Linear Compressor Research Articles

A Long-Life, High-Capacity and High-Efficiency Cryogenic System Developed for High-Tc Superconducting Magnet Applications

Cryogenic system plays a vital role in the field of high-Tc superconducting (HTS) magnet applications. For a variety of HTS magnet applications in energy storage devices, current limiters, generators, and maglevs, the technology itself is relatively mature. However, the problems associated with the used cryocoolers have hampered the advancement of their practical applications. At present, China is developing a new generation of electrodynamic suspension prototype train with HTS magnets. Compared with the existing commercial electromagnetic suspension technology, it has better suspension stability, larger suspension gap, and lower energy consumption. To meet the aimed cooling requirement of the vehicle-mounted HTS magnet, a long-life, high-capacity and high-efficiency cryogenic system based on the Stirling-type pulse tube cryocooler (SPTC) is designed and developed in the authors’ laboratory. The SPTC driven by the linear compressor without any moving component in the cold head has the intrinsic merits of long life and high reliability. The cryogenic system can achieve a minimum no-load cooling temperature of 12 K. Multiple SPTCs will provide a cooling capacity of 100 W at 45 K with the relative Carnot efficiency of 10.8 % to cool the HTS coils inside the magnet. This paper will describe and analyze the overall design approach, integration with the HTS magnet and the arrangement in maglev of the developed cryogenic system, together with the performance characteristics of the SPTC during the laboratory test.

Fundamental motion relation of different solid phase shifters of pulse tube cryocooler

Warm displacer type Stirling pulse tube cryocooler (SPTC) has demonstrated very promising benefits from its strong phase shifting and work recovery functions. Gas driving displacer (the passive displacer), linear motor driving displacer (the active displacer), and moving plug with orifice could be regarded as different operation modes of the general warm displacer. This work intends to fully understand the fundamental motion relation of different solid shifters for designing an effective warm displacer. Hence, starting from the basic momentum equations, the mechanism of the piston motion was studied theoretically, and the function of moving plug with orifice was mentioned in passing. It shows that the gas driving displacer can make the same optimal performance as the linear motor driving displacer does. In case of the mechanical problem of the displacer in the initial design, the gas driving displacer with orifice could be a backup to improve the performance of the SPTC. Based on the theoretical analysis and formula deduction, a simple simulation briefly displays the relationship between the displacer rod diameter ratio, displacer piston mass, and gas spring stiffness for a fixed micro SPTC. A general design method for the warm displacer was worked out eventually and could be extended to the linear compressor design.

Effect of Moving Endwall on Hub Leakage Flow of Cantilevered Stator in a Linear Compressor Cascade

Effect of Moving Endwall on Hub Leakage Flow of Cantilevered Stator in a Linear Compressor Cascade

Inherent capacity modulation of a linear refrigeration compressor

Conventional refrigeration compressors sized for peak cooling load actually operate at partial load mostly which could cause excessive on-off cycling loss. Linear compressor could overcome this challenge by varying the compressor stroke in response to transient cooling load. The present work establishes a numerical model for the capacity modulation of linear compressor using R134a through changing the input voltage to meet the cooling load. The numerical model contains a linear compressor model (including three sub-models describing the in-cylinder thermodynamic process, the piston mechanical dynamic process and the electrical part behaviour, respectively) and an input voltage modulation loop. Experimental data are collected for the validation of the numerical model. The results show that the capacity modulation, aiming to meet the cooling load within the error of 5%, can be achieved through varying the input voltage while fixing temperatures of heat exchangers. An increased voltage brings increased compressor stroke and mass flow rate in a linear trend with the heat exchanger temperature unchanged. Lower input voltage leads to higher motor efficiency due to lower copper loss. At a fixed condenser temperature and cooling load, the inherent capacity modulation can save energy with evaporator temperature increasing as the power consumption decreases. At condenser temperature of 50 °C, 40 °C and 30 °C, the CoP only changes by 1.5%, 0.9% and 2.7% when the cooling load changes from 100 W to 200 W. The inherent capacity modulation will allow the linear compressor to maintain a high annual efficiency for refrigeration.

