Regenerative Cryocoolers Research Articles

Cooling method for the rotor of a superconducting motor

Electrification has been proposed as a route to decarbonizing air travel. Conventional electric motors are too heavy to achieve the power densities required for aviation so superconducting motors for aircraft are being developed. Superconductors however, work at cryogenic temperatures, which indispensably require a reliable and efficient cooling mechanism with a cryocooler. The Robinson Research Institute is developing a 3 MW superconducting motor using an HTS (High Temperature Superconductor) rotor operating at 50 K and MgB2 running at near 20 K for the stator. The motor is intended to run at a shaft speed of 4500-6000 rpm to directly drive a ducted fan. A key challenge is the removal of heat from a cryogenic spinning rotor to a stationary refrigerator so it can be rejected at ambient temperatures. Previous motors have utilized externally pumped or thermos-syphoned cryogens to cool the rotor, which require complex rotary seals. The aircraft application demands a compact and low weight solution. This paper presents a novel cooling design where a stationary cryocooler’s cold heat exchanger penetrates the rotor, and the relative motion of the two creates a pumping motion for cold helium gas to transfer heat from the rotor to the cryocooler. The sealing is low pressure and at ambient temperature and pressure with a temperature gradient in the helium adjacent to the cryocooler regenerator. Design and CFD modelling of the concept are presented showing the system can generate sufficient cold helium flow to transfer heat from the rotor to the cryocooler with a small temperature difference, resulting in a compact, efficient, and low-weight rotor cooling method.

Non-magnetic regenerator material of silver oxide for 4 K cryocoolers

In regenerative cryocoolers, magnetic specific heat is often utilized for heat regeneration at cryogenic temperature, such as below 20 K. Magnetic regenerator materials of rare earth compound which have sufficient specific heat enabled cryocooler to reach below 4 K. However, the magnetic noise emitted from the magnetic materials during the refrigeration cycle affects the performance of noise-sensitive devices. In this study, we propose a non-magnetic regenerator material having “optical phonon degree of freedom” even at cryogenic temperatures, in which silver oxide (Ag2O) is selected. Ag2O decomposes at temperature above 410 K in ambient atmosphere, whereas we succeeded in fabricating a sintered Ag2O bulk and small particles of the size of 0.5 mm with high-density more than 90%. Specific heat measurement down to 100 mK was performed for the sintered bulk sample of Ag2O. Below 10 K, a non-Debye behavior was observed, suggesting the contribution from optical phonon modes. Calculation of the phonon density of state (phDOS) performed by the Evolution Strategy algorithm in addition to the first-principles calculation reveals that phDOS has several distinct peaks at low energies, and the low-energy optical phonons contribute to the non-Debye behavior. The low temperature specific heat of Ag2O enhanced by the optical phonons is larger than conventional non-magnetic regenerator materials when compared in terms of specific heat per volume. Moreover, the calculation of cooling performance of cryocooler using Ag2O particles is found to be reached below 4.2 K. This study shows that non-magnetic material with low temperature specific heat enhanced by optical phonon mode can be used as a regenerator material for regenerative cryocoolers.

Periodic Flow Through Regenerator Fillers Made from Packed Spherical Microparticles

Periodic flow of gases at cryogenic temperatures through microporous structures occur in the regenerators of Stirling-type cryocoolers. For computational fluid dynamics simulation of such flows using the Darcy–Brinkman–Forchheimer model, permeability and inertial coefficients are needed. Experimental pore-level characterization of such regenerators is difficult due to their microporous structure, and the occurrence of near-zero instantaneous flow rates and extraction of these parameters from experimental data can involve significant uncertainty. In this work, by three-dimensional direct simulation, the hydrodynamic parameters are found for flow through a metallic fill powder common to some recent cryocooler regenerators that operate at very low temperatures. The intrinsic permeability and the Forchheimer coefficients are found for parameter ranges of interest to pulse-tube cryocoolers and are compared with reported extracted experimental values. The permeability and Forchheimer coefficients are correlated to be suitable for application in computational fluid dynamics codes.

