Search

This research paper discusses the free-piston Stirling cryocooler (FPSC) in conjunction with a dual-opposed linear compressor. The system was tested under various operational conditions. The developed compressor can produce mass flow with an efficiency exceeding 60%; a loss analysis was conducted with care. The dynamic and thermal aspects of the experimental results are extensively verified via the numerical model. The model describes the physical characteristics of the Stirling cryocooler when a damping effect is applied to the displacer. The damping effect incorporated in the model is expressed through a function of the expansion work at the cold-end. It was also confirmed that the damped motion of the displacer is more suitable than undamped motions for describing real situations. The relevant dynamic characteristics of the piston and the displacer are also explained through the introduction of a phasor diagram. The developed model explains the physics of the Stirling cryocooler well, while accurately explaining the quantity of losses (i.e. heat conduction, imperfection of regenerator and shuttle heat transfer) that are distributed at its cold-end. The thermal load is regulated at various cold-end temperatures of 110 K, 120 K, 150 K and 190 K, and the system is capable of exerting maximum Carnot efficiencies of 25%, 27%, 28% and 29%, respectively.

A compact 154-kV superconducting fault current limiters (SFCL) system has been developed by Korea Electric Power Corporation and led to a primary prototype being tested for three months at Gochang Power Test Center. Due to the fact that the volume of the primary model was still relatively large for application to substations, a compact system was designed. To reduce the volume of the SFCL system, electric field analyses were performed. By carrying out a fine modification of the shape, the design of a compact system was completed, and the volume and footprint of the compact cryostat were 65% and 77% of the primary model, respectively. Following that, a single-phase compact model was fabricated and installed on a dedicated test site at Gochang Power Test Center. Several tests were then performed on the SFCL system, which led to a single-phase voltage of 154 kV being applied to the system. The period covered by the long-term test lasted for one year. This paper presents the results of the one-year testing.

An integral crank driven Stirling cryocooler is solidly based on concepts of direct IR detector mounting on the cryocooler’s cold finger, and the integral construction of the cryocooler and Dewar envelope. Performance factors of the cryocooler depend on operating conditions of the cryocooler such as a cyclic mean pressure of the working fluid, a rotational speed of driving mechanism, a thermal environment, a targeted operation temperature and etc.. At given charging condition of helium gas, the cyclic mean pressure of helium gas in the cryocooler changes with temperatures of the cold end and the environment. In this study, effects of the cyclic mean pressure of helium gas on performances of the Stirling cryocooler were investigated by numerical analyses using the Sage software. The simulation model takes into account thermodynamic losses due to an inefficiency of regenerator, a pressure drop, a shuttle heat transfer and solid conductions. Simulations are performed for the performance variation according to the cyclic mean pressure induced by the temperature of the cold end and the environment. This paper presents P-V works in the compression and expansion space, cooling capacity, contribution of losses in the expansion space.

The superconducting NMR magnets have used cryogen such as liquid helium for their cooling. The conduction cooling method using cryocoolers, however, makes the cryogenic cooling system for NMR magnets more compact and user-friendly than the cryogen cooling method. This paper describes the thermal and structural analysis of a cryogenic conduction cooling system for a 400 MHz HTS NMR magnet, focusing on the magnet assembly. The highly thermo-conductive cooling plates between HTS double pancake coils are used to transfer the heat generated in coils, namely Joule heating at lap splice joints, to thermal link blocks and finally the cryocooler. The conduction cooling structure of the HTS magnet assembly preliminarily designed is verified by thermal and structural analysis. The orthotropic thermal properties of the HTS coil, thermal contact resistance and radiation heat load are considered in the thermal analysis. The thermal analysis confirms the uniform temperature distribution for the present thermal design of the NMR magnet within 0.2 K. The mechanical stress and the displacement by the electromagnetic force and the thermal contraction are checked to verify structural stability. The structural analysis indicates that the mechanical stress on each component of the magnet is less than its material yield strength and the displacement is acceptable in comparison with the magnet dimension.

