Joule-Thomson Refrigerator Research Articles

Binary hydrocarbon mixtures of Joule-Thomson refrigeration cycle based for -70 C freezer: Theoretical and experimental evaluation

Recently, with the increasing worldwide demand for low-temperature freezers, applying natural working medium hydrocarbon refrigerants has been widely concerned. For obtaining a −70 °C freezer, the Joule-Thomson refrigeration cycle of two kinds of binary hydrocarbon mixtures, R1150/R600 and R170/R600, were analyzed and experimentally verified. The theoretical analyses results indicated that the thermodynamic performance of R1150/R600 mixed refrigerant was better than that of R170/R600 when it was used in Joule-Thomson refrigeration system of −70 °C freezer. The coefficient of performance (COP) and the volume cooling capacity of using R1150/R600 was improved by 37.9 % and 50.7 %, respectively, compared with that of R170/R600. For further verify the results of theoretical analysis, two kinds of mixed refrigerants were tested with a low-temperature freezer prototype. The experimental results demonstrate that the lowest cabinet temperature that R170/R600 could reach was −66.9 °C at an ambient temperature of 32 °C with a refrigerant charge of 150g. On the other hand, when the concentration of R1150 is 27.5 %, the average temperature in the cabinet could reach −72 °C, indicating that the application of R1150/R600 in a low-temperature freezer at −70 °C has great potential. In summary, under the existing system configuration, the R1150/R600 was more suitable for −70 °C freezer than the R170/R600. This study could provide theoretical and experimental guidance for the selection of refrigerants for low-temperature freezers based on the Joule-Thomson refrigeration system.

Experimental study on Joule-Thomson refrigeration system with R1150/R290/R601a for ultra-low temperature medical freezer

Since the global outbreak of the COVID-19, the requirement for ultra-low temperature medical freezers has increased rapidly. Joule-Thomson refrigeration systems, known for their simple structure and low cost, are an excellent choice for ultra-low temperature medical freezers. To further improve the performance of Joule-Thomson system for large-capacity ultra-low temperature freezer, this study conducted a series of experiments on the performance of a ternary hydrocarbon mixture R1150/R290/R601a. The impacts of refrigerant charge, capillary tube size, compressor speed, and environmental temperature on the steady-state and dynamic performance were investigated in detail. The results showed that under a fixed system configuration and charge, the freezer can achieve an ultra-low temperature of −82.2 °C and a COP of 0.256 at an environmental temperature of 25 °C, while maintaining a low compressor pressure ratio not exceeding 12. Furthermore, the matching relationship between capillary size and charge was emphatically studied, and it was found that the optimal refrigerant charge increased as the inside diameter of the capillary tube decreased or its length increased. The inside diameter had a more significant effect on system performance than the length. The suitable charge was 225–275 g for capillary tubes with an inside diameter of 1.4 mm, and increased to 400–450 g for an inside diameter of 0.7 mm. In addition, the experiments with various compressor speeds indicated that, before the cabinet temperature dropped to −40 °C, the variation of compressor speed had little effect on the pull-down rate. However, the higher the compressor speed, the lower the cabinet temperature that can be achieved. When the speed was 3500 rpm, the system achieved its lowest daily energy consumption, which was 8.42 kW·h/24 h.

Experimental study on performance characteristics of a −70 °C ultra-low temperature medical freezer with mixed hydrocarbon refrigerant

It is well-known that the −70 °C ultra-low temperature freezers have been widely concerned for vaccine and biomedical products storage due to the outbreak of COVID-19. This paper carries out the experimental investigation on performance characteristics of a −70 °C ultra-low temperature medical freezer with single-stage Joule-Thomson refrigeration system by using binary hydrocarbon mixture R601a/R1150 and ternary mixture R601a/R290/R1150. The aim is to explore and verify hydrocarbon refrigerants that can achieve lower energy consumption, thereby providing a scientific basis for the optimization of ultra-low temperature medical freezers. The experimental results show that both R601a/R1150 and R601a/R290/R1150 with a charge of 250 g could achieve the refrigeration temperature below −70 °C in a 568 L large volume vertical freezer. Comparing binary with ternary refrigerants, it is found that the addition of medium-boiling component R290 is beneficial to reduce the daily energy consumption. When the mass fraction of R601a/R1150 is 0.70/0.30, the lowest daily energy consumption for −70 °C is 9.06 kW h·24 h−1. When the mass fraction of the R601a/R290/R1150 is 0.70/0.10/0.20, the daily energy consumption is 8.42 kW h·24 h−1, which is 7.06 % lower than that when using the R601a/R1150. This indicates that careful selection of refrigerant components and composition optimization can lead to substantial energy savings. Furthermore, it is also observed that among these three components, the low-boiling component R1150 has the most significant influence on the system pressures and operating power, followed by R601a and R290.

