Type Cryocooler Research Articles

Investigation of clearance leakage and piston offset of Joule-Thomson linear compressors at high pressure ratios

Linear compressors are renowned for oil-free operation, no crank mechanism and high efficiency, making them widely adopted in cryocoolers. Typically, high pressure ratio for linear compressor is desirable for Joule-Thomson type cryocoolers to reduce number of compression stage. However, a critical challenge to high pressure ratio is gas leakage across the radial clearance and associated the phenomenon of piston offset. This paper numerically investigated the effect of pressure ratio on clearance leakage and piston offset for a linear compressor. A comprehensive numerical model including a leakage sub-model and two control volumes (cylinder and body) was developed and validated against experimental data using nitrogen as working fluid. Real-time variations of the leakage, body pressure and piston offset were observed. The results show that the leakage due to piston movement is negligible, and the leakage due to pressure difference dominates overall clearance leakage. As the pressure ratio increases from 5 to 7, the body pressure rises by 17.9%, and the piston offset rises by 44.8%. Besides, the adiabatic efficiency decreases by 9.6%.

Improved performance of pulse tube refrigerators using thermoacoustics

A wide variety of science is performed at temperatures near and below 4 K. Such low temperatures are commonly achieved using the pulse tube refrigerator, a type of cryocooler that cyclically compresses and expands helium gas to pump heat from the cold end. Here, we discuss three topics demonstrating that thermoacoustic analysis enables substantial gains in the understanding and performance of these refrigerators. We begin by showing that dynamic acoustic optimization of pulse tube refrigerators can lead to a tremendous increase in their cooldown speed (up to 3.5 times the status quo speed). This is a novel technique that is currently being commercialized. Second, we discuss the development of mass flow meters for the study of pulse tube refrigerator compressor efficiency. The flow meters are used to validate a thermoacoustic expression that more simply finds the flow rate using only pressure transducers. Third, we show that a large amount of cooling power may be extracted from the regenerators of these refrigerators without impacting the cooling power available at the cold end. Thermoacoustic analysis reveals that the real-fluid properties of helium are responsible for this capability.

Fast cooling miniature Joule-Thomson cryocooler with improvement of energy recovery and thermal mass distribution

Open-cycle conical miniature J-T cryocoolers are favored for applications in the infrared detection field, due to their fast cooling and compact lightweight features. In this type of cryocooler, the regenerative heat exchanger for energy recovery plays a vital role in both its miniaturization and fast cool-down, because its geometry greatly determines the distribution of its heat transfer area and thermal mass. To examine and optimize the effects of the geometry on its fast-cooling performance, a validated transient analysis model, being capable of simulating the entire operating process of an open-cycle miniature J-T cryocooler, was applied to analyze and compared the transient heat transfer and flow characteristics, and then the cooling performance under different operating conditions for one cylindrical and three different cone-angle J-T cryocoolers in this study. Researches show that in terms of geometry, it is advisable to increase the heat transfer area near the cold end of heat exchanger while ensuring a relatively small thermal mass for the entire cryocooler. By analysis, a novel structure of heat exchanger with reduced volume and mass was proposed to enhance the fast-cooling performance and miniaturize cryocooler. Studied results indicate that the performance of this novel cryocooler is improved in terms of reducing the cool-down time by 28% and the volume by 50%. Additionally, this optimized structure exhibits excellent fast-cooling performance under broader operating conditions.

