Review of experimental research on Joule–Thomson cryogenic refrigeration system

Applied Thermal Engineering 157 (2019) 113640

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Review of experimental research on Joule–Thomson cryogenic refrigeration system

T

Hui Genga, Xiaoyu Cuia, , Jianhua Wengb, , Hailong Shea, Wenqing Wanga ⁎

a b

⁎

School of Energy and Power Engineering, University of Shanghai for Science and Technology, Shanghai 200093, China College of Energy and Mechanical Engineering, Shanghai University of Electric Power, Shanghai 200090, China

HIGHLIGHTS

cryogenic refrigeration systems have been classified based on their structures. • J-T fluids in J-T cryogenic refrigeration systems have been summarized. • Working of mixed working fluids should be optimized for better refrigeration effect. • Ratio • Micro-channel J-T refrigerator is a promising device for refrigeration and cooling. ARTICLE INFO

ABSTRACT

Keywords: Joule–Thomson Cryogenic refrigerator Refrigerant Experimental research

With the increasing miniaturization of high-powered electronic components, technologies for rapid cooling in small spaces have received more and more attention. Joule-Thomson (J-T) refrigeration is an effective means for rapid cooling. It is widely used in military and medical refrigeration, electronic equipment cooling and many other fields. This paper describes the structural development of J-T refrigerators of the traditional Hampson type, etched microchannel type and others. It reviews experimental studies on refrigeration components and the charge composition of the mixtures in the J-T cryogenic system. It also introduces the main applications of the JT cryogenic system. Finally, it summarizes the above research and puts forward developmental prospects, hoping to provide reference for future experimental design and research.

1. Introduction As early as in 1895, Joule-Thomson (J-T) technology using gas to obtain low temperature has been applied to industrial gas liquefaction. Since the 1950s, micro J-T cryocoolers have been widely used in infrared detectors, military devices [1,2], small electronic devices such as computer chips [3] and frozen-section (FS) medical surgery [4,5] among other equipment and procedures because of their small size, low refrigerating temperature, fast cooling speed and lack of moving parts. Each micro J-T cryocooler consists of a heat exchanger, throttling element and evaporation chamber. Fig. 1 shows a single stage Hampson J-T cryocooler, which is one of the most common types. The micro spiral fin tube is wound around the mandrel and placed in the Dewar tube. The high pressure gas in the finned tube is pre-cooled by low pressure gas flowing back from the tube, and then high pressure gas enters the throttling element to reduce the pressure and temperature. Then the gas evaporates in the evaporation chamber and absorbs heat

⁎

of the heat source at the cold end. At last, it returns the flow to complete the cycle [6]. The J-T cryogenic throttling systems are promoted in various application fields, due to their optimal performance. Advances in processing and manufacturing have led to the rapid development of the J-T refrigerator, which has become smaller, much compact, and much efficient. J-T refrigeration can enhance heat transfer efficiency inside the heat exchanger and the throttling cooling effect by adding sintered powder internally [7], optimizing the fin size [8], or adopting measures such as increasing the number of regenerative heat transfer stages and the number of throttling expansion stages [9,10]. Due to the development of micro-channel etching in recent years, some scholars have applied this technology to the design of J-T refrigerators in order to improve refrigeration efficiency [11,12], that is a new idea for traditional J-T refrigerators The cooling principle of the J-T effect is the differential throttle effect of gas. When the J-T coefficient of the gas is positive, the gas

Corresponding authors. E-mail addresses: [email protected] (X. Cui), [email protected] (J. Weng).

https://doi.org/10.1016/j.applthermaleng.2019.04.050 Received 25 October 2018; Received in revised form 10 March 2019; Accepted 14 April 2019 Available online 15 April 2019 1359-4311/ © 2019 Published by Elsevier Ltd.

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Fig. 1. Single stage Hampson J-T cryocooler.

mass flow rate and heat transfer coefficient are generated based on high pressure during heat transfer. To sum up, the Hampson-type J-T TR has a strong structure and good heat transfer effect, which has been widely used in the early high-pressure system. The typical structure is shown in Fig. 2(a) [18]. The high-pressure gas flows into the fin tube, and outside of the tube is low-pressure reflow. In 1968, Stephens [19] used a small J-T TR to cool down infrared detectors, as shown in Fig. 2(b). The Hymatic Engineering Company made the structure of the refrigerator; the prototypes had the characteristics of the Hampson-type refrigerator. They added an adsorber at the outlet of the throttle valve to prevent clogging which can be caused by phase change of the working fluid during throttling. The performance parameters used to evaluate the J-T TR include cooling capacity and cold end temperature. Many scholars have made attempts to obtain better cooling performance. The mass flow of working fluid is one of the main factors influencing cooling capacity. Hui et al. [20] adopted a parallel double spiral structure to increase cooling. Skye et al. [6] designed a two-stage mixed gas J-T cryocooler in order to obtain a lower cold junction temperature, as shown in Fig. 3. The high pressure gas of the 1st stage is throttled to 240 K, then the conventional vapor compression cycle labeled “1st stage” provides precooling for the “2nd stage,” and the gas enters the throttling element to cool down to 150–180 K. Chien et al. [21] studied a self-adjusting J-T effect refrigerator to solve the problem of temperature fluctuation after throttling caused by unstable mass flow, as shown in Fig. 4. The working principle was that the temperature-sensitive bellows self-adjusted with temperature fluctuations. When the cryocooler was working, the temperature-sensitive bellows fluctuated with temperature changes of the cold end of the refrigerator, driving the needle valve to adjust the flow rate. This method not only ensured the cold end temperature, but also minimized the flow rate of the working medium. Maytal [22] combined the regenerative part of the Hampson-type JT cooler with the throttling section without a separate throttling device. The distributed J-T effect caused by the frictional pressure drop along the tube wall was used to replace the concentrated J-T effect generated by the throttle valve. The single layer and double layer test pieces were tested experimentally. Results showed that the cryocooler with doublelayered and spiral-tube had the fastest cooling rate; it reached the cold end temperature of 66 K. The Hampson-type J-T effect TR has evolved from a single structure to a multi-stage pre-cooling, multi-spiral channel running in parallel to meet the requirements of lower temperature zones and larger cooling capacity. Designers have optimized all aspects; the technology is mature enough to have been successfully promoted and applied commercially. However, the special structure of the spiral finned tube also limits the further increase in flow rate and cooling capacity of the refrigerant. Many applications require use of multiple pieces. These factors affected the further development of the Hampson-type J-T cryogenic TR.

temperature will decrease with the decrease in pressure by the throttling process. At a normal temperature, the optional pure working fluids are nitrogen, argon and carbon dioxide. In the research, non-azeotropic alkanes, mixed with nitrogen or argon, are used to form a multi-component as the agent of throttling refrigeration [13–16]. This meets the requirement of closed cycle refrigeration that has a low pressure ratio and can obtain ideal refrigeration performance. Components of the mixed refrigerant and the optimal ratio between the components can be determined by experimental means. In order to broaden the cooling temperature zone and improve refrigeration performance, many scholars are also exploring new types of throttling refrigerant [17]. This paper reviews the J-T effects of low temperature throttling refrigeration systems (TRS) from three aspects. The first section explores the development of structures of miniature J-T throttling refrigerators (TRs). The second summarizes experimental research and development of throttling refrigerants used in cryogenic TRS. The generalization of J-T refrigeration systems in different application areas is in Section 4. Finally, the paper summarizes and forecasts aspects of the above research. 2. Research of refrigerator in J-T cryogenic refrigeration systems The micro-miniature J-T effect low-temperature (TR) consists of a heat exchanger, a throttling element and an evaporation chamber. Conventional heat exchangers mostly use a type of spiral finned tube. With the expansion of application fields and advances in processing technology, other types of regenerative heat exchangers have also been developed, such as types of tube-in-tube, flat, micro-channel, etc. As the main cooling device in the refrigerator, the throttling element can have many type structures, such as micro-pore, porous powder metallurgy sheets, capillary tube and micro-channel. The evaporation chamber mainly performs heat exchange with the heat source. According to the heat source type, its external structure is generally designed as a column, tower, or flat plate. In addition to the above basic structures, others can be devised to satisfy more application requirements. For example, a lower temperature can be obtained by adding a pre-cooling structure; and control of gas flow can be obtained by adding a self-adjusting device such as described below. According to the development of micro-miniature J-T throttling refrigerator, this chapter first introduces the Hampson-type J-T throttling refrigerator with mature technology, wide application and structural integration; then introduces some other special structures of J-T throttling refrigerator in special fields; finally, the micro-channel J-T throttling refrigerator with more popular research in recent years is introduced in detail. 2.1. Hampson-type J-T throttling refrigerator (TR) The Hampson-type J-T TR is simple in structure. Inside a shell, the capillary tube is wound on the mandrel to form the heat exchanger. At the end of the refrigerator, the refrigerant is throttled through a narrow diameter throttle valve. In the early stage, high pressure refrigerant was used in J-T throttling refrigeration system (TRS). Significant throttling effect can be achieved by large pressure drop. Therefore, the performance requirement of the heat exchanger is not high. In addition, large

2.2. Special structure type J-T throttling refrigerator (TR) J-T TR has the advantages of fast cooling speed, low cooling temperature and simple structure, so it is expected to be applied in a variety 2

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(a) [18]

(b) [19] Fig. 2. Hampson type J-T cooler.

Fig. 3. Geometric schematic of a 2-stage cryoprobe showing primary components and fluid flow paths [6].

Fig. 4. Schematic drawing of bellows control self-regulating J–T cryocooler [21].

of fields. In order to adapt it to the application of different fields, in the development of J-T TRS, many researchers have designed special structures for heat exchanging, throttling and evaporation. Heat exchanger has a great influence on the performance of J-T TR, so many

researches are conducted about it. For example, in order to make the structure more compact, a planar J-T TR was used in the infrared detector [23], In the missile weapon system that needs rapid cooling, a dual-gas J-T TR was designed [24], etc. 3

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a

Internal structure of flat J-T cooler[24]

b

The J-T cooler double gas path[24]

Fig. 5. J-T effect cooler for detector.

2.2.1. Special regenerative heat exchanger type refrigerator The J-T cryogenic TR is used in infrared detectors. Due to the special working environment of the infrared seeker and high precision requirements of the temperature control system, the design of the refrigerator should be short and compact, and able to cool down quickly. The research of early flat J-T refrigerator mainly focused on the vibration of the detector and the compactness of the seeker. In 2000, L’Air Liquide of France developed a flat J-T refrigerator for infrared detectors, as shown in Fig. 5(a). The refrigerator used a single gas double spiral tube, (labeled 4a and 4b), coiled along the involute groove, (labeled 2), to the central expansion chamber (3). High pressure gas, liquid argon, was ejected from the pinhole (6) to cool the detector (9) and low pressure gas formed a reflow along the involute (2) to cool the inlet refrigeration. Finally, liquid argon is ejected from pinhole 6, and the detector can be cooled to working temperature [24]. The heat recovery structure of the traditional Hampson-type refrigerator was a single-gas spiral tube. J-T TRs need to have faster cooling rates in missile weapon systems to ensure weapon response time. In 2003, the German company BGT (Bodenseewerk Geraetetechnik GmbH) developed a dual-gas J-T refrigerator to achieve rapid cooling. Referring to Fig. 5(b), the dualchannel J-T cooler differed in that the main circuit refrigerant was precooled to about 150 K quicker by means of the auxiliary-stage gas path. Thereby higher throttle efficiency and a more specific heat capacity refrigerant, such as R14 (CF4), was used to improve heat transfer efficiency in the auxiliary stage. The test showed that cooling time required

for the double-channel J-T chiller to reach the cold end temperature of 100 K was as follows: (1) when the ambient temperature was 22 °C, less than 1.6 s; (2) when the ambient temperature was 50 °C, less than 2.5 s; (3) the steady state temperature was 87 K ± 3 K [24]. In order to operate superconducting devices, cooling to below the critical temperature is essential. In many of these devices the heat of dissipation is so small that the required cooling power can be reduced to a few milliwatts. In 1998, Holland et al. [25] produced two casing type J-T chillers with different heat recovery lengths (270 mm, 105 mm). The outer tube tip of the countercurrent heat exchanger was closed by a small sealing plug, and the throttle valve was a NiCr wire placed at the top end of the inner tube, as shown in Fig. 6(a). With 10 MPa nitrogen as working fluid, the flow rate of the two refrigerators was 4.2 × 10−6 kg/s and 2.3 × 10−6 kg/s, respectively; the cold end reached the lowest temperature of 82 K. In 2010, Lin et al. [26] studied a glass capillary micro-J-T refrigerator with a casing in the tube. Six hollow glass fiber tubes were built in the glass capillary of the test piece. The glass fiber tubes were filled with high pressure gas. The tube had a low pressure return flow; the top was a flat plate; the throttling element was a J-T expansion valve; and the structure was as shown in Fig. 6(b). Results showed that the mixture with high pressure of 1.6 MPa and low pressure of 0.1 MPa was used as the working medium. When the flow rate was 11 μmol/s, it stabilized to 140 K, and the lowest instantaneous temperature reached was 76 K. In order to adapt microcooler to cryogenic cooling in limited and confined spaces, the researchers proposed a wire-type Joule Thomson