Experimental investigation of the structure effects on the energy recovery and jet impinging process in a fast cooling Joule-Thomson cryocooler

Compared to other Joule-Thomson (J-T) refrigeration systems, open-cycle miniature J-T cryocoolers offer exceptional rapid cooling capabilities, making them ideal for applications such as infrared guidance in missiles. The energy recovery in the heat exchanger enables the refrigerant reach saturation temperature quickly and improve the jet liquefaction rate. The heat transfer intensity of the impinging jet determines the cooling rate of the target. Hence, heat recovery and the impact jet process are the primary factors behind this rapid cooling, with distinct roles that require separate consideration. The structural differences will directly affect the energy recovery and jet impact process. To investigate these affects, an experimental system for rapid cooling J-T cryocoolers was established and three distinct cryocoolers with substantial structural variations were designed. The important structures, including jet height, orifice diameter, enhanced heat transfer treatment of the cold plate, heat exchanger height, and heat exchanger cone angle, were closely studied. In the range of our experiments, it was found that larger heat exchanger cone angle leading better energy recovery performance, while the length of the heat exchanger is limited by the type of refrigerant. Longer heat exchanger actually introduce too much thermal mass for the refrigerant with better energy recovery performance. In the aspect of jet impingement, enhanced heat transfer treatment and larger jet height will improve the jet heat transfer intensity.

Effect of Heat Exchanger and Capillary Geometry on the performance of Joule- Thomson Refrigerators operating with different mixtures

Abstract Joule-Thomson (J-T) refrigerators or J-T cryocoolers are extensively used in many low-temperature applications. J-T refrigerators operating with nitrogen-hydrocarbon (N2-HC) refrigerant mixtures offer several advantages, such as low operating pressures (<20 bar), high exergy efficiency, no moving parts in the cold section, and low cost. The cooling power or cooling capacity of the J-T refrigerator depends on the hardware used as well as the refrigerant composition. The proposed work focuses on estimating the cooling capacity of a mixed refrigerant J-T (MRJ-T) refrigerator of the given hardware and specified refrigerant. An iterative steady-state full-cycle simulation procedure has been presented in this work to simulate the complete system and estimate the cooling capacity, taking into account the possibility of choking of the expansion capillary. Some of the results have been validated against experimental results of an MRJT refrigerator available in the open literature. The details of the simulation model and the results of our studies on the prediction of stable operating range, maximum cooling capacity, the effect of heat exchanger geometry, expansion capillary geometry, mixture composition, and choking of the refrigerant mixture on the performance of an MRJ-T refrigerator are presented in this paper

Thermal analysis of 4 K J-T cryocooler equipped with an ejector

This paper introduces a 4 K Helium Joule-Thomson (J-T) refrigeration system equipped with ejector technology to enhance thermodynamic efficiency, reduce the lowest cold head temperature, and avoid higher compressor suction pressure. The J-T cryocooler, pre-cooled by a Stirling cryocooler, typically requires a high compression ratio when operating at 4 K or below. The ejector serves as a device that converts pressure energy into kinetic energy, enabling it to draw in secondary fluid and then reconvert kinetic energy into pressure energy, thus increasing the pressure of the low-pressure fluid. To investigate the impact of the ejector on J-T refrigerator performance, this study employs a one-dimensional theoretical calculation. The results are used to establish a performance chart of the J-T refrigeration system with an ejector, which is compared to the system without an ejector. The analysis reveals a promising 10-20% improvement in performance with the ejector technology implemented.

