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.     

Stirling-type pulse tube refrigerator (PTR) with cold compression: Cold compressor,colder expander

This research paper focuses on the performance prediction and its validation via experimental investigation of a Stirling-type pulse tube refrigerator (PTR) equipped with a cold linear compressor. When the working gas is compressed at cryogenic temperature, the acoustic power (PV power) can be directly transmitted through the regenerator to the pulsating tube without experiencing unnecessary precooling process. The required PV power generated by the linear compressor, furthermore, can be significantly diminished due to the relatively small specific volume of the working gas at low temperature. The PTR can reach lower temperature efficiently with higher heat lift at the corresponding temperature than other typical single-stage Stirling-type PTRs. Utilizing a cryogenic reservoir as a warm end and regulating the entire operating temperature range of the PTR will enable a PTR to operate efficiently under space environment.In this research, the experimental validation as a proof of concept was carried out to demonstrate the capability of PTR operating between 80 K and 40 K. The linear compressor was submerged in a liquid nitrogen bath and the lowest temperature was measured as 38.5 K. The test results were analyzed to identify loss mechanisms with the simple numerical computation (linear model) which considers the dynamic characteristics of the cold linear compressor with thermo-hydraulic governing equations for each of sub components of the PTR. All the mass flows and pressure waves were assumed to be sinusoidal.     

Vibration reduction of pulse tube cryocooler driven by single piston compressor

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

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.     

Electromagnetic properties of silicon carbide foams and their composites with silicon dioxide as matrix in X-band

Electromagnetic wave absorbing properties of SiC-foams and their composites with SiO₂ as matrix are presented, including theory, numerical analysis, and results/discussion. The reflection coefficients of various SiC-foams and their composites with various dielectric parameters are calculated by numerical simulation. When SiC conductivities are in the range of 2–3 S m⁻¹ in the case of SiC-foams, or 2–5 S m⁻¹ in the case of composites, the minimum reflection coefficients can be obtained in the range of X-band of 8.2–12.4 GHz. These materials are light weight, heat-resistant, and good impedance match with the free space, and therefore, they are a good candidate as a wide-range frequency absorbent medium.     

Characterization of a thermoelectric/Joule–Thomson hybrid microcooler

Micromachined Joule–Thomson (JT) coolers are attractive for cooling small electronic devices. However, microcoolers operated with pure gases, such as nitrogen gas require high pressures of about 9 MPa to achieve reasonable cooling powers. Such high pressures severely add complexity to the development of compressors. To overcome this disadvantage, we combined a JT microcooler with a thermoelectric (TE) pre-cooler to deliver an equivalent cooling power with a lower pressure or, alternatively, a higher cooling power when operating with the same pressure. This hybrid microcooler was operated with nitrogen gas as the working fluid at a low pressure of 0.6 MPa. The cooling power of the microcooler at 101 K operating with a fixed high pressure of 8.8 MPa increased from 21 to 60 mW when the precooling temperature was reduced by the thermoelectric cooler from 295 to 250 K. These tests were simulated using a dynamic numerical model and the accuracy of the model was verified through the comparison between experimental and simulation results. Based on the model, we found the high pressure of the microcooler can be reduced from 8.8 to 5.5 MPa by lowering the precooling temperature from 295 to 250 K. Moreover, the effect of TE cooler position on the performance of the hybrid microcooler was evaluated through simulation analysis.     

Numerical investigation on a 300 Hz pulse tube cryocooler driven by a three-stage traveling-wave thermoacoustic heat engine

A 300 Hz pulse tube cryocooler (PTC) driven by a three-stage traveling-wave thermoacoustic heat engine (TSTHE) has been proposed and studied in this paper. In the configuration, three identical thermoacoustic heat engine units are evenly incorporated in a closed traveling-wave loop, in which three pulse tube cryocoolers are connected to the branch of each thermoacoustic heat engine. Compared with the conventional thermoacoustic heat engine which involves a traveling-wave loop and a long resonator, it has advantages of compact size and potentially high thermal efficiency. A TSTHE–PTC system was designed, optimized and studied in detail based on the thermoacoustic theory. Firstly, numerical simulation was conducted to design the system thus the optimum structure parameters of the system were obtained. With the operating condition of 4 MPa mean pressure and high working frequency, a cooling power of 7.75 W at 77 K and an overall relative Carnot efficiency of 11.78% were achieved. In order to better understand the energy conversion characteristics of the system, distributions of key parameters such as acoustic work, phase difference, dynamic pressure, volume flow rate and exergy loss were presented and discussed. Then, the coupling mechanism of the system was investigated. In addition, influence of coupling position on the system performance was further studied.     