(a) single Structure diagram of the tube-in-tube J-T effect cooler[25]

(b) Multi-casing Structure diagram of the tube-in-tube J-T effect cooler[26]

Fig. 6. Structure diagram of the tube-in-tube J-T effect cooler. 4

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Fig. 7. Wire-type J-T microcooler with a concentric heat exchanger [27].

microcooler with a concentric heat exchanger. In 2010, Widyaparaga et al. [27] produced a micro-miniature J-T effect refrigerator with a regenerative section as a soft-line concentric counter-flow heat exchanger, see Fig. 7. The outer layer was polyetheretherketone and the inner layer was concentric stainless steel. The flow and top temperature of the micro-miniature J-T effect chiller were calculated by heat transfer and flow empirical correlation, and compared with the experimental results. The results showed that the J-T effect had a more important influence on refrigeration than regenerative heat transfer. In 2015, Gong et al. [28] carried out experiments on shell-and-tube and plate-fin heat exchangers as regenerative heat exchangers in TRs. And with the application of oil-lubricated mini-compressors, a closed-cycle, low cost, long lifetime microcooler for electronic devices was developed. Under the same working conditions, the former reached 140 K without heat load; the latter reached 110 K without heat load, and had 4 W cooling capacity at 118 K cold end temperature. Eugeniusz et al. [29] proposed a sintered powder heat transfer J-T refrigerator, as shown in Fig. 8(a). The middle capillary was a highpressure inflow gas, and the low-pressure gas flowed back through the peripheral powder-shaped structure; the particle size of the powder was between 0.4 mm and 0.8 mm. Experiments showed that when the inlet pressure was 15 MPa, the time taken for argon and nitrogen to cool to the lowest temperature (78 K, 90 K) was 230 s and 150 s, respectively. In 2006, Dvornitsyn et al. [30] combined the J-T effect of working fluid with sublimation and proposed a sublimation cooler, as shown in Fig. 8(b). The bottom of the low temperature chamber was solid CO2. Sublimation absorbed the load heat. The sublimated CO2 gas passed

Fig. 9. Pin Fins flow model [31].

through the porous chassis to generate J-T effect secondary refrigeration. The experiment used an open system in which high pressure liquid CO2 was stored in a gas cylinder. Results showed that the refrigerator provided 1–20 W cooling capacity at a low temperature of 200–210 K. In 2015, Wang et al. [31] designed and manufactured a miniature JT TR with a channel size of 0.1 mm based on 3D printing technology. The structure of the regenerative heat exchange section was a nonconnected pin rib type, such as in Fig. 9. This effectively reduced the loss of cooling caused by the axial heat conduction of the refrigerator. Preliminary experiments showed that the mixed gas used as refrigerant reached a temperature of 230 K. 2.2.2. Special throttling and evaporation structure type refrigerator The type of throttling element and evaporator was also an important factor affecting the performance of the cryocooler. In 1968, based on the requirements of rapid cooling, Stephens [19] obtained the fastest cooling rate from a tapered J-T refrigerator with a head at a 90° angle, as shown in Fig. 10(a). Liquefaction was achieved within 2 s with argon at an inlet pressure of 400 atm. In 2004, Paine [32] studied the J-T effect of the microporous channel throttling structure. As shown in Fig. 10(b), the orifice had a diameter of 1.5 mm and a length of 1.8 mm. Results showed that hydrogen at 4.84 MPa was used as the refrigerant in the microporous channel, and cooled to 19 K in 20 to 25 min.

（a）J-T liquefier with sintered heat exchanger[29]

（b）The bottle-sublimation cooler[30]

Fig. 8. Cooler structure diagram in Ref. [29] and [30]. 5

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Micropore channel J-T cooler[33]

High speed minicooler[32]

c

Details of the test probe[34]

Fig. 10. Cooler structure diagram in Refs. [19,32,33].

In 2014, Lee et al. [33] designed a closed-loop J-T cryoablation device for the treatment of great saphenous veins (GSVs), which provided uniform cooling over a large area by using multiple expansion parts along the length. Refrigeration performance of the specimen in the longitudinal direction was studied. Results showed that the temperature difference between the thermocouples at each of the measuring points arranged in accordance with Fig. 10(c) was small.

device is generally required, which also leads to a complicated structure. The pre-cooling device generally includes a structure such as a spiral tube heat exchanger with a long tube length, high heat exchange efficiency, and compact structure. The working medium for pre-cooling mostly uses N2, O2, Ar, and/or H2. The pre-cooled and liquefied throttling medium is generally a low-boiling working medium such as Ne or He. After throttling, the temperature reaches a low temperature zone generally below 4 K. A self-regulating flow device is required in the J-T effect refrigerator to control the flow rate of the working fluid, Bellows were often used as control components. Nowadays, with the advent of memory alloys, they are often used to control flow, combining quick start and automatic adjustment. For example, in 2012, Yao [36] designed a conical throttling cooler for the red focal plane detector, as shown in Fig. 12. The polyimide material itself is deformed and reduced at a low temperature; it is restored to its original state in a normal temperature or a high temperature range (60 °C) to form a displacement amount, thereby achieving the purpose of rapidly controlling the flow rate. The performance indicators of the micro-miniature J-T TR mainly include cooling rate, flow stability, cooling capacity and working temperature range. Based on application fields, there are different requirements for the performance of the chiller. Results showed that the multi-flow channel and the structure of the chiller were designed to achieve rapid cooling. The cryocoolers used regenerative heat exchangers to have a great influence on the cold end temperature and cooling capacity, including such types as double helix, casing, long hose, etc. and various types of throttling structures and evaporation chambers. The pre-cooling device was added to obtain a lower cold-end temperature. The J-T valve was controlled by a bellows or a memory alloy to stabilize the working fluid flow. It was foreseeable that with the expansion of the application field, there would be more J-T effect TRs that could meet some specific requirements such as being flexible, compact and exquisite.

2.2.3. Other special structure refrigerators The cooling of the liquid nitrogen temperature zone, such as optical detectors in space missions, usually requires a pre-cooled J-T TR. The cryocooler adds pre-cooling before the throttling of the refrigerant. The low-boiling working fluid is pre-cooled to the conversion temperature before throttling, then goes through the regenerative heat exchange and finally throttles to reach the liquid helium temperature zone. For J-T throttling fluids with lower conversion temperature, such as helium and neon, pre-cooling at the throttling inlet can improve the cooling performance of the J-T system and reduce throttling and cooling time. For example, Gladun [34] conducted an experiment on a J-T refrigerator; as shown in Fig. 11(a). The throttling refrigerant Ne heat was precooled by a heat exchanger and precooled through a low temperature bath (the pre-cooled working medium was liquid nitrogen, argon or oxygen). Then it passed through two adjustable expansion throttle valves to reach the cold end temperature. This refrigerator reached a low temperature of 30–90 K at an inlet pressure of 200 atm. Daunt et al. [35] designed a J-T liquefier-cryostat, as Fig. 11(b) shows. He3 at the pressure of 4 atm enters from valve 1 and flows through the upper heat exchanger, the spiral pre-cooler, and the J-T heat exchanger (spiral tube pre-cooler and J-T expansion valve). The spiral tube pre-cooler is immersed in He4 at a temperature of 4.2 K. The pre-cooled gas is throttled by a J-T expansion valve, and the liquefied He3 flows down into the liquefaction tank. The liquefier-cryostat has a cooling capacity of 230 mW at a maximum flow rate of 14 L/min and a temperature of 3.2 K. For the pre-cooled J-T TR, a multi-stage pre-cooling heat exchange 6

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Fig. 11. Cooler structure diagram in Ref. [34] and [35].

necessary to improve the heat transfer effectiveness and coefficient to ensure sufficient precooling of working fluids before throttling. The heat exchanger of the Hampson-type J-T TR could not meet the requirements sometimes. The micro-channel heat exchanger has a high heat transfer coefficient and the heat transfer effectiveness could reach 99% [37,38]. Therefore, the J-T TR with micro-channel heat exchanger has been gradually developed and applied in small closed TRS. However, in the application, the cross section geometry of the micro channels and the channel layout have impact on flow resistance and uniformity in the refrigerator, respectively. And many researchers have carried out studies on it [39–45]. With the development of researches, multi-stage micro-channel J-T TRs have emerged, which can achieve lower cooling temperature [12]. In 1982, Little [39] of Stanford University in the United States successfully developed a micro-throttle chiller made of glass based on photolithography technology. The etched micro-channels ranged through several tens of micrometers. The structure and typical cooling performance of the cryocooler are shown in Fig. 13. Lerou and Cao of the University of Twente in the Netherlands optimized the structural parameters of the etched micro-channel J-T refrigerator. In their prototypes, silicon wafers were used as chiller

Fig. 12. Conical self-regulating J-T cooler [36].

2.3. Etching micro-channel type J-T refrigerator With the development of closed J-T throttling refrigeration system, the compressor-driven closed refrigeration system is limited in volume, and it can only provide low pressure ratio. Therefore, the cooling condition after throttling is limited, while the pressure ratio is not high, the flow rate is reduced, so heat exchange is affected adversely. It is

dimension

6.0 1.4 0.2 cm

3

Experimental

Refrigeration

conditions

performance

Workin

Inlet

Tempe

Cooling

pressure

Volume flow rate

Cooling

g fluids

time

rature

power

N2

10 MPa

18 ml/s

7.5 min

83 K

250 mW

Fig. 13. The structure Diagram and its performance parameters of the microcooler in Ref. [39]. 7

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Dimension/mm Heat exchanger

Throttling

Experimental conditions Working fluids

Inlet pressure

Mass flow

Refrigeration performance Outlet pressure

Temper ature

Cooling power loss

18.9×1.5× 0.03

0.083×1× 3×10-4

N2

8 MPa

1 mg/s

0.6 MPa

97 K

4.57 mW

Fig. 14. The structure Diagram and its performance parameters of the microcooler in Ref. [40].

material. In 2004, Lerou [40] designed and manufactured a cross-flow micro-channel J-T cryocooler. The upper layer of low-pressure gas precooled the lower layer of high-pressure gas. The structure and performance are shown in Fig. 14. To make the temperature of the refrigerator reach a lower zone, Cao et al. [11] designed a parallel two-stage etched micro-channel J-T effect refrigerator which consists of three layers of glass sheets. Gold plating on the outside reduced heat loss from the surface of the cooler. The precooling cycle was etched in the middle layer; the refrigeration cycle was etched on the bottom layer. The first stage used N2 as the working medium to pre-cool the secondary throttling medium H2. The cooling performance of the J-T cooler was analyzed by the established dynamic finite element model. Results showed that the optimized overall cooler dimensions were 20.4 × 85.8 × 0.72 mm for a net cooling power of 50 mW at 97 K in the first stage and 20 mW at 28 K in the second stage. Its structure and parameters are shown in Fig. 15. In 2016, Cao [12] produced a J–T cryogenic cooler with parallel two-stage expansion, as shown in Fig. 16. The test piece used N2 as the cycle working medium. The high-pressure gas after passing CFHX I was split into two gas streams. In one, most of the gas flowed through the first restriction to the low-pressure channel of CFHX I. In this case, the expansion along the low-pressure channel had a distributed J-T effect. The second gas stream flowed to CFHX II and then to the second restriction. The mass-flow rate through the second restriction was smaller than the first; however, the gas underwent a first stage pre-cooling before the secondary throttling, which achieved a lower refrigeration temperature. Experiments showed that when the nitrogen pressure was reduced from 8.0 MPa and 8.5 MPa to 0.1 MPa, the time required for the cryocooler temperature to drop from 295 K to 83 K was 12 min and 9 min, and the cooling capacities were 88 mW and 98 mW, respectively. In order to control the flow rate of the working fluid in the refrigeration system, Zhu et al. [41] designed an adaptive J–T cooling system including silicon-microfabricated heat exchanger and microvalve components. As shown in Fig. 17(a) and (b), the heat exchange structure was sequentially laminated with silicon and heat resistant glass; the throttle expansion of the gas was controlled by a piezoelectrically driven micromachined valve. The system had a cooling capacity of 200 mW at 228 K, 1 W at 239 K, and a parasitic heat load between 300 and 500 mW.