A Measuring System for HTS Wires and Coils Properties at Low Temperatures

The use of high temperature superconducting materials (HTS) in various applications, requires knowledge of their critical parameters like critical current, critical magnetic field and critical temperature, in DC applications like high magnetic field generators, superconducting magnetic energy storage system (SMES), magnetic resonance imaging (MRI), magnets for particle accelerators, SC Maglev trains and other applications. Also, these parameters are important for specific properties in alternating current applications like fault current limiters, transformers, generators, motors, and power transmission lines. An important characteristic that needs to be tucked into consideration for AC applications is the power losses that occur in the HTS superconducting materials, which limits the maximum performance that the superconducting materials can attain for these applications. In order to measure these characteristics, which depend on the type of material used, but also on the temperature and other parameters, it is necessary to use an appropriate setup device that allows both obtaining the working temperature range (4.2 K to 300 K and other necessary parameters like high vacuum 10-6 mbar, high current supply (500 A), high magnetic field measurement system (1-10 T), etc. For the cryogenic cooling of the entire system, a closed cycle Gifford-McMahon type cryocooler is used, with two cooling stages, stage I at 50 K and second stage at 4.2 K (liquid Helium temperature). The set-up system used for measurement of the parameters of HTS superconducting materials (YBCO, BSCCO or MgB2) and the various coils made with them, is described in this article together with its functional parameters and some experimental results obtained by testing an YBCO coils system. The maximum generated magnetic flux density was 5 T in the centre of the coils system.

A research of micro-pulse tube cryocooler with displacer phase shifter

• The phase relationships in the whole displacer PTC system were analyzed. • Three different sizes of a displacer type micro PTCs have been designed. • A 2D CFD model was constructed and used to characterize the displacer micro PTC. • A displacer provides a good phase shifting for a small-cooling-power micro PTC. • A cooling power of 1.5 W@80 K and a COP of 20.96% is obtained by the second micro PTC. Investigations of phase shifters and power recovery mechanisms are of sustainable interest for developing Stirling pulse tube cryocoolers (SPTC) with more compact design and higher efficiency. The warm displacer type SPTC has been demonstrated with a very promising theoretical efficiency benefiting from its strong phase shifting and work recovery functions. Extending this same phase shifting approach to miniature pulse tube refrigerators may provide advantages over the other alternatives, since inertance tubes are severely limited in their phase shifting capability when the associated acoustic power decreases below 20 W. A comprehensive investigation regarding the micro-pulse tube cryocooler with a displacer phase shifter is carried out in this work, including the design of the major components of the cryocooler, theoretical analysis regarding the phase relationship in the whole displacer type cryocooler system, and 2D simulation work addressing the performance of the cryocooler systems. In the optimum phase angle conditions of the 3 mm pulse tube size cryocooler, the mass flow rate leads the pressure wave by 28.8° at the regenerator warm end, and lags the pressure wave by 27.7° at the regenerator cold end. A cooling power of 1.5 W at 80 K and a COP (Coefficient of Performance) of 20.96% is obtained with an input power of 7.4 W.

Design, development, and qualification tests of prototype two-channel cryogenic temperature transmitter

A two-channel prototype cryogenic temperature transmitter is developed using an ARM Cortex-M3 series precision analog microcontroller using a rapid prototyping method for use in indigenous developments. The developed prototype utilizes an Arduino compatible baseboard equipped with interfaces for programming/debugging. Additional circuits are fabricated, and embedded application software is developed and tested. The input circuit consists of a low-value high accuracy precision current source to excite the cryogenic temperature sensors of Resistance Temperature Detector (RTD) type and employs a standard four-wire ratiometric measurement technique for accurate resistance measurement. The ratiometric measurement eliminates measurement errors due to current uncertainty. The precision microcontroller is equipped with internal programmable gain amplifiers to accurately scale low-level analog signals from cryogenic temperature sensors. The developed transmitter can interface with two cryogenic RTDs (Cernox® and PT-100 types) and can accurately measure resistance over its calibrated range (300–4 K). The cubic spline interpolation method is employed in application software for converting the measured resistance to temperature. The measured temperature is transmitted to a programmable logic device (via 4–20 mA signals) using the pulse width modulation technique. The developed transmitter is tested for its performance against commercially available transmitters at the liquid nitrogen temperature, liquid helium temperature, and over the entire measurement range using Gifford–McMahon type cryocoolers. The developed transmitter was utilized to assess the impact of the thermal resistance of the cryogenic sensors at the lowest temperature of the cryocooler (∼2.6 K). This paper outlines design details, application software development, experimental setup, measurement uncertainties, and test results.