Effects of pre-pressurization on blast response of clamped aluminum plates

The effects of static pre-pressurization on the blast response of square aluminum plates were studied both experimentally and numerically. Static pre-pressurization values in the range of 0–62.1 kPa (0–9.0 psi) were chosen to represent in-flight loads experienced by the outer skin of a commercial aircraft fuselage due to cabin pressurization. Experimentally, a vacuum vessel system was used to apply a static pressure differential to the test plate. Bare spherical explosive charges were then detonated at a fixed distance from the plate. For load levels that induced only elastic plate deflections, dynamic strains were measured with strain gages located at the center of the test plate. Strain data were recorded for eighteen explosive tests that considered two unique charge sizes. In addition to the experimental work, numerical predictions of the dynamic plate response were determined using finite element analysis. A comparison of the dynamic (elastic) plate strains determined experimentally with those predicted with the finite element method shows good correlation. Only a slight increase (less than 10%) in peak strains was observed as static pre-pressurization increased from 0.0 to 62.1 kPa. For blast load levels that induced plastic deformation in the plates, permanent plate deflections were measured for twenty-four explosive tests at four different blast intensities. A comparison of (plastic) plate deformations determined experimentally with those predicted with the finite element method shows good correlation. For the four explosive load cases studied, no significant change in permanent plate deformations was observed as static pre-pressurization increased from 0.0 to 62.1 kPa.     

On the low cycle fatigue failure of insulated rail joints (IRJs)

Insulated rail joints (IRJs) are safety critical components in the signalling system of railway corridors which provide a break in the continuity of the rail steel to locate trains. IRJs connect the two rail ends at the discontinuity to achieve geometric and mechanical requirements of rail. The bending stiffness of an IRJ is about one third that of continuous rail. As a result, the IRJs, especially those in heavy haul tracks, exhibit early failure predominantly due to ratchetting or alternating plasticity of railhead metal in the vicinity of the endpost insulators.A three-dimensional (3D) finite element numerical simulation is carried out to examine failures of railhead material in the vicinity of the endpost of an insulated rail joint considering high frequency dynamic wheel loading. A dynamic wheel load of 182 kN is applied through a contact patch; the distribution of contact pressure is considered using a non-Hertzian formulation. A 12 m long global IRJ model and a sub-model for localised analysis are employed. The shakedown theorem is employed in this study. Nonlinear isotropic/kinematic elastic–plastic material modelling is employed in the simulation. A peak pressure load lower than the shakedown limit is considered as the input load.The equivalent plastic strain plot for this load case lower than the shakedown limit demonstrates the railhead damage captured through a localised stress analysis in the vicinity of the endpost using the sub-modelling technique. The sub-surface plastic deformation of railhead material extends down to 8 mm from the railhead top surface. The critical crack initiating stress components are at 2–4 mm sub-surface depth. As such, the railhead material fails due to alternating plasticity through low cycle fatigue. Laboratory tests were performed to verify the simulation results and found that test and simulation results correlated well.     

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.     

Modeling of the dynamics of a slug of liquid oxygen in a magnetic field and experimental verification