Optimal absorption of distributed and conductive heat loads with cryocooler regenerators

The second-stage regenerators of pulse tube refrigerators are routinely used to intercept heat in cryogenic systems; however, optimal methods for heat sinking to the regenerator have not been studied in detail. We investigated intermediate cooling methods by densely instrumenting a commercial, two-stage pulse tube refrigerator with thermometers and heaters. We then experimentally emulated heat loads from common sources such as arrays of electrical cables (a single-point conductive load) and 3He return gas for dilution refrigerators (a distributed load). Optimal methods to absorb these heat loads, whether applied independently or simultaneously, are presented. Our study reveals the importance of understanding the response of the regenerator temperature profle for optimal thermal integration of heat loads along the regenerator, i.e., temperatures and heat fows at all heat sink locations. With optimal utilization of regenerator intermediate cooling, 3He fow rates of up to 2 mmol/s can be cooled from 50 K to 3 K and fully condensed using this pulse tube refrigerator; alternatively, the heat leak from over 100 electrical cables can be cooled across that same temperature span while simultaneously condensing 1.4 mmol/s of 3He.

Investigation of regenerator mesh characteristics for a pulse tube cryocooler

An investigation of regenerator mesh characteristics on a Pulse tube cryocooler’s performance is conducted using Sage V11 software. The cryocooler operates between temperatures of 300 K and 80 K. The components of the cryocooler are free piston linear compressor, aftercooler, regenerator, pulse tube, warm heat exchanger, and inertance tube- bounce space as the phase shifter. The performance of the regenerative cryocoolers depends on the regenerator. The regenerator of this cryocooler consists of a stack of stainless steel meshes. In this paper, a comparison of regenerator stainless steel meshes with mesh number #200, #300, #400, #500 and its combinations were analysed. To choose the best multi mesh combination, the length of the individual meshes is varied so that the total length of the regenerator is unchanged. The performance was analysed based on the Coeffcient of Performance of cryocooler for single mesh type and mesh combinations (combination two) at acceptor temperature of 80 K. The maximum performance with single mesh was with #400 mesh with COP of 6.12%. The cryocooler’s maximum Coeffcient of Performance was with the combination of #400 and #500 meshes in the regenerator. The best multi mesh combination gave 4.071 W cooling power with an input power of 61.92 W with a COP of 6.57%. There was an improvement of 7.26% with the use of multiple mesh regenerator.

Junction Temperature of Cmos Electronics Cooled by a Regenerative Cryocooler

Junction Temperature of Cmos Electronics Cooled by a Regenerative Cryocooler

Comparative Study on Performance of Regenerator for Miniature Pulse Tube Cryocooler with Metal Matrix Materials at Ultra High Frequencies

A Miniature Pulse Tube Cryocooler (MPTC) is the first selection for the cooling of IR sensors, infrared detectors, etc. in space technology. The regenerator is one of the key components to operate an MPTC at high efficiency. The objective of this study is to explore the possibilities of the different matrices as regenerator filler materials for MPTC operating at ultra-high frequencies. REGEN 3.3 is one of the best software available for the design and optimization of cryocooler regenerators. We have used REGEN 3.3 for numerical simulations of the three different regenerator matrix materials viz. stainless steel wire mesh screen (SS 635#), brass wire mesh screen (500#) and copper wire mesh screen (500#) at the hot end and cold end temperatures of 300[Formula: see text]K and 80[Formula: see text]K for COP, cooling power, total power losses and pressure losses, at an ultra-high frequency of 100[Formula: see text]Hz and 200[Formula: see text]Hz. The simulation results depict that the regenerator using stainless steel mesh screen shows better results than that of the brass mesh screen and copper mesh screen at 100[Formula: see text]Hz. However, the performance of brass mesh screen and copper mesh screen performs better than the stainless steel at 200[Formula: see text]Hz. Therefore, the proposed matrix materials can be used as regenerator materials for the MPTC at ultra-high frequencies with better performances.

