Intermediate cooling from pulse tube and regenerator in a 4 K pulse tube cryocooler

This paper introduces intermediate cooling by thermally attaching heat exchangers on the second stage pulse tube and regenerator in a commercial 4 K pulse tube cryocooler. Due to the large enthalpy flow in the 2nd stage pulse tube and regenerator, both intermediate heat exchangers on the pulse tube and regenerator can provide cooling capacities in the temperature range of 5–15 K without or with minor effect on the performance of the 4 K stage. Extracting cooling capacity from the pulse tube or regenerator reduces the 1st stage cooling performance in the present study. The joint intermediate heat exchanger on the pulse tube and regenerator has demonstrated promising results for applications.     

Influence of the connecting tube at the cold end in a U-shaped pulse tube cryocooler

In some special applications, the pulse tube cryocooler must be designed as U-shape; however, the connecting tube at the cold end will influence the cooling performance. Although lots of U-shape pulse tubes have been developed, the mechanism of the influence of the connecting tube on the performance has not been well demonstrated. Based on thermoacoustic theory, this paper discusses the influence of the length and diameter of the connecting tube, transition structure, flow straightener, impedance of the inertance tube, etc. on the cooling performance. Primary experiments were carried out in two in-line shape pulse tube cryocoolers to verify the analysis. The two cryocoolers shared the same regenerator, heat exchangers, inertance tube and straightener, and the pulse tube, so the influence of these components could be eliminated. With the same electric power, the pulse tube cryocooler without connecting parts obtained 31 W cooling power at 77 K; meanwhile, the other pulse tube cryocooler with the connecting parts only obtained 27 W, so the connecting tube induced more than a 12.9% decrease on the cooling performance, which agrees with the calculation quite well.     

Effects of biofouling on air-side heat transfer and pressure drop for finned tube heat exchangers

Experimental investigations on the effects of biofouling on air-side heat transfer and pressure drop for three biofouled finned tube heat exchangers and one clean finned tube heat exchanger were performed. Artificial accelerated method of microorganism growth on the fin surface was used for simulating the biofouled finned tube heat exchangers. Experimental results indicate that the effects of biofouling on the air-side heat transfer coefficient decreases 7.2% at 2.0 m/s when the biofouled area ratio is 10%, while it decreases 15.9% at 2.0 m/s when the biofouled area ratio is 60%, and biofouling causes a 21.8% ～ 41.3% increase in pressure drop when the air velocity is between 0.5 and 2.0 m/s. The increase of inlet air velocity is helpful to improve the long-term performance of finned tube heat exchanger. Biofouling makes the hydrophilic coating failure, and the condensation water easily converges on the fin surface where biofouling grows.     

A novel coupled VM-PT cryocooler operating at liquid helium temperature

This paper presents experimental results on a novel two-stage gas-coupled VM-PT cryocooler, which is a one-stage VM cooler coupled a pulse tube cooler. In order to reach temperatures below the critical point of helium-4, a one-stage coaxial pulse tube cryocooler was gas-coupled on the cold end of the former VM cryocooler. The low temperature inertance tube and room temperature gas reservoir were used as phase shifters. The influence of room temperature double-inlet was first investigated, and the results showed that it added excessive heat loss. Then the inertance tube, regenerator and the length of the pulse tube were researched experimentally. Especially, the DC flow, whose function is similar to the double-orifice, was experimentally studied, and shown to contribute about 0.2 K for the no-load temperature. The minimum no-load temperature of 4.4 K was obtained with a pressure ratio near 1.5, working frequency of 2.2 Hz, and average pressure of 1.73 MPa.     

Performances of the 50 mK ADR/sorption cooler

CEA/SBT is currently testing a 50 mK cooler developed in the framework of a European Space Agency Technological Research Program targeted for the Advanced Telescope for High Energy Astrophysics space mission. This cooler is composed of a small demagnetization refrigerator pre cooled by a sorption cooler stage. This Engineering Model is able to produce 1 μW of net heat lift at 50 mK and an additional 10 μW at 300 mK provided by the sorption cooler stage. The autonomy of the cooler is 24 h, and once the low temperature phase at 50 mK is over, it can be recycled in about 8 h with 10 μW and 100 μW available at respectively the 2.5 and 15 K heat sinks. These performances are in agreement with the European Space Agency requirements.In this paper, we present the detailed thermal performances of the cooler in nominal conditions as well as sensitivity measurements of the variation of the heat sink and the cold end temperatures.     