The main advantage of using glass as the material of the refrigerator test piece was that the axial heat conduction of the glass material had less influence on the performance of the refrigerator. At the same time, non-metallic materials such as glass due to limited capacity, could not meet the conditions of high pressure working conditions. The higher the inlet pressure of the working fluid means the better the throttling effect. With the development of printed circuit board technology and atomic diffusion fusion technology, printed circuit board heat exchangers capable of withstanding high pressure have emerged. Mikulin et al. [42] used this technology to make two etched micro-channel J-T TRs. Fig. 18(a) shows a rectangular flat plate refrigerator consisting of three flat plates, a copper plate with a thickness of 0.2 mm between two stainless steel plates of the same thickness of 0.3 mm. Each plate is engraved with zigzag channels 2.25 × 0.12 mm for low-pressure gas, and rectangular channels 0.65 mm × 0.13 mm for high-pressure gas. Fig. 18(b) shows a disk type refrigerator which contains only one etched plate which is covered by a 0.13 mm thick steel plate to form a spiral passage. The cross-section of the heat exchanger and throttling element are 0.5 × 0.135 mm and 0.2 × 0.12 mm, respectively. Narayanan [43] engraved the regenerative, throttling and evaporation zones on a stainless steel sheet, as shown in Fig. 19(a). In order to reduce the length of the channel, the heat exchanger channels were surrounded by a flow structure. The test piece had a thickness of 1–2 mm, a total length of 50–100 mm, a rectangular channel of 200 × 30 μm (width × height). Experiments have shown a cooling capacity of 10–1000 mW at the tip temperature of 50–150 K. Gong [29] used a wire-electrode cutting method to machine parallel rectangular micro-channels on both sides of a stainless steel plate to produce a J-T refrigerator with a plate-fin heat exchanger. The schematic is shown in Fig. 19(b). The low pressure channels of the refrigerator had a fin height of 3 mm and a high pressure passage of 2.5 mm. The fins had a width of 0.3 mm and a dimension between the two fins of 0.2 mm. There was a transverse groove every 30 mm on the heat exchanger to eliminate axial heat transfer. Near the inlets, the lateral groove spacing was 5 mm to reduce inlet flow effects and distribution non-uniformities. A mixed refrigerant composed of hydrocarbon and nitrogen was used as an experimental working fluid; this cryocooler reached a minimum temperature of 120 K. Wenqing Wang et al. [44,45] designed and fabricated stainless steel 8

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First stage

Second stage

Working fluid

N2

H2

pressure drop

80 to 6 bar

40 to 5.7 bar

97 K

28 K

192 mW

35 mW

capacity

50 mW

20 mW

Mass flow

14 mg/s

0.9 4mg/s

Cold-end temperature

Total refrigerating capacity Net refrigerating

Fig. 15. Explosion Diagram of microcooler and its performance parameters in Ref. [11].

The micro-channel type J-T effect refrigerator developed rapidly in recent years due to its large surface area and high heat transfer intensity. The structure of the early microchannel refrigerator was simple; the refrigerator was mostly made by photolithography using silicon or glass. Nowadays, with printed circuit board technology and atomic fusion welding, stainless steel J-T TRs have emerged, as well as other metals. The number of channel layers and the number of channels in the layer can be appropriately increased according to requirements to realize parallel amplification of cooling capacity. Multi-stage cooling can be realized through channel design, and the diversification of the structure of the refrigerator test piece is further realized. However, the axial heat conduction caused by the metal has to be considered, as well as heat loss outside the test piece, and many other factors. In addition, the flow and heat transfer laws of microchannel refrigerators need to be well studied. With the participation of more scholars, there will be more diverse structural types, and it is expected that the microchannel J-T refrigerator can be applied to a wider range of fields.

Fig. 16. Exploded view of the microcooler with parallel two stage expansion [12].

3. Research of refrigerants in the J-T cryogenic refrigeration system

laminated rectangular micro-channels and micro-pin-fin J-T refrigerators. Their schematic diagram was the same as shown in Fig. 20(a). In the rectangular prototypes, six layers of high and low pressure channels were stacked, and six parallel rectangular channels were etched in each layer. Two thin plates were mirrored to form a complete channel, and each channel corresponded to a throttling channel with a smaller cross section. As shown in Fig. 20(b), the channel size of the regenerative section was 0.55 × 0.4 × 100 mm; the throttle section contained a rectangular channel with an equivalent diameter of 0.12 mm and a length of 40 mm. Their experiments showed that with 7 MPa argon as the refrigerant, it reached the cold end temperature of −95 °C. In 2016, the author processed a micro-pin–fin cryocooler with the purpose of reducing axial heat conduction. As shown in Fig. 20(c), the regenerative throttling element of the cooler used a fork row with a center-to-center distance of 0.75 mm. The cylindrical group structure simultaneously realized the pre-cooling and throttling effects. Argon gas was used as the working medium. Under the inlet pressure of 5 MPa, the stable temperature of −82.3 °C was achieved.

Most of the early studies on low temperature TRs were on open-type systems. Nitrogen, argon and other pure working fluids with remarkable throttling effect at room temperature were used as refrigerants, and high-pressure gas cylinders were generally used as gas supply sources. With their promotion, especially in some cooling equipment that requires long-term use, closed-type low-temperature TRS have been developed, but are constrained by the compression ratio of closed systems. The refrigeration effect of pure working fluids in closed systems is not ideal, so the demand for a mixed working medium with low pressure ratio and excellent refrigeration performance was generated. This century, many scholars have conducted in-depth research on various aspects of mixed working fluids composed of nitrogen and hydrocarbons. This chapter summarizes the studies of single pure working fluids and a variety of mixed working fluids in this type system. It deals with mixed refrigerants from three aspects: the components of mixed fluids, the ratio used in experiments, and the effect of mixed working fluids on system performance with system component development. 9

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a

Perforated plate heat exchanger design.

b

Micromachined piezoelectric valve.

Fig. 17. Piezoelectric valve microchannel of the J-T effect cooler [41].

a Rectangular refrigerator plate

demand-flow miniature J-T refrigerator designed for cooling infrared detectors, Yong-JuHong [48] and others studied the cooling temperature and cooling time of the J-T refrigerator with argon and nitrogen under six inlet pressures. Results showed that the mass flow rate in the system increased with the increase of inlet pressure. Under the same pressure, the cooling rate of argon was faster than that of nitrogen; but at the same refrigeration temperature, the cooling capacity of nitrogen was greater than that of argon. In order to investigate the cool-down characteristics of J-T TR at different initial pressures, in 2006, the author [49] studied the effects of different volumes of gas on the refrigeration effect with 8–12 MPa of nitrogen as the refrigerant. Experiments showed that with constant supply pressure or volume of 1000 cm3, the greater the pressure, the greater the mass flow rate, or the lower the cold end temperature. The mass flow rate was proportional to the P/T0.5 even though the temperature and supply pressure were concurrently changed. The humidity of the throttling medium also affected the performance of the cryocooler. A major issue in long-term operation of micro-machined J-T TRs is the clogging of the microchannels and the restriction due to the moisture. Cao [50] selected high-pressure nitrogen with different humidity as the experimental refrigerant, and its humidity was controlled by the inlet filter. Experiments have shown that during the cooling process, the humidity of the inlet gas was higher, the flow rate of the working fluid was slower, and a high cold-end temperature resulted in a high reduction rate of the mass flow rate. Meanwhile, the moisture in the vacuum tank adhered to the surface of the refrigerator, increasing the amount of radiation heat exchange between the refrigerator and outside, and losing more cooling capacity, which led to the increase in clogging rate. J-T TRSs generally use nitrogen or argon as pure working fluids with a cooling effect at normal temperature. When the structure of the TR is determined, its refrigeration performance is mainly determined by the inlet temperature, pressure and working fluid flow of the working fluid. With the decrease of the inlet temperature, the increase of the inlet pressure and the increase of the working fluid flow, the refrigeration performance is enhanced to varying degrees. At the same time, the working fluid humidity will also affect the flow rate, heat transfer intensity, etc. Therefore, the refrigerant is selected according to the cooling demand of the application, and in order to meet the greater cooling requirements of the closed system, mixed refrigerants can be selected.

b Disk refrigerator plate

Fig. 18. The cryocooler structure diagram in Ref. [42].

3.1. Experimental study of pure refrigerant When pure working fluid is used as the refrigerant in the J-T effect TRS, it is possible to compare the cooling performance between different kinds of pure working fluids, for example, refrigeration temperature, cooling time and cooling capacity. There are also experimental studies on the inlet pressure, temperature, humidity and mass flow of pure working fluids. The cooling performance of the J-T refrigerator is related to factors such as working fluid type and inlet pressure. The start up time required for a liquefier to reach its working temperature is a very important parameter. This time should be as short as possible. To study the starting time of throttle-type microliquefiers, Bodio et al. [46] conducted an experiment on a J-T cooler with argon and nitrogen as working fluids. The inlet pressure and cooling time were fitted. It was found that argon cooling time was lower than nitrogen at the same pressure; for the same gas, the higher the pressure results the faster the cooling time, which is consistent with the conclusion reached by Maytal [47]. To characterize the thermal performance of a commercial 10

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Fig. 19. The cryocooler structure diagram in Ref. [29] and [43].

(pressure-driven enthalpy variation at a fixed temperature) over the entire operating temperature range [15]. Therefore, the method to obtain large refrigeration capacity is to maximize the minimum enthalpy difference, that is, the mixture with minimum isothermal enthalpy difference ( HT ) min is the best mixture of refrigerants. Considering other factors in refrigeration system, some scholars have proposed several optimization objectives. For example, Little [51] Hmin proposed optimization objective Q (x1, ...,xn) = Variance . Besides the ( H) minimum enthalpy difference, the uniformity of the enthalpy difference in the temperature range is also considered to ensure the minimization of thermodynamic irreversibility in heat transfer. Gong et al. [52] took the compressor power consumption into account and proposed T T H (xi, T , Ph, Pl) (T ) = 0 T C × as the optimization objective. W C Nevertheless, most optimizations are still based on the minimum enthalpy difference ( HT ) min . When optimizing mixed refrigerants, the physical parameters such as enthalpy of mixed refrigerants at a specific temperature can be obtained by using the physical properties software DDMIX, REFPROP and STRAPP, and the mixed refrigerants can also be calculated according to the gas state equation and the mixing rules of mixed refrigerants [26,53]. In mixed refrigerants, components are selected according to their boiling points and the temperature range of interest. By controlling the amount of different components in a mixture, the enthalpy difference can be made more uniform across the temperature range, and it also can be maximized. The components with different boiling points play different roles in the heat transfer throttling process. The components with high boiling points are evaporated gradually during the heat transfer process. Meanwhile, the components with low boiling points are condensed. Thus, the refrigeration of the pre-cooled low boiling point refrigerants in the low temperature range can be realized. Lower refrigeration temperature can be achieved by using the regenerative heat transfer throttling of the mixed refrigerants. The study of mixed working fluids in low temperature TRS began in the 1930s. American scholar Podbielniak [54] first applied a mixture of hydrocarbons (CH4/C2H6/C3H8/C4H10/C5H12) in a cascading heat exchanger and phase separator. The cycle achieved a low temperature of 103 K, and obtained a national patent. After this study, researchers realized the feasibility of mixed refrigerant and began to explore the components of mixed working fluids. In 1969, German scholar Andrija [21] proposed using nitrogen and hydrocarbon as mixed working fluids

（a）Schematic of multilayer microchannel cooler

（b）Single layer rectangular microchannel J-T cooler

（c）Single layer micro cylindrical group microchannel J-T cooler Fig. 20. Schematic diagram of Multilayer microchannel J-T cooler [44,45].

