• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.12 no.9
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• pp.825-834
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• 2000

A heat exchanger analysis is performed to investigate the heating characteristics of cryogenic nitrogen by ambient air for the purpose of cryogenic automotive propulsion. The heat exchanger is a concentric triple-passage for supercritical nitrogen, and the radial fins are attached on the outermost tube for the crossflow of ambient air. The temperature distribution is calculated for the nitrogen along the passage, including the real gas properties of nitrogen, the fluid convections and the conductions through the tube walls and the fins. Since the wall temperature of the outer (ambient side) tube is very low in most cases, a heavy frost can be formed on the surface, affecting the heat exchange performance. By the method of the similarity between the heat and the mass transfer of moist air, the frost growth and the time-dependent effectiveness of the heat exchanger are calculated for various operating conditions. It is concluded that the frost formation can augment the heating of nitrogen during the initial period because of the latent heat, then gradually degrades the heat exchange because of the increased thermal resistance. Practical design issues are discussed for the flow rate of nitrogen, the velocity and humidity of ambient air, and the sizes of the fin.

• Journal of the Korean Crystal Growth and Crystal Technology
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• v.8 no.4
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• pp.555-562
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• 1998

Under the influence of non-uniform magnetic field, melt flow in steady state and oxygen concentration in unsteady state are numerically investigated. The strength of the applied characteristic magnetic fields are B=0.1T, 0.2T, and 0.3T, respectively. The buoyancy effects due to the crucible wall heating and the thermocapillary effects due to the surface tention at the free surface are suppressed differentially by the non-uniform magnetic fields. As the intensity of characteristic magnetic fields is increasing, the recirculation region in the meridional plane is moving toward the growing crystal, and is diminishing. The oxygen concentration on the growing surface of crystals is decreasing and the uniformity of the oxygen concentration is increasing as the intensity of the magnetic fields is increasing.

• Journal of computational fluids engineering
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• v.15 no.1
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• pp.1-8
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• 2010

RCCS is a passive safety-related system that removes the decay heat of VHTR when normal decay heat removal systems are in failure. Understanding thermo-hydraulics of RCCS is important to design a safer VHTR. RCCS consists of 292 cooling panels, which are placed in the reactor cavity. The layout of RCCS gives an idea that, for CFD simulations, cooling panels can be assumed to be one annulus tube. This assumption can reduce significantly the computational time, especially for the unsteady simulation. To simulate RCCS in an axisymmetric manner, three models were suggested and compared. Each model has (1) the same outer radius, (2) the same cross-sectional area (3) the same pressure drop, respectively, as the RCCS cooling panels. The steady-state simulation was conducted with these three models and the DO radiation model. It is found that over 90% of the heat from the outer wall of the reactor pressure vessel is transported to the RCCS by radiative heat transfer. The simulation with the third model, which has the same pressure drop as the design, estimates the closest wall temperature profiles to a thermo-hydraulic code, GAMMA+, result.

• Journal of computational fluids engineering
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• v.16 no.1
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• pp.22-29
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• 2011

In this study, the glass fiber drawing from a silica preform in the furnace for the optical fiber manufacturing process is numerically simulated by considering the radiative heating of cylindrically shaped preform. The one-dimensional governing equations of the mass, momentum, and energy conservation for the heated and softened preform are solved as a set of the boundary value problems along with the radiative transfer approximation between the muffle tube and the deformed preform shape, while the furnace heating is modeled by prescribing the temperature distribution of muffle tube. The temperature-dependent viscosity of silica plays an important role in formation of preform neck-down profile when the glass fiber is drawn at high speed. The calculated neck-down profile of preform and the draw tension are found to be reasonable and comparable to the actual results observed in the optical fiber industry. This paper also presents the effects of key operating parameters such as the muffle tube temperature distribution and the fiber drawing speed on the preform neck-down profile and the draw tension. Draw tension varies drastically even with the small change of furnace heating conditions such as maximum heating temperature and heating width, and the fine adjustment of furnace heating is required in order to maintain the appropriate draw tension of 100~200 g.

• Proceedings of the Korean Institute of Information and Commucation Sciences Conference
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• 1999.05a
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• pp.425-431
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• 1999

We develope plasma monitoring system which detect plasma signals and store them for Laser welding quality evaluation and analysis using photo detector. The most fundamental and important aspects in such a system are signal restoration fidelity, noise immunity and noise cancelation capability. In this paper, we propose implementation method using distribute processing structure and hybrid digital communication for high noise cancelation capability, immunity and signal fidelity which are poorly presented in other researches. Lab experimental results and welding experimental results show a effectiveness of proposed method and plasma data is stored with 256 kbps without any communication error. we are implementing various welding defect recognition algorithm in this system.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.22 no.3
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• pp.156-164
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• 2010

Combustion instability is a major issue in design of gas turbine combustors for efficient operation with low emissions. Combustion instability is induced by the interaction of the unsteady heat release of the combustion process and the change in the acoustic pressure in the combustion chamber. In an effort to develop a technique to predict self-excited combustion instability of gas turbine combustors, a new stability analysis method based on the transfer matrix method is developed. The method views the combustion system as a one-dimensional acoustic system with a side branch and describes the heat source as the input to the system. This approach makes it possible to use not only the advantages of the transfer matrix method but also well established classic control theories. The approach is applied to a gas turbine combustion system, which shows the validity and effectiveness of the approach.