Design of nonflammable mixed refrigerants based on the partial molar enthalpy difference in mixed-refrigerant Joule-Thomson refrigerators with/no pre-cooling stage

Mixed-refrigerant Joule-Thomson refrigeration (MJTR) is an important cooling method at temperatures from 80 to 230 K. It can be used in cryosurgery, high-temperature superconductivity and sensor cooling etc. However, most of the studies are on nitrogen-hydrocarbon refrigerants, which are not allowed in applications where there is a special need to avoid flammability risks. Therefore, non-flammable mixed refrigerants were investigated in this study.The composition of the mixed refrigerant is a key factor in the system performance (refrigeration temperature, refrigeration capacity, etc). However, the purely mathematical optimization methods lack system knowledge in the optimization of the process, and may have the disadvantages of being a time-consuming process. In this study, the isothermal throttling effect of mixed refrigerants is optimized, and the optimization process is based on the partial molar enthalpy difference of each component. The method is based on thermodynamic properties and is time-saving relative to purely mathematical optimization techniques.Non-flammable mixed refrigerants (NFMR) with refrigeration temperatures from 100 K to 140 K were designed in this study. The results show that the designed mixed refrigerant has a higher COP compared to the reference. Argon has an advantage at refrigeration temperatures from 120 K to 140 K, while nitrogen has an advantage from 100 K to 120 K. In addition, mixed refrigerants for systems with pre-cooling stage were optimized and the results showed that the highest exergy efficiency is achieved at a pre-cooling temperature of 250 K. The exergy efficiencies with pre-cooling stage are nearly twice as high as those without that. Therefore, a pre-cooling stage for nonflammable mixed refrigerants is necessary where there is no requirement for system size.

Influences of distributed J-T effect on the performance of mixed-refrigerant Joule-Thomson refrigeration cycle

In a mixed-refrigerant Joule-Thomson (MRJT) cycle, the working fluid usually produces a throttling effect in the expansion element. However, the high-pressure flow in the recuperator could also generate a throttling effect due to the narrow channels, i.e., the distributed Joule-Thomson effect (DJTE). Previously published studies indicated that the cooling performance of cryocoolers would be improved by DJTE accompanying the heat recovery process. However, most of the current research on this phenomenon is focused on the heat transfer characteristics of the cryocooler itself, with a lack of research on the effect of DJTE on the cycle performance in much detail.Therefore, a detailed investigation of the influence pattern of DJTE on the cycle performance is carried out by combining with the exergy analysis. The simulation is conducted under three quintessential conditions of the recuperator. The results indicate that it is possible to manipulate the DJTE to maintain satisfactory cycle performance under specific conditions. In addition, at a constant minimum temperature difference in the recuperator, the maximum achievable DJTE is closely related to the isothermal throttling effect of the mixed refrigerant. The conclusions of this paper clarify the effect and the application value of DJTE on the cycle efficiency, which provide a reference for determining reasonable operating conditions and ensuring the cycle efficiency in practical applications.

Experimental investigation on a non-flammable mixed refrigerant Joule-Thomson refrigerator for a 100 K cooling temperature

A non-flammable mixed refrigerant Joule-Thomson refrigerator is developed for a cooling temperature of 100 K without freezing of the working fluid itself. A mixture of nitrogen, argon, tetrafluoromethane (R14, CF4), and octafluoropropane (R218, C3F8) is selected as the working fluid of the cascade type Joule-Thomson refrigerator. The optimized composition of the mixed refrigerant components is determined as N2:Ar:R14:R218 = 0.2:0.3:0.3:0.2 which has the freezing temperature of lower than 77 K according to our previous research. The coefficient of performance was obtained as 0.162 with 60 % isentropic efficiency based on the numerical simulation with aforementioned composition in the cascade configuration. As a result, the lowest temperature of 98.5 K is realized, and the maximum cooling capacity is 15 J/g at 103 K, which is the normalized value with respect to the mass flow rate of the working fluid. Additionally, the inefficiency of the experimental apparatus is analyzed and the possibility of further improving the coefficient of performance is proposed. Additionally, the inefficiency of the experimental apparatus is analyzed and the possibility of further improving the COP is proposed. This paper demonstrates that the non-flammable mixed refrigerant Joule-Thomson refrigerator can achieve cooling temperature of 100 K, below the freezing point of its pure components. It confirms the potential of using non-flammable mixed refrigerant Joule-Thomson refrigerators in large-scale industries, such as high-temperature superconducting transmission cable industries, semiconductor manufacturing, or bio-preservation fields.

