phase shifters, 21.Reservoir

Table 1 Main system parameters of the thermal compressor

Component Parameter Value

Driving motor Mover mass 3.4kg Stiffness coefficient 68 kN/m Damp 50 Ns/m Resistance 0.87Ω Inductance 18.5 mH Regenerator Diameter 40 mm Length 156 mm

Filler 60# stainless steel mesh hot heat exchanger

Diameter 40 mm

Length 52 mm

width of gas gap 1 mm

Cold heat exchanger

Diameter 18 mm

Length 25 mm

width of gas gap 0.5 mm

Displacer

Diameter of hot side 59.16 mm Diameter of cold side 60 mm Maximum displacement 7 mm

Table 2 Main system parameters of low temperature stage pulse tube cryocooler

Component Parameter Value

Regenerator Diameter 18 mm Length 80 mm Filler HoCu2 Sphere diameter 0.3 mm Porosity 0.39

Cold heat exchanger Diameter 18 mm

Length 8 mm

Pulse tube Diameter 8 mm

Length 380 mm

Fig.2 Illustration of system and thermometer positions

Fig.2 illustrates the setup inside the vacuum chamber and temperature sensor locations. The whole system has 12 calibrated resistance thermometers placed at different positions of the system. Specially, two Lake Shore CernoxTM thermometers (T9 and T10) are used to monitor temperatures at the cold head and the cold end of the pulse tube of the added lower stage. The following section preliminary results with the test rig are discussed. Unless specified in the experimental results, the displacer always moves with a maximum allowable displacement of 7 mm.

3. EXPERIMENTAL RESULTS AND DISCUSSION

3.1 Cooling performance of the Stirling type pulse tube cryocooler

Table 3 lists typical performance of the Stirling type pulse tube cryocooler which a 4 kW pressure wave generator. The two cold heads could reach lowest no-load temperature around 55 K with a slight difference. At a frequency of 70 Hz, an average pressure of 3.7 MPa and an input acoustic power of 2 kW, the two cold heads can provide a total cooling power of about 80 W @77 K. A larger input power is hindered by current limit of the pressure wave generator.

Table 3 Typical performance of the Stirling type pulse tube cryocooler

Frequency Average pressure Electricity power Acoustic power Pressure ratio Total cooling power 70 Hz 3.7 MPa 3151 W 2013 W 1.18 ~80 W@77 K

Table 4 Temperature in K of thermal bridge and cold heads

T1 T2 T3 T4 T5 T6 T7 T8

61.8 61.0 64.5 63.2 66.6 60.1 59.2 62.4

To evaluate the performance of the thermal compressor and the static heat losses inside the system, we observed the temperature distribution when the thermal compressor is not operating. Table 4 shows the values of the temperature recordings. T2 and T7 at the cold ends of the Stirling type pulse tube cryocooler reaches 61 K and 59.2 K. The temperature of cold end of thermal compressor is about 65.6 K. The temperature difference between them is about 4-6 K. According to the previous experimental results,

Subsystem A

Subsystem C Subsystem B

the Stirling type cryocooler can provide the cooling power of about 9 W @ 60 K. This provides an insight into the thermal resistance along the thermal bridge as well as the static thermal loss.

3.2 System performance General performance

Figure 3 shows the cool-down curves of the system. Due to small pressure ratio provided by the thermal compressor, it approximately takes about 2～3 hours for the cold head to reach a lowest temperature which settles below 30 K.

Fig.3 System cool-down curves, operation of the thermal compressor starts when the Stirling type pulse tube cryocooler reaches a relatively stable temperature

Performance at 5 Hz

Table 5 shows the operating parameters of thermal compressor at 5 Hz. The average pressure is 2.39 MPa. When the thermal compressor operates at the temperature difference between 300 K and 100 K and the displacement of displacer is 7 mm, the pressure ratio is 1.18. Fig.4 and Fig.5 show the dependence of lowest temperature on valve openings. Valve 1 refers to orifice and Valve 2 and 3 connected in series refers to double inlet valve. Using two valves for double inlet allows for a finer adjustment of flow resistance. Fig.4 shows the dependence of temperature on the Valve 3 when Valve 1 opens by 2 turns and Valve 2 opens by 3.25 turns respectively. Fig.5 shows dependence of temperature on Valve 1 when Valve 2 opens by 3.25 turns and Valve 3 opens by 12 turns, respectively.

Table 5 The main operating parameters of thermal compressor at 5 Hz

Frequency Average pressure Displacer

displacement Temperature difference

Pressure ratio 5 Hz 2.39 MPa 7 mm 300K～100K 1.18 0 1 2 3 4 5 6 7 8 9 10 11 20 40 60 80 100 120 140 160 180 200 220 240 260 280 300 T em p era tu re /K Time/h T1 T2 T3 T4 T5 T6 T7 T8 T11 T12 T9 T10 Open thermal compressor

T11 T12

9 10 11 12 13 14 15 26 28 30 32 34 T em p era tu re /K Turns 1.0 1.5 2.0 2.5 3.0 3.5 26 28 30 32 34 T em p era tu re /K Turns

Fig.4 Dependence of temperature T9 on the Valve 3

Fig. 5Dependence of temperature T9 on the Valve1

As shown in the figures above, we can see the lowest temperature T9 is 28.1 K at the frequency of 5 Hz, when Valve1 opens by 2 turns, Valve 2 opens by 3.25 turns and Valve 3 opens by 12 turns, respectively. The temperature of the pulse tube cold end is about 32.2 K, and the slope gets larger as the temperature increases.

