• 한국초전도ㆍ저온공학회논문지
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• 제19권4호
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• pp.40-44
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• 2017

This paper analyzed the fault current limiting characteristics of the previously proposed transformer superconducting fault current limiter (TSFCL) interruption system according to its transformer type. The TSFCL interruption system is an interruption technology that combines a TSFCL, which uses a transformer and a superconductor, and a mechanical DC circuit breaker. This technology first limits the fault current using the inductance of the transformer winding and the quench characteristics of the superconductor. The limited fault current is then interrupted by a mechanical DC circuit breaker. The magnitude of the limited fault current can be controlled by the quench resistance of the superconductor in the TSFCL and the turns ratio of the transformer. When the fault current is controlled using a superconductor, additional costs are incurred due to the cooling vessel and the length of the superconductor. When the fault current is controlled using step-up and step-down transformers, however, it is possible to control the fault current more economically than using the superconductor. The TSFCL interruption system was designed using PSCAD/EMTDC-based analysis software, and the fault current limiting characteristics according to the type of the transformer were analyzed. The turns ratios of the step-up and step-down transformers were set to 1:2 and 2:1. The results were compared with those of a transformer with a 1:1 turns ratio.

• Progress in Superconductivity
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• 제4권1호
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• pp.90-93
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• 2002

The sintering process of BSCCO 2223 tapes is a complex process that is very sensitive to parameters, such as temperature, oxygen partial pressure, heating and cooling rate and holding time. During the first heat treatment, 2212 phase of precursor powder is partially transformed into 2223 phase and some residual secondary phases, such as $(Bi,Pb)_2$$Sr_2$CuO／sub y／(2201), $(Ca,Sr)_2$CuO／sub y／(2/1AEC), (Ca,Sr)／sub 14／Cu／sub 24／O／sub 41／(14／24 AEC) etc. The secondary phases are difficult to be removed from the BSCCO 2223 matrix on the heat treatment. These secondary phases degrade the critical current density. In order to minimize the amount and size of alkaline earth cuprate(AEC) particles step heat treatment is applied during the first heat treatment under the varying atmosphere. Experimental results showed that by adapting the step heat treatment process, the amount and particle size of the secondary phases in the final tapes are decreased. Consequently, the BSCCO 2223grain texture and Jc properties are improved.

• 설비공학논문집
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• 제14권11호
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• pp.914-922
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• 2002

Thermo-acoustic waves can be thermally generated in a compressible flow field by rapid heating and cooling, and chemical reaction near the boundary walls. This mechanism is very important in the space environment in which natural convection does not exist. Also this may be a significant factor for heat transfer when the fluids are close to the thermodynamic critical point. In this study, the generation and transmission characteristics of thermo-acoustic waves in an air-filled confined domain with two-step pulsed heating are studied numerically. The governing equations are discretized using control volume method, and are solved using PISO algorithm and second-order upwind scheme. For the purpose of stable solution, time step was set to the order of $1\times10_-9s,\;and\;grids\;are\;50\times2000$. Results show that temperature and pressure distributions of fluid near the boundary wall subjected to a rapid heating are increased abruptly, and the induced thermo-acoustic wave propagates through the fluid until it decays due to viscous and heat dissipation. Pressure waves have sharp front shape and decay with a long tail in the case of step heating, but these waves have sharp pin shape in the case of pulsed heating.

• 대한금속재료학회지
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• 제49권1호
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• pp.9-16
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• 2011

To develop a 6061 aluminum alloy with low residual stress and high tensile strength, a cryogenic treatment process was investigated. Compared to the conventional heat treatment process for precipitation hardening with artificial aging, the cryogenic treatment process has two additional steps. The first step is cryogenic quenching of the sample into liquid nitrogen, the second step is up-hill quenching of the sample into boiling water. The residual stress for the sample was measured by the $sin^2{\psi}$ method with X-ray diffraction. The 6061 aluminum alloy sample showed 67% relief in stress at the cryogenic treatment process with artificial aging at $175^{\circ}C$. From this study, it was found that the optimum cryogenic treatment process for a sample with low residual stress and high tensile strength is relatively low cooling speed in the cryogenic quenching step and a very high heating speed in the up-hill quenching step.