Experimental investigation and regression analysis on the characteristics of the JT refrigerator operating with natural refrigerant mixtures

This paper presents the results of the analysis of the experimental characteristics of the closed-cycle Joule-Thomson (JT) refrigerator operating on a nitrogen-hydrocarbon refrigerant mixture. Designing such energy-efficient refrigeration machines involves the calculation and experimental determination of the optimal values of the “charge” composition of the mixture components. The design process must take into account the change in the “circulating” composition of the mixture components. In the paper, we present the results of regression analysis of the “circulating” composition of the mixture components of a JT low-performance refrigerator (compressor volumetric flow rate less than 25 m3/h). Such refrigerators can be used for thermostatting objects at the temperatures down to −160 °C. The results are given in the form of relationships for specific refrigeration capacity and a number of efficiency indicators. In the considered component concentration range, the relative root-mean-square regression error was less than 12.6 %. Finally, we determined the optimal concentrations of the mixture components of the JT refrigerator having a compressor with a volumetric flow rate of 4.36 m3/h which corresponds to the maximum value of specific refrigerating capacity Qt/Ve [19,87 W/(m3/h)] at the temperature of the evaporator inlet of −150 °C. The results of approbation the proposed regression method on a prototype low-temperature freezer based on a JT refrigerator operating with MMR and the limits of applicability of the developed regression models are determined. These relationships provide valuable insights for designing stock-produced devices of varying standard sizes, reducing development time and cost while leveraging comparable operating cycles and working body compositions.

Compositions optimization method based on the effective refrigeration effect for Mixed-refrigerant Joule-Thomson refrigerators cooling distributed-temperature heat loads

Mixed-refrigerant Joule-Thomson refrigeration (MJTR) is the dominant technology for cooling distributed-temperature heat loads, such as natural gas liquefaction. The mixed refrigerant determines the refrigeration cycle's performance (cooling temperature and efficiency, etc.). However, the current mathematical methods for mixed refrigerants’ optimization are time-consuming. In this paper, a mixed refrigerant optimization method based on the effective refrigeration effect is proposed. The equivalent enthalpy (EH˙) of temperature difference is proposed to predict the temperature difference distribution in the recuperator without cycle simulation. The optimization objective is to optimizeEH˙much closer to zero. In each step, only one component is adjusted, which is determined through its’ effective refrigeration effect, i.e., the partial molar enthalpy difference, in the optimization process. This method is applied to natural gas liquefaction (LNG), nitrogen liquefaction (LN2) and Cryo-compressed hydrogen storage (CcH2) systems. The exergy efficiencies in LNG, LN2 and CcH2 systems are 0.462, 0.390 and 0.421, which increase about 23.9%, 22.4% and 23.5% compared to references, respectively. The optimization time is less than 10 min per case in this study. This method can optimize mixed refrigerants in MJTR cooling various distributed-temperature heat loads.

Optimization method for mixed refrigerants in Joule–Thomson refrigerators with fixed-temperature heat loads

Mixed-refrigerant Joule–Thomson refrigeration is a widely used and effective method for refrigeration in the temperature range of 80–230 K. The demand for cooling heat loads at fixed temperatures, such as biological storage and cryogenic treatment of materials, is significant. The mixed refrigerant determines the refrigeration temperature and capacity in many cases; therefore, optimizing the mixed refrigerant composition is essential. The current mathematical approach for optimizing mixed refrigerants is not well-defined, owing to the challenge of determining how to adjust the concentrations of the components in the optimization process. Therefore, the current approach may exhibit poor robustness or a longer optimization time. This study proposes an alternative optimization method based on the effective refrigeration effect. The method aims to overcome the challenges associated with complex algorithms typically used in optimizing mixed refrigerants. The maximum refrigeration capacity was achieved by optimizing the isothermal throttling effect in the full-temperature zone without cycle simulations. Only one component with a significant refrigeration effect was changed at each optimization step. The proposed method was applied to a cryogenic refrigerator at 120 K, achieving a higher coefficient of performance than a genetic algorithm and other references. The study determined that varying the initial concentrations of the refrigerant components changed the performance coefficient to less than 0.002, indicating good robustness of the proposed optimization approach. Thus, our proposal is reliable for optimizing the composition of mixed refrigerants and can be extended to all mixed-refrigerant Joule–Thomson refrigeration systems with fixed-temperature heat loads.

Thermodynamic analysis of a novel vapor-injection Joule-Thomson refrigeration cycle with a binary hydrocarbon mixture for −86 °C freezer