Performance characteristics comparison of major types of mechanical cryocoolers for aerospace applications

The three major types of mechanical cryocoolers in aerospace applications, namely reverse-Brayton cryocoolers, Stirling/pulse tube cryocoolers and Joule-Thomson cryocoolers, have significantly different performance characteristics. Some of these differences are due to the nature of their thermodynamic cycles; others come from their drastic differences in mean operating pressures and pressure ratios. This paper first provides an overview of the performance characteristics of these three types of cryocoolers, their unique performance benefits and main applications. It then derives an analytical expression for a cryocooler’s Carnot efficiency, considering the effects of the system pressure ratio drop from the warm end to the cold end and heat leak due to the convective flow and axial conduction. The convective heat leak is related to the pressure drop and the figure of merit of the heat transfer surface using the Colburn-Chilton analogy. Based on this result, the paper discusses the key control parameters affecting the size and mass of the heat exchanger and thus the overall cooler system in each type of cryocooler.

Progress of the 4 K-class Vuilleumier type cryocooler and its further applications

Vuilleumier cycle was first patented in 1918. It was usually regard as the thermal-driven Stirling refrigeration cycle, which combined the low-frequency working pattern like Giffod-Mcmahon (GM) cryocooler and the compactness of Stirling-type cryocooler. With the continuous development in the past 100 years, the Vuilleumier-type (VM-type) cryocoolers have to been proved to generate cooling power from ambient temperature to liquid nitrogen temperature and even to liquid helium temperature. In recent decades, some efforts were made on traditional displacer-type VM cryocooler, Vuilleumier hybrid pulse tube cryocooler (VM-HPTC) and the VM-type pulse tube cryocooler (VM-type PTC) to obtain the liquid helium temperature. Successfully, not only the 4 K was obtained, but also the limit temperature of oscillating cryocooler by He4 was hit. This paper presents the progress of the 4 K-class VM-type cryocooler and analyzed the possible applications of 4 K class VM-type cryocooler.

On the magnetic field stability of cryogen-free magnets for magnetic resonance applications

The temporal magnetic field variation associated with Cold Head operation in cryogen-free magnets was studied. Three different mechanisms for such variations were tested separately and rated by their importance. It was found that mechanical displacement of the magnet inside the cryostat is the main issue of magnetic field perturbation. In a cryostat with the Gifford- McMahon type of cryocooler, motion of the displacer with magnetic material inside also produces significant field modulation. The temperature variation of the magnet, although noticeable, leads to smaller field distortions compared to the previous two factors. It was shown that the temporal magnetic field variation could be reduced down to below 20 ​ppb level that could be acceptable for MRI and MAS NMR applications. It was also shown that a single cryogen-free magnet could be easily used at different fields on a day-to-day basis without compromising the field stability unlike magnets housed in a liquid helium reservoir.

Study on the output pressure amplitudes of the linear compressor based on the complex vector analysis method

The advent of high-performance linear compressor has promoted the rapid development of Stirling type cryocoolers due to its advantages of low vibration, long life and compactness. At present, most of the studies on the usage of the linear compressor are focus on the high efficiency and output acoustic power (pV power) for the cold head. However, besides the above two parameters, the high-performance operation of the cold head also requires enough input pressure amplitude, which can be defined as the quality of the required power. In this study, the output pressure amplitudes of the linear compressor were graphically investigated with the help of the complex vector method. Compared to harmonic oscillation analysis, the complex vector diagram of the piston stress and velocity can provide an easier understanding for analyzing the pressure amplitude output characteristics of the compressor. Equations for estimating the output maximum pressure amplitude and the pressure amplitude under the maximum pV power of a linear compressor were deduced based on the vector diagram, which can be used for the design and selection of linear compressors. Experiments were done to verify the theoretical analysis. Influences of different operating and structure parameters on the output pressure amplitudes of the linear compressor were calculated and compared. It shows that the frequency and piston area are the optimal parameters for improving the output pressure amplitudes, which also benefits the compactness of the compressor.