• 한국정밀공학회지
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• 제22권1호
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• pp.97-102
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• 2005

The procedure of semi-solid forming is composed of heating a billet, forming, compression holding and ejecting step. There are several methods to heat a billet during semi-solid forming process such as electric heating and induction heating. Usually in semi-solid forming process, induction heating has been adopted to achieve more uniform temperature of semi-solid material. Although induction heating is better method than any others, however, there is still difference of temperature between internal part and surface part of semi-solid material. Worse yet, in case of high liquid fraction of semi-solid material, liquid of the billet will flow down though solid of the billet still remains, which is very difficult to handle. In the present study, induction heating of semi-solid material with compulsive surface cooling has been performed to obtain uniform distribution of temperature. Distribution of temperature of the billets was measured and compared with that of conventional distribution of temperature. By this new induction heating method, not only temperature over the whole billet become uniform, but also control of temperature is possible.

• 연구논문집
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• 통권29호
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• pp.5-15
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• 1999

Present study deals with performance analysis of an inert gas generator (IGG) which is to be used as an effective mean to suppress the fire. The IGG uses a turbo jet cycle gas turbine engine to generate inert gas for fire extinguishing. It is generally known that a lesser degree of oxygen content in the product of combustion will increase the effectiveness of fire suppressing. An inert gas generator system with water injection will bring advantages of suffocating and cooling effects which are considered as vital factors for fire extinguishing. As the inert gas is injected to the burning site, it lowers the oxygen content of the air surrounding the flame as well as reduces the temperature around the fire as the vapour in the inert gas evaporates during the time of spreading. Some important aspects of influencing parameters, such as, air excess coefficient. $\alpha$, compressor pressure ratio, $ pi_c$, air temperature before combustion chamber, $T_2$, gas temperature after combustion chamber, $T_3$, mass flow rate of water injection, $M_w$, etc., on the performance of IGG system are investigated. Calculations of total amount of water needed to reduce the turbine exit temperature to pre-set nozzle exit temperature employing a heat exchanger were made to compare the economics of the system. A heat exchanger with two step cooling by water and steam is considered to be better than water cooling only. Computer programs were developed to perform the cycle analysis of the IGG system and heat exchanger considered in the present study.

• 열처리공학회지
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• 제21권6호
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• pp.281-286
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• 2008

In the 1.1 Mn steel containing boron, effects of the 0.1 V addition and processing condition were studied. In the $550^{\circ}C$ interrupted cooling where the main structure is (ferrite + pearlite), the impact toughness decreased as the tensile strength increased by the 0.1 V addition. The $800^{\circ}C$ rolling including two step rolling of $800-770^{\circ}C$, exhibited better strength-toughness balance, as compared to the $770^{\circ}C$ rolling. This seems to be kind of conditioning effect at higher temperature, e.g., more uniform deformation effect. In the accelerated cooling after the $750^{\circ}C$ rolling in a dual phase range, the impact toughness was enhanced, despite a large increase in tensile strength. This is believed to be related to the change of main structure from (ferrite + pearlite) to (ferrite + bainite).

• 한국초전도ㆍ저온공학회논문지
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• 제13권1호
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• pp.36-40
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• 2011

This paper deals with the design on iron-core shape for superconducting hybrid electromagnet (SH-EM). A proto-type SH-EM was developed by Yonsei University. It is the first step study on the fundamental technology for advanced railroad system development. We proposed the method that reduce the number of cooling systems in order to improve operating efficiency of the SH-EM. The results was obtained by changing of iron-core shape were verified by the FEM analysis. This paper can be expected to suggest useful data for an advanced SH-EM.

• 한국주거학회논문집
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• 제14권3호
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• pp.119-126
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• 2003

The purpose of this study is to evaluate the natural ventilation performance for variable external wind speed as a preliminary step to determining the seasonal operating modes of the Double-skin Facade System applied to apartment buildings. For this purpose, two simulation programs are used to compare the Double-Skin Facade System with the Double Sash Window. First, TAS is used to plan a schedule for natural ventilation during the intermediate season and to analyze the cooling loads. Second, CFD is used for a more detailed airflow analysis on a typical floor plan of the model building. The results of the simulations on natural ventilation performance show that the Double-Skin Facade System can reduce the cooling load by 10.5% compared to the Double Sash Window.

• 한국항공우주학회지
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• 제30권8호
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• pp.112-119
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• 2002

주기적으로 배열된 액체 로켓엔진의 연료 분사기 배열에 대한 3차원 열전달 수치해석을 수행하였다. 해석에는 SIMPLE 알고리즘을 사용한 유한체적법이 적용되었으며 2차원 후향 계단에 대해서 기존의 문헌과 일치하는 열전달 결과를 주었다. 분사기 요소 사이의 거리가 가까울수록 누셀 수와 압력강하가 증가하였다. 레이놀즈 수가 증가할 경우, 누셀 수는 증가 하였으며 무차원 압력강하는 약간 감소하였다.