3.2. Experimental study of mixed refrigerant In J-T TRS, the high-pressure refrigerant is recirculated, heat exchanged, and throttled to complete the refrigeration cycle. When mixed refrigerants being used for throttling refrigeration, the selection of refrigerant components and the ratio of refrigerants have a great influence on the performance of refrigeration system. In the mixed refrigerants, firstly the components of mixed refrigerants should be selected according to the refrigeration temperature range, that is, the components with boiling point in the temperature range should be selected. Secondly, to obtain larger refrigeration capacity, the by proportion of different components in the mixture should be defined correctly. These two optimizations are the basic considerations for the selection of mixed refrigerants. In ideal JT systems, the refrigeration capacity per unit mass flow rate is the minimum enthalpy difference between high pressure and low pressure refrigerant. In other words, the maximum possible refrigeration capacity per unit mass flow rate is the minimum J-T effect ( HT ) min 11

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to obtain a low temperature of 103 K at a pressure ratio of 40:1. With the deepening of research, scholars have added other working fluids such as argon, helium, neon and hydrofluorocarbons to achieve better results. Since 2000, many scholars studied the mixed working fluids used in the cooling temperature zone from 80 to 200 K. Many results showed the influence of mixed working fluids and mixed working fluids with different ratios on refrigeration performance, The flammability of hydrocarbons may cause danger in applications. Therefore, some researchers added flame retardants to the mixed working fluid. Little [55] proposed and verified that the flame retardant CF3Br could be added to a nitrogen and hydrocarbon mixed working fluid to make the medium non-flammable. In addition, Freon was widely studied and used at one time and can achieve the desired cooling effect in low-temperature TRS. With the ban of Freon use in countries around the world, scholars began to study alternative refrigerants, such as the more environmentally friendly R404a [17] and new refrigerants such as carbon dioxide [56]. Some progress has been made. It was found that the development of mixed working fluids met the refrigeration demand in terms of cooling temperature and cooling capacity. The adoption of environmentally friendly components was an inevitable trend. With the development of compressors and improvement of performance, the optimization of the composition and ratio of mixed working fluids also required further research.

of 2 MPa, the cold junction temperature was stabilized at 121 K, and when the heat load was 5 W, it reached a low temperature of 123 K. The specific heat ratio of nitrogen and methane (k = CP/CV) was larger than other gases, so authors speculated that the compressor problem of overheating was caused by low boiling point working fluids such as N2 and CH4. If a suitable compressor pressure ratio was used and the problem of overheating controlled, then the commercial scroll compressor could be applied in cryogenic TRS. Through the above research, it can be concluded that the different components of the mixed working fluid have a significant influence on the temperature distribution, time of startup process and cooling temperature of the refrigerator. Experimental studies comparing highboiling point and low-boiling point mixed working fluids found that low-boiling components such as nitrogen, helium or methane were generally used in mixed working fluids to facilitate rapid start-up and to reach low temperature zones of liquid nitrogen (77 K) and below. However, it is also necessary to control the proportion of low-boiling components, not too large to avoid overheating of the compressor. Adding high-boiling components such as propane, butane and isobutane can increase the refrigeration capacity of the refrigeration system, and is suitable for the application of the cooling temperature zone of about 200 K. (b) Effect of adding specific components on system performance.

3.2.1. Effect of components on system performance For the purpose of research on the effects of different components of working fluids on the performance of the refrigeration system, these components have been studied. They are rich and comprehensive, involving a variety of high and low boiling components such as nitrogen, argon, helium, carbon, hydrogen compounds and hydrofluorocarbons.

Furthermore, some scholars have focused on the variation of refrigeration characteristics after the addition of certain types of working fluids. For example, a study was conducted by adding helium gas in a mixed working medium composed of nitrogen and hydrocarbon. In 2002, Gong et al. [13] used N2/CH4/C2H6/C3H8/iC4H10 and added another mixed ratio of Ne/iC5H12 as refrigerant (Ne/N2/CH4/C2H6/ C3H8/iC4H10/iC5H12). They studied the effect of the mixing ratio of non-azeotropic mixture on the temperature distribution in the counterflow heat exchanger of the TR. The cold-end temperature of the mixed working fluid consisting of 5 components was 120 K, and the cold-end temperature of the mixed working fluids consisting of 7 components was 80 K. The experimental results and the thermal properties of the working fluid said that adding helium to the mixing medium had a lower cooling temperature. In 2010, Lakshmi Narasimhan [14] used 25 different ratios of N2/CH4/C2H6/C3H8 as mixed working fluids. In order to explore the effect of Ne on the concentration shift of the mixed working fluid in the refrigeration cycle, five groups of mixed working fluids with the addition of Ne were added, and the above mixed working fluids were studied in different parts of the single-stage TRS. The concentration changes of the five working fluids, including Ne, were evaluated under the 10 W heat load. It was found that the addition of Ne had no significant effect on the changes of other working fluid concentrations. These studies showed that the addition of helium with a lower boiling point in the mixed working medium lowered the cold end temperature of the cryogenic refrigeration system. However, the addition of helium did not significantly affect the concentration shift of the components of the mixed working fluid in the cryogenic refrigeration system. Therefore, helium can generally be added as a low boiling point refrigerant to reduce the cooling temperature. Some scholars added hydrofluorocarbons to the mixed working fluids to study the refrigeration characteristics. For example, in 2010, Walimbe et al. [17] used three groups of refrigerants. The first group was N2/Ne mixed with hydrocarbons (CH4/C2H6/C3H8/iC4H10); the second group was N2/Ne mixed with hydrofluorocarbon (C2H2F4/R404a/CHF3); and the third group was mixed with CH4 in the second group of mixed working fluid. They studied the refrigeration performance of the three groups in the single-stage low-temperature TR. The law of refrigeration performance can be seen in the T-h and P-h diagrams. Results showed that the first group of refrigerants mainly composed of hydrocarbons had the best refrigeration performance, obtained the temperature of 65 K, and cooling capacity of 6 W when the cooling temperature was 80 K. The

(a) Effect of high and low boiling components on system performance. Many studies have covered the division of different working fluids by the boiling point in the J-T TRS. The low boiling point working fluid can obtain a lower cooling temperature; and the high boiling point working medium can obtain a larger cooling capacity because of its larger specific heat. Therefore, the cryogenic TRS often uses different gases with high and low boiling points as refrigerants. In 2002, Gong [57] and others studied the performance of non-azeotropic mixed working fluids in a single-stage low-temperature TRS. The first group of mixed working fluids included low boiling point gas N2/CH4 and high boiling point gas C2H6/C3H8/iC4H10. The second group added a mixture of low boiling point gas, Ne, and high boiling point gas, iC5H12. The first group and the second group used two different ratios. Studies were done on the temperature distribution of the working fluids in the countercurrent heat exchanger of low temperature TRS. Results showed that the mixed working fluid had different positions of the maximum and minimum temperature differences in the heat exchange section because of the difference of high and low boiling point components and the proportion. The temperature pinch in the heat exchanger generally appeared near the boiling point of the relatively small component of the mixed working fluid. The desired temperature distribution was obtained in the heat exchanger by optimizing the mixture components. In 2009, Andrey Rozhentsev [58] used three ratios of CH4/C2H6/C3H8 as mixed working fluids to study the refrigeration performance of the TR in the Linde cycle. Results showed that during the startup process, the compressor pressure ratio, power consumption and output pressure of the three components were 1.5–2 times larger than the pure working fluid R12. The low boiling component CH4 played an important role in the startup process. In 2013, Ji Sung Lee [59] used low-boiling N2/CH4 and high-boiling C2H6/C3H8/iC4H10 as a mixed refrigerant in a lowtemperature TRS. Lee applied a commercial scroll compressor. Overheating of the compressor during operation was studied. In the experiment, the compressor appeared to overheat at the beginning of system operation, but was alleviated by water cooling. At the pressure 12

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second group consisting of hydrofluorocarbons, nitrogen and helium obtained 4 W cooling capacity at 143 K. The third group of mixed working fluids with the same cooling capacity had a cold end temperature of 120 K. In the pressure range and low temperature range of the study, the mixed working fluid composed of hydrocarbons had better cooling effect than the hydrofluorocarbon. Hydrofluorocarbons have a higher boiling point, most above 200 K. At lower throttling temperatures, the cooling capacity will be lower than that of hydrocarbons, which is more suitable for applications at higher temperatures. And with the constraints of environmental issues, the use of HFCs should be reduced.

and cooling capacity requirements, combined with the performance of different components in the refrigeration system. 3.2.2. Effect of working ratio on system performance After determining the composition of the mixed working fluid, the proportion of the various components in the mixed working fluid also had a great influence on the refrigeration performance. Optimizing the ratio of mixed working fluids can clarify the role of various components in the refrigeration system, obtain a more suitable working fluid ratio of the working fluid, and improve the refrigeration performance of the low temperature TRS. The use of different refrigeration cycles will have an impact on the optimum ratios of mixed working fluids. In 2004, Gong [61] used a mixed working medium composed of nitrogen and hydrocarbons (CH4/C2H6/C3H8/iC4H10). According to the boiling point of the components, the mixing ratios of the six different proportions were compared. The ratios are 60/25/10/3/2, 10/15/15/25/35, 30/35/2/ 5/28, 10/15/55/15/5, 22.78/32.38/6.8/20.9/17.2 and 19.5/36.8/ 8.82/14.22/20.72. The thermal performance of single-stage low-temperature throttling refrigeration cycle and automatic cascade cryogenic refrigeration cycle was studied. Results showed that in the ratio of high boiling component to large ratio (10/15/15/25/35), the temperature of gas phase transformation was high. According to the high, medium and low boiling point of the gases, the proportion of certain components was large, then in the regenerative section, the maximum temperature difference occurred in the liquefaction temperature region of this component. When the proportion of high-boiling components was large, the automatic cascade cycle improved the refrigeration performance, and vice versa. It was seen that there was a difference in the optimal ratio of the mixed working fluid in the single-stage cycle and the automatic cascade cycle, so it was necessary to determine the application before optimizing the ratio of the working fluid. In 2010, Wang et al. [62] experimentally studied the cooling effects of six different molar mass ratios of two-component mixed refrigerants in a single-stage Hampson-type refrigerator; they were R23/R134a (20/ 80), R23/R227ea (20/80), R23/R236ea (45/55), R170/R290 (20/80), R170/R600a (40/60), R170/R600 (40/60), respectively. At a specific composition and pressure ratio, the optimum ratio is that of the maximum temperature difference between the hot end and the cold end of the regenerator; this is also the ratio when the system obtains the maximum COP. Results showed that the mixed gas consisted of R23 and R236ea; and the mole percentage of R23 accounted for 55% and 60% respectively. They were the most promising non-combustible fuels in medium and low pressure suction compressors. When the R170 mole percentage was 60% and 65%, the mixed gas of R170/R600 was the most promising binary refrigerant with global warming potential. In the same year, Zhang et al. [63] used mixed working fluids (CH4/CHF3/ C4H10) or R600a, in the auto-cascade cryoprobe with four different molar mass ratios (37/21/42, 46/14/40, 45/13/42, 39/23/38). Experiments showed that the molar ratio was 39/23/38, which had better refrigeration performance. At this ratio, the freezing probe was as low as −100 °C, and 8 W cooling capacity was obtained at −80 °C. When the probe was immersed in a water bath at 37 °C, it formed an ice ball of 11.6 mm diameter. In 2010, Rajesh Reddy et al. [64] simulated the refrigeration characteristics of a particular component in the system when it was reduced or missing in the cycle, so that the leakage of working fluid could be replenished to the optimal ratio according to the refrigeration characteristics. The experiment studied the refrigeration performance of the mixed working fluids composed of N2/CH4/C2H6/ C3H8, in which ethane was missing to varying degrees. Maximizing the exergy efficiency of the system to obtain the optimum molar mass ratio was 21.48/22.34/17.96/38.22. The molar mass of other components was unchanged; the molar mass of ethane was reduced by 1/2 to 3/4 and all on the basis of the optimal ratio. The composition ratios of 23.6/ 24.54/9.87/41.99, 24.82/25.82/5.19/44.17 and 26.18/27.23/0/ 46.59, respectively, and the complete absence of methane were tested. In the temperature-time diagram of the throttling process, two points of

