• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.16 no.10
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• pp.889-894
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• 2004

Infrared (IR) detectors are widely used for many applications, such as temperature measurement, intruder and fire detection, robotics and industrial equipment, thermoelstic stress analysis, medical diagnostics, and chemical analysis. Quantum detectors commonly need to be refrigerated below 80 K, and thus a cooling system should be equipped together with the detector system. The cooling load, which should be removed by the cooling system to maintain the nominal operating temperature of the detector, critically depends on the insulation efficiency of the cryochamber housing the detector. Thermal analysis of cryochamber includes the conduction heat transfer through a cold well, the gases conduction and gas outgassing, as well as radiation heat transfer, The transient cooling characteristics of an infrared detector cryochamber are investigated experimentally in the present study. The transient cooling load increases as the gas pressure is increased. Gas pressure becomes significant as the cooling process proceeds. Cool down time is also increased as the gas pressure is increased. It is also found that natural convection effects on cool down time become significant when the gas pressure is increased.

• Progress in Superconductivity and Cryogenics
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• v.12 no.4
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• pp.41-45
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• 2010

Miniature Joule-Thomson refrigerators have been widely used for rapid cooling of infrared detectors, optoelectronic device, and integrated circuits of micro electronics. The typical J-T refrigerator consists of the recuperative heat exchanger with the double helical tube and fin configuration, J-T nozzle, a mandrel, Dewar and a compressed gas storage bottle. In this study, to predict the thermodynamic behaviors of the refrigerator with a compressed gas storage bottle during the cool-down time, numerical study of transient characteristics for a J-T refrigerator was developed. A simplified transient one.dimensional model of the momentum and energy equations was simultaneously solved to consider the thermal interactions of the each component of the refrigerator. To account for effects of the thermal mass of the solid, the heat capacities of the tube, fins, mandrel and Dewar are considered. The results show the charged gas pressure of the gas storage bottle has significant effects on the performance of the J-T refrigerator. At the elevated gas pressure of the gas storage bottle, the large capacity of the compressed gas storage does not need to get the fast cool-down performance of the J-T refrigerator in the cool-down stage.

• Progress in Superconductivity and Cryogenics
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• v.8 no.4
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• pp.30-33
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• 2006

Several crucial issues are discussed in the design of cryogenic cooling system for high field magnets. This study is mainly motivated by our ongoing program to develop a 21 T Fourier Transform Ion Cyclotron Resonance Mass Spectrometer (FT-ICR MS). The magnets of this system will be built horizontally to accomplish the requirement of user friendliness and reliability, and the replenishment of cryogen will not be necessary by a closed-loop cooling concept. The initial cool-down and safety are basically considered in this paper. The effects of the helium II volume and the gap distance of the weight load relief valve (or safety valve) on the cool-down time and temperature rising during an off-normal state are discussed. The total amount of cryogenic cooling loads and the required helium flow rate during cool-down are also estimated by a relevant heat transfer analysis. The temperatures of cryogen-free radiation shield are finally determined from the refrigeration power of a cryocooler and the total cryogenic loads.

• Transactions of the Korean hydrogen and new energy society
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• v.34 no.6
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• pp.641-649
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• 2023

This study analyzes the cool-down process of liquid hydrogen storage tanks, which have advantages in terms of large-capacity transfer, storage, and utilization as hydrogen demand increases. A hydrogen liquefaction plant is selected for analysis and an efficient tank cooling method is sought by comparing the time required for the cool-down process with the gas consumption in connection with the gassing-up process required for the operation of the liquid hydrogen storage tank. The results of this study can be referred to in the operation process after the initial start-up and maintenance of the hydrogen liquefaction plant.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.10 no.5
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• pp.600-609
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• 1998

The experimental investigation of thermal response characteristics by the air flow through the porous media has been carried out. The packed spheres of steel or glass were considered as the porous media in the present study. Temperature distributions of the fluid in the porous media as well as pressure drops through the porous media were measured. The transient temperature variations in the porous media are compared favorably with the analytical results in the high Reynolds number ranges. However, in the low Reynolds number ranges, the experimental data deviate from the analytical results, due to the dominant heat conduction penetration to the upstream direction, which is not considered in the analytical model. The cool-down response of porous media is found to be dependent upon the specific dimensionless time considering the material property and air velocity. The heat discharge process is recommended to be operated until a certain time, considering the cost efficiency.

