• 설비공학논문집
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• 제21권9호
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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.

• 한국추진공학회지
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• 제23권2호
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• pp.27-37
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• 2019

쉘-튜브 열교환기가 나로우주센터 추진기관종합시험장(PSTC)에 설치되었으며, 이 열교환기는 극저온의 헬륨을 고온의 열매유와 열교환하여 약 500 K 까지 가열시키는 역할을 한다. 열교환기에서 토출되는 헬륨의 온도가 설계보다 100 K 낮게 나옴에 따라, 성능저하의 원인으로 열매유의 격막효과가 지목되었다. CFD 해석을 통해 격막효과의 유무를 확인하였으며, 격막효과에 의한 열교환기 성능저하는 미미한 것으로 판단되었다. 추가적으로 열교환기의 성능을 증가시키기 위하여 열매유 교체에 따른 열교환기 성능 변화를 알아보았다. 열매유를 사용하는 열교환기의 성능향상을 위해서는 500 K 부근에서 점성이 낮아야 하고, 열전도도가 높아야 한다는 것을 확인할 수 있었다. 추진기관종합시험장에서 운용된 극저온 헬륨과 고온 열매유의 열교환 시스템의 시험 결과를 본 논문에서 확인할 수 있다.

• 한국추진공학회지
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• 제24권3호
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• pp.59-70
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• 2020

본 논문에서는 기존 적층형 배관의 총 열전달 계수 경험식을 활용할 때 발생하는 한계점을 해결하고자, 외부 총 열전달 계수의 강제 대류 열전달 계수 항을 독립적으로 도출하는 간소화된 모델링을 제안하고, 이를 극저온 환경의 실험 결과로 확인하였다. 액체 산소 냉각 나선형 열교환기가 액체 질소와 열교환하는 실험 장치를 구성하고 열교환기의 열전달량을 계측하여, 외부 총 열전달 계수를 도출하였다. 측정된 외부 총 열전달 계수가 모델링으로 예측 곡선과 일치함을 확인하였다.

• 한국초전도ㆍ저온공학회논문지
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• 제9권3호
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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.

• 대한조선학회논문집
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• 제55권6호
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• pp.514-520
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• 2018

Polyurethane foam has excellent mechanical strength and insulation performance, and has been adopted as an insulation material for $-163^{\circ}C$ liquefied natural gas carrier. In this study, Kevlar Polyurethane Foams(K-PUF) were synthesized by adding Kevlar pulp with excellent mechanical strength for the purpose of improving the performance of existing polyurethane foam. Since polyurethane foam has mechanical performance depending on the proportions of Kevlar pulp added, the mechanical strength of the K-PUF with ratios of fiber0.2wt.%, 0.4wt.%, 0.6wt.%, 0.8wt.% and 1.0wt.%) was evaluated. The compression tests were performed on the 4 different temperatures($20^{\circ}C$, $-50^{\circ}C$, $-110^{\circ}C$ and $-163^{\circ}C$) in consideration of the environmental characteristics as a cryogenic insulation used in LNG carrier. Besides, the effects of the fiber addition on polyurethane foam with closed cell structure were evaluated in a phenomenological approach through SEM analysis. All the results were compared to Neat-polyurethane foam. As a results, 0.8wt.% K-PUF showed the improved mechanical strength, and the addition of Kevlar pulp in a certain ratio improves the mechanical performance by enhancing the compression resistance.

• 대한조선학회논문집
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• 제58권4호
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• pp.262-270
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• 2021

Consumption of Liquefied Natural Gas (LNG) has increased due to environmental pollution; therefore, the need for LNG carriers can efficiently transport large quantities of LNG, is increased. In various types of LNG Cargo Containment System (CCS), Membrane-type MARK-III composed of composite materials is generally employed in the construction of an LNG carrier. Among composite materials in a Mark-III system, glass-fiber composites act as a secondary barrier to prevent the inner hull structure from leakage of LNG when the primary barrier is damaged. Nevertheless, several cases of damage to the secondary barriers have been reported and if damage occurs, LNG can flow into the inner hull structure, causing a brittle fracture. To prevent those problems, this study conducted the applicability assessment of composite material manufactured by bonding glass-fiber and aluminum with epoxy resin and increasing layer from three-ply (triplex) to five-ply (pentaplex). Tensile tests were performed in five temperature points (25, -20, -70, -120, and -170℃) considering temperature gradient in CCS. Scanning Electron Microscopy (SEM) and Coefficient of Thermal Expansion (CTE) analyses were carried out to evaluate the microstructure and thermos-mechanical properties of the pentaplex. The results showed epoxy resin and increasing layer number contributed to improving the mechanical properties over the whole temperature range.