This paper proposes a novel vapor-injection Joule-Thomson refrigeration cycle with mixed-refrigerant. The introduction of the compressor with vapor-injection can reduce the pressure ratio of each compression stage, thus reducing the compressor power and discharge temperature. The thermodynamic analysis results show that the performance of the novel cycle is better than that of the basic Joule-Thomson cycle. In addition, this paper studies the binary hydrocarbons applied in −86 °C freezers, and find a binary hydrocarbon R601a/R1150 with Superior performance. The mass fractions of R1150 in R601a/R1150 corresponding to the maximum COP of the novel cycle and basic cycle are 0.45 and 0.44, respectively. Moreover, the maximum COP of the novel cycle is 19.25% higher than that of the basic cycle. The comparison results demonstrate that the novel cycle has a great energy-saving potential. In addition, at different condenser outlet temperatures of 16 °C, 25 °C and 32 °C, both cycles have the optimal condensation pressure of 300 kPa, 500 kPa and 650 kPa with respect to COP. For the novel cycle, the compressor injection ratio has little effect on COP. The increase of intermediate pressure can improve cycle performance, but too high intermediate pressure is not conducive to the normal operation of the system. Furthermore, the exergy analysis result shows that the location of the largest exergy destruction rate is in the compressors for both cycles, followed by the recuperators, condensers, evaporators, and capillary tubes.

EXPERIMENTAL INVESTIGATION ON FLOW CONDENSATION HEAT TRANSFER OF METHANE/ETHYLENE MIXTURE INSIDE A HORIZONTAL SMOOTH TUBE

Methane (R50) and ethylene (R1150) are important components in mixed-refrigerant Joule-Thomson (MRJT) refrigeration systems. Among many efforts to improve MRJT refrigerators, the work focusing on the properties of condensing heat transfer in mixtures is essential. Therefore, this paper aims to investigate these characteristics of high-temperature glide mixture R50/R1150 (26.7/73.3% molar fraction) inside a horizontally placed smooth tube. The experiments were done for varying saturation pressures (1.7-2.3 MPa), mass fluxes (101-391 kg m<sup>-2</sup> s<sup>-1</sup>), and the entire range of vapor qualities. The results indicate that higher heat transfer coefficients (HTCs) occur at larger mass fluxes and smaller saturation pressures. In addition, the effect of temperature glide was discussed by comparing the HTCs of R50/R1150 with those of R50/R170 reported in the previous literature under the same test condition. It is indicated that higher temperature glide contributes to worse heat transfer performance. Moreover, six existing correlations combined with the equilibrium model were employed to predict the HTCs of R50/R1150. All predictions exhibit an underestimation of the HTCs, and only Dobson and Chato correlation and Shah correlation presented reasonable predictions with a mean absolute relative deviation (MARD) under 30%. Therefore, a new and improved heat transfer correlation was developed based on the pure fluid correlation proposed by Shah. The correlation takes into account the effects of temperature glide and mass transfer resistance and draws on experimental data for hydrocarbon mixtures reported in the existing literature. Comparison with the experimental data shows that the new correlation has good applicability to hydrocarbon mixtures, providing satisfactory predictive values of R50/R1150 with a MARD of 14% and 95.3% of the data within ± 30% deviation.

The effective refrigeration temperature zone expressed by partial molar enthalpy difference of each component in mixed-refrigerant Joule-Thomson refrigerators

Mixed-refrigerant Joule-Thomson refrigerators (MJTRs) are widely used in the fields of energy, chemical engineering and medical treatment. Mixed refrigerants generally employ a mixture of 4–8 components, which provide refrigeration effects in different temperature zones. It is essential to reveal the effective refrigeration temperature zones of different components for the selection of components and adjustment of their concentrations. In this study, the partial molar enthalpy difference is used to characterize the contribution of components to the isothermal throttling effect. The results show that there is an obvious peak in the graph of the contribution of each component to the isothermal throttling effect with temperature, whose height and width represent the refrigeration effect and the effective refrigeration temperature zone of the component, respectively. The temperature zone where the partial molar enthalpy difference of the component is greater than the isothermal throttling effect of the mixed refrigerant is defined as the effective refrigeration temperature zone. The effective refrigeration temperature zone of the component with highest boiling point lies between the high-pressure and low-pressure dew points, and that of the component with lowest boiling point lies between the high-pressure and low-pressure bubble points. Further, the other components provide the refrigeration effect in the temperature zone between low-pressure dew point and high-pressure bubble point, thereby playing the connecting role.