• Nuclear Engineering and Technology
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• v.51 no.2
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• pp.402-409
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• 2019

When a loss of coolant accident (LOCA) occurs in a nuclear power plant, accident scenarios which can prevent core damage are defined based on break size. Current probabilistic safety assessment evaluates that core damage can be prevented under small-break LOCA (SBLOCA) and steam generator tube rupture (SGTR) with rapid cool down (RCD) strategy when all safety injection systems are unavailable. However, previous research has pointed out a limitation of RCD in terms of initiation time. Therefore, RCD success criteria estimation based on allowable coping time under a SBLOCA or SGTR when all safety injection systems are unavailable was performed based on time-line and thermal-hydraulic analyses. The time line analysis assumed a single emergency operating procedure flow, and the thermal hydraulic analysis utilized MARS-KS code with variables of break size, cooling rate, and operator allowable time. Results show while RCD is possible under SGTR, it is impossible under SBLOCA at the APR1400's current cooling rate limitation of 55 K/hr. A success criteria map for RCD under SBLOCA is suggested without cooling rate limitation.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.21 no.9
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• pp.505-512
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• 2009

A thermosiphon is utilized as a thermal shunt to reduce the cool-down time of a cryogenic system cooled by a two stage cryocooler. The cool-down time reduction by the thermosiphon is determined by the type of working fluid which is directly related to the operating temperature range of the thermosiphon. A mixed working fluid has a potential to widen the operation temperature range of the thermosipohon. In this study, the thermosiphon using N$_2$ and CF$_4$ mixture as the working fluid is fabricated and tested to verify its transient heat transfer performance. The thermosiphon with the mixed working fluid has no noticeable reduction of cool-down time compared with that of the thermosiphon with pure working fluid in this experiment. However, it seems that the thermosiphon with mixed working fluid may have an advantage according to the cooling capacity of a cryocooler, the cooling target temperature and the size of a cooling object.

• Proceedings of the Korea Institute of Applied Superconductivity and Cryogenics Conference
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• 2003.10a
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• pp.320-323
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• 2003

This paper presents the results of a series of performance tests for the Stilting cryocooler. A free piston and free displacer(FPFD) Stilting cryocooler for cooling infrared and cryo-sensors is currently under development at KIMM. Our coolers are specifically designed to work in the thermal imaging device and to meet requirements such as cooling capacity, COP and high reliability. In this work, Stilting cryocooler is designed, manufactured and fabricated, and performance characteristics for the cooling capacity / applied input power and cool down time are investigated. This cooler delivers approximately 0.9W cooling at 80K for 30W ~ 40W of input power. And, It takes approximately 2 minutes to cool down to 80K at the ambient temperature of 23$^{\circ}C$.

• Nuclear Engineering and Technology
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• v.47 no.6
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• pp.746-755
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• 2015

To investigate the effect of hydrogen and oxygen contents on hydride reorientations during cool-down processes, zirconium-niobium cladding tube specimens were hydrogen-charged before some specimens were oxidized, resulting in 250 ppm and 500 ppm hydrogen-charged specimens containing no oxide and an oxide thickness of $0.38{\mu}m$ at each surface. The nonoxidized and oxidized hydrogen-charged specimens were heated up to $400^{\circ}C$ and then cooled down to room temperature at cooling rates of $0.3^{\circ}C/min$ and $8.0^{\circ}C/min$ under a tensile hoop stress of 150 MPa. The lower hydrogen contents and the slower cooling rate generated a larger fraction of radial hydrides, a longer radial hydride length, and a lower ultimate tensile strength and plastic elongation. In addition, the oxidized specimens generated a smaller fraction of radial hydrides and a lower ultimate tensile strength and plastic elongation than the nonoxidized specimens. This may be due to: a solubility difference between room temperature and $400^{\circ}C$; an oxygen-induced increase in hydrogen solubility and radial hydride nucleation energy; high temperature residence time during the cool-down; or undissolved circumferential hydrides at $400^{\circ}C$.

• Proceedings of the KSME Conference
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• 2004.11a
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• pp.1329-1334
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• 2004

The most compact and convenient pulse tube cryocooler for practical applications is the coaxial type. It can replace Stirling cryocooler without any change to the Dewar or the connection to the cooled devices. The experimental results of the coaxial inertance tube pulse tube cryocooler for cooling superconductor RF filter are presented in this paper. To find optimal conditions of inertance tube pulse tube cryocooler, no load temperature according to the variations of inertance tube volume, reservoir volume are measured, and the cool down characteristics at the particular conditions are presented. In case of the coaxial type inertance tube pulse tube refrigerator, cool down time is the lowest in the inertance tube diameter of 1.3 mm and inertance tube length of 1900 mm and lowest temperature is 112K. This results are not satisfactory for practical applications. So, We propose vacuum insulation between regenerator and pulse tube in the Stirling type coaxial pulse tube cryocooler. Stirling type coaxial pulse tube cryocooler with the vacuum insulation between regenerator and pulse tube was designed and manufactured by KIMM(Korea Institute of Machinery and Materials). The optimal conditions will be tested for Stirling type coaxial pulse tube cryocooler with the vacuum insulation between regenerator and pulse tube.