• Title/Summary/Keyword: critical phenomena

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Thermal Shock Resistance of $Al_2$TiO$_5$ Ceramics Prepared from Electrofused Powders (전기용융 분말로부터 합성된 $Al_2$TiO$_5$ Ceramics의 열충격 저항성)

  • ;Constantin Zografou
    • Journal of the Korean Ceramic Society
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    • v.35 no.10
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    • pp.1061-1069
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    • 1998
  • The thermal instability of Al2TiO5 Ceramics was contrlled by solid solution with MgO SiO2 and ZrO2 through electrofusion in an arc furnace. The thermal expansion properties of Al2TiO5 composites show the hysteresis due to the strong anisotropy of The crystal axes of these material. These phenomena are ex-plained by the opening and closing of microcracks. The difference in microcracking temperatures e.g 587.6(ATG2), 405.9(ATG3) and 519.7$^{\circ}C$(ATG4) is caused by the difference in grain size and stabilizer type. The thermal shock behaviour under cyclic conditions between 750-1400-75$0^{\circ}C$ show no change in mi-crostructure and phase assemblage for all three stabilized specimens. After the thermal loading test at 110$0^{\circ}C$ for 100hrs. ATG1 and ATG2 materials decomposes completely to its components corundum and ru-tile in both cases. However with approximatelly 20% retention of the Al2TiO5 Thus in order to prevent decomposition of the stabilized material in the critical temperature range 800-130$0^{\circ}C$ it must be traversed within a short period of time.

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Insulation Ageing Diagnosis Using HFPD Pattern Analysis (HFPD 패턴분석을 이용한 절연열화 진단)

  • Kim, Deok-Keun;Yeo, In-Sun;Lim, Jang-Seob;Lee, Jin
    • Proceedings of the KIEE Conference
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    • 2003.07c
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    • pp.1726-1728
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    • 2003
  • The aging diagnosis method using partial discharge measurement detects discharge signals that critical cause of failure in insulation material operated a long time and can diagnose aging state of insulation materials with an aging analysis algorithm. The HFPD measurement method is a technique to analyze aging state of high voltage insulation materials and detect higher frequency signals than conventional PD measurement method therefore it takes less noise effect and could execute active line measurement. It is possible to analyze main discharge phenomena and obtain access to aging progress occurred in insulation materials through accumulation of HFPD signals during determined interval and expression of fractal dimension using statistical process of accumulated signals. The HFPD signals that occurred in each applied voltages are measured during 180 cycles and accumulated to the same phase of one cycle. These patterns that made by previous method are normalized with logarithm function and than inputted to neural networks. The aging diagnosis of insulation material was possible and the recognition ratio of neural network appeared very high.

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M&S Case Study for Information Sharing Enabled Combat Entities (전투 개체간의 정보 공유가 가능한 모델링 및 시뮬레이션 사례 분석)

  • Kho, Younghoon;Lim, Byungyoun;Park, Sangchul;Kwon, Yongjin
    • Journal of the Korea Institute of Military Science and Technology
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    • v.17 no.4
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    • pp.395-403
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    • 2014
  • Recent technological advancement has a profound effect on the ways that the war is being conducted and fought. The advanced communications, information, computing and sensor technologies enable the combat units to be integrated in the battlefield management network. By exchanging and sharing real-time battlefield information that is critical for the successful outcome of military engagement, the legacy forces are becoming much more effective and lethal than ever before, The bigger picture of such phenomena can be summarized as the concept of Network Centric Warfare(NCW). The main purpose of this study is to compare the outcome of regional combat engagement between the legacy forces and the future combat systems(FCS). The FCS capitalizes on the advanced technologies within the frame of NCW. This study uses the modeling and simulation methodology to assess the effectiveness of two different combat forces. The simulation results show that the FCS is more effective, hence vindicating the superiority of technologically advanced combat units.

