• International Journal of Korean Welding Society
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• 제3권1호
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• pp.17-22
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• 2003

During the laser welding, weldments are suddenly heated and cooled by laser beam of high density energy. This phenomenon gives an occasion to complex welding residual stresses, which have a great influence on structural instability, in laser welds. However, relevant researches on this field are not sufficient until now and residual stress measurements have experimental and practical limitations. From these reasons, a numerical simulation may be attractive in order to solve the residual stress problem. For clarifying the distribution of heat and welding residual stresses in laser welds with the nail-head shape, authors conduct the finite element analysis (two-dimensional unstationary heat conduction & thermal elastic and plastic analysis). From the results, we can confirm the stress concentration occurs at the place of melting line shape changed in laser welds with the nail-head shape.

• 대한기계학회논문집B
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• 제27권5호
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• pp.668-676
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• 2003

The effect of nozzle height on heat transfer of a hot steel plate cooled by an impinging liquid jet is not well understood. Previous studies have been based on the dimensionless parameter z/d. To test the validity of this dimensionless parameter and to investigate gravitational effects on the jet, stagnation velocity of an impinging liquid jet were measured and the cooling experiments of a hot steel plate were conducted for z/d from 6.7 to 75, and an inverse heat conduction method is applied for the quantitative comparison. Also, the critical instability point of a liquid jet was examined over a range of flow rates. The experimental velocity data for the liquid jet were well correlated with the dimensionless number 1/F $r_{z}$$^2$based on distance. It was thought that the z/d parameter was not valid for heat transfer to an impinging liquid jet under gravitational forces. In the cooling experiments, heat transfer was independent of z when 1/F $r_{z}$$^2$< 0.187(z/d = 6.7). However, it was found that the heat transfer quantity for 1/F $r_{z}$$^2$=0.523(z/d = 70) is larger 11% than that in the region for 1/F $r_{z}$$^2$=0.187. The discrepancy between these results and previous research is likely due to the instability of liquid jet.uid jet.

• 한국초전도ㆍ저온공학회논문지
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• 제16권4호
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• pp.44-48
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• 2014

A liquid nitrogen-cooled prepolarization ($B_p$) coil made for ultra-low field nuclear magnetic resonance and magnetic resonance imaging (ULF-MR) designed to generate 7 mT/A was fabricated. However, with suspected internal insulation failure, the coil was investigated in order to find out the source of the failure. This paper reports detailed build of the failed $B_p$ coil and a number of analysis methods utilized to figure out the source and the mode of failure. The analysis revealed that pyrolytic graphite sheet linings put on either sides of the coil for better thermal conduction acted as an electrical bridge between inner and outer layers of the coil to short out the coil whenever a moderately high voltage was applied across the coil. A simple model circuit simulation corroborated the analysis and further revealed that the failed insulation acted effectively as a damping resistor of $R_{d,eff}=6{\Omega}$ across the coil. This damping resistance produced a 50 ms-long voltage tail after the coil current was ramped down, making the coil not suitable for use in ULF-MR, which requires complete removal of magnetic field from $B_p$ coil within milliseconds.

• 한국자기학회지
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• 제8권5호
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• pp.317-332
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• 1998

소자화 랭동법은 전자 또는 핵의 자기 모벤트를 등온적으로 자화시킨 다음 단열상태에서 자화를 없앰으로써 온도를 내리는 방법이다. 전자의 소자화 냉동기는 교석 랭동기의 출현으로 거의 사용되지 않고 있으나 핵의 소자화 냉동기는 ${\mu}K$ 이하의 극저온을 얻을 수 있는 유일한 방법으로 극저온에서의 $^{3}He$액체 또는 $^{3}He$ 고체의 상전이, 극저온 초전도 현상, 핵의 자발질서 현상, 스핀 유리 상전이등의 연구에 많이 사용되고 있다. 핵의 소자화 냉동기는 대개 물체(시료) 전체의 온도를 내리는데 사용하나 빠른 소자화를 통해 핵만의 온도를 내리는데도 사용된다. 핵만의 온도를 내리는 냉동방법은 핵 자기 현상의 연구에 많이 이용되고 있다. 이러한 극저온을 얻기 위한 냉동법은 최첨단 기술이나 현재 국내에서는 극저온을 중점적로 연구하는 곳이 없는 실정이다. 본 논문에서는 소자화 냉동에 관한 현재의 취세를 소개하고 앞으로의 연구방향을 제시하고자 한다.

• Bulletin of the Korean Chemical Society
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• 제9권6호
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• pp.333-337
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• 1988

Spinel $CoFe_2O_4$ solid solutions containing up to 50 mol% CoO were synthesized with spectroscopically pure CoO and ${\alpha}-Fe_2O_3$ polycrystalline powders. The spinel structures of the $CoFe_2O_4$ solid solutions were analyzed from XRD patterns and the Mossbauer spectra showed that the quenched $CoFe_2O_4$ had a partially inversed spinel structure ($Co_{0.23}Fe_{0.77}$) < $Co_{0.77}Fe_{1.23}$ > $O_4$, while the slowly cooled $CoFe_2O_4$ was completely inversed spinel ($Co_{0.04}Fe_{0.96}$) <$Co_{0.96}Fe_{1.04}$ > $O_4$. The $CoFe_2O_4$ specimens containing 10, 20, 30 and 40 mol% CoO turned to be a mixture of corundum and spinel structures. Electrical conductivities were measured as a function of temperature from 300 to $900^{\circ}C$ under oxygen partial pressures from $10^{-3}$ to 1 atm. The temperature dependencies of the electrical conductivity show different behaviors in the low- and high-temperature regions. The average activation energies are 0.23 eV and 0.80 eV in the low- and high-temperature regions, respectively. It is suggested that $Co^{2+} {\to} CO^{3+} + e^-$ and $Fe^{2+} {\rightleftharpoons} Fe^{3+} + e^-$ are the main conduction mechanisms responsible for the electronic conduction in the low- and high-temperature regions, respectively.