Application of pre-positioning method in compressor cascade measurement with pressure-sensitive paint

The pressure-sensitive paint (PSP) technique has shown superiorities in its large coverage area capability and high spatial resolution. The process of the PSP technique usually requires sufficient illumination that covers the entire test object, and a camera that acquires the particular excited fluorescence. Applications in internal flow study, like the compressor cascade, may fail as the optical access is heavily limited due to the closely adjacent components, which creates a high demand for spatial positioning arrangement of optical devices in the visualization system. In this work, a pre-positioning method was proposed to help determine the arrangement of optical devices of an internal flow PSP measurement system in advance. It was applied to the measurement of a linear compressor cascade with large solidity. The results showed that with the optical layout arrangement determined using this pre-positioning method, a full-field pressure distribution of the cascade blade can be acquired successfully.

Thermo-economical sensitivity analysis of a linear compressor

The linear compressor is a new kind of reciprocating compressor in which the reciprocating motion is provided by means of a linear motor that replaces the crank mechanism, allowing a reduction in friction, oil-free operation and a continuous control of the cooling capacity without ON/OFF operations. A detailed compressor model has been implemented with the purpose to design a compressor for household refrigerators, using R600a as working fluid and having a cooling power of 200 W. Geometrical and design parameters have been identified, and a sensitivity analysis is carried out to quantify the influence of each parameter on the compressor global and volumetric efficiencies, and costs. All the simulated solutions have been then compared, in terms of global efficiency and costs, to the traditional reciprocating and the linear compressors currently available on the market. It is shown that this technology has generally a higher efficiency but also higher costs. However, a careful design of all the influencing parameters could lead to significant improvements for both objective functions.

Numerical Aeroacoustic Analysis of a Linear Compressor Cascade with Tip Gap

A wall-resolved compressible large-eddy simulation is performed on a linear compressor cascade with tip gap to investigate the tip-leakage noise sources. The cascade is composed of eight 4%-thick modified circular-arc blades with a 254 mm chord and a 235 mm spacing. The chord-based Reynolds number is , and the Mach number is 0.07. The gap considered in this study is 4.0 mm (1.6% chord). An aerodynamic analysis of the tip-leakage flow coupled with a Ffowcs-Williams and Hawkings analogy to compute the noise in the far field allows identifying the main tip-noise mechanisms. The acoustic contribution of the tip-leakage flow is highlighted by comparison with the trailing-edge noise contribution obtained with a 25 mm slice configuration at midspan without tip gap. This small-span configuration is also simulated using large-eddy simulation and allows computing the blade trailing-edge noise. A dynamic mode decomposition performed on the full configuration close to the tip highlights the main noise source, located around 75% chord with a propagation frequency between 5 and 6 kHz. The acoustic waves propagate mostly downstream of the cascade and on the pressure side. This study provides the first high-fidelity wall-resolved compressible simulation of a linear compressor cascade comprising a comprehensive investigation of the tip-leakage noise sources, which is a necessary first step toward fan tip-noise prediction.

Experimental Study on Performance of Transonic Compressor Cascade with Microgroove Polyurethane Coatings

Due to the harsh operating environment of aero-engines, a surface structure that provides excellent aerodynamic performance is urgently required to save energy and reduce emissions. In this study, microgroove polyurethane coatings fabricated by chemical synthesis are investigated in terms of their effect on aerodynamic performance, which is a new attempt to investigate the impact on aerodynamic performance of compressor cascade at transonic speeds. This method reduces manufacturing and maintenance cost significantly compared with traditional laser machining. Wake measurements are conducted in the high-speed linear compressor cascade wind tunnel to evaluate the performance of cascade attached with different microgroove polyurethane coatings. Compared with the Blank case, the microgroove polyurethane coatings have the characteristic of reducing flow loss, with a maximum reducing rate of 5.87% in the area-averaged total pressure loss coefficient. The mechanism of flow loss control is discussed through analyzing the correlation between the total pressure distribution and turbulence intensity distribution. The results indicate that a large quantity of energy loss in the flow field due to turbulence dissipation and the reduction in viscous drag by microgroove polyurethane coatings relates to its effect on turbulence control. This paper demonstrates a great perspective on designing micro-nano surface structure for aero-engine applications.