Baseline design of a sorption-based Joule-Thomson cooler chain for the METIS instrument in the E-ELT

METIS, the Mid-Infrared E-ELT Imager and Spectrograph, is one of the proposed instruments in E-ELT (European Extremely Large Telescope). Its infrared detectors require multiple operating temperatures below 77 K. Therefore, active coolers have to be deployed to provide sub-liquid-nitrogen (sub-LN2) temperature cooling. However, the sensitive imaging optical detecting system also demands very low levels of vibration. Thus, the University of Twente proposed a vibration-free cooling technique based on physical sorption. In this paper, we describe the baseline design of such a sorption-based Joule-Thomson cooler chain for the METIS instrument, that is able to deliver cooling powers of 0.4 W at 8 K, 1.1 W at 25 K and 1.4 W at 40 K from a 70-K heat sinking. This design is based on working fluid selection, cascading cooler stages and operating parameter optimization. Also, the performance of the resulting cooler design is analyzed.     

High-capacity 60 K single-stage coaxial pulse tube cryocoolers

A high-capacity single-stage coaxial pulse tube cryocooler operating at around 60 K has been developed to provide the appropriate cooling for the next-generation very-large-scale long wave infrared focal plane arrays under development. The application background and cooler design process are described, and the performance characteristics are presented. At present, the cooler typically provides 4.06 W at 60 K with the input power of 180 W at 300 K reject temperature. 4.72 W can also be achieved when the input power increases to 200 W, and over 9.4% of Carnot efficiency at 60 K has been realized. The larger pulse tube diameter of 14.2 mm is used and the evident orientation sensitivity is observed in the range of 55–65 Hz. The experiments also observe the obvious reject temperature dependence.     

3He/4He dilution refrigerator with high cooling capacity and direct pulse tube pre-cooling

In the article, a ³He/⁴He dilution refrigerator (DR) is described which is pre-cooled by a commercial two-stage pulse tube refrigerator (PTR); cryo-liquids are not necessary with this type of milli-kelvin refrigerator. The simple design of the condensation stage of this so-called dry DR is novel and explained in detail. In most dry DRs the circulating ³He gas is cooled by a two-stage PTR to a temperature of about 4 K. In the next cooling step, the ³He flow is cooled and partially liquefied in a Joule–Thomson circuit, before it is run to the dilution refrigeration unit. The counterflow heat exchanger of the Joule–Thomson circuit is cooled by the cold ³He gas pumped from the still of the DR. In the DR described here, the heat exchanger of the Joule–Thomson stage was omitted entirely; in the present design, the ³He gas is cooled by the PTR in three different heat exchangers, with the first one mounted on the first stage of the PTR, the second one on the regenerator of the second stage, and the third one on the cold end of the second stage. The heat load caused by the ³He flow is mostly absorbed by the first two heat exchangers. Thus the ³He flow presents only a small heat load to the second stage of the PTR, which therefore operates close to its base temperature of 2.5 K at all times. A pre-cooling temperature of 2.5 K of the ³He flow is sufficiently low to run a DR without further pre-cooling. The simplified condensation system allows for a shorter, compacter and more economical design of the DR. Additionally, the pumping speed of the turbo pump is no longer obstructed by the counterflow heat exchanger of the Joule Thomson stage as in our earlier DR design.     

Design and prototyping of a large capacity high frequency pulse tube

This document describes the design and the prototyping performed at CEA/SBT in partnership with AIR LIQUIDE of a high frequency large cooling power pulse tube. Driven at 58 Hz by a 7.5 kW flexure bearing pressure wave generator, this system provides a net heat lift of 210 W at 65 K. The phase shift is obtained by an inertance and a buffer volume. This type of cryogenic cooler can be used for on site gas liquefaction or drilling site and for high temperature superconductivity power device cooling (transmission lines, large generators, fault current limiters).In this paper, we focus on two essential points, the regenerator and the flow straightener. The regenerator is a key component for good performance of the pulse tube cooler. It must have a large thermal inertia, a low dead volume, a good heat transfer gas/matrix and at the same time, small pressure drop. In the present case and unlike typical moderate cooling power pulse tubes, the regenerator is very compact. However, the resulting conductive losses remain negligible compared to the cooling power targeted. The goal of the flow straightener is to avoid as much as possible any jet stream effect and to guarantee the uniformity of the velocity field at both ends of the pulse tube. Indeed multi-dimensional flow effects can significantly impact the performances of the machine.     

