Study on the phase shift characteristic of the pneumatic Stirling cryocooler

Due to entire pneumatic connection between free piston and free displacer, the motion parameters of them including amplitude and phase shift can actually impact the cooling capacity and overall performance of cryocooler obviously. In this study, the procedure of design and manufacture pneumatic free piston and free displacer (FPFD) Stirling cryocooler had firstly been described in details. Then in order to accomplish study, the experimental bench has been set up based on 80 K@1 W Stirling cryocooler. The effect of the thermodynamic and pneumatic parameters including charging pressure, natural frequency of displacer, damping coefficient of displacer, working frequency on the pressure, displacement and displacer phase shift has been investigated, respectively by means of experimental and theoretical method. In particular, the variation of damping is realized by adjusting the width of clearance cut on the additional damping component, which is screwed on the displacer rod. Similarly, natural frequency of displacer is changed by the extra mass connected on the displacer. Due to the results of experimental study, the optimum working conditions of this Stirling cryocooler for 80 K cold tip temperature are as follows: charge pressure 15 bar, natural frequency of displacer 46 Hz, width of clearance 300 μm and working frequency 43 Hz. In agreement with the optimum working conditions, neighborhood interval of 90° is the ideal working domain for displacement phase shift. Meanwhile, the displacer phase shift should approach to 0°as near as possible and pressure phase shift should also be as small as possible, which have linear relation with non-dimensional damping characteristic of compressor. In view of theoretical study, the expressions of three phase shifts deduced from thermodynamic equation of piston and displacer respectively are expressed as the functions of working parameters, which are verified by the experimental data and consequently can be used as the powerful guidance to optimum seeking.     

Opposed piston linear compressor driven two-stage Stirling Cryocooler for cooling of IR sensors in space application

A two-stage Stirling Cryocooler has been developed and tested for cooling IR sensors in space application. The concept uses an opposed piston linear compressor to drive the two-stage Stirling expander. The configuration used a moving coil linear motor for the compressor as well as for the expander unit. Electrical phase difference of 80 degrees was maintained between the voltage waveforms supplied to the compressor motor and expander motor. The piston and displacer surface were coated with Rulon an anti-friction material to ensure oil less operation of the unit. The present article discusses analysis results, features of the cryocooler and experimental tests conducted on the developed unit. The two-stages of Cryo-cylinder and the expander units were manufactured from a single piece to ensure precise alignment between the two-stages. Flexure bearings were used to suspend the piston and displacer about its mean position. The objective of the work was to develop a two-stage Stirling cryocooler with 2 W at 120 K and 0.5 W at 60 K cooling capacity for the two-stages and input power of less than 120 W. The Cryocooler achieved a minimum temperature of 40.7 K at stage 2.     

Operating characteristics of a single-stage Stirling cryocooler capable of providing 700 W cooling power at 77 K

High cooling capacity Stirling cryocooler generally has hundreds to thousands watts of cooling power at liquid nitrogen temperature. It is promising in boil-off gas (BOG) recondensation and high temperature superconducting (HTS) applications. A high cooling capacity Stirling cryocooler driven by a crank-rod mechanism was developed and studied systematically. The pressure and frequency characteristics of the cryocooler, the heat rejection from the ambient heat exchanger, and the cooling performance are studied under different charging pressure. Energy conversion and distribution in the cryocooler are analyzed theoretically. With an electric input power of 10.9 kW and a rotating speed of 1450 r/min of the motor, a cooling power of 700 W at 77 K and a relative Carnot efficiency of 18.2% of the cryocooler have been achieved in the present study, and the corresponding pressure ratio in the compression space reaches 2.46.     