• 한국방재학회:학술대회논문집
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• 2011.02a
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• pp.84-84
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• 2011

터널화재의 위험요소에 대한 해석을 위해서는 실제 상황을 재현한 실대형 실험이 가장 유용하겠지만 현실적으로 시간적, 공간적, 경제적인 제약이 따르기 때문에 CFD Modeling 기술의 이용 및 검증이 필요하고, 실제 상황에 가까운 현상의 재현을 위해서는 시뮬레이션의 정확도에 대한 향상이 필수적이다. 또한, CFD Modeling을 터널화재에 적용할 때 시뮬레이션의 질에 영향을 미칠 수 있는 요소들에 대한 결정이 선행되어야 한다. 우선, 터널의 기하학적 구조와 경계조건의 확립이 필요한데 필요한 정보를 얻기 위해서 어느정도 길이의 터널이 적절한지에 대해 생각할 필요가 있으며, 단면변화에 대한 결정을 통해 모델링을 수행하여야 한다. 모델링 작업이 선행된 후에 화재의 위치, 성장률, 최대 크기, 환기시스템 사항 등의 고려가 필요한데 이러한 조건들은 CFD Modeling의 결과에 직접적인 영향을 주기 때문에 충분한 사전조사가 이루어져야 하고, 각 사항들의 변수를 고려하여 다양한 화재시나리오의 도출이 가능할 수 있다. 마지막으로, 화재에서 발생된 열중 약 30%가 복사에 의해 주위 벽으로 전달될 수 있고 열은 연기가 가득찬 영역내에서 재분배될 수 있는데, 열전달 및 연기의 유동 등에 관한 자료를 기초로 화재현상에 대한 분석이 가능하다. 이러한 과정들을 통해 실제 상황에 가까운 설계화재 시나리오를 예측할 수 있다. 본 연구에서는 우리나라 최장대터널인 죽령터널에 대해 합리적인 가정을 통한 설계화재 시나리오를 기초로 화재시뮬레이션은 FDS(Fire Dynamics Simulator) 프로그램을 사용하여 화재 및 연기의 이동 양상을 분석하고, 피난시뮬레이션은 SIMULEX 프로그램을 사용하여 피난시간을 예측 함으로써 터널화재의 CFD Modeling에 의한 피난안전성을 검토하고자 한다.

• Proceedings of the Korean Society of Propulsion Engineers Conference
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• 2003.10a
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• pp.238-241
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• 2003

In case of liquid rocket using turbopump, the inner pressure of liquid oxygen tank is maintained low, so vaporization of LOX is generally occurred. This vaporization tendency increases as the inlet helium gas temperature is higher. For estimating the amount of helium in the rocket system, the LOX vaporization phenomena should be carefully considered. In this paper, Inner process of LOX tank is analyzed by two phase flow modeling. the vaporization rate and required Helium mass is investigated with varying inlet helium temperature and heat transfer coefficient.

• Journal of the Korean Institute of Gas
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• v.9 no.1 s.26
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• pp.26-32
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• 2005

This paper presents the design safety of metal seals in pressure vessels. For a high-pressure vessel, a metal seal is usually used as a primary sealing, and an elastomeric rubber O-ring is adopted as a secondary sealing unit. The FEM computed results show that an aluminium material for sealing a gas leakage is superior to a steel one because of the thermal expansion rate. The deformation and stress distributions on the metal seal and pressure vessel structures are mainly dominated by transferred temperature compared to those of the gas pressure in which is supplied by an external pump. Thus, the temperature of a metal seal material should be restricted to under $200^{\circ}C$.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.28 no.9
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• pp.1051-1057
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• 2004

A numerical optimization procedure for the shape of three-dimensional channel with angled ribs mounted on one of the walls to enhance turbulent heat transfer is presented. The response surface method is used as an optimization technique with Reynolds-averaged Wavier-Stokes analyses of flow and heat transfer. SST turbulence model is used as a turbulence closure. Computational results for local heat transfer rate show reasonable agreements with experimental data. The pitch-to-height ratio of the rib and rib height-to-channel height ratio are set to be 9.0 and 0.1, respectively, and width-to-rib height ratio and attack angle of the rib are chosen as design variables. The objective function is defined as a linear combination of heat-transfer and friction-loss related terms with weighting factor. Full-factorial experimental design method is used to determine the data points. Optimum shapes of the channel have been obtained in the range from 0.0 to 0.1 of weighting factor.