Experimental investigation of the discharge valve dynamics in an oil-free linear compressor for Joule-Thomson throttling refrigerator

• A visual and real-time test system was set up to investigate dynamics of the linear compressor discharge valve. • The principle of the valve movement characteristics on exhaust loss is discussed. • The effect of valve dynamics on the thermodynamic efficiency of the free-piston oil-free linear compressor is studied. As the core component of the linear compressor which is a research hotspot of Joule-Thomson (J-T) throttling refrigerators for space applications, the discharge valve has a significant impact on their reliability and efficiency. Therefore, studying the dynamic characteristics of the discharge valve is indispensable which contributes to improving the compressor efficiency. Based on the oil-free linear compressor independently developed by the laboratory, a visual and real-time test system was set up to investigate the dynamics of the discharge valve and the compressor performance. While varying the parameters including piston stroke, valve stiffness, and operating frequency, the movement characteristics of the discharge valve and the thermodynamic efficiency of the compressor were measured and studied. The detailed principle of the valve movement characteristics on exhaust loss is discussed in the analysis of the results. The better discharge valve thickness for the linear compressor was selected. The experimental results indicated that the thermodynamic efficiency with the valve thickness of 0.152 mm is higher than 0.102 mm and 0.203 mm. The research in this paper will provide a valuable reference to design a discharge valve with high-efficiency and positive behavior for the linear compressor.

A closed-cycle miniature Joule–Thomson cooler for cooling cold electronics

There is growing interest in operating electronics at low temperatures; however, the complexity of the cryogenic burden imposed by large coolers impedes the widespread usage of low-temperature electronics. In this study, we present a Hampson-type miniature Joule–Thomson (JT) cold stage driven by a linear compressor, and the performance of the cooler operating with a ternary mixture of methane, ethane and neopentane is investigated experimentally and numerically. Gas chromatography analysis shows that the composition of the circulated mixture is different from the charged mixture due to the liquid hold-up of neopentane. The cooler has a cold-end temperature of 143 K in steady state when operated between 0.15 MPa and 1.92 MPa, and a net cooling power of 198 mW at 145 K, 212 mW at 150 K, and 173 mW at 155 K. A dynamic numerical model is developed to predict cooler performance, and the feasibility of the numerical model is validated through the comparison between measured and calculated net cooling powers. The model can be used to estimate cooler performance at different operating conditions. This study is conducive to promote the widespread use of cryogenic electronics in the fields of military, space science, and biomedicine among others. • A miniature Joule–Thomson cooler driven by a linear compressor is built. • A ternary mixture of methane, ethane and neopentane for 145 K cooling is proposed. • The Joule–Thomson cooler achieves 143 K and has a cooling power of 198 mW at 145 K. • A model of Hampson-type Joule–Thomson cooler operating with mixture is developed.

Vortical characteristics of the corner separation flow in a high-speed compressor cascade

In aero-engine compressors, corner separations cause blockage, loss generation, and flow instabilities. The aim of the current study is to gain better knowledge of the vortical structures in corner separation flows and their resultant blockage and loss. Numerical simulations at different incidences have been performed for a high-speed linear compressor cascade. First, vortical structures have been visualized with the help of the Q-criterion and eigenvector method, and those associated with the corner separation have been focused on. It was found that the corner separation occurring on either the pressure surface or suction surface corner region was characterized by a large-scale streamwise vortex, named as the corner separation vortex in this paper. The pressure-side corner separation vortex is observed at a great negative incidence initiated from a focus point on the pressure surface, while the suction-side corner separation vortex observed at all concerned incidences originated from a focus on the endwall near the suction surface. Then, the mechanism by which corner separation vortices resulted in blockage and loss has been uncovered. The results showed that low-momentum fluids inside the core region of corner separation vortices led to the flow blockage and the viscous friction between them, and mainstream was responsible for the entropy generation. One major contributor of low-momentum fluids in the suction-side corner region was an off-wall reversed-flow region which was attributed to the breakdown of the suction-side corner separation vortex. Finally, multi-vortex structures in the leading region of the suction-side corner separation at two high positive incidences have been analyzed. It was found that the horseshoe vortex influenced the formation of the suction-side corner separation vortex, which induced a multi-vortex structure, and prevented the onset location of suction-side corner separation vortex from moving upstream with the increased incidence.

