• 한국정밀공학회지
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• 제34권3호
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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.

• 한국추진공학회:학술대회논문집
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• 한국추진공학회 2011년도 제37회 추계학술대회논문집
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• pp.709-716
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• 2011

추진제탱크 내의 극저온 추진제는 발사체의 비행 과정동안 주변으로부터 에너지를 흡수하여 온도가 상승한다. 비행 종료 시점에 있어 터보펌프 입구 요구조건 이상으로 온도가 상승된 추진제는 사용할 수 없는 잔류추진제로 남게 된다. 본 논문에서는 극저온 추진제 상층부의 온도변화를 살펴보기 위하여 추진제 표면 근처에서의 열전달계수를 구해보고자 하였다. 추진제 상층부의 열전달을 전도로 단순화하여 열전달계수를 예측하는 방법을 제시하였다. 이를 통해 얻어진 추진제 상층부의 온도를 시험데이터와 비교하여 열전달계수 예측 방법의 적용 가능성을 확인하였다.

• 한국해양공학회지
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• 제27권1호
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• pp.74-79
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• 2013

In the context of the structural performance of an LNG hold, the mechanical characteristics of the insulation material are considered to be a critical design factor under cryogenic temperatures. This paper presents the thermal elasto-plastic behavior of the reinforced polyurethane foam (RPUF) adapted for the insulation material of a membrane-type LNG carrier via both experiments and numerical simulations realizing the cryogenic condition. The experiments are carried out to investigate the thermal transfer and thermal elasto-plastic deformation characteristics of an actual RPUF specimen. The heat transfer simulations based on the finite element method (FEM) include a forced convection analysis. The results of heat transfer analyses are compared with the experimental results. Reasonable cryogenic conditions for RPUF are reviewed based on both the analysis and experimental results.

• 한국초전도ㆍ저온공학회논문지
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• 제15권4호
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• pp.10-14
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• 2013

We have clearly investigated with respect to the survival of the insulator at cryogenic temperature under the electrical stress. The breakdown and voltage-time characteristics of turn-to-turn models for point contact geometry and surface contact geometry using copper multi wrapped with polyimide film for an HTS transformer were investigated under AC and impulse voltage at 77 K. Polyimide film (Kapton) 0.025 mm thick is used for multi wrapping of the electrode. As expected, the breakdown voltages for the surface contact geometry are lower than that of the point contact geometry, because the contact area of the surface contact geometry is lager than that of the point contact geometry. The time to breakdown t50 decreases as the applied voltage is increased, and the lifetime indices increase slightly as the number of layers is increased. The electric field amplitude at the position where breakdown occurs is about 80 % of the maximum electric field value. The relationship between survival probability and the electrical stress at cryogenic temperature was evident.

• 한국유체기계학회 논문집
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• 제11권2호
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• pp.13-19
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• 2008

Butterfly valves are widely used as control valves for industrial process. For the definition of optimum configuration of the valve, wide range of related studies has been actively conducted in the case of working fluids of water or air under the normal temperature. Recently, internal flow and performance characteristics of cryogenic butterfly valve for LNG carrier take a growing interest in the field of research and development. Therefore, present study is aimed to investigate the internal flow and performance characteristics of the cryogenic butterfly valve because the study result for the valve can be hardly found at present. Part 1 of this paper describes the study result of a butterfly valve model under the condition of the normal temperature. Succeeding Part 2 of this paper will describe the internal flow characteristics of a cryogenic butterfly valve for LNG carrier. The results of Part 1 show that pressure loss coefficients and flow rate coefficients obtained by the present experiment and CFD analysis agree well each other. Moreover, internal flow visualization for the valve by CFD analysis and PIV measurement have revealed complicated flow patterns of the internal flow field in detail.

• 한국추진공학회:학술대회논문집
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• 한국추진공학회 2010년도 제34회 춘계학술대회논문집
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• pp.61-65
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• 2010

초저온 추진제를 사용하는 액체로켓용 인젝터의 개발을 위해서는 단일 인젝터의 수류/연소시험을 통해 인젝터의 수력학적 특성 및 혼합, 미립화 등의 분무특성에 대한 연구가 필수적이다. 본 연구에서는 대학실험실 급에서 운용할 수 있는 범위의 가압식 초저온 추진제 공급장치 및 LabVIEW를 이용한 Cyclogram 제어 및 데이터 수집장치 등을 구축하였다. 제작된 시험장치를 이용하여 스월-동축인젝터의 $LN_2-GN_2$ 초저온 수류시험 및 분무 가시화를 수행하였다. 또한 LOX-$GCH_4$ 추진제 조합의 연소시험을 위한 연소기와 추력대를 설계 및 제작하였다.