This study presents the theoretical basis for the dynamics of a slug of liquid oxygen in a quartz tube when displaced by a pulsed magnetic field. The theoretical model calculated slug movement by balancing the forces due to magnetism, pressure, and damping and was verified with experimental data for a slug 1.3 cm long and 1.9 mm in diameter. During the experiments, the hidden slug length and damping factor were unknown, but quantifiable through the numerical solution. The hidden slug length accounted for the mass of LOX which cannot be seen during the experiment and was calculated as 10–14.5 cm. The damping factor was an empirical augmentation to represent increased damping from various phenomena and was calculated as 5.76–6.3. The experiments generated damped pressure waves of 6–8 Hz with maximum amplitudes of 0.8–1.3 kPa. Outside these ranges, the model indicated that the oscillation frequency decreased logarithmically with the hidden slug length, and the maximum amplitude decreased logarithmically with the damping factor. Measurement uncertainties of the visible length and slug initial position (0.8 mm) were also evaluated for their effects on the frequency and amplitude of the oscillations. The visible slug length did not seem to significantly affect the pressure waves, but the initial position strongly altered the amplitudes and mean of the oscillations. The predictive model matched the experiment well and could be used to design advanced flow control systems for cryogenic applications.     

An UHV system to study gassing and outgassing of metals under friction

An UHV dynamic expansion system to study gassing and outgassing of metals under friction in vacuum has been developed. The vacuum chamber of the system has two main parts separated by a wall with a central orifice of nominal conductance of 5.27 l s⁻¹ for N₂ at 300 K. The main part of the system is equipped with a quadrupole mass-spectrometer, ion gauge and the gas line supply. The chamber closer to the pumping systems is also equipped with an ion gauge. The pumping of the system is performed by: (I) a turbomolecular and (II) a cryogenic pump, which can operate either independently or in parallel. The ultimate pressure of chambers 1 and 2 are respectively 5.2.10⁻¹¹ mbar and 1.10⁻⁹ mbar. The friction cell allocated in chamber 2 consists of a sample holder and a linear motion feedthrough with a steel ball indent allows the friction experiments. The sample holder lets to apply a load to the sample surface with a normal force in the range of 0,02 N to 0,85 N. Heating of the sample up to 500°C is provided by electron bombardment. The temperature of the sample as well as the electrical resistance between the indent and the sample surface can be measured. The results show that pressure increase caused by the deformation of the sealing bellow of the friction cell, i.e. without friction, is 1.7×10⁻¹¹ mbar (0.6% of the base pressure). The outgassing of the sample caused by friction depends on the normal force as well as on the sweep frequency. The behavior of the total pressure is characterised by an oscillating current supported on a continuous current, which is not possible to explain at present time.     

Experimental investigation of two-phase flow characteristics of HFC-134a through short-tube orifices

The two-phase flow characteristics of HFC-134a, including flow pattern, mass flow rate, pressure distribution and temperature distribution through short-tube orifices are experimentally investigated. Short tube diameters ranging between 0.605 and 1.2 mm with length-to-diameter ratios ranging between 8.3 and 33 are used in the experiments. The test runs are performed at upstream pressure ranging between 900 and 1300 kPa, downstream pressure ranging between 300 and 400 kPa, and degree of subcooling ranging between 1 and 12 °C. Two groups of short-tube orifices are used in the experiment. The first is used to visualise the flow pattern. The second is used to measure temperature and pressure distributions along the tube. The results from the present experiment show that metastable flow and choked flow phenomena exist inside the short-tube orifices over the whole range of experimental conditions. The metastable liquid flow region increases with increasing degree of subcooling and upstream pressure. The mass flow rate is directly proportional to upstream pressure and degree of subcooling. The results of pressure distribution inside the short-tube orifices indicate that accelerational pressure drop at the inlet and outlet has a significant effect on the total pressure drop across the short-tube orifice.     

Effect of pressure on the vacuum cooling of iceberg lettuce

Vacuum cooling is known as a rapid evaporative cooling technique for any porous product which has free water. The aim of this paper is to apply vacuum cooling technique to the cooling of the iceberg lettuce and show the pressure effect on the cooling time and temperature decrease. The results of vacuum cooling are also compared with conventional cooling (cooling in refrigerator) for different temperatures. Vacuum cooling of iceberg lettuce at 0.7 kPa is about 13 times faster than conventional cooling of iceberg lettuce at 6 °C. It has been also found that it is not possible to decrease the iceberg lettuce temperature below 10 °C if vacuum cooling method is used and vacuum pressure is set to 1.5 kPa.     