The characterization of a cascade thermoelectric cooler in a cryosurgery device

An experimental investigation using a Peltier thermoelectric cooler (TEC) to cool down a cryoprobe for cryosurgery was performed. Two prototypes of cryosurgery devices consisting of 5- and 6-stage TEC modules were analyzed using a variety of electrical voltages, circulating thermostatic bath (CTB) temperatures, and heat exchanger configurations to obtain an optimum cold side temperature and temperature differences between sides of the modules. To increase the heat exchanges at the hot side, a heat pipe system with a water block was used. Using an electric voltage of 12 V and a CTB temperature of 273.55 K, a cryogenic temperature of 177.09 K and a temperature difference of 99.87 K were achieved. These results indicate that the TEC module can be an effective cooling source for cryosurgery. The prototype has shown potential for clinical trials.     

A closed-cycle 1 K refrigeration cryostat

A 1 K closed-cycle cryostat has been developed to provide continuous cooling to a photon detector below 2 K. A two-stage 4 K pulse tube cryocooler is used to liquefy evacuated vapor from a 1 K pumping port to form a closed-cycle refrigeration loop. A 1 K instrumentation chamber, attached to the 1 K cooling station, is designed to operate with helium inside and provide more uniform cooling. The design of the cryostat has no direct mechanical contact between the pulse tube cryocooler heat exchangers and the 1 K cooling station resulting in almost no vibration transfer to instrumentation chamber. The cryostat can reach a no-load temperature of 1.62 K and provide 250 mW cooling power at 1.84 K.     

Effect of the geometry of flattened tube on the thermal performance of a high temperature generator

The present study numerically investigated the effect of the geometry of flattened tube on the thermal performance of a high temperature generator (HTG) with the pre-mixed surface flame burner of the double effect LiBr–water absorption system. The heat transfer tubes of the HTG were consisted with a set of circular and flattened tubes in series. FLUENT, as a commercial code, was applied for estimating the thermal performance of the HTG. Key parameters were the aspect ratio of flattened tubes, rib transversal length, and the rib pitch ratio on the flattened tube of the HTG. The maximum heat transfer rate of the HTG was obtained at the aspect ratio of 6.8 for the flattened tube. The heat transfer rate for the flattened tube was increased by 4.2% as the rib transversal length was increased from 2 mm to 3 mm. The heat transfer rate of the flattened tube with the rib pitch ratio of 15.3 was higher by the maximum 5.8% than that without rib. The heat transfer rate of the HTG with the rib of the rib transversal length of 3 mm and the rib pitch ratio of 15.3 was higher by 3.4% than that without the rib. It led that the exhaust gas temperature of the HTG with the rib was lower by 23 °C than that without the rib.     

Theoretical and experimental investigations on the partial scaling method for the Oxford-type moving-coil linear compressor

This paper puts forward the partial scaling method of the Oxford-type moving-coil linear compressor for pulse tube cryocoolers and analyzes the related principles. The systematic experimental investigations are further made to verify the analyses. One of the typical compressors developed in the authors’ laboratory is chosen to be scaled, and then coupled with the original pulse tube cold finger. At the typical operating temperature of 80 K for the pulse tube cold finger, the scaled compressor’s maximum input electric power increases from 236.7 W to 370.0 W, and the cooling power is enhanced from 10.0 W to 15.0 W. The motor efficiency decreases from 78% to 73%, but the average cooling efficiency slightly increases from 11% to 12% of Carnot efficiency due to a better match between scaled compressor and original cold finger. The rationality and feasibility of the partial scaling method have been verified by the theoretical analyses and experimental investigations.     

Innovative phase shifter for pulse tube operating below 10 K

Stirling type pulse tubes are classically based on the use of an inertance phase shifter to optimize their cooling power. The limitations of the phase shifting capabilities of these inertances have been pointed out in various studies. These limitations are particularly critical for low temperature operation, typically below about 50 K. An innovative phase shifter using an inertance tube filled with liquid, or fluid with high density or low viscosity, and separated by a sealed metallic diaphragm has been conceived and tested. This device has been characterized and validated on a dedicated test bench. Operation on a 50–80 K pulse tube cooler and on a low temperature (below 8 K) pulse tube cooler have been demonstrated and have validated the device in operation. These developments open the door for efficient and compact low temperature Stirling type pulse tube coolers. The possibility of long life operation has been experimentally verified and a design for space applications is proposed.     

Properties of solid wood and laminated wood lumber manufactured by cold pressing and heat treatment

Physical, mechanical, and morphological properties of solid wood lumbers which were cold pressed in a press and then heat treated in a kiln. Two different kinds of domestic thinning small-diameter softwood (Ginko biloba L.) and hardwood (Tilia amurensis Rupr.) were used in this study. First 50 mm thick lumbers were cold pressed until 35 mm (30% of control lumber) using a stopper for 5 min. Then the cold pressed lumbers were heat treated in an electric kiln at 180 °C for 6, 12, 24, or 48 h. To increase the utilizability of woods, the LVLs were produced from 4 mm thick veneers prepared from the heat treated lumbers using a veneer saw. Each LVL sample consisted of 5 layers which were subsequently 48 h-, 24 h-, 12 h-, and 6 h-treated veneers and untreated veneer (from top layer to bottom layer). The shrinkage rates of softwood and hardwood were considerably decreased with increasing temperature. The mechanical properties of heat treated samples were better than those of unpressed control samples. The bending strength and modulus of elasticity of the LVLs manufactured from cold pressed and then heat treated lumbers were slightly lower than those of untreated woods. The colour values obtained from the heat treated wood samples showed a clear effect of the temperature on the colour changes.     

