• Proceedings of the Korean Society of Propulsion Engineers Conference
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• 2011.11a
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• pp.263-267
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• 2011

This paper presents a numerical investigation of the deflagration to detonation transition (DDT) of flame acceleration by a shock wave filled with an ethylene-air mixture in bent tube. A model consisting of the reactive compressible Navier-Stokes equations and the ghost fluid method (GFM) for complex boundary treatment is used. A various intensities of incident shock wave simulations show the generation of hot spots by shock-flame interaction and the accelerated flame propagation due to geometrical effect. Also the first detonation occurs nearly constant chemical heat release rate, 20 MJ/($g{\cdot}s$). Through our simulation's results, we concentrate the complex confinement effects in generating strong shock wave, shock-flame interaction, hot spot and DDT in pipe.

• Journal of Korea Foundry Society
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• v.24 no.2
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• pp.101-107
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• 2004

회주철 및 구상흑연주철에 있어서 압탕방안 및 합금원소가 수축결함의 생성에 미치는 영향을 연구하였다. 두 종류의 압탕방안으로 실린더형상의 계단상시편을 제조하였으며 회주철의 경우 5조성(ISO 150, 200, 250, 300, 350), 구상흑연주철의 경우 6조성(SG 10, 20, 30, 40, 50, 60)을 사용하였다. 회주철 및 구상흑연주칠 공히, 1차 압탕방안의 경우 액상수축에 의한 1차수축결함이 후육부의 표면에 발생하였으며 수축결함의 내면은 매끄러웠다. 회주철의 경우 응고수축에 의한 2차수축결함은 생성되지 않았으나 구상흑연주철의 경우 모든 시편의 내부열점에 2차수축결함이 발생하였고 그 내면은 거칠었다. 2차압탕방안의 경우 회주철의 모든 시편에서는 1차 및 2차수축결함이 발생되지 않았다. 그러나 구상흑연주철의 경우 탄화물 생성원소가 첨가된 SG 40, 50 및 60의 3조성에서 2차수축결함이 열점에 생성되었다. 견고한 ���V주형을 사용하였기 때문에 주형벽이동으로 인한 표면팽창은 어느 경우에도 관찰되지 않았다.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.36 no.3
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• pp.315-324
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• 2012

This paper presents a numerical investigation of the deflagration-to-detonation transition (DDT) of flame acceleration by a shock wave filled with an ethylene/air mixture as the combustible gas, considering geometrical changes by using obstacles and bent tubes. The model used consists of the reactive compressible Navier-Stokes equations and the ghost fluid method (GFM) for complex boundary treatment. Simulations with a variety of bent tubes with obstacles show the generation of hot spots through flame and strong shock-wave interactions, and restrained or accelerated flame propagation due to geometrical effects. In addition, the simulation results show that the DDT occurs with a nearly constant chemical heat-release rate of 20 MJ/($g{\bullet}s$) in our numerical setup. Furthermore, the DDT triggering time can be delayed by the absence of unreacted material together with insufficient pressures and temperatures induced by different flame shapes, although hot spots are formed in the same positions.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.20 no.7
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• pp.2148-2158
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• 1996

Uncoupled, quasi-static and linear thermoviscoelastic problems are analyzed in time domain by the finite element approximation which is developed using the principle of virtual work and viscoelasticity matrices instead of shear and bulk relaxation functions as in usual formulations. The material is assumed to be isotropic, homegeneous and thermorheologically simple, which means that the temperature-time equivalence postulate is effective. The stress-strain laws are expressed by relaxation-type hereditary integrals. In spatial and time discritizations, isoparametric quadratic quadrilateral finite elements and linear time variations are adopted. For explicit derivations, the viscoelastic material is assumed to behave standard linear solid in shear and elastically in dilatation. Two-dimensional examples are solved under general temperature distributions T = T(x, t), and compared with other opproximate solutions to show the versatility of the presented analysis.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.34 no.7
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• pp.721-727
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• 2010

The spontaneous ignition of coal stockpile causes serious safety and economic problems. Such spontaneous ignition occurs in coal stockpile when the rate of heat released by the oxidation of coal is greater than the rate of heat lost to the surroundings. In this study, a two-dimensional unsteady model is adopted for studying spontaneous ignition and the numerical results are compared with experimental results. The numerical results are in a good agreement with the experimental ones. Depending on the porosity, the internal maximum temperature, pressure, and oxygen mass fraction during spontaneous ignition are investigated. On the basis of the numerical results, the transient temperature variations for several shapes of coal stockpiles are analyzed. Further, the physical mechanisms of hot-spot formation and spontaneous ignition are analyzed.