Experimental investigation on the start-up behaviors of a Joule–Thomson refrigeration system with pressure control

• Start-up behaviors of the J-T refrigeration system with pressure control were explored. • Reducing the pressure control time could accelerate pull-down rate. • Increasing control pressure and expansion valve opening decreases pull-down time. • Reducing compressor speed and R1150 concentration could reduce pull-down time. The excessively high discharge pressure of mixture-based Joule–Thomson (J–T) refrigeration system for low-temperature freezers at the start-up process without active control leads to the reduction of operation reliability of compressors. Start-up characteristics of a J–T refrigeration system with an expansion tank for pressure control were investigated in this study. The binary mixture R1150/R600a was selected as the working fluid, and influences of the control pressure, compressor speed, throttle valve opening, and mixture concentration on the system’s dynamic performances were experimentally explored. Results demonstrated that the discharge pressure could be effectively limited below the compressor’s safety pressure with an expansion tank. Meanwhile, the close relationship between pull-down rate and time of the pressure control phase was associated with the control pressure, compressor speed, throttle valve opening, and mixture concentration. The pull-down rate can be accelerated by increasing the control pressure and throttle valve opening and reducing the compressor speed and the R1150 concentration at the start-up process. The excessively rich R1150 concentration caused the continuous action of the pressure switch and the malfunction of the low-temperature freezing at an acceptable time range. The build-up of the heat recuperation played a significant role in pressure control time and freezing temperature reduction rate.

Isothermal Vapor–Liquid Equilibrium for the Tetrafluoromethane (R14) + Octafluoropropane (R218) Binary Mixture at Five Temperatures from 173.150 to 213.150 K

Tetrafluoromethane (R14) and octafluoropropane (R218) are essential nonflammable refrigerants for mixed-refrigerant Joule Thomson refrigerators. In this paper, the vapor–liquid equilibrium (VLE) data were acquired for the R14 + R218 binary mixture from 173.150 to 213.150 K by a vapor-phase single-cycle method. The VLE data were regressed by the Soave–Redlich–Kwong (SRK) equation of state plus van der Waals (vdW) mixing rule model (SRK-vdW) and the Soave–Redlich–Kwong (SRK) equation of state combined with modified Huron–Vidal first-order (MHV1) mixing rule plus non-random two liquids (NRTL) activity coefficient model (SRK-MHV1-NRTL). The maximum average absolute relative deviation of pressure (AARDp) and the maximum average absolute deviation of vapor-phase mole concentration (AADy) are 2.42% and 0.0019 for the SRK-vdW model, respectively. A better correlation result was obtained by the SRK-MHV1-NRTL model, and the maximum AARDp and AADy are 1.04% and 0.0019, respectively. Based on the experimental and calculated results, a strong non-azeotropic behavior was found.

Development of a scanning tunneling microscope for variable temperature electron spin resonance.

Recent advances in improving the spectroscopic energy resolution in scanning tunneling microscopy (STM) have been achieved by integrating electron spin resonance (ESR) with STM. Here, we demonstrate the design and performance of a homebuilt STM capable of ESR at temperatures ranging from 1 to 10K. The STM is incorporated with a homebuilt Joule-Thomson refrigerator and a two-axis vector magnet. Our STM design allows for the deposition of atoms and molecules directly into the cold STM, eliminating the need to extract the sample for deposition. In addition, we adopt two methods to apply radio-frequency (RF) voltages to the tunnel junction: the early design of wiring to the STM tip directly and a more recent idea to use an RF antenna. Direct comparisons of ESR results measured using the two methods and simulations of electric field distribution around the tunnel junction show that, despite their different designs and capacitive coupling to the tunnel junction, there is no discernible difference in the driving and detection of ESR. Furthermore, at a magnetic field of ∼1.6T, we observe ESR signals (near 40GHz) sustained up to 10K, which is the highest temperature for ESR-STM measurement reported to date, to the best of our knowledge. Although the ESR intensity exponentially decreases with increasing temperature, our ESR-STM system with low noise at the tunnel junction allows us to measure weak ESR signals with intensities of a few fA. Our new design of an ESR-STM system, which is operational in a large frequency and temperature range, can broaden the use of ESR spectroscopy in STM and enable the simple modification of existing STM systems, which will hopefully accelerate a generalized use of ESR-STM.

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.

운전조건 변화에 따른 혼합냉매 Joule-Thomson 냉동기의 성능특성

Joule-Thomson refrigerators have the advantages of simple structure and no moving component at the cold part, which should result in high reliability. Change in refrigeration capacity were investigated by varying the compressor discharge pressure, evaporator inlet pressure, and aftercooler outlet temperature to analyze the influence of operating conditions on the performance of Joule-Thomson refrigerators. The results show that a refrigeration capacity was significantly affected by compressor discharge pressure, evaporator inlet pressure, and aftercooler outlet temperature. Furthermore, the aftercooler outlet temperature has the greatest influence on refrigeration capacity.