Numerical study on a two-stage large cooling capacity stirling cryocooler working at 20 K

Large cooling capacity regenerative cryocoolers working at 20 K is indispensable in developing superconducting technology and hydrogen energy systems. Two-stage Stirling cryocooler is promising for its advantages such as large cooling capacity, high efficiency, fast cool-down process, and compact configuration. A two-stage large cooling capacity Stirling cryocooler is designed and numerically studied. The influence of the main parameters of the regenerators and working conditions on the cooling performance is discussed. Acoustic field distribution in the cryocooler is revealed for a deep understanding of the internal working mechanism. The performance of the cryocooler, including the available energy losses and cooling capacity, is systematically analyzed. it is found that the phase of the pressure wave always leads the volumetric flow rate in the two regenerators of the cryocooler, which is quite different from the general view. The ambient heat exchanger causes the most substantial part of available energy losses. Under the mean pressure of 3 MPa, a no-load refrigeration temperature of 11.81 K is achieved while the temperature of the first stage is 46.02 K. With no heat load on the first stage, the cryocooler is capable of providing 147 W cooling power at 20 K with a relative Carnot efficiency of 27.2%. When the two stages work at 77 K and 20 K respectively, the cryocooler can simultaneously provide 274.8 W cooling power at the first stage and 130.2 W cooling power at the second stage, with an exergy efficiency of 0.346.

Hydrodynamic resistance parameters of regenerator filler materials at cryogenic temperatures

Prototypical cryocooler regenerators were assembled with several common regenerator filler materials and were tested under periodic helium flow at room and cryogenic temperatures. The mass flow and pressure drop data were analysed to determine the friction factor, Darcy Permeability and Forchheimer coefficients. The results are compared to previous investigations, and the relevance of room-temperature models and correlations to cryogenic temperatures is assessed. It is shown that within the range of parameters tested, room-temperature hydrodynamic parameters such as friction factor can be used at cryogenic temperatures. Empirical correlations with improved accuracy are developed for the friction factor for each of the tested filler materials.

Study on the use of porous materials with adsorbed helium as the regenerator of cryocooler at temperatures below 10 K

Cryocoolers play a vital role in ground low-temperature experiments and space explorations. The regenerator is an important part of the cryocooler, which is used to store the cold produced by the expansion of the working fluid helium and absorb the heat produced during the next cycle of compression. However, the specific heat of helium at temperatures below 10 K is higher than that of almost all currently known regenerator materials, resulting in insufficient cold storage capacity and severely restricting the improvement of the cryocooler cooling performance. In this paper, the study of using porous material to absorb helium as the regenerator material for cryocoolers is carried out. An experimental system was built to test the helium adsorption capacity of super activated carbon in the temperature range of 3–10 K, and it was found that the activated carbon has a significant increase in specific heat compared with the current conventional materials. A thermodynamic calculation model was further established to investigate the dynamic cryogenic adsorption characteristics and the effect of adsorption/desorption heat on the performance of the regenerator. This work is an exploration in the research of low-temperature regenerators, which can provide useful help for the selection and use of regenerator materials for cryocoolers working below 10 K. It also has great application prospects in the fields of energy storage, low-temperature superconductivity, and sub-K refrigeration systems.

Correlation between cooling power and heat quantity of Er-Ho binary nitride as regenerator of 4K-GM cryocooler

Advanced regenerator materials of erbium and holmium binary nitrides, Er x Ho1-x N (x = 0 - 1) were studied. Its specific heat vs. temperature curve, possesses a peak due to the magnetic phase transition at their Curie temperature. The cooling power at 4.2 K of a commercial 4K-GM cryocooler filled with the binary nitride of x = 0.625, 0.75, 0.875 or 1 was evaluated. It was found that the cooling power at 4.2 K has a good linear correlation with the heat quantity, which is calculated by integrating C with respect to T in the region of 4.2 - 7.0 K. Three binary nitrides (x = 0.625, 0.875 and 1) filled in series along the regenerator column gave 26% higher cooling power than that filled with HoCu2.

Active suppression of temperature oscillation from a pulse-tube cryocooler in a cryogen-free cryostat: Part 1. Simulation modeling from thermal response characteristics

A cryogen-free cryostat cooled using a 4 K commercial GM or pulse tube cryocooler (PTC) displays temperature oscillations caused by the intrinsic working principle of the regenerative cryocooler. To dampen such oscillations usually requires either a large heat capacity or a large thermal resistance. To understand this phenomenon better and suppress it more effectively, both the step response characteristic and the intrinsic oscillation characteristic of cryostat have been used to obtain the complete transfer functions of a simulation model. The latter is used to test and optimize traditional PID feedback control. The results showed this approach has almost no effect on the temperature oscillation amplitude. Based on this simulation model, a novel active method was proposed and tested numerically. Simulation results predict the method should suppress the amplitude of the original temperature oscillation by a factor of two.