A new method of semi-continuous casting of AZ80 Mg alloy billets by a combination of electromagnetic and ultrasonic fields

The influence of low frequency electromagnetic field and power ultrasonic field on the microstructure of AZ80 Mg alloy billets was studied. The magnetic flux density and time average electromagnetic volume force density were obtained by numerical simulation. The acoustic pressure distribution was also numerically calculated in order to characterize the ultrasonic field propagation. After comparison of the different effects of grain refinement by low frequency electromagnetic casting (LFEC) and ultrasonic casting (UC), a new method (LFEC + UC) was developed by simultaneously applying ultrasonic vibration to the melt during low frequency electromagnetic semi-continuous casting of AZ80 Mg alloy billets. With the application of the compound field under the optimum conditions, the as-cast macrostructure and microstructure were refined and homogeneously distributed. An obvious improvement of mechanical properties was finally obtained. The mechanisms of grain refinement and interaction of low frequency electromagnetic field and power ultrasonic field with the melt were discussed.     

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

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

Analysis and optimisation of a mixed fluid cascade (MFC) process

A mixed fluid cascade (MFC) process that comprises three refrigeration cycles has great capacity for large-scale LNG production, which consumes a great amount of energy. Therefore, any performance enhancement of the liquefaction process will significantly reduce the energy consumption. The MFC process is simulated and analysed by use of proprietary software, Aspen HYSYS. The effect of feed gas pressure, LNG storage pressure, water-cooler outlet temperature, different pre-cooling regimes, liquefaction, and sub-cooling refrigerant composition on MFC performance are investigated and presented. The characteristics of its excellent numerical calculation ability and the user-friendly interface of MATLAB™ and powerful thermo-physical property package of Aspen HYSYS are combined. A genetic algorithm is then invoked to optimise the MFC process globally. After optimisation, the unit power consumption can be reduced to 4.655 kW h/kmol, or 4.366 kW h/kmol on condition that the compressor adiabatic efficiency is 80%, or 85%, respectively. Additionally, to improve the process further, with regards its thermodynamic efficiency, configuration optimisation is conducted for the MFC process and several configurations are established. By analysing heat transfer and thermodynamic performances, the configuration entailing a pre-cooling cycle with three pressure levels, liquefaction, and a sub-cooling cycle with one pressure level is identified as the most efficient and thus optimal: its unit power consumption is 4.205 kW h/kmol. Additionally, the mechanism responsible for the weak performance of the suggested liquefaction cycle configuration lies in the unbalanced distribution of cold energy in the liquefaction temperature range.     

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

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

Broad bandwidth of thin composite radar absorbing structures embedded with frequency selective surfaces

The three-layer ultrathin radar absorbing structure (RAS) involving a frequency selective surface (FSS) exhibiting excellent broad bandwidth properties is designed and fabricated. The EW and flaky carbonyl iron powders were used to produce two kinds of silicone rubber matrix magnetic composites for the top and the bottom layer, respectively. The electromagnetic parameters of the composites were measured in the frequency range of 2–18 GHz. The middle layer is an FSS in the form of double-square loops with four micro-split gaps in the middle of the outer loop. The results show that the proposed RAS can provide a 10 dB absorbing bandwidth of 13.2 GHz from 4.8 to 18 GHz (1.7 mm thickness) and a 10 dB absorbing bandwidth of 14.1 GHz from 3.9 to 18 GHz, covering C-band, X-band and Ku-band (2.0 mm thickness). A good match between simulation and measurement results demonstrates the validity of our design.     

Study on a 10 W/90 K in-line pulse tube cryocooler

A Stirling-type in-line pulse tube cryocooler (PTC) has been designed, built and tested at Shanghai Institute of Technical Physics (SITP), Chinese Academy of Sciences. This PTC prototype can obtain a low-noise cooling capacity of more than 10 W at around 90 K cold head temperature and is used for cooling a space-borne infrared photo detector. In order to achieve a highly efficient PTC, a simplified numerical simulation model has been established for design and optimization. The simulation results of the regenerator, pulse tube and inertance tube are analyzed in detail. Besides, some key parameters of the PTC are listed in the paper. The PTC’s performances are tested at different operating frequencies from 42 Hz to 55 Hz and its reject temperature dependence is observed in the range of 290 K to 320 K. Furthermore, the map of the PTC’s performance characteristics is presented.