Modeling of Liquid Entrainment and Vapor Pull-Through in Header-Feeder Pipes of CANDU

  • Cho Yong Jin;Jeun Gyoo Dong
    • Nuclear Engineering and Technology
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    • v.36 no.2
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    • pp.142-152
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    • 2004
  • The liquid entrainment and vapor pull-through offtake model of RELAP5/MOD3 had been developed for SBLOCA (Small Break Loss of Coolant Accident). The RELAP5/MOD3 model for horizontal volumes accounts for the phase separation phenomena and computes the flux of mass and energy through a branch when stratified conditions occur in the horizontal pipe. In the case of CANDU reactor, this model should be used in the coolant flow of 95 feeders connected to the reactor header component under the horizontal stratification in header. The current RELAP5 model can treat the only 3 directions junctions; vertical upward, downward, and side oriented junctions, and thus improvements for the liquid entrainment and vapor pull-through model were needed for considering the exact angles. The RELAP5 off-take model was modified and generalized by considering the geometric effect of branching angles. Based on the previous experimental results, the critical height correlation was reconstructed by use of the branch line connection angle and validation analyses were also performed using SET. The new model can be applied to vertical upward, downward and angled branch, and the accuracy of the new correlations is more improved than that of RELAP5.

Subwavelength Focusing of Light From a Metallic Slit Surrounded by Grooves with Chirped Period

  • Yoon Jaewoong;Choi Kiyoung;Song Seok Ho;Lee Gwansu
    • Journal of the Optical Society of Korea
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    • v.9 no.4
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    • pp.162-168
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    • 2005
  • Extraordinary phenomena related to the transmission of light via metallic films with subwavelength holes and grooves are known to be due to resonant excitation and interference of surface waves. These waves make various surface structures to have optically effective responses. Further, a related study subject involves the control of light transmitted from a single hole or slit by surrounding it with diffractive structures. This paper reports on the effects of controlling light with a periodic groove structure with Fresnel-type chirping. In Fresnel-type chirping, diffracted surface waves are coherently converged into a focus, and it is designed considering the conditions of constructive interference and angular spectrum optimization under the assumption that the surface waves are composite diffracted evanescent waves with a well-defined in-plane wavenumber. The focusing ability of the chirped periodic structures is confirmed experimentally by two-beam attenuated total reflection coupling. Critical factors for achieving subwavelength foci and bounds on size of focal spots are discussed in terms of the simulation, which uses the FDTD algorithm.

Effect of Main Operating Conditions on Cathode Flooding Characteristics in a PEM Unit Fuel Cell (고분자전해질형 단위 연료전지의 주요 작동 조건이 공기극 플러딩 현상에 미치는 영향)

  • Min Kyoung-Doug;Kim Han-Sang
    • Transactions of the Korean Society of Mechanical Engineers B
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    • v.30 no.5 s.248
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    • pp.489-495
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    • 2006
  • Proton exchange membrane (PEM) should be sufficiently hydrated with a careful consideration of heat and water management. Water management has been a critical operation issue for better understanding the operation and optimizing the performance of a PEM fuel cell. The flooding on cathode side resulting from excess water can limit the fuel cell performance. In this study, the visual cell was designed and fabricated fur the visualization of liquid water droplet dynamics related to cathode flooding in flow channels. The experiment was carried out to observe the formation, growth and removal of water droplets using CCD imaging system. Effects of operating conditions such as cell temperature, air flow rate and air relative humidity on cathode flooding characteristics were mainly investigated. Based on this study, we can get the basic insight into flooding phenomena and its two-phase flow nature. It is expected that data obtained can be effectively used fur the setup and validation of two-phase PEM fuel cell models considering cathode flooding.

27Al and 87Rb Nuclear Magnetic Resonance Study of the Relaxation Mechanisms of RbAl(CrO4)2·2H2O Single Crystals