Vibration reduction of pulse tube cryocooler driven by single piston compressor

The development of pulse tube coolers has progressed significantly during the past two decades. A single piston linear compressor is used to in order to reduce the size and mass of a high frequency pulse tube cryocooler. The pulse tube achieved a no-load temperature of 61 K and a cooling power of 1 W@80 K with an operating frequency of 80 Hz and an electrical input power of 50 W. By itself, the single piston compressor generates a large vibration, so a set of leaf springs with an additional mass is used to reduce the vibration. The equation relating the mass, the elasticity coefficient of leaf spring and the working frequency is obtained through an empirical fit of the experimental data. The vibration amplitude is reduced from 55 mm/s to lower than 5 mm/s by using a proper leaf spring. This paper demonstrates that a single piston compressor with vibration reduction provides a good choice for a PTC.     

1-D transient numerical model of a regenerator in a novel sub Kelvin Active Magnetic Regenerative Refrigerator

A sub Kelvin Active Magnetic Regenerative Refrigerator (AMRR) is being developed at the University of Wisconsin – Madison. This AMRR consists of two circulators, two regenerators, one superleak, one cold heat exchanger, and two warm heat exchangers. The circulators are novel non-moving part pumps that reciprocate a superfluid mixture of ⁴He–³He in the system. Heat from the mixture is removed within the two regenerators of this tandem system. An accurate model of the regenerators in this AMRR is necessary in order to predict the performance of these components, which in turn helps predicting the overall performance of the AMRR system. This work presents modeling methodology along with results from a 1-D transient numerical model of the regenerators of an AMRR capable of removing 2.5 mW at 850 mK at cyclic steady state.     

A two-stage Stirling-type pulse tube cryocooler with a cold inertance tube

A thermally coupled two-stage Stirling-type pulse tube cryocooler (PTC) with inertance tubes as phase shifters has been designed, manufactured and tested. In order to obtain a larger phase shift at the low acoustic power of about 2.0 W, a cold inertance tube as well as a cold reservoir for the second stage, precooled by the cold end of the first stage, was introduced into the system. The transmission line model was used to calculate the phase shift produced by the cold inertance tube. Effect of regenerator material, geometry and charging pressure on the performance of the second stage of the two-stage PTC was investigated based on the well known regenerator model REGEN. Experimental results of the two-stage PTC were carried out with an emphasis on the performance of the second stage. A lowest cooling temperature of 23.7 K and 0.50 W at 33.9 K were obtained with an input electric power of 150.0 W and an operating frequency of 40 Hz.     

Experimental investigation and optimization of small-scale helium liquefaction with multi-cryocoolers

Small-scale helium liquefiers using regenerative cryocoolers with cooling power up to 1.5 W at 4.2 K could be used to re-liquefy evaporated helium gas of small- and medium-sized cryogenic devices such as MEG and PPMS. A serial–parallel-path helium liquefier with a liquefaction rate of 83 Litres per day (L/d) using five 4 K G-M cryocoolers is developed, and has been applied to the Wuhan National High Magnetic Field Center (WHMFC) in China. Different from parallel-path helium liquefier, the helium gas is effectively, stepwise precooled by heat exchangers on multi-cold flanges, and thus the additional purifier and precooling coil heat exchangers on the thinner part of the cold head cylinder containing the 2nd stage displacer could be removed to simplify the construction. Through theoretical calculation and conclusive analysis, an optimum configuration is proposed and makes a reference to the design of serial–parallel-path helium liquefier with multi-cryocoolers.     

Low cryogen inventory,forced flow Ne cooling system with room temperature compression stage and heat recuperation

We present design and commissioning results of a forced flow cooling system utilizing neon at 30 K. The cryogen is pumped through the system by a room-temperature compression stage. To decouple the cold zone from the compression stage a recuperating counterflow tube-in-tube heat exchanger is used. Commissioning demonstrated successful condensation of neon and transfer of up to 30 W cooling power to the load at 30 K using only 30 g of the cryogen circulating in the system at pressures below 170 kPa.