• 한국추진공학회지
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• 제19권4호
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• pp.1-7
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• 2015

캐비테이션 벤츄리는 후단 압력에 상관없이 액체의 유량을 일정하게 유지시켜주는 장치로, 일정한 추진제 유량의 공급을 필요로 하는 액체로켓엔진 시스템에 성공적으로 사용되고 있다. 본 연구에서는 축소되는 유입각과 팽창하는 출구각 만이 다른 네 개의 캐비테이션 벤츄리를 설계, 제작하였다. 압력을변경시켜가며 벤츄리를 통과하는 유량과 전/후단의 압력을 측정하였다. 실험결과로부터 각 벤츄리에 대한 유량계수와 임계 압력비를 계산하였다. 캐비테이션 벤츄리의 입구각과 출구각은 유량계수에 영향을 주었으며, 출구각은 임계 압력비에도 영향을 주는 것을 확인하였다.

• Structural Monitoring and Maintenance
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• 제8권4호
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• pp.345-360
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• 2021

This paper presents an analytical approach to calculate the buckling load of the cylindrical ring structures subjected to both hydrostatic and hydrodynamic pressures. Based on the conservative law of energy and Timoshenko beam theory, a theoretical formula, which can be used to evaluate the critical pressure of buckling, is first derived for the simplified cylindrical ring structures. It is assumed that the hydrodynamic pressure can be treated as an equivalent hydrostatic pressure as a cosine function along the perimeter while the thickness ratio is limited to 0.2. Note that this paper limits the deformed shape of the cylindrical ring structures to an elliptical shape. The proposed analytical solutions are then compared with the numerical simulations. The critical pressure is evaluated in this study considering two possible failure modes: ultimate failure and buckling failure. The results show that the proposed analytical solutions can correctly predict the critical pressure for both failure modes. However, it is not recommended to be used when the hydrostatic pressure is low or medium (less than 80% of the critical pressure) as the analytical solutions underestimate the critical pressure especially when the ultimate failure mode occurs. This implies that the proposed solutions can still be used properly when the subsea vehicles are located in the deep parts of the ocean where the hydrostatic pressure is high. The finding will further help improve the geometric design of subsea vehicles against both hydrostatic and hydrodynamic pressures to enhance its strength and stability when it moves underwater. It will also help to control the speed of the subsea vehicles especially they move close to the sea bottom to prevent a catastrophic failure.

• 드라이브 ㆍ 컨트롤
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• 제10권4호
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• pp.29-33
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• 2013

Pneumatic system is widely applied in various industry because it have a many advantage(low cost, high safety, etc..). For design of pneumatic system, accurate flow measurement is required. In this study, compressible fluid flow measurement apparatus was designed and built. It uses an isothermal chamber that can approximate isothermal condition. Therefore, it can be measured for flow-rate using pressure response of isothermal chamber. As a result, this apparatus can be measured for sonic conductance and critical pressure ratio of pneumatic components and it required less time and energy than conventional flow meter. The effectiveness of the designed apparatus is proved by experimental result.

• Nuclear Engineering and Technology
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• 제34권4호
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• pp.382-395
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• 2002

An experimental study on transient critical heat flux (CHF) under flow coastdown has been performed for the water flow in a non-uniformly heated vertical annulus under low flow and a wide range of pressure conditions. The objectives of this study are to systematically investigate the effect of the flow transient on the CHF and to compare the transient CHF with steady-state CHF The transient CHF experiments have been performed for three kinds of flow transient modes based on the coastdown data of a nuclear power plant reactor coolant pump. At the same inlet subcooling, system pressure and heat flux, the effect of the initial mass flux on the critical mass flux can be negligible. However, the effect of the initial mass flux on the time-to- CHF becomes large as the heat flux decreases. The critical mass flux has the largest value for slow flow reduction rate. There is a pressure effect on the ratio of the transient CHF data to steady-state CHF data. Except under low system pressure conditions, the flow transient CHF was revealed to be conservative compared with the steady-state CHF data. Bowling CHF correlation and thermal hydraulic system code MARS show promising results for the prediction of CHF occurrence .

• 대한기계학회논문집
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• 제17권2호
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• pp.371-379
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• 1993

The wrinkling in the electromagnetic tube compression with a mandrel is remarkably smaller than that of the process without it. To analyze this phenomenon, the critical forming parameters such as the ratio of the clearance to the shell radius, the ratio of the thickness to the shell radius, and the ratio of the applied pressure to the standard pressure are introduced tp consider the effect of the mandrel, in addition to those of the thickness of shell and applied loads. The amplification ratio is also used to observe the magnitude of amplification. The results obtained by 2-D finite element method show that the initial imperfection embedded in the radius of cylindrical shell is the dominant factor to determine the final shape of the tube compression, and that the amplification ratio tends to have smaller values with the smaller clearance ratio and also with the larger thickness and pressure ratios.