Cascade pulse-tube cryocooler using a displacer for efficient work recovery

Expansion work is generally wasted as heat in a pulse-tube cryocooler and thus represents an obstacle to obtaining higher Carnot efficiency. Recovery of this dissipated power is crucial to improvement of these cooling systems, particularly when the cooling temperature is not very low. In this paper, an efficient cascade cryocooler that is capable of recovering acoustic power is introduced. The cryocooler is composed of two coolers and a displacer unit. The displacer, which fulfills both phase modulation and power transmission roles, is sandwiched in the structure by the two coolers. This means that the expansion work from the first stage cooler can then be used by the second stage cooler. The expansion work of the second stage cooler is much lower than the total input work and it is thus not necessary to recover it. Analyses and experiments were conducted to verify the proposed configuration. At an input power of 1249 W, the cascade cryocooler achieved its highest overall relative Carnot efficiency of 37.2% and a cooling power of 371 W at 130 K. When compared with the performance of a traditional pulse-tube cryocooler, the cooling efficiency was improved by 32%.     

Numerical analysis on performance and contaminated failures of the miniature split Stirling cryocooler

A mathematical model based on thermodynamic theory of variable mass is developed for the split Stirling refrigerator, in which, the whole machine is considered by one-dimensional approach while the processes in the regenerator are simulated by two-dimensional approach. First, the influence of the ideal frost layer distributions on the flow and heat transfer in the regenerator and the performance of the Stirling cryocooler are simulated. Then, the distribution of the contaminated water vapor and its coagulated and deposited process is qualitatively analyzed. Finally, the lifetime of the refrigerator is evaluated based on the calculated data. The results show that when the refrigerator is operated at uniform distribution of the water vapor partial pressure in the regenerator, the cooling capacity is reduced over 10% at about 631 h, and the power consumption of compressor is increased over 20% at about 1168 h. However, for the linear distribution of water vapor partial pressure, the refrigerator can work properly because the frost never reaches the criterion of failure. Also, it is found that when the Stirling cryocooler restarts after a shutdown, the cooling capacity is reduced by 10% once the frost mass is over 7.05 mg, and there is no cooling capacity once the frost mass reaches 41.2 mg.     

Advances on a cryogen-free Vuilleumier type pulse tube cryocooler

This paper presents experimental results and numerical evaluation of a Vuilleumier (VM) type pulse tube cryocooler. The cryocooler consists of three main subsystems: a thermal compressor, a low temperature pulse tube cryocooler, and a Stirling type precooler. The thermal compressor, similar to that in a Vuilleumier cryocooler, is used to drive the low temperature stage pulse tube cryocooler. The Stirling type precooler is used to establish a temperature difference for the thermal compressor to generate pressure wave. A lowest no-load temperature of 15.1 K is obtained with a pressure ratio of 1.18, a working frequency of 3 Hz and an average pressure of 2.45 MPa. Numerical simulations have been performed to help the understanding of the system performance. With given experimental conditions, the simulation predicts a lowest temperature in reasonable agreement with the experimental result. Analyses show that there is a large discrepancy in the pre-cooling power between experiments and calculation, which requires further investigation.     

Cryogenic infrared mission “JAXA/SPICA” with advanced cryocoolers

Since the next cryogenic infrared mission “JAXA/SPICA” employs advanced mechanical cryocoolers with effective radiant cooling in place of cryogen, the primary mirror, 3.5 m in diameter, and the optical bench can be maintained at 4.5 K for at least 5 years. First, the feasibility of the thermal design of the cryogenic system is presented. A 20 K-class Stirling cryocooler was then improved in cooling capacity and reliability for the mission, and the effects of contaminated working gas or new regenerator materials on cooling performance were investigated. Development of a new ³He-JT (Joule–Thomson) cryocooler for use at 1.7 K is also described, along with the successful results of a cooling capacity higher than the required 10 mW. A 4 K-class cryocooler was modified and developed for higher reliability over a five-year operational life and a higher cooling capacity exceeding the current 30 mW. Finally, we discuss a system for heat rejection from cryocoolers using thermal control devices.     