• 대한기계학회논문집B
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• 제21권7호
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• pp.928-938
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• 1997

The end winding is an important part in induction motor for thermal analysis. But there is little information on the heat transfer coefficient of that surfaces because of geometrical complexity. So our experimental object is to know the heat transfer coefficient of end winding and find the optimum design parameter of rotor fan. Carbon coated papers were used for a uniform heat generating surfaces which were easy to fabricate. The experiments of some parameters were performed as varying rotation speed of rotor fan. We obtained the local and average Nusselt number of the end winding surfaces by correcting radiation and conduction losses errors. The results showed that the average Nusselt number increased with rotor fan blade number and width but decreased with end winding length. However, the increasing limits existed in the case of rotor fan width and blade number. So optimum design value were obtained for rotor fan width and blade numbers.

• 태양에너지
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• 제13권2_3호
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• pp.53-64
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• 1993

본 연구는 용융된 파라핀을 채운 수직원관 내의 상변화물질의 초기온도와 수직원관의 벽면온도를 변화시켰을때 관 내에서 일어나는 열전달현상을 다루었다. 자연대류의 효과는 초기과열된 액상영역 내에서 응고초기 짧은 시간에 걸쳐 일어났고, 그 후 전도열전달이 paraffin 전 영역을 지배하였다. 실험에서 관찰한 응고 형태는 상부표면에서 밀도 증가에 의한 수축공간이 발생하였으며, 그 공간의 크기는 냉각이 진행됨에 따라 증가하였다. 자연대류가 끝나자. 상경계면 상에서 수지상 결정과 mush-zone이 발견되었다. 액상 paraffin의 초기과열은 실험 전반부의 응고질량과 응고두께를 감소시키는 경향을 보였으며, 초기액상과열도와 벽면 과냉도가 큰 경우에 크게 나타났다.

• 한국초전도학회:학술대회논문집
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• 한국초전도학회 1999년도 High Temperature Superconductivity Vol.IX
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• pp.7-11
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• 1999

The recent progress in new cooling techniques of the high Tc superconductor(HTS) systems is reported and discussed with some practical examples. At the beginning stage of the HTS development in research laboratories, liquid nitrogen(LN$_2$) is the standard medium for an effective cooling. The success of HTS in many different application areas, however, has required a variety of need in the cooling temperature and the cooling capacity with specific design restrictions. While the utilization of alternative liquid cryogens such as liquid neon (LNe) or liquid hydrogen (LH$_2$) has been tired in some of them, even solid cryogens such as solid nitrogen (SN$_2$) or solid hydrogen (SH$_2$) may be another option in special applications. The gaseous helium cooled by a cryogenic refrigerator has also been a good candidate in many cases. One of the best cooling methods for the HTS is the direct conduction-cooling by a closed-cycle refrigerator with no cryogen at all. The refrigeration may be based on Joul-Thomson, Brayton, Stirling, Gifford-McMahon, or pulse tube cycles. The pros and cons of the newly proposed cooling methods are described and some significant design issues are presented.

• Journal of the Optical Society of Korea
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• 제16권1호
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• pp.53-56
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• 2012

We have developed a slab-type Nd:YAG gain module based on the techniques of conduction cooling and end pumping. The Nd:YAG slab is end-capped on both ends by undoped pure YAG and is pumped through the end-caps by stacked arrays of laser diode bars. The slab's surfaces of total internal reflection are in contact on both sides with microchannel cooling blocks which are cooled by water circulation. The power oscillator based on the gain module generates more than 400 W at 1-kW pumping with a slope efficiency of 55%. The small-signal gain of the gain module is 10 in a single zig-zag pass, and the amplified beam shows a near diffraction-limited beam quality.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2001년도 추계학술대회논문집B
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• pp.211-216
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• 2001

Flow motion and variation of thermal field around a bubble which attached at the upper cooled solid wall in a $B\dot{e}nard$ convection flow is studied experimentally using thermo-sensitive liquid-crystal tracers and image processing for flow visualization and analysis. The air is injected gradually by $0.1m\ell$ to make the bubble. As the growing of the bubble in a $B\dot{e}nard$ convection flow, the variation of temperature field and surface tension along the bubble, which in turn cause to change the thermal field patterns and the flow direction and patterns. 6 cells flow pattern is transformed into diverse flow pattern. At the large size of a bubble, it's only conduction mechanism under the region of the bubble because of low Ra number 1137, but the convection flow both sides of the bubble leads to another convection flow in the bubble influence area which has been remained stable stagnation.