• 소성∙가공
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• 제7권1호
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• pp.66-71
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• 1998

The present study is concerned with the hydrostatic extrusion process of copper-clad aluminium bar to investigate the bonding conditions as well as the basic flow characteristics. Considering the bonding mechanism of bi-metal contact surface as cold pressure welding the normal pressure and the contact surface expansion are selected as process parameters governing the bonding conditions, in this study the critical normal pressure required for the local extrusion-the protrusion of virgin surfaces by the surface expansion at the interface-is obtained using a slip line method and is then used as a criteron for the bonding. A rigid plastic finite element method is used to analyze the steady state extrusion process. The interface profile of bi-metal rod is predicted by tracking the paths of two particles adja-process. The interface profile of bi-metal rod is predicted by tracking the paths of two particles adja-cent to interface surface. The contact surface area ration and the normal pressure along the interface are calculated and compared to the critical normal pressure to check bonding. It is found that the model predictions are generally in good agreement with the experimental observations. The compar-isons of the extrusion pressure and interface profile by the finite element with those by experi-ments are also given.

• Progress in Superconductivity
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• 제4권2호
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• pp.172-175
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• 2003

Critical current ratio and n-value of Bi(2223)/Ag superconducting joint tape were measured as a function of uniaxial pressure. In the superconducting joint method, MM and MSM joint were used ; MM joint is direct connection of two -multi filamentary tapes, and MSM is connection of them by using a single -filamentary tape. It was observed that the critical current ratio(CCR) for jointed tapes was not dependent on the uniaxial pressure but joining methods. The n-value of jointed tapes has similar trends with that of the CCR. Especially, double MSM joint showed the highest electrical properties as 63.4-76.0%(CCR) and 3.5-5.1 (n-value). It is considered that the improvements are due to the better inter connections of multifilaments by two single filamentary tapes on both sides.

• 동력기계공학회지
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• 제11권1호
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• pp.76-81
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• 2007

The objective of this study is to find the optimum design of a snubber using CFD analysis. Several dimensions such as snubber height(H), snubber diameter(D), buffer width and buffer angle are considered in this study. The present study shows that the CFD can be applied to study the pressure characteristics inside the snubber. The objective of the snubber design optimization are to minimize a pressure loss and the pulsation ratio. Numerical results such as particle track, pressure distribution and turbulent kinetic energy are used to analyze the critical area and pressure behavior inside the snubber. As a result, snubber model with H/D ratio of 3.23 and buffer angle of $40^{\circ}$ has a minimum pressure loss. On the other hand, snubber model with H/D ratio 4.41 and buffer angle $10^{\circ}$ has a minimum pulsation ratio.

• 대한설비공학회:학술대회논문집
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• 대한설비공학회 2006년도 하계학술발표대회 논문집
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• pp.885-890
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• 2006

This study presents analysis results for the hybrid system combining solid oxide fuel cell and gas turbine. Two different system layouts(an ambient pressure system and pressurized system) are considered and their design performance are comparatively investigated taking into account critical design factor, the most critical parameter such as turbine inlet temperature, gas turbine pressure ratio, temperature difference at the fuel cell and fuel cell operating temperature are considered as design constraints. Performance variations according to system layout and design parameters are examined in energetic view point.

• 한국철도학회:학술대회논문집
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• 한국철도학회 2010년도 춘계학술대회 논문집
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• pp.139-150
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• 2010

최근 한국에서는 세계적인 녹색기술을 맞아 향후 차세대 교통 시스템으로서 튜브 트레인 시스템의 본격적인 연구가 진행되고 있다. 한국 철도기술연구원(KRRI, Korea Railroad Research Institute)에서 튜브 트레인 시스템의 건설을 위한 목적으로 연구가 진행되고 있다. 본 논문에서는 기초연구의 일환으로 축대칭하며 긴 형상의 수송체가 가지는 다양한 튜브(터널) 내부의 압력, 막힘비율, 운행속도를 파라미터로 선정하여 전산해석을 수행하였다. 세부사항으로는 동일 형상의 운송체가 동일 운행속도를 가지고 일반적 압력의 개활지(오픈 시스템, 개활지 운행)를 운행할 때와, 다양한 환경(튜브 내 압력, 막힘비율, 운행속도)의 튜브 내부를 운행할 때에 대하여 튜브 트레인의 공력특성 연구를 수행하였다. 이를 통하여 다양한 운행속도-막힘비율 별로 개활지와 동일 에너지 효율 나오는 튜브 내 압력(P-D 관계)을 계산하였고, 막힘비율 증가에 따른 튜브 내 감압 정도(P-${\beta}$ 관계), 다양한 막힘비율-튜브 내 압력 별로 운행속도에 따른 전체 항력 양상(D-V 관계)을 보여 주었다. 그리고 개활지 운행시와 튜브 내부 운행시의 에너지 효율(주행저항)을 비교하였고, 튜브 내부 운행 시 트레인이 갖는 효율과 관계되는 충격파 발생의 임계속도(critical V-B 관계) 및 한계속도(V-P 관계)를 얻을 수 있었다. 이러한 연구의 결과는 튜브 시스템 설계 및 건설에 꼭 필요한 것이며 가이드라인을 제시했다.