Numerical Thermal Analysis and 2-D CFD Evaluation Model for An Ideal Cryogenic Regenerator.

Regenerative cryocoolers such as Stirling, Gifford–McMahon, and pulse tube cryocoolers possess great merits such as small size, low cost, high reliability, and good cooling capacity. These merits led them to meet many IR and superconducting based application requirements. The regenerator is a vital element in these closed-cycle cryocoolers, but the overall performance depends strongly on the effectiveness of the regenerator. This paper presents a one-dimensional numerical analysis for the idealized thermal equations of the matrix and the working gas inside the regenerator. The algorithm predicts the temperature profiles for the gas during the heating and cooling periods, along with the matrix nodal temperatures. It examines the effect of the regenerator’s length and diameter, the matrix’s geometric parameters, the number of heat transfer units, and the volumetric flow rate, on the performance of an ideal regenerator. This paper proposes a 2D axisymmetric CFD model to evaluate the ideal regenerator model and to validate its findings.

A Second Law Study of the Regenerators in Cryocoolers based on Pore-level Analysis of Entropy Generation

Regenerators are key components of regenerative cryocoolers and losses in regenerators have a significant effect on the performance of cryocoolers. In previous studies, the efficiency of regenerators has been characterized mostly based on the first law of thermodynamics, and second-law analyses have been based on exergy considerations or irreversibility associated with macroscopic flow models. In this work, we investigate the entropy generation in regenerators based on detailed pore-level simulations. Computational Fluid Dynamics (CFD) simulations are used to model two-dimensional regenerator geometries, and examine the microscopic flow and heat transfer phenomena that cause irreversibility. A CFD approach and a volume averaging semi-analytical approach are developed to analyze the entropy generation in pore level. A modified Bejan number (Re conv) and a Performance Evaluation Factor (PEF) are propose as indicators for the level of entropy generation in regenerators. The predicted entropy generation rates at various Reynolds numbers and geometric dimensions are analyzed. The results indicate that the proposed semi-analytical approach can be a convenient alternative to CFD simulations when empirical data of convective heat transfer and friction factor are available. The analysis of entropy generation using Reconv and PEF can be used to minimized the irreversibility of regenerators.

Entropy generation in the woven mesh regenerator filler of cryocoolers

The regenerator is a key component of a pulse tube cryocooler, and is also a major contributor to irreversibility and losses in the cryocooler. In this study a method for the assessment of losses in cryocooler regenerators based on entropy generation is proposed. Pore-level CFD simulations are performed for woven mesh regenerator filler by defining periodically repeating unit cells. Simulations are performed for steady-state (uni-directional) as well as periodic flows. It is shown that entropy generation in periodic flow is higher than in steady flow, and is sensitive to operating parameters as well as small geometric irregularities. It is proposed that by optimization of the microstructure of regenerator fillers the entropy generation and losses can be minimized.

Adsorption characteristics of helium on an activated carbon at 4-10 K and its prospective application in 4 K-class regenerative cryocoolers

Adsorption characteristics of helium on an activated carbon at 4-10 K and its prospective application in 4 K-class regenerative cryocoolers

Porosity and heat transfer characteristics analysis of Stirling cryocooler regenerator using darcy permeability and forchheimer coefficient

Porosity and heat transfer characteristics analysis of Stirling cryocooler regenerator using darcy permeability and forchheimer coefficient

A two-stage Stirling cryocooler capable of providing more than 100 W cooling power at 30 K