>> This paper describes the continuing effort to analysis and design on dynamic and electrical behaviorof gamma-type free piston Stirling engine/generator with dual–opposed linear generator for domestic micro-CHP (Combined Heat and Power) system. The double acting Stirling engine/generator has one displacer and two power piston which are supported by flexure springs. Two power pistons oscillate with symmetric sinusoidal displacementand are connected with moving magnet type linear generators for power generation. To operate Stirling engine/ generator, combustion heat of natural gas is supplied to hot-end and heat is rejected from cold-end by coolingwater. The temperature difference across the displacer induces the oscillating motion, and it can be explained withmass-spring vibration system. The purpose of this paper is to describe the design process of linear generator for the double acting free-piston Stirling engine. Key words : Double acting free piston Stirling engine/alternator(양방향 자유 피스톤 스털링엔진 /발전기), Lineargenerator(선형발전기), Piston(피스톤), Displacer(변위기), Operating frequency(작동주파수)

This research paper describes the design methodology of the free-piston Stirling engine (FPSE) and its demonstration. The FPSE is equipped with a dual-opposed linear alternator. The design and experiment are mainly focused on understanding how the FPSE dynamically behaves. Based on the data obtained from experiments, the operational issues regarding the output and dynamic characteristics are discussed in detail. The FPSE can accept approximately 4.2 kW at the engine head with a temperature of 525 °C. As a heat source, city-supplied natural gas is used. The linear alternator stably produces 961 We-electricity at a resonance frequency of 60 Hz. The deduced efficiency of the linear alternator is 70–75% of the thermodynamic work generated by the engine. The integrated system, FPSE, has an overall efficiency of 23.0% when comparing the heat supplied to the electrical output. The FPSE was also proven with a direct connection to the grid, so it can generate and sell electricity back to the grid. The equipped resonant capacitor enables this by compensating for the power factor. The experiment results show that the design methodology is able to adequately explain the behavior of the FPSE.

Cryogenic cooling system for HTS electric power devices requirea reliable and efficient highs -capacity cryocooler. A Striling cryocooler with a linear compressor can be a good candidate. It has advantages of low vibration and long maintenance cycle compared with a kinematic-driven Stirling cryocooler. In this study, we developed dual-opposed linear compressor of 12 kW electric input power with two 6 kW linear motors. Electrical performance of fabricated linear compressor is verified by experimental measurement of thrust constant. The developed Stirling cryocooler has gamma-type configuration. Piston and displacer are supported with flexure spring. A slit -type heat exchanger is adopted for cold and warm -end, and the generated heat is rejected by cooling water. In cooling performance test, waveforms of voltage, current, displacement and pressure are obtained and their amplitude and phase difference are analysed. Moreover, temperatures of cooling water, housing and l inear motor are recorded and electric power parameters of driving circuit are also obtained. The developed Stirling cryocooler reaches to 47.8 K within 23.4 min. with no -load. From heat load tests, it shows cooling capacity of 440 W at 78.1 K with 6.45 kW of electric input power and 19.4 of % Carnot COP.

A Superconducting Fault Current Limiter is an electric power device which limits the fault current immediately in a power grid. The SFCL must be cooled to below the critical temperature of high temperature superconductor modules. In general, they are submerged in sub-cooled liquid nitrogen for their stable thermal characteristics. To cool and maintain the target temperature and pressure of the sub-cooled liquid nitrogen, the cryogenic cooling system should be designed well with a cryocooler and coolant circulation devices. The pressure of the cryostat for the SFCL should be pressurized to suppress the generation of nitrogen bubbles in quench mode of the SFCL. In this study, we tested the performance of the cooling system for the prototype 154 kV SFCL, which consist of a Stirling cryocooler, a subcooling cryostat, a pressure builder and a main cryostat for the SFCL module, to verify the design of the cooling system and the electric performance of the SFCL. The normal operation condition of the main cryostat is 71 K and 500 kPa. This paper presents tests results of the overall cooling system.