(c) Effect of non-azeotropic refrigerant components on system performance. Researchers represented by M. Q. Gong found that the composition shift of a mixed-gas J-T refrigerator is also worthy of attention. The concentration of the mixed gas changes during the refrigeration cycle, causing the low temperature TRS to fail in achieving the cooling effect of original ratios. In 2002, Gong et al. [57] studied dynamic changes of the concentration of three groups of non-azeotropic mixed gas in three typical temperature zones in a closed-type TRS. It was driven by an air conditioner compressor in which mixed working fluid CF4/C2H6/C3H8/ iC4H10/iC5H12 was used in the temperature range of 180–200 K and N2/ CH4/C2H6/C3H8/iC4H10 was used in the temperature range of 120–150 K. Ne was added to the second group used in the 80–100 K temperature zone. Results showed that the maximum variation of the mixed working fluid components reached 6%. In addition, when the refrigerator was shut down, the mixture ratio of the components in the system was as high as 24%, and the concentration of high boiling components in the circulating refrigerant decreased. However, the composition of the mixture with different ratios had different concentration changes. The article proposed a solution to increase the highboiling components according to the ratio when formulating the working medium. Subsequently, the author continued to study this problem in 2007 [20], in the 120 K temperature zone, the low temperature TRS was driven by an oil-free compressor. The concentration shift problem of mixed gas composed of nitrogen and hydrocarbons was investigated due to liquid retention in the two-phase region. Both experimental and calculation results showed that high-boiling components were greatly reduced due to the liquid accumulation of the mixed refrigerant in the low temperature section of the refrigeration system. Lakshmi Narasimhan [14] also studied this issue in 2010. The study used a mixture of nitrogen, helium and hydrocarbons. Thirty different ratios were used in the study of Gong et al. [13,60,61]. By studying the different components of the mixed gas in the single-stage TRS and the law of concentration deviation in different working conditions, the linear equation of concentration corresponding to the various components under the 10 W thermal load was obtained. It was concluded that the non-azeotropic mixture underwent a concentration shift in the low temperature refrigeration cycle where phase change occurred. The high-boiling component of the mixed hydrocarbons decreased. One possible reason is that it had better solubility with the compressor lubricating oil, and dissolved in the lubricating oil during the cycle to reduce the content. Another reason could be that the high-boiling component phase changed in the heat exchange zone, and liquid accumulation occurred in the low temperature section and could not participate in the refrigeration cycle. Of course, it is possible that the refrigerant leaked during the cycle and the mixed working fluid concentration changed. Through the above research, it was found that mixed gases used in the low temperature TRS were mainly hydrocarbons, such as CH4, C2H6, C3H8, C4H10, C5H12. At the same time, low-boiling gas N2, Ne and other working fluids were added for application in the low temperature zone below the liquid nitrogen temperature. The specific component selection needed to be determined according to the cooling temperature 13

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the slope changed greatly, and the corresponding temperatures were the dew point and bubble point temperature, respectively, of the mixed working fluid. The refrigeration curve corresponding to the different missing ratio of ethane was obtained. Therefore, the working ratio was estimated according to the refrigeration characteristics under certain working conditions, and the working medium was adjusted to the optimized ratio. Results also showed that the low temperature of the first four ratios was 200 K, the fifth one was methane, and nitrogen was more than the other four. The mixed gas with the highest proportion of propane achieved a cooling effect of 150 K. In 2014, Tzabar et al. [65] used N2/C2H6 with molar ratios of 40/60, 55/45 and 70/30 and N2/ C3H8 with ratio of 60/40 as mixed refrigerants. Their study calculated the cooling temperature of the binary mixed working fluid in the refrigeration cycle according to the physical properties of the working fluid. They compared it with the experimental results to study the mixture of working fluids to achieve a ratio of stable cold end temperature in the gas-liquid equilibrium state. Results showed that methane was not suitable for applications where the cold end temperature was stabilized. Mixed refrigerant with N2/C2H6 had better refrigeration performance. When the target stable cooling temperature was above 100 K, it was recommended that the nitrogen ratio be larger than 55%. In 2016, Lee [16] used ternary mixed working fluid (Ar/R14/R218), and the proportion of the three components changed from 0.1 to 0.8, respectively, for a total of 36 kinds. The experiment of low temperature visualization was carried out, and then compared with the refrigeration effect of the two-component or pure refrigerants in the previous literature. It was found that when the molar percentage of R14 in the mixed gas is higher than 50%, the mixed working medium will not condense even at 77 K. In addition, the molar mass ratio between R14 and R218 is a key parameter for reducing the condensation temperature of the ternary mixed working fluid in the experiment. If the molar mass ratio between R14 and R218 is higher than 2, even in a liquid nitrogen environment, there is no state in which the working medium is condensed. According to the above research, it was found that the mixed working fluid components required in different throttling refrigeration cycle systems were different. Mixed working fluids of various components had different optimized ratios under different heat exchange structures and different demand conditions. Under the specific conditions, the optimal ratio of the mixed working fluid was used to achieve the best cooling effect.

refrigeration temperature was 228.4 K and the cooling capacity was 7.8 mW. With the diversification of compressors in cryogenic TRS, the composition of the mixed working fluids used is also more diverse, for example, the mixture of suitable working fluids for small compressors. When using a commercial scroll compressor in a cryogenic TRS, a mixed working medium suitable for a high-pressure ratio may be selected, and it is necessary to avoid overheating of the compressor. Other advancements, such as pre-cooling of the system, make the refrigeration effect of the cryogenic TRS better. So, the mixed working medium used in the system can adopt relatively simple components. In particular, the compressor pressure ratio and efficiency are the main influencing factors in the refrigeration system. With the industrial application of high pressure ratio, high efficiency and oil-free compressors, the mixed working fluid components suitable for this type of compressor will also be developed rapidly. Table 1 shows the research of the mixed refrigerant in the cryogenic J-T TRS; it lists the details of the research literature on mixed working fluids after 2000. It includes the composition and the ratio of the mixed working fluid and the analysis of the ratio of the selected components used in the study. In addition, the research results are listed from the perspectives of refrigeration temperature and cooling capacity for reference. Summarizing the recent research, the different ratios of mixed working fluids used in the literature are mainly selected according to the system cooling requirements and the working characteristics of different components. After 2000, nitrogen gas became the low-boiling working medium of the mixed working fluid which was always used in the cryogenic cooling temperature range of 80–200 K. Helium gas with a lower boiling point was also added to obtain a lower cooling temperature, or argon with larger throttling effect at high pressure. Hydrocarbon-based high-boiling components can increase the refrigeration capacity of the refrigeration system. Therefore, mixed refrigerants with comprehensive cooling performance such as refrigeration temperature and cooling capacity are generally combined with nitrogen, helium and high-boiling hydrocarbons. Considering the temperature pinch point in the heat exchanger, medium and low boiling point hydrocarbons such as methane and ethylene are also added to fill the temperature gap between the high and low boiling points. In order to obtain a cooling temperature lower than the liquid nitrogen temperature, a large proportion (50% or more) of a low boiling point working medium can be used.

3.2.3. Effect of mixed refrigerant on system performance with system development In recent years, research on different components of cryogenic TRS has progressed. For example, the compressors in refrigeration systems have become diversified, and pre-cooling structures in systems has improved. These advances have led to a more diversified study. For example, in the 2013 study by Lee et al. [59], the commercial air conditioning scroll compressor was applied to a low-temperature TRS. They used N2/CH4/C2H6/C3H8/iC4H10 as mixed refrigerant and found that the overheating problem of the compressor may have been caused by N2/CH4 that had the larger specific heat ratio. The authors used a water injection cooling system for the compressor to alleviate temperature overshooting. In 2014, the author [66] applied a binary working fluid consisting of nitrogen and helium to a TRS with a highpressure ratio commercial scroll compressor, and precooled with liquid nitrogen. Experiments showed that when the compression ratio reached 31 and the heating load was 35.9 W, the minimum temperature was 63.6 K. For another example, Ryan Lewis [67] used the mixed refrigerant of nitrogen-hydrogen (CH4/C2H6/C3H8/iC4H10/iC5H12) which optimized the ratio with the software NIST4, to study the refrigeration performance of the system when using a small low-pressure compressor of 0.4–0.1 MPa. They found that the refrigeration temperature drop in the refrigeration cycle composed of the small compressor was less than that in the refrigeration of the general compressor, and the lowest

4. Application of the J-T cryogenic refrigeration system After the 1990s, many countries successively put J-T cryogenic TRs into production and application. In 1994, Longsworth [75] of APD Cryogenics used a commercial oil-lubricated compressor to drive a closed-type throttling refrigeration cycle, using nitrogen and hydrocarbons. When the high pressure was about 2 MPa, the 1 W cooling capacity was obtained at the 80 K cold end temperature. In 1995, APD company's Cryoiger® series of TRs achieved a minimum cooling temperature of 68 K, of which the refrigerator had a cooling capacity of 7 W at 80 K and an input power of 450 W. China's research and application of TRS closely followed. Researchers Luo Ercang and Gong [76] of the Low Temperature Technology Experimental Center of the Chinese Academy of Sciences developed a single-stage oil-lubricated air-conditioning compressor drive in 1999. The throttling and cooling prototype used nitrogen and hydrocarbon as the mixed working fluid, which was applied to infrared detection. The cooling temperature was below 80–85 K, and the available cooling capacity was between 0.1 and 10 W. The technical indicators of a mixed working fluid TR developed by the 16th Research Institute of China Electronics Technology Group were superior to those of the same type manufactured in Russia and form small batch production [3]. After several years of development, J-T low temperature TRs were used in a wide range of applications, including 14

15

Lakshmi. NaRasimhan

Gong et al

2004 [61]

2010 [14]

Gong et al.

2002 [13]

Gong et al.

Gong et al.

2002 [57]

2007 [60]

Author

Time

CH4/C2H6/ C3H8

CH4/C2H6/ C3H8/iC4H10

N2

N2

CH4/C2H6/ C3H8/C4H10/ C5H12

CH4/C2H6/ C3H8/iC4H10

CH4/C2H6/ C3H8/iC4H10 CH4/C2H6/ C3H8/iC4H10/ iC5H12

C2H6/C3H8/ iC4H10/iC5H12 CH4/C2H6/ C3H8/iC4H10 CH4/C2H6/ C3H8/iC4H10

hydrocarbon

N2

N2

N2

N2

N2

N2

No

N2

No Ne

No

No

No

Ne

No

Ne

No

CF4

Other composition

Refrigerant composition

Table 1 Summary table of mixed workplace documents.

25 different ratios 5 kinds of ratio

22.78/32.38/6.8/ 20.9/17.2

10/15/55/15/5

30/35/2/5/28

10/15/15/25/35

27/38/8.7/8.7/8.8/ 8.8 60/25/10/3/5

5/10/15/15/25/30

5/15/55/15/7/3

16.7/16.7/16.7/ 16.7/16.7/16.7 65/20/7/5/2/1

19.5/36.8/8.82/ 14.22/20.72

22.78/32.38/6.8/ 20.9/17.2

10/15/55/15/5

30/35/2/5/28

10/15/15/25/35

60/25/10/3/2

5.0/38.2/27.3/14.0/ 15.5 30.1/33.4/9.9/8.6/ 7.9/3.7/6.4 51.3/11.4/7.9/5.7/ 3.75/1.25/18.6

20.42/29.39/8.62/ 8.79/32.78 2.85/32.42/27.81/ 16.51/20.41 3.43/27.64/15.11/ 12.13/8.96/32.73

Molar mass ratio of refrigerant/%

–

Evenly distribute the components Added low boiling point components Added medium boiling point gases Added high boiling point component The optimization ratio of 120 K Reduce high boiling point gases Increase high boiling point gases Reduce medium boiling point gases Increase the medium boiling point Optimization ratio of single-stage cooling cycle

Reduce high boiling point gas Increase high boiling point gas Reduce medium boiling point gas Increase medium boiling point gas The optimization ratio of single -stage refrigeration cycle The optimization ratio of automatic cascade cycle

They are the great ratio of high, medium and low boiling components, and study the variation of different ratios

High boiling point gases are used in high temperature, N2/Ne are added in low temperature zone

Proportioning analysis

70–170

120

120

80

80

120

80–100

120–150

180–200

Refrigeration temperature/K

Results

10 W

–

–

–

–

Cooling capacity

Effects of composition

Effects of composition

Experimental study on optimizing the ratios

Effects of composition

Effects of composition

Research aspects

Concentration shift

Concentration shift due to liquid retention when generating two-phase flow

The relationship between ratio and different throttling refrigeration cycles

Temperature distribution in the heat exchanger

Concentration shift

Focus

(continued on next page)

The effect of adding Ne

Oil-free compressor; using the PR equation to study the solubility of the working fluid in the lubricating oil during gasliquid equilibrium

Thermal Performance of two kinds of refrigeration system

–

–

Others

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Applied Thermal Engineering 157 (2019) 113640

16

Walimbe et al.

2010 [17]

Lakshmi Narasimhan et al.

Rajesh Reddy et al.