Diaphragm forming of continuous fibre reinforced thermoplastics: influence of temperature,pressure and forming velocity on the forming of Upilex-R® diaphragms

In the diaphragm forming process, the thermoplastic composite sheet is clamped between two high temperature thermoplastic diaphragms. In the present study, the influence of temperature, pressure and forming rate on the deformation of high temperature PI diaphragms (Upilex-R^®, Ube Industries) is described. At temperatures below 275 °C the upper diaphragm slides over the bottom diaphragm and shows a more global deformation, above 305 °C, the upper diaphragm cannot slide over the bottom diaphragm and deforms in the same manner. The region 275–305 °C is a kind of transition region between the previous two temperature ranges. A hydrostatic pressure of 1 bar turned out to be sufficient to deform the diaphragms, therefore, no influence of pressure was observed. The deformation of the bottom diaphragm is independent of forming rate, while the upper diaphragm showed some dependence.     

Raytheon long life cryocoolers for future space missions

Over the last several years, Raytheon has made significant advances on two long-life cryocoolers designed for efficient operation on space platforms. The first is the Low-Temperature Raytheon Stirling/Pulse Tube 2-stage (LT-RSP2) hybrid cryocooler, which is capable of providing simultaneous cooling at 55 K and 10 K nominal first and second stage temperatures. The LT-RSP2 design was finalized in mid-2009, with fabrication of the prototype unit taking place in late 2009 and early 2010 and execution of the production program in 2011–2015. During this period the LT-RSP2 has undergone extensive characterization testing and has successfully been integrated with an optical bench. The second cryocooler is the Raytheon Advanced Miniature (RAM) cryocooler, a flight packaged single stage pulse tube cooler with an integrated surge volume and inertance tube. It has been designed for high frequency operation and has been fully optimized to make use of the Raytheon Advanced Regenerator, resulting in improved efficiency relative to previous Raytheon pulse tube coolers. In this paper, aspects of both the LT-RSP2 and RAM mechanical and thermodynamic designs will be presented as well as information regarding their capabilities and performance.     

Parameter effect analysis for a Stirling cryocooler

In this paper, an isothermal model is used for modeling the Stirling cryocooler. Various losses including regenerator imperfection thermal loss, piston finite speed loss, gas spring hysteresis loss, displacer shuttle heat loss, clearance heat pump loss, heat conduction loss, and flow viscosity loss are taken into consideration at the same time step, as they could interact with each other. Energy and exergy balance analysis of the cryocooler shows that the mechanical friction loss is the biggest mechanical loss; conduction loss is the biggest heat loss. Effects of parameters consisting of cold end temperature, hot end temperature, average pressure, rotation speed, displacer clearance size, phase shift between piston and displacer, and ratio between diameter and stroke of piston on the cryocooler's performance are investigated. It shows that, there is optimum displacer clearance size, optimum phase shift between piston and displacer, and optimum ratio between diameter and stroke of piston for the studied cryocooler. The isothermal model was verified by the PPC-102 Stirling cryocooler.     

Numerical study on effect of CuO-water nanofluid on cooling performance of two different cross-sectional heat sinks

In this paper, effect of CuO nanoparticles in distilled water on heat dissipation from electronic components is investigated numerically. Computational Fluid Dynamics (CFD) simulations are carried out to study the rectangular and circular cross-sectional shaped heat sinks, and influence of their sectional geometry on the flow and heat transfer characteristics. The three-dimensional governing equations for fluid flow and heat transfer are solved using finite volume method. The two-phase and single-phase models are used to simulate the nanofluid flow. Comparisons of the numerical results with corresponding experimental data show that the two-phase model is more accurate than the single-phase model. Also, effect of various nanoparticle volume fraction on thermal and hydrodynamic characteristics of the heat sink are discussed in details for two different geometry of channel. The results show that the heat sink with nanofluid has better heat transfer rate in comparison with the water-cooled heat sink. A comparison between rectangular and circular channels at the same Reynolds number and cross section area shows that the heat sink with rectangular channels has lower thermal resistance.     