It is one of important design issues in the high-temperature superconducting fault current limiter (HTS-FCL) to estimate how much liquid nitrogen vaporizes during its quench process. This paper describes the small scale experiment about the vaporization of subcooled liquid nitrogen by instantaneous heat generation corresponding to the quench in HTS-FCL. In the experiment, the heat is given liquid nitrogen by Joule heating of the stainless steel strip for the short time of about 100 ms. The time variations of temperature and pressure in the liquid nitrogen test chamber are measured for different subcooling conditions of liquid nitrogen and various heat flux conditions. In addition, the amount of vaporization of liquid nitrogen is calculated from experimental results according to the subcooling of liquid nitrogen. The experimental results show that the vaporization of liquid nitrogen strongly depends on heat flux and subcooling conditions. The amount of vaporization linearly decreases with the increase in the subcooling of liquid nitrogen. The calculation results about the amount of vaporization suggest that the subcooling of more than 20 K is necessary to suppress bubble generation for the heat flux condition of 46 W/cm 2 or more.

Stirling type pulse tube cryocoolers are very attractive for cooling of diverse application because it has it has several inherent advantages such as no moving part in the cold end, low manufacturing cost and long operation life. A moving magnet type linear motor of dual piston configuration is designed, and this compressor could be operated with the electric power of 100 W at the operating frequency of 50 Hz. A pulse tube cold finger with a double segmented inertance tube was designed for the desired cooling capacity. The coiled inertance tube was assembled inside the reservoir. In experiment, the pulse tube cryocooler is capable of providing the cooling capacity of 2.0 W at 80 K with power consumption of 99.1 W. Random and sinusoidal vibration tests have been conducted to evaluate their performance characteristics and structural integrity.

Abstract : In this paper, we carried out the basic design of 36 MTD natural gas BOG re-liquefaction system to recover the generated natural gas during performance test of LNG pump and natural gas compressor. The re-liquefaction process of natural gas is designed to have 1500 kg/h of liquefaction rate with reverse Brayton refrigeration cycle. With the designed process, the variat ion of liquefaction rate is calculated for various inlet conditions of feed gas. From results, the liquefaction rate is more sensitive for inlet temperature than gas composition. The specifications of equipments such as gas blower, natural gas compressor, cryogenic heat exchanger and nitrogen compander are determined on the basis of the designed process. The requirement of power consumption and cooling water are also determined through the basic design.

This paper describes experimental study on GM-type pulse tube refrigerator (PTR). In a PTR, the pulse tube is only filled with working gas and there exists secondary flow due to a large temperature difference between cold-end and warm-end. The stability of secondary flow is affected by orientation of cold-head and thus, the cooling performance is deteriorated by gas mixing due to secondary flow. In this study, a single stage GM-type pulse tube refrigerator is fabricated and tested. The cooing performance of the fabricated PTR is measured as varying cold-head orientation angle and the results are used as reference data. Then, we divided interior space of pulse tube into three segments, and fixed the various size of screen mesh at interface of each segment to suppress the performance degradation due to secondary flow. For various configuration of pulse tube, no-load test and heat load test are carried out with the fixed experimental condition of charging pressure, operating frequency and orifice valve turns. From experimental results, the fine screen mesh shows the effective suppression of performance degradation for the large orientation angle, but the use of screen mesh cause the loss of cooling capacity rather than the case of no insertion into pulse tube. It should be compromised whether the use of screen mesh in consideration of the installation limitation of a GM-type pulse tube refrigerator.

