• 한국소성가공학회:학술대회논문집
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• 한국소성가공학회 1999년도 압출 및 인발 심포지엄
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• pp.24-37
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• 1999

A comprehensive methodology to consolidate the hypereutectic Al-27Si-X alloy via spray forming was investigated in an attempt to judge the feasiblilty of this alloy in applying wear resistant components. Billets having desired shape and microstructures were fabricated using forming parameters obtained from numerical calculations. Prior to tube extrusion of the spray formed billets, effects of various extrusion conditions, such as extrusion ratio, die temperature, and die configuration, on microstructures of the billet were studied. Based on results obtained from the preliminary extrusion tests, the formed billets were then hot extruded into a tubular shape. Various material properties of the extruded billet were measured and compared with the other candidate materials for anti-wear applications.

• Journal of Advanced Marine Engineering and Technology
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• 제38권10호
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• pp.1200-1205
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• 2014

The effect of nanofluids on the heat transfer performance of a pulsating heat pipe has been experimentally investigated. Water-based diamond nanofluid and aluminium oxide ($Al_2O_3$) nanofluid were tested in the concentration range of 0.5-5%. The pulsating heat pipe was constructed using clear Pyrex tubes of 1.85 mm in inner diameter in order to visualize the pulsating action. The total number of turns was eight each for heated and cooled parts. The supply temperatures of heating water and cooling water were fixed at $80^{\circ}C$ and $25^{\circ}C$ respectively. The liquid charging ratio of the nanofluid was 50-70%. The test results showed that the case of 5% concentration of diamond nanofluid showed 18% increase in heat transfer rate compared to pure water. The case of 0.5% concentration of $Al_2O_3$ nanofluid showed 24% increase in heat transfer rate compared to pure water. But the increase of $Al_2O_3$ nanofluid concentration up to 3% did not show further enhancement in heat transfer. It is also observed that the deposited nanoparticles on the tube wall played a major role in enhanced evaporation of working fluid and this could be the reason for the enhancement of heat transfer by a nanofluid, not the enhanced thermal conductivity of the nanofluid.

• 대한방사선기술학회지:방사선기술과학
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• 제30권2호
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• pp.113-119
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• 2007

인체의 유방은 조직 간의 엑스선 흡수차가 작기 때문에 엑스선관 Target/Filter의 적절한 조합으로 피사체의 대 조도를 높이는 일이 중요하다. 최근 장비들은 다양한 유방두께와 유선조직 밀도에 따라서 Target/Filter의 조합을 변화시켜 환자의 피폭 선량 경감 및 진단에 알맞는 화질을 얻고 있다. 본 실험은 Target/Filter의 조합에 따라 해상력 챠트를 이용하여 MTF 영상의 평가와 유방 팬톰 영상을 평가하여 다음과 같은 결론을 얻었다. 1. 아크릴 두께(2 cm, 3 cm, 4 cm)의 조합(Mo/Mo, Mo/Rh. Mo/Al, Rh/Rh, Rh/Al)에 따라 고유원자번호에 의해 엑스선 에너지에서 큰 차이를 보였다. 2. MTF 곡선에서 선예도를 나타내는 MTF 0.5에서의 Lp/mm 값은 아크릴 2 cm일 때 Mo/Mo은 2.4 Lp/mm, 4 cm일 때 Mo/Rh은 2.63 Lp/mm, 6 cm일 때 Rh/Rh은 2.9 Lp/mm로 높게 나타났다. 3. 분해능을 나타내는 MTF 값은 아크릴 2 cm일 때 Mo/Mo은 6.0 Lp/mm, 4cm일 때 Rh/Al은 4.60 Lp/mm, 6 cm일 때 Rh/Al은 6.03 Lp/mm로 MTF 값이 높게 나타났다. 4. 시각적 식별이 가능한 2.5 Lp/mm에서 MTF 값은 2 cm일 때 Mo/Mo은 0.48 Lp/mm, 4 cm일 때 Mo/Rh은 0.53 Lp/mm, 6 cm일 때 Rh/Rh은 0.59 Lp/mm로 높게 나타났다. 5. 유방 팬톰 영상평가에서는 Mo/Mo일 때 12점, Mo/Rh일 때 11점, Rh/Rh일 때 10.5점, Mo/Al일 때 10점, Rh/Al일 때 9점 순으로 나타났다.