Numerical study on the heat transfer characteristics of oscillating flow in cryogenic regenerators

The heat transfer characteristics of oscillating flow in regenerator have significant impacts on the system performance of regenerative cryocoolers such as pulse tube, Stirling and G-M type cryocoolers. In this paper, the hydrodynamics and heat transfer in porous media featuring periodically configured arrays of cylinders are simulated under periodically oscillating flow conditions. The effects of operating parameters including working frequency, velocity amplitude and location on the heat transfer coefficient are evaluated. The simulation results reveal that the Nusselt number is significantly different between steady and oscillating flow conditions with the magnitude strongly depending on the oscillation frequency and velocity amplitude. A correlation for the cycle averaged Nusselt number is proposed, which yields deviations of 7% from the simulation results of the oscillating flow in the cylinder array.

A novel method to hit the limit temperature of Stirling-type cryocooler

The Stirling-type cryocooler with its compact size and high efficiency is always expected to obtain its temperature limit of below 3 K. However, the pressure drop losses caused by high-frequency oscillation create large obstacles for this objective. This paper reports a novel thermal-driven Stirling-type cryocooler to obtain the lowest temperature of a Stirling-type cryocooler. The advantages of a thermal-driven cryocooler (Vuilleumier cryocooler) and pulse tube cryocooler are combined with a new type of cryocooler, called the Vuilleumier gas-coupling pulse tube hybrid cryocooler (VM-PT). A prototype of the VM-PT was recently developed and optimized in our laboratory. By using helium-4 as the working gas and magnetic regenerative materials (HoCu2 and Er3Ni), the lowest temperature of 2.5 K was obtained, which can be regarded as an important breakthrough for the Stirling-type cryocooler to achieve its limit temperature of below 3 K. It can supply >30 mW cooling power at 4.2 K and >500 mW cooling power at 20 K simultaneously. Theoretically, it is feasible to use this VM-PT for cooling the superconducting devices in space applications.

Experimental progress of a 4K VM/PT hybrid cryocooler for pre-cooling 1K sorption cooler

Sub-kelvin refrigerator has many applications in space detector and manned space station, such as for the transition-edge superconducting (TES) bolometers operated in the 50 mK range. In order to meet the requirement of space applications, the high efficient, vibration free and high stability refrigerator need to be designed. VM/PT hybrid cryocooler is a new type cryocooler capable of attaining temperature below 4K. As a low frequency Stirling type cryocooler, it has the advantages of high stability and high efficiency. Combined with the vibration free sorption cooler and ADR refrigerator, a novel sub-kelvin cooling chain can be designed for the TES bolometer. This paper presents the recent experimental progress of the 4K VM/PT hybrid cryocooler in our laboratory. By optimizing of regenerators, phase shifters and heat exchangers, a lowest temperature of 2.6K was attained. Based on this cryocooler, a preliminary sorption cooler could be designed.

Numerical study of a VM type multi-bypass pulse tube cryocooler operating at 4K

VM cryocooler is one kind of Stirling type cryocooler working at low frequency. At present, we have obtained the liquid helium temperature by using a two-stage VM/pulse tube hybrid cryocooler. As a new kind of 4K cryocooler, there are many aspects need to be studied and optimized in detail. In order to reducing the vibration and improving the stability of this cryocooler, a pulse tube cryocooler was designed to get rid of the displacer in the first stage. This paper presents a detail numerical investigation on this pulse tube cryocooler by using the SAGE software. The low temperature phase shifters were adopted in this cryocooler, which were low temperature gas reservoir, low temperature double-inlet and multi-bypass. After optimizing, the structure parameters and the best diameters of orifice, multi-bypass and double-inlet were obtained. With the pressure ratio of about 1.6 and operating frequency 2Hz, this cryocooler could supply above 40mW cooling power at 4.2K, and the total input power needs no more than 60W at 77K. Based on the highest efficiency of 77K high capacity cryocooler, the overall efficiency of this VM type pulse tube cryocooler is above 0.5% relative Carnot efficient.