This paper describes experimental study on the orientation dependence of GM-type pulse tube refrigerator with helium and neon as working gas. A pulse tube refrigerator generates refrigeration work with gas expansion by gas displacer in the pulse tube. The pulse tube is only filled with working gas and there exists secondary flow due to large temperature difference between cold-end and warm-end. The stability of secondary flow is affected by orientation of cold-head and thus cooling performance is deteriorated by gas mixing due to secondary flow. In this study, a single stage GM-type pulse tube with orifice valve as a phase control device is fabricated and tested. The fabricated pulse tube refrigerator is tested with two different working gases of helium and neon. First, optimal valve opening and operating frequency are determined with experimental results of no-load test. And then, the variation of no-load temperature as orientation angle of cold-head is measured for two different working gases. Effect of orientation dependence of cold-head as working gas is discussed with experimental results.

This paper describes experimental investigation on neon refrigeration system using commercial helium compressor. In this paper, neon refrigeration cycle is calculated with assumption of ideal heat exchanger. From analysis, 32.6 K of the lowest temperature and 0.945 of quality after expansion are predicted. Cryogenic heat exchangers for pre cooler and main heat exchanger are designed and fabricated with configuration of tube-in-tube heat exchanger. In experiments, cooling performance test are performed as variation of charging pressure and orifice hole diameter. From experimental results, the lowest temperature of 44.0 K was measured with 500 orifice and 1500 kPa of charging pressure.

Stirling type pulse tube cryocoolers are very attractive for cooling of diverse application because it has it has several inherent advantages such as no moving part in the cold end, low manufacturing cost and long operation life. To develop the Stirling-type pulse tube cryocooler, we need to design a linear compressor to drive the pulse tube cryocooler. A moving magnet type linear motor of dual piston configuration is designed and fabricated, and this compressor could be operated with the electric power of 100 W and the frequency up to 60 Hz. A single stage coaxial type pulse tube cold finger aiming at over 1.5 W at 80K is built and tested with the linear compressor. Experimental investigations have been conducted to evaluate their performance characteristics with respect to several parameters such as the phase shifter, the charging pressure and the operating frequency of the linear compressor.

In this research, a heater using eddy current for generator is designed and some analysis is performed to validate the system. FEM (finite element method) is used for analysing eddy current phenomena at the various speed of rotor with permanent magnet. A real heater system is fabricated. Some experiment is also performed to validate the analysis result. Through the experiment, the FE analysis is validated and we found that the efficiency of developed heater is over 94%.

HTS superconducting magnetic energy storage (SMES) systems need cryogenic cooling systems. Conduction cooling system has more effective, compact structure than cryogen. In general, two stage GM cryocoolers are used for conduction cooling of HTS SMES system. First stages of cryocoolers are used for the cooling of current leads and radiation shields, and second stages of cryocoolers for HTS coil. The cryocooler has a limited cooling capacity. So radiation shields are used to minimize the heat leak to the HTS coil. For the effective conduction cooling of the HTS SMES system, the temperature difference between the cryocooler and HTS coil should be minimized. In this paper, a cryogenic conduction cooling system for HTS SMES is analyzed to evaluate the performance of the conduction cooling system. The heat transfer analysis is carried out in steady state with the heat generation of the HTS coil and effects of the thermal contact resistance. The results show the effects of the heat generation and thermal contact resistance on the temperature distribution.

A Stirling type pulse tube refrigerator with an active phase control has been experimentally investigated. A phase shifter, which controls the phase angle between the mass flow and the pressure inside a pulse tube, plays a key roll in the performance of pulse tube refrigerators. In this study, an electrically driven and mechanically damped linear compressor, which is directly connected at the warm end of the pulse tube using a connecting tube, is used as the active phase controller (APC). Therefore, this active phase control pulse tube refrigerator (APCPTR) has no reservoir. Amplified electric signals of a function generator are supplied to both the main linear compressor, which is used as the pressure wave generator (PWG), and the APC. The type of these two linear compressors is a dual-opposed piston. The advantage of this phase sifter is easy to control the electric input power and the phase angle between the PWG and the APC. In order to clarify the characteristics of the APCPTR, the cold end temperature and the gas pressure have been measured.