2010 [64]

2011 [69]

Zhang et al

2010 [63]

Wang et al.

Rozhentsev

2010 [68]

2010 [62]

Author

Time

Table 1 (continued)

N2

No No No No No No

CH4/C2H6/ C3H8

No No No No No No

CH4

No

N2

N2

CH4/C2H6/ C3H8/iC4H10

CH4/C2H6/ C3H8

CH4

CH4/C2H6/ C3H8

hydrocarbon

N2

N2

No

No

N2

No

CHF3/C2H2F4 CHF3/C3HF7 CHF3/C3H2F6 C2H6/C3H8 C2H6/R600 C2H6/R600a

Ne/C2H2F4/ R404a/CHF3

Ne

No

CHF3/ C4H10(R600a)

No

Other composition

Refrigerant composition

18.11/17.81/28.59/ 35.49 20.00/17.97/17.29/ 44.74 21.81/19.13/21.48/ 37.58 22.07/20.00/18.97/ 38.96 24.75/21.99/23.71/ 29.55 25.00/19.33/25.67/ 30.00 27.13/21.46/21.05/ 30.36

20/80 20/80 45/55 20/80 40/60 40/60

Ne/N2/CH4 and CH4/ C2H6/C3H8/iC4H10 account for 53 and70 Low boiling point N2 account for 43 Low boiling point working fluid (N2 and CH4) accounted for 48

21.48/22.34/17.96/ 38.22 23.6/24.54/9.87/ 41.99 24.82/25.82/5.19/ 44.17 26.18/27.23/0/ 46.59 27.66/0/23.13/ 49.21

37/21/42 46/14/40 45/13/42 39/23/38

13.5/29.5/57 30/35/35 4.5/74.5/21

Molar mass ratio of refrigerant/%

200 150

Completely missing ethane Completely missing propane

Proportion of theoretical optimization in patents

–

100–130

100–125

100–130

100–120

100–130

100–125

105–135

213

105–125

130–145

65–110

200

Ethane reduced by 3/4

In order to obtain a low temperature of less than 80K, N2/Ne are required. To increase the cooling capacity, hydrocarbons or HFCs are required

200

200

182–198 180–198 176–198 173–198

193–203

Refrigeration temperature/K

Results

Maximize the exergy efficiency Ethane reduced by 1/2

–

–

Proportioning analysis

125 K, 19 W 125 K, 18 W 12 5 K, 22 W 125 K, 22 W 125 K, 20 W 125 K, 23 W 125 K, 17 W

–

143 K, 4 W 120 K, 4 W

80 K, 5 W

–

3.5 W 4W 7W 8W

10 W

Cooling capacity

Effects of composition

Experimental study on optimizing the ratios

Effects of composition

Experimental study on optimizing the ratios

Experimental study on optimizing the ratios

Effects of composition

Research aspects

Components and throttling components have common impact on refrigeration performance

Suitable ratio for medium to low-pressure compressors

Refrigeration temperature and cooling power

Temperature change when a component is missing to varying degrees

Refrigeration temperature and cooling power

Power and consumption during the startup

Focus

–

(continued on next page)

Optimized with maximum cop

–

–

Automatic throttling cryoprobe

–

Others

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Applied Thermal Engineering 157 (2019) 113640

17

Satya Mehera, Venkatarathnama

2018 [74]

Ben-Zion Maytal

2015 [71]

Lingxue Jin Cheonkyu Lee etc.

Tzabar

2014 [65]

2018 [73]

Jisung Lee

2014 [66]

Wang et al.

Jisung Lee

2013 [59]

2017 [72]

Ryan Lewis et al.

2013 [71]

Cheonkyu Lee

Xu, Liu et al.

2011 [70]

2016 [16]

Author

Time

Table 1 (continued)

N2

N2

N2

No

No

N2

N2

N2

No

No

N2

CH4/C2H6/ C3H8

CH4/C2H6/ iC4H10/nC5H12

CH4/C2H6/ C3H8/iC4H10/ iC5H12

No

No

C2H6/C3H8

No

CH4/C2H6/ C3H8/iC4H10

CH4/C2H6/ C3H8/iC4H10/ iC5H12

C3H8/iC4H10

hydrocarbon

No

No

Ne/CF4

Ar/CF4/C3F8

Ar/O2

No

Ne

No

No

No

Other composition

Refrigerant composition

Study the temperature of the mixed working medium freezing point under different ratios of binary mixed working fluid

In order to increase the triple point temperature of argon

Explore mixed working fluids which are suitable for obtaining a stable cold temperature

It was expected to obtain a low temperature of 77K, so N2 and Ne were used

–

With the maximum (△h|T)min as the optimization target, the software NIST4 optimized working ratio

In order to observe the concentration change of the high and low boiling point gases, the average ratio of the two is used

Proportioning analysis

20.28/30.06/12.37/ 37.29 20.38/27.48/17.16/ 35.02 33.55/15.91/15.54/ 35.00

29.57/22.3/15.01/ 33.12

LN2 precooling 0.33/0.35/0.14/0.17/0.02

20.28/12.24/10.00/9.01/9.01/7.61/0/31.85 25.77/10.08/7.75/10.11/8.76/8.82/0/28.71 30.09/13.21/5.17/8.38/12.44/8.95/0/21.76 30.37/10.18/2.50/8.89/13.40/7.51/6.73/20.42 24.10/2.70/1.00/17.73/12.56/5.04/20.07/16.81

The proportion of the three components varies from 10 to 80 to 10 in order, with a total of 36 ratios

80/20

40/60/0 55/45/0 70/30/0 60/0/40

42/58

15/30/30/10/15

34/20/18/16/12 34/22/20/12/12 24/36/14/10/16 8/46/14/4/28

54.8/45.2 50.4/49.6

Molar mass ratio of refrigerant/%

90–100

78

80–120

77–104

77–87

90–150

Precooling to 101.5, and throttling to 63

121

261.7 241.7 245.6 228.4

–

Refrigeration temperature/K

Results

10 W

–

–

–

300 mW

–

63.6 K, 35.9 W

123 K, 5 W

4.67 mW 6.63 mW 5.53 mW 7.89 mW

–

Cooling capacity

Effects of composition

Effects of composition

Effects of composition

Experimental study on optimizing the ratios

Effects of composition

Experimental study on optimizing the ratios

Effects of composition

Effects of composition

Effects of composition

Effects of composition

Research aspects

Experiment results show that the homogenous model is the most suitable model for a J-T refrigerator with nitrogen-hydrocarbon mixtures

Refrigeration temperature and cop

Cyclic exergy analysis and unit volume cooling capacity

temperature freezing point temperature

Inhibition of argon gas phase change

Gas-liquid equilibrium

High-pressure ratio system with liquid N2 precooling

Compressor overheating problem

Refrigeration temperature and cooling power

Study on the concentration shift of two-phase flow

Focus

Different J-T cycles

Cooling performance of mixed working fluid in MJTR and RBC

At high altitude

–

Compression ratio of 31

Scroll compressor

Compressor with a pressure ratio of 0.1–0.4

–

Others

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aerospace equipment cooling, clinical medical refrigeration, electronic equipment cooling, vacuum freeze drying and cryogenic biology. In the early days, J-T TRs were mainly used in gas liquefaction. Later, with the demand of different application fields and the progress of processing technology, J-T TRs with more perfect structure and better performance were applied in many fields. In space exploration, infrared guidance, etc., the working accuracy and efficiency of electronic devices can be guaranteed only in the cryogenic temperature range. It is found that the TRs with low boiling temperature such as nitrogen and neon can reach the cryogenic temperature. The J-T TR with special structure such as tapered J-T TR is designed to meet the demand, and ensure the fast cryogenic refrigeration in these fields. Surgical treatment of cancer requires a refrigerator that can quickly achieve low temperature, so that cancer cells can be killed quickly and less damage is caused to healthy cells. When high pressure argon or other refrigerants is used in J-T throttle cooler, low temperature can be achieved quickly. Moreover, J-T throttle cooler has simple structure and small cold end. Therefore, the medical of J-T cryoprobe can help to complete the operation quickly and successfully. With the miniaturization of electronic equipment operation, a large amount of heat is generated by electronic devices of small size. The increase of operation temperature of electronic devices will reduce by lifetime and lower the reliability of equipment. Therefore, cooling device is needed in electronic equipment for heat dissipation. In the smallscale closed J-T TRS, the pressure ratio of working fluid is limited by the compressor, and the application of mixed refrigerant can not only achieve lower temperature, but also obtain larger refrigeration capacity. Therefore, the mixed refrigerant J-T TR is applied in electronic equipment to ensure the stable and timely loss of heat of electronic equipment and the efficient operation of equipment. Many biomaterials need to be kept in the temperature range of 70–80 K in order to maintain their activity. J-T TR can be operated stably in the low temperature region, so it used in the fields of cryopreservation. J-T TR has the advantages of fast throttling, low refrigeration temperature, simple structure, and low noise, and it has been applied in many cryogenic applications. With the development of refrigeration structure and working fluids, J-T TR will be applied in more fields.

demagnetization refrigeration (ADR) is used to cool the x-spectrum probe, where the J-T cooler is used as the second pre-cooling stage of the ADR to pre-cool the refrigerant to 1.7 K. The researchers of Japan Aerospace Exploration Agency (JAXA) combined a 4 K J-T cooler with a liquid helium tank and 2 two-stage Stirling coolers to cool the ADR’s heatsink to 1.3 K or less in orbit for several years [85]. 4.2. Application in the medical field From the perspective of clinical treatment [86], cryotherapy is a common medical treatment for killing tumors. In order to get enough cooling capacity to kill tumors in a small working environment, the American company ENDCARE developed the first generation argon boring system in the 1990 s. The device utilized the J-T effect of argon and helium to rapidly freeze and thaw the tumor in a narrow area of the tip. It can not only enhance the killing effect, but also greatly reduce or even protect the surrounding normal tissues [87]. In the treatment of multiple tumors such as lung cancer, liver cancer, prostate cancer and breast cancer, minimally invasive cryotherapy is widely used and has many advantages [88–90]. In order to determine the cold treatment area, the researchers measured the refrigeration range of the cryoprobe through the hockey test [91,92]. Mohammed Shurrab et al. [93] obtained the cryoprobe cooling treatment range by studying the ice hockey growth of the J-T effect refrigerator. 4.3. Other multiple applications In terms of various electronic devices, J-T cryogenic TRs have also been gradually applied, including in low-temperature electronic technology. The temperature of computer chips can be reduced by smallscale low-temperature throttling cooling, which can increase the operation speed by more than 30% [3]. It also involves television programs broadcast by communication satellites, using a low-temperature throttling cooler to reduce the temperature of the electronic device, so that it maintains high sensitivity and high-speed response, so as to quickly obtain a clear TV picture [36]. J-T low-temperature TRs are also used in vacuum freeze-drying technology, cryogenic biology, etc. Bangma et al. [94] and Rijpma et al. [95] used J-T effect coolers to cool embryonic heart monitoring sensors. Gong et al. [96] used a mixed working fluid J-T effect refrigeration cycle in a cryogenic vessel for preserving biological materials, compared to conventional liquid nitrogen cryopreservation vessels. Mechanical low temperature chamber cooling rate was faster and reached −180 °C in just 2.5 h. Yang Junling et al. [97] used the open J-T refrigeration system in the mobile mine refuge chamber to consume less power and reduce the temperature of the refuge chamber. In addition to the above-mentioned applications in several major fields, the micro-miniature J-T cryogenic TR is also used in some scientific research applications. Daly et al. [98] designed a cryogenic refrigerator for scientific research, which provided 3.5 K low temperature in the third stage of J-T refrigeration. Yukio Morii et al. [99] developed an mK-class cryogenic refrigerator for neutron scattering research with a minimum temperature of 41 mK. Gong [100] used a J-T refrigerator to cool the superconducting material, and the cooling temperature was lower than 80 K. As described above, the J-T cryogenic TR has been applied to various fields. With the research and development of cryogenic TRS, cryogenic TRs have large refrigeration capacity, fast cooling speed, large COP and wide refrigeration temperature range. The refrigeration advantages will be more prominent as it is applied in more cryogenic refrigeration fields.