• 한국초전도ㆍ저온공학회논문지
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• 제11권1호
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• pp.16-19
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• 2009

Recently, for improvement of the magnetic field of high temperature superconductor (HTS) apparatus, many studies investigating on operating in the range of $20{\sim}65K$ with liquid helium or the conducting method using cryocooler is actively reviewed. Also, the cooling method using solid nitrogen as cryogen is being suggested. Since the nitrogen has very large specific heat in solid state, it is expected that it can enable long time operation without a continuous supply of cooling energy. However, there is still insufficient data on the characteristics of solid nitrogen such as thermodynamic properties and liquid-solid phase change. Especially, there was almost no study done on the electrical insulation properties of solid nitrogen so far. In this study, solid nitrogen to find the electrical characteristics was made by using cryocooler and cryostat, and investigated the flashover discharge and breakdown. The results of this study will be useful as a basic data for electrical insulation design of the HTS system using solid nitrogen as cryogen.

• 소성∙가공
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• 제32권2호
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• pp.74-80
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• 2023

A deformation behavior of commercially pure titanium (CP-Ti) is highly dependent on material and processing parameters, such as deformation temperature, deformation direction, and strain rate. This study aims to predict the multivariable and nonlinear tensile behavior of CP-Ti using machine learning based on three algorithms: artificial neural network (ANN), light gradient boosting machine (LGBM), and long short-term memory (LSTM). The predictivity for tensile behaviors at the cryogenic temperature was lower than those in the room temperature due to the larger data scattering in the train dataset used in the machine learning. Although LGBM showed the lowest value of root mean squared error, it was not the best strategy owing to the overfitting and step-function morphology different from the actual data. LSTM performed the best as it effectively learned the continuous characteristics of a flow curve as well as it spent the reduced time for machine learning, even without sufficient database and hyperparameter tuning.

• 대한조선학회논문집
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• 제59권4호
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• pp.207-213
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• 2022

LNG CCS which is a special type of cargo hold operated at -163℃ for transporting liquefied LNG is composed of a primary barrier, plywood, insulation panel, secondary barrier, and mastic. Currently, glass fiber is used to reinforce polyurethane foam. In this paper, we evaluated the possibility of replacing glass fiber-reinforced polyurethane foam with basalt fiber-reinforced polyurethane foam. We conducted a thermal conductivity test to confirm thermal performance at room temperature. To evaluate the mechanical properties between basalt and glass-fiber-reinforced polyurethane foam which is fiber content of 5 wt% and 10 wt%, tensile and an impact test was performed repeatedly. All of the tests were performed at room temperature and cryogenic temperature(-163℃) in consideration of the temperature gradient in the LNG CCS. As a result of the thermal conductivity test, the insulating performance of glass fiber reinforced polyurethane foam and basalt fiber reinforced polyurethane foam presented similar results. The tensile test results represent that the strength of basalt fiber-reinforced polyurethane foam is superior to glass fiber at room temperature, and there is a clear difference. However, the strength is similar to each other at cryogenic temperatures. In the impact test, the strength of PUR-B5 is the highest, but in common, the strength decreases as the weight ratio of the two fibers increases. In conclusion, basalt fiber-reinforced polyurethane foam has sufficient potential to replace glass fiber-reinforced polyurethane foam.

• 대한조선학회논문집
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• 제56권4호
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• pp.291-297
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• 2019

Polyurethane foam is the most efficient, high-performance insulation material, used for liquefied natural gas carrier (LNGC) insulation. Because LNGC is exposed to sloshing impact load due to ship motion of 6 degrees of freedom, polyurethane foam should be sufficient dynamic properties. The dynamic properties of these polyurethane foam depends on temperature and density. Therefore, this study investigates the dynamic response of polyurethane foam for various temperature($25^{\circ}C$, $-70^{\circ}C$, $-163^{\circ}C$) and density($90kg/m^3$, $113kg/m^3$, $134kg/m^3$, $150kg/m^3$) under drop impact test with impact energy of 20J, 50J, and 80J. For dynamic response was evaluated in terms of peak force, peak displacement, absorb energy, and the mechanical property with minimized density effects. The results show the effect of temperature and density on the polyurethane foam material for the dynamic response.