Influence of minor geometric features on Stirling pulse tube cryocooler performance

Minor geometric features and imperfections are commonly introduced into the basic design of multi-component systems to simplify or reduce the manufacturing expense. In this work, the cooling performance of a Stirling type cryocooler was tested in different driving powers, cold-end temperatures and inclination angles. A series of Computational Fluid Dynamics (CFD) simulations based on a prototypical cold tip was carried out. Detailed CFD model predictions were compared with the experiment and were used to investigate the impact of such apparently minor geometric imperfections on the performance of Stirling type pulse tube cryocoolers. Predictions of cooling performance and gravity orientation sensitivity were compared with experimental results obtained with the cryocooler prototypes. The results indicate that minor geometry features in the cold tip assembly can have considerable negative effects on the gravity orientation sensitivity of a pulse tube cryocooler.

Study on the stroke amplitude of the linear compressor

Linear compressor is widely used to drive Stirling type cryocooler due to its advantages of low vibration, long life and compactness. In practice, it is often expected to operate at or near the resonant state under which it can reduce more mechanical loss and Joule heat loss, and transfer more electric energy into thermal acoustic energy of the gas. It is significant to explore the working characteristics of the linear compressor for realizing its high-efficient operation in the cryocooler system. In this study, considering the effects of both acoustic and mechanical impedances, a mass-gas-spring model is built to investigate the piston stroke amplitude of the linear compressor. Different with previous models that contain some uncertain parameters, an expression of the stroke amplitude that only based on easily obtainable parameters has been deduced for the first time. The calculated curve of the stroke amplitude at different operating frequencies provides a direct indication of whether the compressor is working at its resonant state or not. An existing high power linear compressor has been used to test and verify the equations. The theoretical stroke amplitude curve matches the experimental data quite well with a deviation of less than 10%. Further analyses about the influences of the operating parameters and intrinsic parameters of a compressor on its resonant characteristics have been given, which can be used as guidance of how to well utilize and design a linear compressor for a cryocooler system.

Measured and predicted heat transfer coefficients for boiling zeotropic mixed refrigerants in horizontal tubes

The use of mixed gas working fluids has become common in Joule–Thomson type cryocoolers for a variety of applications. However, there is a scarcity of data currently available with supporting theory capable of predicting the heat transfer coefficients associated with two-phase, multi-component mixtures at cryogenic temperatures. This paper aims to fill this void by providing experimental data for the heat transfer coefficient associated with of multicomponent zeotropic mixtures boiling in small channels over temperatures ranging from 100K to room temperature. The sensitivity of the measured heat transfer coefficient to parameters such as heat flux, mass flux, pressure, tube diameter, and mixture composition is also presented. The results indicate that the heat transfer process is driven, principally, by convective boiling; however, composition, diameter, and surface roughness affect the measured heat transfer coefficient. Evaporating pressure has less relevance compared to the other parameters.The actual experimental data collected as part of this effort and additional data from Nellis et al. (2005) are used to evaluate models to characterize the heat transfer process for boiling zeotropic mixtures in horizontal tubes. The heat transfer coefficient data is predicted well using correlations described by Granryd (1991) and Little (2008). The Granryd correlation is the recommended correlation to predict heat transfer coefficient of zeotropic mixtures because it shows the best accuracy with an Absolute Average Deviation (AAD) of 16% and predicts 83% of the data with a relative error lower than 25%. The Little correlation exhibits greater accuracy at high Reynolds number, high thermodynamic quality, or both; however, its accuracy is substantially reduced for low qualities and low Reynolds number.