Cool-down characteristics of a miniature J–T refrigerator with a pressurized vessel, which has different initial pressures of nitrogen gas, were investigated by experiment. The supply and exhaust pressures, the temperatures on the surface of the cryostat, and the mass flow rate were measured simultaneously to analyze the cool-down characteristics of the J–T refrigerator with a pressurized vessel or reservoir. The results show the effect of the supply pressure on the cold end temperature of the J–T refrigerator. The influence of the supply pressure and the temperature on the mass flow rate during the cool-down stage is discussed in details.

This paper presents design, manufacturing process and test results in the optimum operating condition for the free piston free displacer (FPFD) type Stirling cryocooler designed and manufactured by Korea Institute of Machinery and Materials. FPFD Stirling cryocooler is currently under development for cooling infrared detector. The compressor in the Stirling cryocooler uses opposed linear motors to drive opposed pistons. The performance of FPFD Stirling cryocooler is evaluated as a function of charging pressure and operating frequency. In general, as the charging pressure is increased, optimum-operating frequency of the compressor is increased but resonant frequency of the displacer is almost constant. The prototype has achieved no load temperature of 49 K and cooling power of 0.5 W at 72 K.

The purpose of this study is to analyze the characteristics of the linear compressor for Stirling cryocooler. To ensure high performance and long life of the free piston, free displacer Stirling cryocooler, the operating parameters of the linear compressor should be optimized. The experimental results show the operating frequency of the linear compressor has significant effect to the input power characteristics, and the compressor with higher charging pressure of working fluid has high pressure ratio. For the high performance of the linear compressor, compressor has uniform magnetic force with different stroke, and the optimal operating frequency with respect to the charging pressure.

A conduction-cooled 3 T 100 mm winding bore multi-width and no-insulation (NI) all-REBCO magnet was designed, constructed and tested at 13 K. The magnet consists of a stack of double pancake (DP) coils wound with, for the first time, REBCO tapes having a 1 μm thick layer of stainless steel, named 'metallic cladding', that surrounds the tapes in a hermetic way to substantially reduce the NI charging delay. After construction, the magnet was cooled down to the target operating temperature of 13 K using a two-stage pulse-tube cryo-cooler. During charging–discharging tests up to 200 A, magnetic center field, voltage of each DP coil, power supply current, and magnet temperature were monitored. The charging time constant of the magnet was measured to be about 10.1 s, 13 times smaller than that of its NI counterpart. The magnet experienced, due to an unexpected power supply trip, a sudden discharge at a peak coil current density of 353 A mm2, yet it survived without any degradation. The results demonstrated strong potential of the metallic cladding NI-REBCO magnet for significant charging-delay reduction and self-protecting operation.

A nuclear magnetic resonance (NMR) magnet composed of a high-temperature superconductor (HTS) is expected to contribute to a smaller size and a higher magnetic field (>1 GHz) of the NMR magnet due to its high critical current density under high magnetic field and better mechanical properties. The high operating temperature of HTS NMR magnets also makes conduction cooling by a cryocooler available. The conduction cooling system combined with a cryocooler is compact and user-friendly but has critical issues about the design of conduction cooling passages and the mechanical vibration. The small-scale HTS magnet with the conduction cooling system is fabricated and tested to check the issues and establish the design basis as a preliminary step for the development of a 400-MHz HTS NMR magnet. Thermal analysis for heat loads and thermal resistances at main joints is performed from the temperature distribution of the cooling system. The vibration levels at the crycooler and the vacuum chamber are discussed in comparison to the vibration criterion of NMR magnets. In addition, the design approach to the conduction cooling system for the HTS NMR magnet is discussed.