  • Kim, Jae Sung;Lim, Ae Ran
    • Journal of the Korean Magnetic Resonance Society
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    • v.16 no.2
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    • pp.111-121
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    • 2012
  • The spin-lattice relaxation times, $T_1$, and spin-spin relaxation times, $T_2$, of the $^{27}Al$ and $^{87}Rb$ nuclei in $RbAl(CrO_4)_2{\cdot}2H_2O$ crystals were investigated. The presence of only one resonance line for the $^{27}Al$ nuclei indicates that the results in a dynamical averaging of the crystal electric field that produces a cubic symmetry field. The changes in the temperature dependence of $T_1$ are related to variations in the symmetry of the octahedra of water molecules surrounding $Al^+$ and $Rb^+$. The $T_1$ values for the $^{27}Al$ and $^{87}Rb$ nuclei are different due to differences in the local environments of these ions. We also compared these $^{27}Al$ and $^{87}Rb$ NMR results with those obtained for $RbAl(CrO_4)_2{\cdot}2H_2O$ crystals. The relaxation mechanisms of $RbAl(XO_4)_2{\cdot}nH_2O$ (X=Cr and S) crystals are characterized by completely different NMR behaviors.

Study of the Ignition with TEAL for an Oxygen Rich Preburner (산화제 과잉 예연소기를 위한 TEAL 점화연구)

  • Moon, In-Sang;Moon, Il-Yoon;You, Jae-Han;Lee, Sun-Mee;Lee, Soo-Yong
    • Proceedings of the Korean Society of Propulsion Engineers Conference
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    • 2011.04a
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    • pp.97-100
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    • 2011
  • It is critical to set up the starting sequence of liquid rocket engines because not carefully arranged process can lead the engine damages. Thus, many efforts were made to prevent the hard start at the ignition. Hypergolic fuels are frequently used to ignite LRE and TEAL, one of the hypergolic fuel is also used for kerosene-LOx LRE ignition. However, since we are still lack of experiences igniting oxygen rich preburners of the staged combustion cycle engines, it would be helpful to estimate the TEAL ignition phenomena before the actual tests.

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Modeling flow instability of an Algerian sand with the dilatancy rule in CASM

  • Ramos, Catarina;Fonseca, Antonio Viana da;Vaunat, Jean
    • Geomechanics and Engineering
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    • v.9 no.6
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    • pp.729-742
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    • 2015
  • The aim of the present work was the study of instability in a loose sand from Les Dunes beach in Ain Beninan, Algeria, where the Boumerdes earthquake occurred in 2003. This earthquake caused significant structural damages and claimed the lives of many people. Damages caused to infrastructures were strongly related to phenomena of liquefaction. The study was based on the results of two drained and six undrained triaxial tests over a local sand collected in a region where liquefaction occurred. All the tests hereby analyzed followed compression stress-paths in monotonic conditions and the specimens were isotropically consolidated, since the objective was to study the instability due to static loading as part of a more general project, which also included cyclic studies. The instability was modeled with the second-order work increment criterion. The definition of the instability line for Les Dunes sand and its relation with yield surfaces allowed the identification of the region of potential instability and helped in the evaluation of the susceptibility of soils to liquefy under undrained conditions and its modeling. The dilatancy rate was studied in the points where instability began. Some mixed tests were also simulated, starting with drained conditions and then changing to undrained conditions at different time steps.

Investigation of Temperature-Dependent Microscopic Morphological Variation of PEEK Powder for a 3D Printer using Dissipative Particle and Molecular Dynamics Simulations (소산입자동역학과 분자동역학을 이용한 3D 프린터용 PEEK 분말에 대한 온도에 따른 미시적 구조변화에 대한 연구)

  • Kim, Namwon;Yi, Taeil
    • Journal of the Korean Society of Manufacturing Process Engineers
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    • v.17 no.5
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    • pp.117-122
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    • 2018
  • 3D printing technology and its applications have grown rapidly in academia and industry. We consider a 3D printing system designed for the selective laser sintering (SLS) method, which is one of the powder bed fusion (PBF) techniques to build up the final product by layering sintered powder slices. Thermal distortion of printing products is a critical challenge in 3D printing. This study investigates temperature-dependent conformational behaviors of 3D printed samples of sintered poly-ether-ether-ketone (PEEK) powders using molecular dynamics simulations. The wear and chemical resistance properties of PEEK are understood, as it is a well-known biocompatible material used for implants. However, studies on physical phenomena at nanoscale in PEEK are rarely published in public. We simulate dissipative particle dynamics to elucidate how a cavity regime forms in PEEK at different system temperatures. We demonstrate how PEEK structures deform subject to the system temperature distribution.