Performance at 3 Hz

Table 6 shows the main operating parameters of thermal compressor at the frequency of 3 Hz. The average pressure is 2.4 MPa. When the displacement of displacer is 7 mm and thermal compressor operates at the temperature difference between the 300 K and the 90 K, the pressure ratio is also 1.18. Fig.7 and Fig.8 show dependence of lowest temperature on valve openings.. Fig.7 shows dependence of temperature on the Valve 3 when Valve 1 opens by 2 turns and Valve 2 opens by 3.25 turns, respectively. Fig.8 shows dependence of temperature on Valve 1 when Valve 2 opens by 3.25 turns and Valve 3 opens by 14 turns, respectively.

Table 6 The main operating parameters of thermal compressor

Frequency Average pressure Displacer displacement Temperature

difference Pressure ratio

6 8 10 12 14 16 20 22 24 26 28 30 32 34 36 38 40 42 44 T em p era tu re /K Turns 1.0 1.5 2.0 2.5 3.0 20 25 30 35 40 45 50 T em p era tu re /K Turns

Fig.6 Dependence of temperature on the Valve 3 Fig.7 Dependence of temperature on the Valve 1

As shown in the above figures, lowest temperature attained is 24.3 K at 3 Hz, when the Valve1 has 2 turns, Valve2 has 3.25 turns and Valve3 has 14 turns.

4. CONCLUSIONS

This paper presents a modified configuration of a low temperature pulse tube cryocooler driven by a VM thermal compressor. This configuration could be one of the candidates for future portable, dry and more efficient cryocooler operating at liquid helium temperature. From the preliminary experiments, lowest temperature attained is about 24 K at a frequency of 3Hz and about 28K at a frequency of 5 Hz. So far, maximum pressure ratio can only be 1.18 and the sink temperature falls in the range of 80-100 K. We estimate that more than 80 W cooling power is consumed, which is far more than our estimation through numeric model. The reason could be the poor performance of thermal compressor or the lower stage regenerator. A systematical study including experimental and numerical simulations is currently underway to investigate these losses, reduction of which may bring down the sink temperature and improve the system performance with a larger pressure ratio.

ACKNOWLEDGMENT

This work is financially supported by National Natural Science Foundation with project number 51376187 and the Institute Supervisor Fund of Technical Institute of Physics and Chemistry of Chinese Academy of Sciences.

REFERENCE

Dai, W., Matsubara, Y., Kobayashi, H., 2003. Experimental results on VM type pulse tube refrigerator. Cryocoolers 12. 331-336

Dai, W., Wang, X., Zhao, Y., 2015. Characteristics of a VM type thermal compressor for driving a pulse tube cooler. Cryocooler 18.

Hashimoto, T., Ogawa, M., Hayashi, A., 1992. Recent progress on rare earth magnetic regenerator material. In Advances in Cryogenic Engineering, Vol. 37. Plenum Press, New York, p. 859.

Matsubara, Y., Gao, J.L., 1994. Novel configuration of three-stage pulse tube refrigerator for temperatures below 4 K. Cryogenics 34. 259.

Mikulin, E.L., Tarasov, A.A., Shkrebyonoch, M.P., 1984. Low temperature expansion pulse tubes In Advances in Cryogenic Engineering. Vol.29. Plenum Press, New York, p. 629.

Qiu, L.M., Cao, Q., 2011. A three-stage Stirling pulse tube cryocooler operating below the critical point of helium-4. Cryogenics 51. 609–12.

Qiu, L.M., Cao, Q., Zhi, X.Q., 2012, “Operating characteristics of a three-stage Stirling pulse tube cryocooler operating near 5 K,” Cryogenics 52, 382–8.

Radebaugh, R., Huang, Y.H., O’Gallagher A, 2008. Calculated regenerator performance at 4 K with helium-4 and helium-3. Cryogenics 53. 225–34.

Sun, D.M., Dietrich, M., Thummes, G., 2009. High-power Stirling-type pulse tube cooler working below 30 K. Cryogenics 49. 457–462.

Thummes, G., Bender, S., Heiden, C., 1996. Approaching the 4He lambda line with a liquid nitrogen prccooled two-stage pulse tube refrigerator,” Cryogenics 36. 709.

Wang, C., 1997. Numerical analysis of 4 K pulse tube coolers: Part I. Performances and internal processes. Cryogenics 37. 207-213.

Wang, C., 1997. Numerical analysis of 4 K pulse tube coolers: Part II. Performances and internal processes. Cryogenics 37. 215-220.

Wang, C., Thummes, G., Heiden, C., 1997. A two stage pulse tube cooler operating below 4 K. Cryogenics 37.159.

Xu, M., Takayama, H., Nakano, K., 2011. Development of High Efficiency 4K Two-Stage Pulse Tube Cryocooler. Cryocooler 16.

Zhu, S.W., Wu, P.Y., Chen, Z.Q., 1990. Double inlet pulse tube refrigerator: an important improvement.Cryogenics30. 514.

Zhao, Y., Wang, X., Dai, W., 2015. Experimental study and numerical simulation of thermal compressor

performance in VM refrigerator in liquid helium temperature region. CRYOGENICS (Chinese journal), No.6. 11-15.

Zhi, X.Q., Han, L., 2013. A three-stage Stirling pulse tube cryocooler reached 4.26 K with He-4 working fluid. Cryogenics 58. 93–96.