Description and validation of the Little correlation for boiling zeotropic mixtures in horizontal tubes from cryogenic to room temperature

The use of mixed gas working fluids has become common in Joule-Thomson (JT) type cryocoolers for a variety of applications operating in temperatures ranging from 80 to 230 K. The thermal efficiency of mixed gas JT cryocoolers is dependent on the optimization of the gas mixture composition. Most optimization methodologies focus on thermodynamic criteria of the cycle because there are very little data or theory currently available regarding the heat transfer coefficients associated with these multi-component mixtures in two-phase regimes at cryogenic temperatures. A generally accepted correlation to predict the local heat transfer coefficient (htc) for mixtures during the boiling process does not exist. Little [1] proposed a correlation to be used on horizontal tubes that shows good agreement with Nellis et al. [2] experimental data of nitrogen-hydrocarbon mixtures. However, it is not clearly shown how the correlation is obtained and how it should be applied. This paper provides a more complete description of the Little correlation and also expands its validation using the experimental data provided by Barraza [3]. The new experimental data include measurements of the local heat transfer coefficient for mixtures comprising 2 component (binary) up to 5 components in the temperature range between 100 K and room temperature. These mixtures are formed from nitrogen-hydrocarbon and argon-fluorocarbon mixtures and evaporate in horizontal tubes with diameters from 0.5 to 3.0 mm for different heat flux, mass flux, evaporating pressure, and composition.

Hampson’s type cryocoolers with distributed Joule–Thomson effect for mixed refrigerants closed cycle

Most previous studies on Joule–Thomson cryocoolers of mixed refrigerants in a closed cycle focus on the Linde kind recuperator. The present study focuses on four constructions of Hampson’s kind miniature Joule–Thomson cryocoolers based on finned capillary tubes. The frictional pressure drop along the tubes plays the role of distributed Joule–Thomson expansion so that an additional orifice or any throttle at the cold end is eliminated. The high pressure tube is a throttle and a channel of recuperation at the same time. These coolers are tested within two closed cycle systems of different compressors and different compositions of mixed coolants. All tests were driven by the same level of discharge pressure (2.9MPa) while the associated suction pressures and the associated reached temperatures are dependent on each particular cryocooler and on the closed cycle system. The mixture of higher specific cooling capacity cannot reach temperatures below 80K when driven by the smaller compressor. The other mixture of lower specific cooling capacity driven by the larger compressor reaches lower temperatures. The examined parameters are the cooldown period and the reachable temperatures by each cryocooler.

A Commercial HTS Dipole Magnet for X-Ray Magnetic Circular Dichroism (XMCD) Experiments

A commercial HTS magnet was designed, built and tested by HTS-110 Ltd for X-ray Magnetic Circular Dichroism (XMCD) experiments. The magnet was integrated with a XMCD UHV (Ultra High Vacuum) chamber and installed at an existing soft X-ray beamline at Singapore Synchrotron Light Source (SSLS). The dipole magnet has a bore of 40 mm in diameter and a gap of 90 mm to accommodate the XMCD chamber with nine access ports for sample manipulation and XMCD analysis. To achieve a UHV of 10-10 mbar inside the chamber, an independent bakeout system was developed for this magnet to allow the XMCD UHV chamber to be baked out at a temperature over 100 degrees Celsius without affecting the HTS magnet. A small GM type cryocooler is employed to cool the magnet coils down to 17 K by thermally intercepting the heat load from the 2nd stage to the 1st stage. The magnet can provide variable magnetic fields up to 2.16 T and with the field homogeneity better than 0.1% over a 1 cm diameter sphere. Limited by the peak voltage of the bipolar power supply used for the magnet, the minimum time of full field energizing is around 20 seconds and the minimum time of full field reversal is around 40 seconds. The technical issues related to UHV compatibility and ramping rate in developing similar HTS applications are discussed in this paper.