In this study, we developed neon refrigeration system using commercial helium compressor which was originally designed for GM cryocooler. We performed this research as precedent study before developing neon refrigeration system for small-scale hydrogen liquefaction system. The developed system is based on precooled Linde-Hampson system with liquid nitrogen as precoolant. Design parameters of heat exchangers are determined from thermodynamic cycle analysis with operating pressure of 2 MPa and 0.4 MPa. Heat exchangers have concentric-tube heat exchanger configuration and orifice is used as Joule- Thomson expansion device. In experiments, pressure, temperature, mass flow rate and compressor input power are measured as charging pressure. With experimental results, the characteristics of heat exchanger, Joule-Thomson expansion and refrigeration effect are discussed. The developed neon refrigeration system shows the lowest temperature of 43.9 K.

Miniature Joule-Thomson refrigerators have been widely used for rapid cooling of infrared detectors, probes of cryosurgery, thermal cameras, missile homing head and guidance system, due to their special features of simple configuration, compact structure and rapid cool-down characteristics. Typical performance factors of the Joule-Thomson refrigerator are cool-down time, temperature of the cold end, running time and gas consumption. Those depend on operating conditions such as the pressure of the gas, thermal environment and etc.. In this study, experimental study of a miniature Joule- Thomson refrigerator with the gas pressure up to 12 MPa were performed to investigate the effects of the thermal environment (-40 ~ 50 °C). In experiments, to obtain the information of cool-down time, gas consumption and etc., the temperature of the cold end, mass flow rate and pressure of the argon gas are simultaneously measured. The Joule-Thomson refrigerator in cold thermal environment has rapid cool-down characteristics and small gas consumption. In the cold environmental condition, the Joule-Thomson refrigerator has high mass flow rate during cool-down process and in steady state.

A Superconducting Fault Current Limiter is an electric power device which limits the fault current immediately in a power grid. The SFCL must be cooled to below the critical temperature of high temperature superconductor modules. In general, they are submerged in sub-cooled liquid nitrogen for their stable thermal characteristics. To cool and maintain the target temperature and pressure of the sub-cooled liquid nitrogen, the cryogenic cooling system should be designed well with a cryocooler and coolant circulation devices. The pressure of the cryostat for the SFCL should be pressurized to suppress the generation of nitrogen bubbles in quench mode of the SFCL. In this study, we tested the performance of the cooling system for the prototype 154 kV SFCL, which consist of a Stirling cryocooler, a subcooling cryostat, a pressure builder and a main cryostat for the SFCL module, to verify the design of the cooling system and the electric performance of the SFCL. The normal operation condition of the main cryostat is 71 K and 500 kPa. This paper presents tests results of the overall cooling system.

Cooling capacity and cool‐down time of a Joule‐Thomson (J‐T) refrigerator depends on mass flow rates of refrigerants and their enthalpy changes. It is obvious that the mass flow rate through the J‐T refrigerator depends on supply pressure; it increases as the supply pressure increases. Furthermore the mass flow rate increases as the temperature of the cold end decreases when the supply pressure does not change during the cool‐down process. In this study, the cool‐down characteristics of a miniature J‐T refrigerator (fixed orifice / demand flow type) with different pressures of nitrogen and argon gas, were investigated by experiment. To analyze the cool‐down characteristics of the refrigerator, the supply and exhaust pressure, the temperatures on the surface of the cryostat, and the mass flow rate were measured simultaneously. The results show the effect of the supply pressure on the performance of the refrigerator. The influence of the supply pressure and the temperature on the mass flow rate during the cool‐down stage is discussed in detail.

Miniature Joule Thomson refrigerators have been widely used for the cooling of infrared detectors, cryosurgery, thermal cameras, missile homing heads, and guidance systems because they have a simple, compact structure and rapid cooldown characteristics.

Publisher Summary This chapter investigates the influences of the radial clearance on the performance of Stirling refrigerator by numerical simulation. The Stirling refrigerator has the clearance in the compressor and cold finger. Its cooling capacity comes from the relation of the pressure in the expansion space and motion of displacer. In the radial clearance of the cold finger, the parasitic dynamic losses from the leakage flow and shuttle heat losses are inevitably occurred. Therefore, to get the maximum performance of the refrigerator, the radial clearance should be optimized. The SAGE is used to investigate the effects of the operating frequency, the stroke of the displacer and the radial clearance between the seal rings and the expander on the cooling performance. The calculation results show that the enthalpy flow through the refrigerator is a dominant heat loss. The leakage flow through the clearance has the effects on the cooling capacity through the gross refrigeration, the enthalpy flow, and the shuttle heat loss. The enthalpy flow through the clearance depends on the operating frequency and the radial clearance of the cold end, whereas the shuttle loss depends on the stroke of the displacer.