Experimental investigation on helium valved linear compressors with different active offsets

Piston offset in the helium valved linear compressor (VLC) deteriorates the cooling performance and operating life of Joule-Thomson (JT) refrigerators, especially in aerospace applications. To clarify the influence of piston offset on the VLC performance, an experimental investigation was carried out. Based on a laboratory developed VLC, the effects of piston offset on the suction and discharge pressure, pressure ratio, mass flow rate, motor efficiency, volume efficiency and adiabatic efficiency are systematically studied under two piston motion states (fixed piston displacements and full strokes). The results show that when the piston stroke is constant, the offset will increase the discharge pressure and mass flow rate, and decrease suction pressure. When the piston moves at full stroke, the offset will reduce the discharge pressure and mass flow rate, and increase suction pressure. Regardless of the working conditions, the existence of offset will affect the efficiency of the compressor, hence increasing the power consumption.

Investigation of Water Rivulets Formation on the Pressure Side of a Linear Compressor Cascade Under Wet Compression

Investigation of Water Rivulets Formation on the Pressure Side of a Linear Compressor Cascade Under Wet Compression

An Approach to Intelligent Fault Diagnosis of Cryocooler Using Time-Frequency Image and CNN.

Cryocooler plays an essential role in the field of infrared remote sensing. Linear compressor, as the power component of the cryocooler, will directly affect the normal operation and performance of the detector if there is a fault. Therefore, the intelligent fault diagnosis of the linear compressor is of great significance. An intelligent fault diagnosis method based on time-frequency image and convolutional neural network is proposed to solve the problems of piston and cylinder friction, mass imbalance, and plate spring distortion in the linear compressor. Firstly, the wavelet transform time-frequency analysis method is used to generate the corresponding time-frequency image. Convolutional neural network (CNN) is used to automatically extract features of time-frequency images, so as to realize the classification of various fault modes. The results of simulation experiments show that the method can identify several fault modes of the linear compressor with 95% accuracy.

Experimental research on performance of an oil-free moving-coil symmetrical dual-piston linear compressor for the J - T throttle refrigerator

In order to improve the efficiency of the Joule-Thomson (J-T) throttling refrigerator, an oil-free symmetrical dual-piston moving-coil linear compressor was developed, and its principle was demonstrated in detail. As the core component of the J-T throttling refrigerator, the linear compressor provides the pressure ratio and mass flow rate required by the throttling cycle. The linear compressor prototype was tested with nitrogen to evaluate its performance at different pressure ratios, strokes, and driving frequencies. Key performance parameters have been obtained, and the experimental results show that the input power is 175.2 W, the mass flow is 1.128 g/s, the unit power mass flow is 6.438 mg/s/W, the motor efficiency, volumetric efficiency, isentropic efficiency, and isothermal efficiency is 87.7%, 81.5%, 74.9%, and 63.7% at the design parameter (pressure ratio of 3.0, stroke of 10.0 mm, frequency of 60.0 Hz), respectively. The maximum mass flow rate is 1.742 g/s, and the maximum motor efficiency, volumetric efficiency, isentropic efficiency, and isothermal efficiency is 88.1%, 94.0%, 81.2%, and 73.4%, respectively. The symmetrical dual-piston linear compressor brings large-capacity and high efficiency, which provide a useful attempt and reference for the optimization of the linear compressors for the J-T throttling refrigerator.