• KIEE International Transactions on Electrophysics and Applications
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• 제4C권1호
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• pp.15-20
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• 2004

This paper discusses electrical insulation characteristics at cryogenic temperature, especially focusing on partial discharge (PD) inception characteristics, for high temperature superconducting cables. In liquid nitrogen (L$N_2$) / polypropylene (PP) laminated paper composite insulation system, PD inception strength (PDIE) was evaluated in terms of volume effect and V-t characteristics. Different kinds of butt gap condition were applied in the experiments, using parallel plane and coaxial cylindrical cable samples. Experimental results revealed that the volume effect on PDIE could be evaluated by the statistical stressed liquid volume (SSLV) taking account the discharge probability not only in the butt gap but also in the other thin layers between PP laminated papers. Furthermore, the indices n of V-t characteristics at PD inception were estimated to be 80∼110, irrespective of the butt gap condition.

• 한국초전도ㆍ저온공학회논문지
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• 제21권3호
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• pp.1-5
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• 2019

The superconductor stability theories are consistently described by the integral formula. If the defined stability function is a simple decreasing function, it becomes a cryogenic stability condition. If the stability function has a maximum value and a minimum value, and the maximum value is less than 0, then it is a cold-end recovery condition. If the maximum value is more than 0, it can be shown that the unstable equilibrium temperature, that is, the MPZ (minimum propagation zone) temperature distribution can exist. The MPZ region is divided into two regions according to the current ratio. At the low current ratio, the maximum dimensionless temperature is greater than 1, and at the relatively high current ratio, the maximum dimensionless temperature is less than 1. In order to predict the minimum quench energy, the dimensionless energy was obtained for the MPZ temperature distribution. In particular, it was shown that the dimensionless energy can be obtained even when the MPZ maximum temperature is 1 or more.

• 한국추진공학회:학술대회논문집
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• 한국추진공학회 2008년도 제31회 추계학술대회논문집
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• pp.153-156
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• 2008

고공 환경에서의 점화 연소 특성을 확인하기 위해 선행되어야 할 저압/저온 환경 모사 및 연료 유량 제어 실험을 수행하였다. 저압 환경을 모사하기 위하여 초음속 디퓨저를 이용하였고, 공기 유량 공급 및 디퓨저 1차 전단 노즐 압력에 따라 다양한 고도의 저압 환경을 조성할 수 있음을 확인하였다. 또한 저온 환경을 모사하기 위해 액체 질소를 이용한 열교환기를 활용하였고, 혼합 탱크로 유입되는 극저온/상온 공기 온도 조건을 일정하게 유지할 경우 다양한 공기 유량 조건에서 혼합 공기의 온도는 극저온/상온 공기의 혼합비에 의해 결정됨을 알 수 있었다. 이에 따라 본 연구에서 구축한 고고도 환경 모사 시스템을 활용하여 다양한 고도 조건에서의 점화 및 연소 특성 실험 수행이 가능함을 입증하였다.

• 대한기계학회논문집B
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• 제26권8호
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• pp.1095-1101
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• 2002

An experimental apparatus was prepared to investigate thermal and hydrodynamic characteristics of regenerator at cryogenic temperature under pulsating pressure condition. The regenerator was pressurized and depressurized by a compressor with various operating frequencies. Cold end of the regenerator was maintained around 100 K by means of a liquid nitrogen heat exchanger. Instantaneous gas temperature and mass flow rate were measured at both ends of the regenerator during the whole pressure cycle. Pulsating pressure drop across the regenerator was also measured to see if it could be predicted by a friction factor at steady flow condition. The operating frequency of pressure cycle was varied between 3 and 60 Hz, which are typical operating frequencies of Gifford-McMahon, pulse tube, and Stilting cryocoolers. First, the measured friction factor for typical wire screen mesh regenerator was nearly same as steady flow friction factor for maximum oscillating Reynolds number up to 100 at less than 9 Hz. For 60 Hz operations, however, the discrepancy between oscillating flow friction factor and steady flow one was noticeable if Reynolds number was higher than 50. Second, the ineffectiveness of regenerator was directly calculated from experimental data when the cold-end was maintained around 100 K and the warm-end around 293 K, which simulates an actual operating condition of cryogenic regenerator. Influence of the operating frequency on ineffectiveness was discussed at low frequency range.