• Progress in Superconductivity and Cryogenics
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• v.9 no.3
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• pp.72-82
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• 2007

This paper relates to reducing the temperature of a cryogenic liquid by contacting it with gas bubbles, which can be characterized by diffusion-driven evaporative cooling, The characteristic of diffusion-driven evaporative cooling is thoroughly examined by theoretical. analytical and experimental methods specifically for the case of helium injection into liquid oxygen. The results reveal that if the gaseous oxygen partial pressure in helium bubbles is lower than the liquid oxygen vapor pressure, cooling occurs autonomously due to diffusion mass transfer. The method of lowering the injected helium temperature turns out to be very effective for cooling purpose.

• Journal of the Korean Society for Precision Engineering
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• v.34 no.3
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• pp.185-189
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• 2017

Recently, titanium alloys have been widely used in aerospace, biomedical engineering, and military industries due to their high strength to weight ratio and corrosion resistance. However, it is well known that titanium alloys are difficult-to-cut materials because of a poor machinability characteristic caused by low thermal conductivity, chemical reactivity with all tool materials at high temperature, and high hardness. To improve the machinability of titanium alloys, cryogenic cooling with LN2 (Liquid Nitrogen) and nanofluid MQL (Minimum Quantity Lubrication) technologies have been studied while turning a Ti-6Al-4V alloy. For the analysis of turning process characteristics, the cutting force, the coefficient of friction, and the surface roughness are measured and analyzed according to varying lubrication and cooling conditions. The experimental results show that combined cryogenic cooling and nanofluid MQL significantly reduces the cutting forces, coefficients of friction and surface roughness when compared to wet condition during the turning process of Ti-6Al-4V.

• Proceedings of the Korean Society of Precision Engineering Conference
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• 2001.04a
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• pp.1048-1051
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• 2001

This paper presents an analytical and experimental study of a cryogenic machining for precision hard turning. A cryogenic circulation system is designed and mounted on the top of the tool insert. The machining process used is facing operation on a CNC turning center with dry and cryogenic conditions. The tool temperature and cutting forces are measured by the K-type thermocouple and by a three-component Kistler dynamometer, respectively. Both data are fed into the data acquisition program through an A/D card. Surface roughness and form accuracy of the machined surface are measured by WYKO NT2000. It is also found that surface roughness and form accuracy with cryogenic cooling are better than those with no coolant.

• Journal of the Korean Society for Precision Engineering
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• v.34 no.4
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• pp.237-241
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• 2017

Cryogenic machining uses liquid nitrogen (LN2) as a coolant. This machining process can reduce the cutting temperature and increase tool life. Titanium alloys have been widely used in the aerospace and automobile industries because of their high strength-to-weight ratio. However, they are difficult to machine because of their poor thermal properties, which reduce tool life. In this study, we applied cryogenic machining to titanium alloys. Orthogonal cutting experiments were performed at a low cutting speed (1.2 - 2.1 m/min) in three cooling conditions: dry, cryogenic, and cryogenic plus heat. Cutting force and friction coefficients were observed to evaluate the machining characteristics for each cooling condition. For the cryogenic condition, cutting force and friction coefficients increased, but decreased for the cryogenic plus heat condition.

• 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.

• Proceedings of the Korea Institute of Applied Superconductivity and Cryogenics Conference
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• 2003.10a
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• pp.234-236
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• 2003

The cryogenic system for inductive superconducting fault current limiter (SFCL) has been investigated recently. In this investigation, the sub-cooled nitrogen cryogenic system was adopted to enhance the performance of DC reactor for 6.6㎸/200A inductive SFCL. In sub-cooled nitrogen state at 64K, the critical current value and the thermal conductivity are larger than those of saturated nitrogen state at 77K and the electrical insulation capacitance should be remarkably enhanced. The solenoid type of 84mH superconducting DC reactor was fabricated and cooled down to 64K by using sub-cooled cooling method with GM-cryocooler and rotary pump. The fabrication techniques of cryogenic system and some experimental results such as cooling down characteristic are introduced in this study. Moreover, the sub-cooled nitrogen cryogenic system was detailedly introduced in this paper.

• Proceedings of the KSME Conference
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• 2007.05b
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• pp.1959-1963
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• 2007

SMES systems need cryogenic cooling systems. Conduction cooling system has more effective, compact structure than cryogen. In general, 2 stage GM cryocoolers are used for conduction cooling of HTS SMES system. 1st stages of cryocoolers are used for the cooling of current leads and radiation shields, and 2nd stages of cryocoolers for 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 cooling system. The analysis is carried out for the 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.

• Journal of computational fluids engineering
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• v.20 no.3
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• pp.8-14
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• 2015

More and more, spaces are decreasing which satisfy multiple requirements for wind power plants. However, areas which have excellent wind resources and are free to civil complaints occupy a large space, although they are exposed to the cryogenic environment. This study conducted a thermal-fluid analysis of a cryogenic chamber for testing large wind turbine parts exposed to the cryogenic environment. The position of supply air is placed to the upper area to compare each cooling performance for each location of various outlets in mixing ventilated conditions. The study carried out CFD analysis for the chamber both with and without a test object. For the cases without the test object, the air temperature of the upper supply and down extract type chamber was cooled faster by 5-100% than the others. However, for the cases with the test object, the object temperature of upper supply and center extract on the opposite side type chamber was cooled faster by 33-132% than the others. The cooling performance by the air inside the chamber and the test object did not show the same pattern, which implicates the need to consider the cooling performance by not only the air but also the test object in the large cryogenic chamber design for testing large parts.

• Progress in Superconductivity and Cryogenics
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• v.22 no.3
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• pp.20-24
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• 2020

The purpose of this analytic study is to design and examine an efficient hydrogen liquefaction cycle by using a pre-cooler. The liquefaction cycle is primarily comprised of a pre-cooler and a refrigerator. The fed hydrogen gas is cooled down from ambient temperature (300 K) to the pre-cooling coolant temperature (either 77 K or 120 K approximately) through the pre-cooler. There are two pre-cooling methods: a single pre-coolant pre-cooler and a cascade pre-cooler which uses two levels of pre-coolants. After heat exchanging with the pre-cooler, the hydrogen gas is further cooled and finally liquefied through the refrigerator. The working fluids of the potential pre-cooling cycle are selected as liquid nitrogen and liquefied natural gas. A commercial software Aspen HYSYS is utilized to perform the numerical simulation of the proposed liquefaction cycle. Efficiency is compared with respect to the various conditions of the heat exchanging part of the pre-cooler. The analysis results show that the cascade method is more efficient, and the heat exchanging part of the pre-coolers should have specific UA ratios to maximize both spatial and energy efficiencies. This paper presents the quantitative performance of the pre-cooler in the hydrogen liquefaction cycle in detail, which shall be useful for designing an energy-efficient liquefaction system.

• Progress in Superconductivity and Cryogenics
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• v.17 no.3
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• pp.57-61
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• 2015

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.