Numerical analysis and experimental research of a 2W/35 K Stirling-type pulse tube cryocooler

Cooling to the temperature of liquid nitrogen to liquid hydrogen is a necessary working prerequisite for some infrared detectors. The development of compact and efficient miniature cryocoolers is of great significance for the space application of the detectors. The pulse tube cryocooler driven by a dual-opposed linear compressor is prospective in space applications due to the advantages of no low-temperature moving parts, compact structure, and high reliability. In this paper, a 2W/35K Stirling-type pulse tube cryocooler with a single-stage structure has been designed and tested. The influences of structural parameters of the phase shifters such as inertance tube, as well as the operating parameters such as operating frequency and charging pressure, on the cooling performance were investigated through numerical calculations and experiments.

Development of a 2∼4 K closed-cycle JT cryocooler for space application

Cryogenic system proving cooling capacity at liquid helium temperatures is one of the key technologies for space exploration missions. A closed-cycle Joule-Thomson cryocooler capacity of providing different cooling powers at 2∼4 K is developed in our lab. The cryocooler is precooled by a two-stage thermal coupled pulse-tube cryocooler with active phase shifter and driven by multi-stage valved linear compressors. The lowest temperature about 2.5 K and more than 50 mW at about 4.0 K can be obtained by adapting the suited charging pressures. The two-stage pulse tube cryocooler can provide pre-cooling capacity at 17 K and 76 K, respectively with an input power of about 350 W. A large pressure ratio of about 26 and a mass flow rate of 6.7 mg/s can be provide by the multi-stage valved linear compressors with a charge pressure of 0.6 MPa and a total electrical power of less than 110 W. The cooling-down time for the cryocooler from room temperature to about 4 K is less than 10 hours. The cooling system can provide a required cryogenic environment for future space applications.

Effect of vorticity transport on flow structure in the tip region of axial compressors

Numerical simulations are carried out to investigate the flow structure in the blade tip region of axial compressors. Various tip clearance heights and end wall motion conditions in a linear compressor cascade are studied to assess the effect of vorticity transport on the tip leakage flow (TLF). Moreover, the effect of vorticity transport on the TLF in a compressor rotor at different operating conditions is studied using delayed detached eddy simulation. The results show that the vorticity transport at both the blade tip and the end wall plays an important role in the roll-up and evolution of the tip leakage vortex (TLV), resulting in great impacts on the loss and stability of the TLV. It is found that the TLV is composed of a two-layer structure. The inner vortex core region formed by the vorticity transport from the blade tip shear layer to the TLV has a great effect on the strength and loss of the vortex, and the structure of the outer shear layer is altered by the secondary vortex formed by the vorticity transport from the end wall shear layer and thus affects the stability of the TLV. By the mechanism of the vorticity transport, the effects of the clearance height, the end wall motion, and the non-uniform clearance as a control method can be explained uniformly. The new understanding of the TLF structure and the vorticity transport mechanism helps to improve the performance of axial compressors by controlling the vorticity transport of the TLF.

An 880 mW@15 K thermal coupled pulse tube cryocooler with active phase shifter

A two-stage Stirling-type pulse tube cryocooler (SPTC) operating in 15 K is developed in our lab for cooling infrared detectors and precooling helium JT cryocooler. For easy adjustment, the driven compressor of the SPTC is designed as two independent linear compressors. The second stage cold finger uses an active warm displacer (AWD) as phase shifter to maximize the cooling performance. The variation of pre-cooling temperature and swept volume of the AWD will influence the cooling capacity of the SPTC. The cooling performance are experimentally tested and analyzed. With the pre-cooling temperature of 80 K, the second stage cold finger can obtain 470 mW at cooling temperature of 15 K with a total electric input power of 363 W. It can also obtain 880 mW at cooling temperature of 15 K with full input power of 527 W. A no-load temperature of 9.9 K can be obtained with an electric power of 180 W and 170 W for first and second stage. The experimental results also show that there exists an optimal pre-cooling temperature and swept volume of the AWD to achieve the maximum cooling capacity for 20 K applications.

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.