A pulse tube cryocooler (refrigerator) has no moving parts in its cold section, and, when driven by a linear compressor, has the potential of achieving higher reliability and lower vibration than other small refrigerators. The most compact and convenient pulse tube cryocooler for practical applications is the coaxial type. It can often replace Stirling cryocoolers without any change to the dewar or to the connection to the cooled device. The purpose of this study is to analyze the characteristics of a coaxial type inertance pulse tube refrigerator (IPTR), and to understand the principal factors governing its performance.

Publisher Summary The purpose of this chapter is to analyze the characteristics of in-line type inertance tube pulse tube refrigerator (IPTR), and to get main factor to improve the performance of the inline type IPTR. The Experimental results of the in-line type inertance tube pulse tube cryocooler for cooling superconductor RF filter are presented. Design parameters of the in-line IPTR are discussed by Ames Research Center Orifice Pulse Tube Refrigerator (ARCOPTR) program of NASA's Ames Research Center and then the effects of the charging pressure, the volume of the regenerator and the diameter and length of the inertance tube on the cooldown characteristics of the pulse tube cryocooler are investigated by experiment. The optimal conditions of in-line type IPTR, cool down characteristics according to the variations of the charging pressure, inertance tube volume, regenerator volume, and pulse tube volume are measured by the experiment. In the analysis by ARCOPTR program, as the diameter of the regenerator decreased, maximum cooling capacity and COP increased and volume ratio at the maximum cooling capacity and COP decreased, but the range of the volume ratio at the maximum cooling capacity and COP was very narrow, therefore the IPTR was operated stably in the large volume of the regenerator. As volume of the regenerator increased, the performances of the lowest temperature, cooling capacity and COP were improved.

The cooling load, which should be removed by the cooling system to maintain the nominal operating temperature of a cryochamber, depends on the thermal insulation efficiency of the cryochamber. Therefore, to decrease and minimize the heat loss of the cryochamber has become a big problem in the design stage, in considering the very low thermal efficiency and small cooling capacity of the cryocooler. Radiation and conduction heat transfer occur simultaneously in a cryochamber. The radiation heat transfer from the hot surface to the cold surface depends on the emissivity, temperature, area and radiation exchange factor.In the present work, the energy equation, which includes the rarefied gas conduction, radiation between the boundary surfaces and solid conduction, is solved to evaluate the steady cooling loads at the cold finger of a small cryocooler. The surface to surface radiation model is used for the calculation of the radiation exchange factor. This paper presents the effects of the emissivity of the ma...

This paper describes the thermal response of a high-temperature superconducting (HTS) wire model to instantaneous heat generation in a pressurized liquid nitrogen chamber for an HTS fault current limiter. A dc impulse power input to a stainless steel strip is adopted to simulate the quench state of the HTS wire. The test sample is submerged in the liquid nitrogen, which maintains a 77 K temperature with operating pressures of 101, 250, 400, and 600 kPa. Three different levels of dc current are supplied to the test sample during 50 ms for each operating condition. The boiling phenomena are captured with a high-speed camera and the surface temperature of the sample strip is measured to investigate the recovery process. From the captured video, the suppression of bubble generation is clearly observed as increasing operating pressure, especially for the lower heat flux condition. From the measurement of temperature, temperature rise of sample strip during heat generation decreases with increasing operating pressure except for the higher heat flux. For the recovery process, increasing operating pressure delays the recovery of the sample strip, but recovery time is within a few seconds for all cases.