Regenerative cryocoolers with large cooling capacity at 30 K have a promising prospect in the fields of superconducting technology and gas liquefaction. Compared with other types of cryocoolers, Stirling cryocoolers have the advantages of high cooling capacity, high efficiency, fast cool-down process and compact configuration. A two-stage Stirling cryocooler with large cooling capacity driven by the crank-rod mechanism is developed based on the Sage simulation results, and the preliminary experimental results are presented. Firstly, the pressure characteristics under different working conditions are investigated. The amplitude of the pressure and the pressure ratio at the compression chamber increases as the charging pressure increases. Increasing the refrigeration temperature of the second stage results in the decrease of the pressure amplitude, and the decreasing rate increases as the heat load on the first stage increases. Then, the performance of the ambient heat exchanger is studied. As the charging pressure increases from 2.2 to 2.6 MPa, the heat rejected from the ambient heat exchanger increases from 7.57 to 9.19 kW when there are no heat loads at the two stages of the cryocooler. The heat rejected from the ambient heat exchanger increases by 400 W with every increase of 0.1 MPa of charging pressure. With the increase of the refrigeration temperature of the second stage, the heat rejected from the ambient heat exchanger decreases, and the decreasing rate increases as the heat load on the first stage increases. Furthermore, the influence of the variation of the charging pressure and heat loads at the two stages on the cooling performance is introduced. The temperature of the second-stage cold end heat exchanger drops to 30 K at a nearly constant cooling rate after the startup of the cryocooler. As the charging pressure increases from 2.2 to 2.6 MPa, the cooling rate increases from 16.59 to 19.46 K/min, and the cooling rate increases by about 0.72 K/min for every increase in pressure of 0.1 MPa. Under the charging pressure of 2.6 MPa, the second stage of the cryocooler reaches a no-load refrigeration temperature of 19.83 K, while the temperature of the first stage is 71.2 K. The no-load refrigeration temperature has a weak relationship with the charging pressure. As the charging pressure increases, the increasing rate of cooling capacity and COP with the refrigeration temperature increases. The cryocooler is capable of providing 110 W cooling power at 30 K with a relative Carnot efficiency of 10.96%. This is the best result ever reported in China. What is more, it is found that the cooling power at the two stages has little influence on the cooling performance of each other, which means the cooling power from the first stage can be applied in the pre-cooling process with little influence on the second stage. These results lay a solid foundation for future applications and performance improvement of the cryocooler.

Adsorption characteristics of helium on an activated carbon at 4–10 K and its prospective application in 4 K-class regenerative cryocoolers

Adsorption data of helium on activated carbon at low temperatures are essential for its use in sorption coolers, gas-gap heat switches and the regenerators of cryocoolers. Especially when an activated carbon is used as the regenerator material in a cryocooler, the high specific heat caused by the high amount of adsorbed helium is expected to solve the bottleneck problem that the conventional 4 K cryocooler regenerator material suffers. It also has the advantage of being non-magnetic. However, the amounts of helium adsorbed on on the activated carbon in the temperature range of 4–10 K are low. In this paper, the adsorption isotherms of helium on a coconut shell-based activated carbon in the pressure-temperature range (4–10 K, 0.5–3.5 MPa) were measured with a self-built adsorption apparatus, from which the isosteric heats of adsorption and the specific heats of the activated carbon with adsorbed helium were calculated. The helium flow resistance through the activated carbon was also measured to evaluate its feasibility in a cryocooler. Results indicate that the isotherms are type I and can be fitted by the Langmuir adsorption model. The isosteric heats of adsorption are around 279 J/mol and change little with the amount of adsorbed helium. The specific heats of the activated carbon with adsorbed helium depend on the adsorption temperature and pressure, and their average values at 4–10 K are the highest among the commonly used solid regenerator materials (lead, SS, Er3Ni and HoCu2) and gaseous helium under the pressures investigated, which are ascribed to the high amount of helium adsorbed and the high heat of adsorption of helium on the activated carbon. The helium flow resistance through a material is related to its size and surface roughness and the value through the activated carbon is moderate among the four commonly used solid regenerator materials. Activated carbon with adsorbed helium is suitable as a regenerator material of cryocoolers because of its high specific heat and moderate flow resistance.

Theoretical analysis on pressure drop across porous cryocooler regenerator in evaluating the optimum regenerator porosity

ABSTRACT The regenerator used in Stirling cryocoolers is the crucial component, which has a flow conduit through the porous matrix material. Various parameters affect the performance of the regenerator, of which pressure drop is the main parameter needs detailed analysis. Theoretical analysis of pressure drop across the length of Stirling cryocooler regenerator has been carried out at various mass flow rates (ranging from 0.4 g/s to 1.2 g/s) and at various charge pressures (ranging from 18 to 24 bar), using ANSYS 16.0 Computational Fluid Dynamics (CFD) and REGEN 3.3 numerical program for Regenerators. The paper mainly focuses on the pressure drop analysis of porous cryocooler regenerator in evaluating the optimized porosity value for the regenerator.