Operating modes and cooling capabilities of the 3-stage ADR developed for the Soft-X-ray Spectrometer instrument on Astro-H

A 3-stage adiabatic demagnetization refrigerator (ADR) (Shirron et al., 2012) is used on the Soft X-ray Spectrometer instrument (Mitsuda et al., 2010) on Astro-H (Takahashi et al., 2010) [3] to cool a 6 × 6 array of X-ray microcalorimeters to 50 mK. The ADR is supported by a cryogenic system (Fujimoto et al., 2010) consisting of a superfluid helium tank, a 4.5 K Joule–Thomson (JT) cryocooler, and additional 2-stage Stirling cryocoolers that pre-cool the JT cooler and cool radiation shields within the cryostat. The ADR is configured so that it can use either the liquid helium or the JT cryocooler as its heat sink, giving the instrument an unusual degree of tolerance for component failures or degradation in the cryogenic system. The flight detector assembly, ADR and dewar were integrated into the flight dewar in early 2014, and have since been extensively characterized and calibrated. This paper summarizes the operation and performance of the ADR in all of its operating modes.     

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.     

Experimental study of one-stage VM cryocooler operating below 8 K

The Vuilleumier (VM) refrigerator, known as heat driven refrigerator, is one kind of closed-cycle Stirling type regenerative refrigerator. The VM refrigerator with power being supplied by liquid nitrogen was proposed by Hogen and developed by Zhou, which shows great potential for development below 10 K. This paper describes the experimental development of a VM cryocooler operating below 8 K, which was achieved by using liquid nitrogen as a heat sink of middle cavity. The regenerator was optimized by using a part of metallic magnetic regenerator material Er₃Ni to replace the lead sphere and a no-load temperature of 7.8 K was obtained. Then all the lead spheres were replaced by Er_0.6Pr_0.4 material and a no-load temperature of 7.35 K was obtained, which is the lowest temperature for this kind of refrigerator reported so far. The cooling power at 10 K is about 500 mW with a pressure ratio near 1.6 and a charge pressure of 1.8 MPa. Especially, the magnetic material Er_0.6Pr_0.4 was found to be a potential substitution for the conventional lead.     

CFD simulation of a miniature coaxial Stirling-type pulse tube cryocooler operating at 128 Hz

A two-dimensional axis-symmetric CFD model of a miniature coaxial Stirling-type pulse tube cryocooler with an overall weight of 920 g operating at 128 Hz is established, and systematic simulations of the performance characteristics at different temperatures are conducted. Both thermal equilibrium and non-equilibrium mechanisms for the porous matrix are considered, and the regenerator losses including the gas and solid conduction, the pressure drop and the imperfect interfacial heat transfer are calculated, respectively. The results indicate that the pressure drop loss is dominant during the first 85% and 78% of regenerator length for the thermal equilibrium and non-equilibrium models, respectively, and it decreases monotonously from warm to cold end due to the steadily decreasing Darcy and Forchheimer terms, whereas other entropy generations share similar changing tendencies, going up gradually near the warm end, increasing dramatically from about 60% of length and then decreasing sharply near the cold end. The reasons for these entropy variations are discussed.     

Numerical analysis of wall shear patterns on the external wall of an API 5L X42 natural gas pipe

Natural gas pipeline is designed and constructed according to stringent international codes and standards, thus it hardly to rupture or leaks. However, erosive water jet from leaked water pipeline is able to erode the pipe surfaces and lead to its failure. Due to complications in understanding the subsurface hydrodynamics of buried pipe condition, Computational Fluid Dynamics (CFD) is commonly used to predict its characteristics to provide multifaceted overview of the erosion phenomenon. In this study, CFD simulation of multiple water leakage surface of 0.05 m diameter at 9 atm water pipeline pressure was performed with separation distance of 0.05 m, using standard k–epsilon model (SKE) of turbulent model. Simulation results were obtained using discrete random walk to determine the instantaneous fluid velocity. It was found that different region of water shear and sand shear on natural gas pipelines was formed due to the strikes of erosive slurry at different angles. The CFD simulation results showed that sand was the primary cause of the natural gas pipe wall shear, causing cutting and deformation region on the surface of gas pipe. The erosion pattern of the three ruptured holes from the incident sites had a similar trend with those from simulated CFD results of water jetting source at angles of 45°, 60°, and 75°.     