4.1. Application in the aviation field J-T effect refrigerators have been used to cool aerospace equipment such as infrared telescopes, spectral detectors and x-spectral probes because of their small size and fast cooling rate. Burt Zhang [1] and others used the J-T loop to cool the infrared telescope focal plane arrays (FPA) located a few meters away from the spacecraft. Levenduski [2] designed a J-T effect cryocooler code-named engineering development model (EDM) to cool the electronics in the spacecraft. The structure is rugged and suitable for a wide range of thermal loads. In avionics refrigeration systems, J-T cryocoolers are mostly used for the second or third stage of refrigeration, Considering to meet the two-year use of the Along Track Scanning Radiometer (ATSR) and the Improved Stratospheric and Mesospheric Sounder (ISAMS) probes, the multi-stage refrigeration system is used to replace the cryogenic liquid refrigerant cooling by Orlowska [77], et al… The system consists of second-order Stirling and first-order J-T cooling. Bhatia [78] et al. designed a device called Polatron for cooling infrared probes in 2001. J-T effect refrigeration was used in the third stage of the refrigeration of the device to obtain a low temperature of 4 K. In 2002, the author [79] tested the Rutherford Appleton Laboratory (RAL) 4 K J-T effect chiller, which was mainly used to preheat the space infrared telescope heat radiation probe. Katshiro Narasaki et al. [80] used a combination of second-order Stirling pre-cooling plus first-order J-T cooling to cool the probe of the space infrared telescope to 1.7 K, providing 10 mW of cooling capacity. In the Astro-H space program [81–84], the secondary Adiabatic

5. Summary and outlook In this paper the research on the structural evolution of microminiature J-T cryogenic refrigerators in recent years, the research on 18

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H. Geng, et al.

the influence of refrigerant on the composition and the ratio of working fluid in the system, and the application of micro-miniature J-T effect cryogenic refrigerators are investigated. The following conclusions and outlooks have been obtained:

one hand, it can enrich various structures in the refrigerator. For example, with the development of micro-etching and soldering processing, it can enrich the development of multi-channel parallel and multistage cooling micro-channel refrigerators. At the same time, the refrigeration system can be enriched by adding multiple stages of regenerative heat transfer, multi-stage pre-cooling and multi-stage J-T throttling. On the other hand, the use of multiple mixed working fluids in the system can get a wider range of cooling temperatures. By enriching the J-T effect cryogenic TRS through the above methods, the refrigeration advantages of large cooling capacity, fast cooling rate, large COP and wide refrigeration temperature range will be more prominent. The research and development of mixed working fluids in JT cryogenic refrigeration systems will be more inclusive, diverse and environmentally friendly. With the optimization of the refrigerators’ structures and refrigerants in refrigeration cycles, the J-T cryogenic refrigeration system will be applied in electronic equipment heat dissipation, low-temperature biology and many other cryogenic refrigeration fields.

(1) With the development of application fields and processing technology, J-T refrigerators have various structural types. The development of Hampson-type refrigerators is relatively mature and has been successfully promoted and applied commercially. From the single structure, multi-stage pre-cooling and multi-spiral channel parallel structures have been developed. The optimization of the size parameters such as the height and spacing of the spiral finned tubes, and the optimization and improvement of various throttling components have been carried out. However, the spiral finned tube structure limits the further increase of the flow rate and the cooling capacity. In this respect, there is more room for imagination and development. In recent years, with the development of microetching and soldering processes, multi-channel parallel and multistage cooling microchannel J-T cryocoolers have emerged. Multilayer and multi-channel parallel can amplify the cooling capacity through large flow and parallel, while maintaining a small overall size. In addition to the more widely used micro-rectangular channel structure, current research also has micro-column group and microorifice plate structure. There are not only two or three layers of single-stage refrigeration microchannel TRs, but also a single test piece to complete two-stage refrigeration and a plurality of stacked micro-channel J-T cryogenic refrigerators. (2) In the cryogenic TRS, mixed working fluid can overcome the defects of pure working fluid in thermal properties. Mixed working fluid can not only meet the requirements of long-term recycling and low pressure ratio of closed systems, but also improve temperature distribution of the cryocooler and improve refrigeration performance. (a) The selection of the mixed refrigerant components in the cryogenic TRS is primarily determined by specific refrigeration requirements and the performance of different components of the refrigeration system. When the cooling temperature zone is lower than 200 K, the working fluid components used are mainly alkane compounds, such as CH4, C2H6, C3H8, C4H10, C5H12, etc. In order to obtain a cooling temperature lower than 80 K, a low boiling point working medium such as nitrogen, helium or argon may be added. The performance of the compressor in a low temperature TRS is also a major factor affecting the choice of working fluid. For example, use of an oil-free compressor can reduce the concentration deviation of the mixed working fluid, so the high-boiling component can be appropriately reduced. Application of the high pressure ratio compressor can improve the refrigeration performance of the cryogenic TRS. Therefore, the high and low boiling point working components are simpler. (b) Optimize the ratio of mixed working fluids through experimental research combined with empirical analysis. A low boiling point working fluid is selected from nitrogen, helium, argon, etc., in order to obtain a cooling temperature lower than the liquid nitrogen temperature, a large proportion (≥50%) of low boiling point working medium can be used. High-boiling components such as hydrocarbons can increase the refrigeration capacity of the system. The mixing fluid components required in different throttling refrigeration cycle systems are different. The mixed working fluids of various components are different in different heat-exchange structures. There are different optimization ratios under demand conditions, and the optimal cooling ratio can be achieved under specific conditions using the optimized ratio corresponding to the mixed working fluid.

Funding This work was supported by the Natural Science Foundation of Shanghai [Grant No. 14ZR1429100]. Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.applthermaleng.2019.04.050. References [1] B. Zhang, M. Larson, J. Rodriguez, Passive coolers for pre-cooling of JT loops for deep space infrared imaging applications, Cryogenics 50 (9) (2010) 628–6322009. [2] R. Levenduski, R. Scarlotti, Joule-Thomson cryocooler for space applications, Cryogenics 36 (10) (1996) 859–866. [3] Q. Chen, Theoretical and Experimental Research on a Small-Scale Mixed Refrigerant J-T Throttling Refrigerator, Zhejiang University, Hangzhou, 2001. [4] L.L. Pisters, A.C. von Eschenbach, S.M. Scott, D.A. Swanson, C.P. Dinney, C.A. Pettaway, R.J. Babaian, The efficacy and complications of salvage cryotherapy of the prostate, J. Urol. 157 (3) (1997) 921–925. [5] A. Zhang, L.X. Xu, G.A. Sandison, et al., A microscale model for prediction of breast cancer cell damage during cryosurgery, Cryobiology 47 (2) (2003) 143–154. [6] H. Skye, G. Nellis, S. Klein, Modeling and optimization of a cascaded mixed gas Joule-Thompson cryoprobe system, Ashrae Trans. 115 (2009) 966–983. [7] B. Eugeniusz, C. Maciej, W. Marta, et al., Miniature Joule - Thomson liquefier with sintered heat exchanger, Cryogenics 32 (1) (1992) 13–16. [8] P.K. Gupta, P.K. Kush, A. Tiwari, Design and optimization of coil finned-tube heat exchangers for cryogenic application, Cryogenics 47 (5–6) (2007) 322–332. [9] S. Gygax, Cheap helium liquifiers, Phys. B+c 126 (1–3) (1984) 134–137. [10] K. Narasaki, Analysis of two-stage Joule-Thomson expansion, Cryogenics 74 (2016) 59–65. [11] C. Huang, G. Zhao, H.W. Zhang, et al., Design and optimization of a two-stage 28 K Joule-Thomson microcooler, Cryogenics 52 (1) (2012) 51–57. [12] H.S. Cao, S. Vanapalli, H.J. Holland, et al., A micromachined Joule-Thomson cryogenic cooler with parallel two-stage expansion, Int. J. Refrig. 69 (2016) 223–231. [13] M.Q. Gong, E.C. Luo, J.F. Wu, et al., On the temperature distribution in the counter flow heat exchanger with multicomponent non-azeotropic mixtures, Cryogenics 42 (12) (2002) 795–804. [14] N.L. Narasimhan, G. Venkatarathnam, A method for estimating the composition of the mixture to be charged to get the desired composition in circulation in a single stage JT refrigerator operating with mixtures, Cryogenics 50 (2) (2010) 93–101. [15] F. Keppler, G. Nellis, S. Klein, Optimization of the composition of a gas mixture in a Joule-Thomson cycle, HVAC&R Res. 10 (2) (2004) 213–230. [16] C. Lee, J. Yoo, J. Lee, et al., Visualization of the solid–liquid equilibria for nonflammable mixed refrigerants, Cryogenics 75 (2016) 26–34. [17] N.S. Walimbe, K.G. Narayankhedkar, M.D. Atrey, Experimental investigation on mixed refrigerant Joule-Thomson cryocooler with flammable and non-flammable refrigerant mixtures, Cryogenics 50 (10) (2010) 653–659. [18] K.C. Ng, H. Xue, J.B. Wang, Experimental and numerical study on a miniature Joule-Thomson cooler for steady-state characteristics, Int. J. Heat Mass Transf. 45 (3) (2002) 609–618. [19] S.W. Stephens, Advanced design of Joule-Thomson coolers for infra-red detectors, Infrared Phys. 8 (1) (1968) 25–35. [20] T.C. Hui, X. Wang, H.Y. Teo, A numerical study of the Hampson-type miniature Joule-Thomson cryocooler, Int. J. Heat Mass Transf. 49 (3–4) (2006) 582–593.