• The Journal of Korea Institute of Information, Electronics, and Communication Technology
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• v.10 no.6
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• pp.477-486
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• 2017

For the analysis of the optical characteristics of electrodeless lamps, all the lamp surface temperatures have been treated the same. However, the interpretation of optical properties in this way has not been sufficient in terms of accuracy. In this paper, to overcome this problem, we divided the inside of the bulb into two parts, hot spot and cold spot, and analyzed the density difference of mercury by different temperatures. Here, it is assumed that the distribution of temperature and density is linear. The effect of optical characteristics through redistribution of hot spot and cold spot density was analyzed. It was also confirmed that the ratio of the density of the redistributed discharge gas has a great influence on the saturation of the optical characteristics. Therefore, it is proved that the design method through the domestic setting is very useful in the actual design, and the method for shortening the time for stabilizing the optical characteristics is obtained.

• New & Renewable Energy
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• v.6 no.1
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• pp.38-45
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• 2010

Abstract Under the physical stress on photovoltaic (PV) module, it will be warped according to elongation of the front glass and then micro-crack will be occurred in the thermally sealed solar cell. This micro-crack leads to drop of short circuit current of the PV module. This is because of increase of resistance component by micro-crack. Micro-crack at specific solar cell in the module lessens the durability of PV module with reduced output, hot-spot caused by solar cell output mismatch and increased resistance component. This study shows the relation between electrical characteristics and micro- cracks due to mechanical stress on PV module.

• Computational Structural Engineering
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• v.9 no.1
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• pp.101-113
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• 1996

In this paper, the finite element formulation for the thermal analysis of quasi-static, uncoupled, homogeneous, isotropic and linear viscoelastic problems is presented based on the principle of virtual work. The viscoelastic material is assumed to be thermorheologically simple, which is well known material property in a large class of high polymers. The variational formulation and the finite element equation in matrix from are derived. Effective generation and storage of the hereditary stiffness matrices are given in detail especially for the case of the steady state temperature distribution T=T(x). Some numerical examples are given and compared with published results to show the versatility of the derived finite element formulations.

• Proceedings of the Korean Nuclear Society Conference
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• 1996.11a
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• pp.427-434
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• 1996

OECD-NEA 주관으로 수행된 TMI-2의 압력용기 변형연구의 결과, 하반부의 creep해석에 많은 문제점이 제기되어 있다. 본 논문은 TMI-2 노심용융 사고에 대한 기존 구조해석에서 creep 상관식의 형태, 적용방법 및 FEM 해석절차상의 상이점을 밝혀내고 이에 따라 압력용기 하반부의 파손확률이 크게 다르게 결정됨을 보였다. 기존의 TMI-2 구조해석에서 주 오차의 요인으로서 시간의 변화에 따른 국부열점 및 이를 포함한 재배치된 용융노심의 열경계조건의 불확실도와 압력용기강의 creep strain을 시간 및 온도에 대하여 불충분하게 묘사한 점을 밝혔다. 또한 creep-rupture 예측에 사용된 Larson-Miller Parameter도 해석을 지나치게 보수적인 결과로 유도하였다. 중대사고시 압력용기 하반부 천공방어를 위한 방안인 용기하부 외벽 냉각방식을 적용하였을 때 TMI-2 사고를 재해석한 결과, 압력용기의 건전성이 충분한 보수성을 가지고 유지됨을 보였다.

• Proceedings of the Korean Nuclear Society Conference
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• 1998.05a
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• pp.818-823
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• 1998

Three Mile Island Unit-2 (TMI-2)의 사고 후 OECD-NEA 주관의 연구에 의하면 압력용기 하부의 노즐이 국부열점(hot spot) 영역의 경우 거의 압력용기 바닥까지 용융되었음이 조사되었다. [1]. 이러한 재배치된 용융노심의 열속에 의하여 압력용기의 외부와 통하는 penetration tube weld(노즐 용접부)가 파손된다면 내부의 고압상태로 인해 penetration tube ejection 사고 및 이에 따르는 용융노심의 압력용기 외부로의 유출 가능성까지 배제할 수 없을 것이다. 본 연구의 출발점은 중대사고시 이러한 압력 및 열속에 따르는 노즐 용접부의 파손확률을 결정하는데 있다. 크리프 파출시 기존의 해석에서 쓰인 deterministic approach를 개선하여 probabilistic approach를 개발하였다. 또한 기존의 해석에서 쓰인 단순한 안전 여유도(margin-to-failure)의 개념과 비교하여 용접부에서의 파손확률을 계산하였다.