Novel compact sorption generators for car air conditioning

A prototype compact generator using the activated carbon–ammonia pair based on the plate heat exchanger concept has been designed and built at Warwick University. The novel generator has low thermal mass and good heat transfer. The heat exchanger uses nickel-brazed shims and spacers to create adsorbent layers only 4 mm thick between pairs of liquid flow channels of very low thermal mass. The prototype sorption generator manufactured was evaluated under EU car air conditioning test conditions.The prototype sorption generator is described and its experimental performance reported. While driven with waste heat from the engine coolant water (at 90 °C), a pair of the current prototype generators (loaded with about 1 kg of activated carbon) operating out of phase has produced an average cooling power 1.6 kW with about 2 kW peaks. The typical average COP obtained is 0.22.     

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.     

Design and performance of a fast thermal response miniature Chromium Potassium Alum (CPA) salt pill for use in a millikelvin cryocooler

The design and performance of a fast thermal response miniature (24 mm outer diameter by 30 mm long) Chromium Potassium Alum (CPA) salt pill is described. The need for a fast thermal response has been driven by the development of a continuously operating millikelvin cryocooler (mKCC) which uses 2 T superconducting magnets that can be ramped to full field in 30 s. The consequence of magnetising and demagnetising the CPA pill in such a short time is that thermal boundary resistance and eddy current heating have a significant impact on the performance of the pill, which was investigated in detail using modelling. The complete design of a prototype CPA pill is described in this paper, including the methods used to minimise thermal boundary resistance and eddy current heating as well as the manufacturing and assembly processes. The performance of the prototype CPA pill operated from a 3.6 K bath is presented, demonstrating that a complete CPA cycle (magnetising, cooling to bath and demagnetising) can be accomplished in under 2.5 min, with magnetisation and demagnetisation taking just 30 s each. The cold finger base temperature of the prototype varies with demagnetisation speed as a consequence of eddy current heating; for a 30 s demagnetisation, a base temperature of 161 mK is obtained, whilst for a 5 min demagnetisation, a base temperature of 149 mK was measured (both from a 3.6 K and 2 T starting position). The measured hold times of the CPA pill at 200 mK, 300 mK, and 1 K are given, proving that the hold time far exceeds the recycle time and demonstrating the potential for continuous operation when two ADRs are used in a tandem configuration. The ease and speed at which the CPA pill temperature can be changed and controlled when stepping between operating temperatures in the range of 200 mK to 4 K using a servo control program is also shown, once again highlighting the excellent thermal response of the pill. All of the test results are in good agreement with the modelling used to design the CPA pill, giving good confidence in our ability to understand and estimate the effects of eddy current heating and thermal boundary resistance. To conclude, the design for the CPA pill to be used in the mKCC (which is heavily based on the design of the prototype) is presented.     

CFD modeling and experimental verification of a single-stage coaxial Stirling-type pulse tube cryocooler without either double-inlet or multi-bypass operating at 30–35 K using mixed stainless steel mesh regenerator matrices

This paper presents the CFD modeling and experimental verifications of a single-stage inertance tube coaxial Stirling-type pulse tube cryocooler operating at 30–35 K using mixed stainless steel mesh regenerator matrices without either double-inlet or multi-bypass. A two-dimensional axis-symmetric CFD model with the thermal non-equilibrium mode is developed to simulate the internal process, and the underlying mechanism of significantly reducing the regenerator losses with mixed matrices is discussed in detail based on the given six cases. The modeling also indicates that the combination of the given different mesh segments can be optimized to achieve the highest cooling efficiency or the largest exergy ratio, and then the verification experiments are conducted in which the satisfactory agreements between simulated and tested results are observed. The experiments achieve a no-load temperature of 27.2 K and the cooling power of 0.78 W at 35 K, or 0.29 W at 30 K, with an input electric power of 220 W and a reject temperature of 300 K.