In summary, in order to obtain a more ideal cooling effect, on the 19

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H. Geng, et al. [21] S.B. Chien, L.T. Chen, Two-phase coexistence analysis of the bellows control mechanism for a J-T cryocooler, Cryogenics 39 (4) (1999) 359–365. [22] B.Z. Maytal, Hampson’s type cryocoolers with distributed Joule-Thomson effect for mixed refrigerants closed cycle, Cryogenics 61 (2014) 92–96. [23] J. Guichard, A. Cottereau, D. Chazot, Cryogenic characterization of low-cost JouleThomson coolers, Proc. SPIE – Int. Soc. Opt. Eng. 4130 (2000) 496–506. [24] L.V. Changshui, Development situation and tendency of the fast-cool-down joulethomson cryocoolers used for IRFPA, Infrared Laser Eng. 36 (s2) (2007) 521–525. [25] H.J. Holland, J.F. Burger, N. Boersma, et al., Miniature 10–150 mW Linde-Hampson cooler with glass-tube heat exchanger operating with nitrogen, Cryogenics 38 (4) (1998) 407–410. [26] M.H. Lin, P.E. Bradley, M.L. Huber, et al., Mixed refrigerants for a glass capillary micro cryogenic cooler, Cryogenics 50 (8) (2010) 439–442. [27] A. Widyaparaga, M. Kuwamoto, A. Tanabe, et al., Study on a wire-type Joule Thomson microcooler with a concentric heat exchanger, Appl. Therm. Eng. 30 (16) (2010) 2563–2573. [28] M.Q. Gong, J.F. Wu, B. Yan, et al., Study on a miniature mixed-gases JouleThomson cooler driven by an oil-lubricated mini-compressor for 120 K temperature ranges, Phys. Procedia 67 (2015) 405–410. [29] B. Eugeniusz, C. Maciej, W. Marta, et al., Miniature Joule-Thomson liquefier with sintered heat exchanger, Cryogenics 32 (1) (1992) 13–16. [30] A. Dvornitsyn, V. Naer, A. Rozhentsev, Experimental investigation of a bottle-sublimation cooler, Int. J. Refrig. 29 (1) (2006) 101–109. [31] A. Wang, M.Q. Gong, W.U. Jian-Feng, Experimental investigation on a 3D print made microminiature mixed refrigerant J-T cooler, J. Eng. Thermophys. 36 (3) (2015) 600–604. [32] C.G. Paine, Characterization of porous sinter materials as Joule-Thomson restrictors for the Planck sorption cooler, Cryogenics 44 (6–8) (2004) 425–429. [33] C. Lee, S. Baek, J. Lee, et al., Development of a closed-loop J-T cryoablation device with a long cooling area and multiple expansion parts, Med. Eng. Phys. 36 (11) (2014) 1464–1472. [34] A. Gladun, The Joule-Thomson effect in neon, Cryogenics 6 (1) (1966) 31–33. [35] J.D. Daunt, E. Lerner, A closed-cycle Joule-Thomson liquefier and cryostat for He3, Cryogenics 10 (6) (1970) 476–479. [36] Q.H. Yao, Application research of tapered self-regulated J-T cryocoolers, Laser & Infrared (2012). [37] E.D. Marquardt, R. Radebaugh, Compact High Effectiveness Parallel Plate Heat Exchangers, Cryocoolers 12. R. Ross, Jr., Springer US, 2002, pp. 507–516. [38] S. Baek, et al., Effect of flow maldistribution and axial conduction on compact microchannel heat exchanger, Cryogenics 60 (2014) 49–61. [39] W.A. Little, Microminiature refrigeration—Small is better, Phys. B+c 109 (1982) 2001–2009. [40] P.P.P.M. Lerou, H. Jansen, G.C.F. Venhorst, et al., Progress in micro Joule-Thomson cooling at twente university, Cryocoolers 13, Springer US, 2005, pp. 489–496. [41] W. Zhu, J.M. Park, M.J. White, et al., Experimental evaluation of an adaptive JouleThomson cooling system including silicon-microfabricated heat exchanger and microvalve components, J. Vacuum Sci. Technol. A Vacuum Surfaces Films Off. J. Am. Vacuum Soc. 29 (2) (2011) 21005. [42] E. Milulin, J. Shevich, T. Danilenko, et al., The miniature Joule-Thomson refrigerator, Cryogenics 32 (1) (1992) 17–19. [43] S.P. Narayanan, G. Venkatarathnam, Analysis of performance of heat exchangers used in practical micro miniature refrigerators, Cryogenics 39 (6) (1999) 517–527. [44] G.E.N.G. Hui, C.U.I. Xiaoyu, W.A.N.G. Wenqing, et al., Experimental study on multilayer rectangular micro-channel Joule-Thomson cryocooler with nitrogen, J. Refrig. 38 (5) (2017) 93–98. [45] W.A.N.G. Wenqing, Study on Printed Circuit Board Joule-Thomson Effect Cryocooler, University of Shanghai for Science and Technology, 2017. [46] E. Ballico, Analysis of the starting time of the Joule-Thomson microliquefier, Cryogenics 21 (12) (1981) 704–706. [47] B.Z. Maytal, Performance of ideal flow regulated Joule-Thomson cryocooler, Cryogenics 34 (9) (1994) 723–726. [48] Y.J. Hong, S.J. Park, H.B. Kim, et al., The performance of Joule Thomson refrigerator, Cryocoolers 13, Springer US, 2005. [49] Y.J. Hong, S.J. Park, H.B. Kim, et al., The cool-down characteristics of a miniature Joule-Thomson refrigerator, Cryogenics 46 (5) (2006) 391–3952. [50] H.S. Cao, S. Vanapalli, H.J. Holland, et al., Sensitivity of micromachined JouleThomson cooler to clogging due to moisture, Phys. Procedia 67 (2015) 417–422. [51] W.A. Little, Method for efficient counter-current heat exchange using optimized mixtures: US, US5644502[P], 1997. [52] M.Q. Gong, Y. Zhou, Optimization principle of mixture throttling cooling machine, Cryo. Supercond. 24 (3) (1996) 1–6. [53] M.Q. Gong, E.C. Luo, Y. Zhou, et al., Optimum composition calculation for multicomponent cryogenic mixture used in Joule-Thomson refrigerators, Vacuum Cryogenics 45 (1999). [54] W.J. Podbielniak, Art of refrigeration: US, US2041725[P], 1936. [55] W.A. Little, I. Sapozhnikov, T. Thompson, et al., Cryosurgical system and method: US, US 6306129 B1[P], 2001. [56] C.L. He, Experimental Research on CO2 Refrigeration Based on Joule-Womson Effect, Dalian Maritime University, Dalian, 2013. [57] M.Q. Gong, Y.F. Qing, Q.G. Hu, et al., Experimental study on concentration variation of concentration of working components in multi-component mixed refrigerants throttling chiller, Low Temp. Specialty Gases 20 (2) (2002) 16–19. [58] A. Rozhentsev, V. Naer, Investigation of the starting modes of the low-temperature refrigerating machines working on the mixtures of refrigerants, Int. J. Refrig. 32 (5) (2009) 901–910. [59] J. Lee, K. Lee, S. Jeong, Experimental study of a mixed refrigerant Joule-Thomson

[60] [61] [62] [63] [64] [65] [66] [67] [68] [69] [70] [71] [72] [73] [74] [75] [76] [77] [78] [79] [80] [81] [82] [83] [84] [85] [86] [87] [88] [89] [90] [91] [92] [93] [94]

20

cryocooler using a commercial air-conditioning scroll compressor, Cryogenics 55–56 (5) (2013) 47–52. M.Q. Gong, Z.B. Deng, J.F. Wu, Composition shift of a mixed-gas Joule-Thomson refrigerator driven by an oil-free compressor, Cryocoolers (2007). M.Q. Gong, J.F. Wu, E.G. Luo, Performances of the mixed-gases Joule-Thomson refrigeration cycles for cooling fixed-temperature heat loads, Cryogenics 44 (12) (2004) 847–857. Q. Wang, K. Cui, T.F. Sun, et al., Performance of a single-stage Linde-Hampson refrigerator operating with binary refrigerants at the temperature level of-60°C, J. Zhejiang Univ. Sci. A 11 (2) (2010) 115–127. S.Z. Zhang, B. Dong, et al., Experimental study on a cryosurgery apparatus, J. Zhejiang Univ. Sci. A 11 (2) (2010) 128–131. K.R. Reddy, S.S. Murthy, G. Venkatarathnam, Relationship between the cooldown characteristics of J-T refrigerators and mixture composition, Cryogenics 50 (6) (2010) 421–425. N. Tzabar, Binary mixed-refrigerants for steady cooling temperatures between 80K and 150K with Joule-Thomson cryocoolers, Cryogenics 64 (2014) 70–76. J. Lee, H. Oh, S. Jeong, Investigation of neon–nitrogen mixed refrigerant JouleThomson cryocooler operating below 70 K with precooling at 100 K, Cryogenics 61 (5) (2014) 55–62. R. Lewis, Y. Wang, P.E. Bradley, et al., Experimental investigation of low-pressure refrigerant mixtures for micro cryogenic coolers, Cryogenics 54 (2) (2013) 37–43. A. Rozhentsev, V. Naer, Investigation of the starting modes of the low-temperature refrigerating machines working on the mixtures of refrigerants, Int. J. Refrig. 32 (5) (2009) 901–910. N.L. Narasimhan, G. Venkatarathnam, Effect of mixture composition and hardware on the performance of a single stage JT refrigerator, Cryogenics 51 (8) (2011) 446–451. X.W. Xu, J.P. Liu, L. Cao, et al., Analysis of concentration migration of non-azeotropic mixtures in refrigeration cycle, J. Chem. Ind. Eng. 62 (11) (2011) 3066–3072. B.Z. Maytal, Argon-oxygen mixtures for Joule-Thomson cryocooling at elevated altitude, Int. J. Refrig. 60 (2015) 54–61. H.C. Wang, G.F. Chen, M.Q. Gong, X. Li, Performance comparison of single-stage mixed-refrigerant Joule-Thomson cycle and reverse Brayton cycle for cooling 80 to 120 K temperature-distributed heat loads, Mater. Sci. Eng. 278 (2017) 012051. L. Jin, C. Lee, S. Baek, et al., Design of high-efficiency Joule-Thomson cycles for high-temperature superconductor power cable cooling, Cryogenics (2018). R.S. Meher, G. Venkatarathnam, Estimation of performance of a J-T Refrigerators Operating With Nitrogen-Hydrocarbon Mixtures and a Coiled tubes-in-tube heat exchanger, Cryogenics (2018). R.C. Longsworth, Cryogenic refrigerator with single stage compressor: US, US5337572[P], 1994. E.C. Luo, M.Q. Gong, L. Shi, et al., Comprehensive report on the research of throttling refrigeration system for low temperature mixed refrigerants with different uses, Cryogenics Eng. 4 (1999) 43–49. A.H. Orlowska, T.W. Bradshaw, J. Hieatt, Closed cycle coolers for temperatures below 30 K, Cryogenics 30 (3) (1990) 246–248. R.S. Bhatia, S.T. Chase, W.C. Jones, et al., Closed cycle cooling of infrared detectors to 250 mK, Cryogenics 42 (2) (2002) 113–122. R.S. Bhatia, P.A.R. Ade, T.W. Bradshaw, et al., The effects of cryocooler microphonics, EMI and temperature variations on bolometric detectors, Cryogenics 41 (11–12) (2001) 851–863. K. Narasaki, S. Tsunematsu, K. Ootsuka, et al., Development of 1 K-class mechanical cooler for SPICA, Cryogenics 44 (6) (2012) 375–381. P. Shirron, M. Kimball, D. Wegel, et al., ADR design for the Soft X-ray Spectrometer instrument on the Astro-H mission, Cryogenics 50 (9) (2010) 494–499. Y. Sato, H. Sugita, K. Mitsuda, et al., Development of mechanical cryocoolers for Astro-H/SXS, Cryogenics 50 (9) (2010) 500–506. P.J. Shirron, M.O. Kimball, B.L. James, et al., Design and predicted performance of the 3-stage ADR for the Soft-X-ray Spectrometer instrument on Astro-H, Cryogenics 52 (4–6) (2012) 165–171. P.J. Shirron, M.O. Kimball, B.L. James, et al., Operating modes and cooling capabilities of the 3-stage ADR developed for the soft-X-ray spectrometer instrument on Astro-H, Cryogenics 74 (2016) 2. S. Yoshida, M. Miyaoka, K. Kanao, et al., In-orbit performance of a helium Dewar for the soft X-ray spectrometer onboard ASTRO-H, Cryogenics 91 (2018). W.L. Holman, M.D.J. Ikeshita Jret al., Cardiac cryosurgery: effects of myocardial temperature on cryolesion size, Surgery 93 (2) (1983) 268–272. Y.T. Hua, W.H. Wu, Z.Y. Xin, et al., Measurement and analysis of operation performance of the “Endocare” cryoprobe system, Space Med. Med. Eng. (2003). J.R. Caso, M. Tsivian, V. Mouraviev, et al., Complications and postoperative events after cryosurgery for prostate cancer, BJU Int. 109 (6) (2012) 840–845. L.L. Pisters, A.C. von Eschenbach, S.M. Scott, et al., The efficacy and complications of salvage cryotherapy of the prostate, J. Urol. 157 (3) (1997) 921–925. J. Hachisuka, K. Doi, M. Furue, Combination cryosurgery with hyperthermia in the management of skin metastasis from breast cancer: A case report, Int. J. Surg. Case Reports 3 (2) (2012) 68. J.G. Baust, A.A. Gage, D. Clarke, et al., Cryosurgery—a putative approach to molecular-based optimization, Cryobiology 48 (2) (2004) 190–204. A.J. Sheen, G.J. Poston, D.J. Sherlock, Cryotherapeutic ablation of liver tumours, Br. J. Surg. 89 (11) (2002) 1396. M. Shurrab, H. Wang, N. Kubo, et al., The cooling performance of a cryoprobe: Establishing guidelines for the safety margins in cryosurgery, Int. J. Refrig. 67 (2016) 308–318. M.R. Bangma, A.P. Rijpma, E.D. Vries, et al., Interference characterisation of a

Applied Thermal Engineering 157 (2019) 113640

H. Geng, et al. commercial Joule-Thomson cooler to be used in a SQUID-based foetal heart monitor, Cryogenics 41 (9) (2001) 657–663. [95] A.P. Rijpma, H.J.M.T. Brake, E.D. Vries, et al., A high-Tc SQUID-based sensor head cooled by a Joule-Thomson cryocooler, Phys. C Superconduct. Appl. 372 (12) (2002) 209–212. [96] M. Gong, J. Wu, et al., Development of a −186 °C cryogenic preservation chamber based on a dual mixed-gases Joule-Thomson refrigeration cycle, Appl. Therm. Eng. 36 (1) (2012) 188–192. [97] J. Yang, L. Yang, J. Wei, et al., Study on open-cycle carbon dioxide refrigerator for

movable mine refuge chamber, Appl. Therm. Eng. 52 (2) (2013) 304–312. [98] E.F. Daly, T.J. Dean, A new heat pump cycle and its application to a 3·5 K refrigerator, Cryogenics 10 (2) (1970) 123–135. [99] Y. Morii, Y. Koike, T. Igarashi, et al., A liquid-He-free dilution refrigerator for neutron scattering, Phys. B Phys. Condensed Matter s284–288 (2002) 2014–2015. [100] M.Q. Gong, J.F. Wu, A 70 K J-T cryocooler based on the fractional mixed-refrigerant cycle, Proceedings of the Twentieth International Cryogenic Engineering Conference (ICEC20), Elsevier Ltd, 2005, pp. 173–176.

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