• 조명전기설비학회논문지
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• 제20권7호
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• pp.58-64
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• 2006

최근 탈취, 탈색, 정수처리 및 배연가스의 정화에 많이 이용되고 있는 오존의 효과적인 발생에 많은 관심이 모아지고 있다. 전기 방전에 의한 오존 발생 방법 중 유전체 배리어 방전이 보다 낮은 전력을 소모하며, 또한 저전압 동작이 가능하여 가장 효과적인 것으로 생각되고 있다. 본 연구에서는 오존발생농도 및 에너지효율을 개선하기 위하여 알루미나 유전체 및 알루미나 유전체와 알루미나 비드를 충진한 무성방전관을 제작하여 무성방전특성을 연구하였다. 또한 오존발생특성을 무성방전특성과 연관하여 논하였으며, 특히 방전시 유전체에 축적되는 벽전하 및 소비전력특성의 관점에서 논하였다. 결과, 비드를 가진 방전관의 경우 축적되는 벽전하량은 비드가 없는 방전관에 비하여 약 2.5배 정도였으며, 발생되는 오존농도 및 에너지 수율 또한 개선되었다.

• 한국세라믹학회지
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• 제51권6호
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• pp.544-548
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• 2014

Aluminum nitride was synthesized using a carbothermal method from mesoporous alumina having a high surface area (> $1,000m^2/g$) as an aluminum source and CNTs (carbon nano tubes) as a carbon source. In this case the mesoporous alumina was used as the starting material instead of ${\alpha}-Al_2O_3$ with the expectation that the mesopores in mesoporous alumina act as channels for N2 gas and elimination of CO generated as by-product. It is also expected that the synthetic temperature should be lower compared to the use of ${\alpha}-Al_2O_3$ as a starting material due to its high surface area. The crystallinity of the produced aluminum nitride was studied by XRD and FT-IR, and the microstructure was investigated by FE-SEM. Also the purity of the aluminum nitride was analyzed through N/O determinator and ICP analysis.

• Journal of Advanced Marine Engineering and Technology
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• 제31권6호
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• pp.707-714
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• 2007

The heat transfer coefficient and Pressure drop during gas cooling process of $CO_2$ (R-744) in inclined helical coil copper tubes were investigated experimentally. The main components of the refrigerant loop are a receiver. a variable-speed pump. a mass flow meter, a pre-heater and a inclined helical coil type gas cooler (test section). The test section consists of a smooth copper tube of 2.45mm inner diameter. The refrigerant mass fluxes were varied from 200 to $600[kg/m^2s]$ and the inlet Pressures of gas cooler were 7.5 to 10.0 [MPa]. The heat transfer coefficients of $CO_2$ in the inclined helical coil tubes increases with the increase of mass flux and gas cooling pressure of $CO_2$. The pressure drop of $CO_2$ in the gas cooler shows a relatively good agreement with those Predicted by Ito's correlation developed for single-phase in a helical coil tube. The local heat transfer coefficient of $CO_2$ agrees well with the correlation by Pitla et al. However, at the region near pseudo-critical temperature. the experiments indicate higher values than the Pitla et al. correlation. Therefore. various experiments in the inclined helical coil tubes have to be conducted and it is necessary to develop the reliable and accurate prediction determining the heat transfer and pressure drop of $CO_2$ in the inclined helical coil tubes.

• Nuclear Engineering and Technology
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• 제47권4호
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• pp.398-406
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• 2015

In this study, R-123 flow boiling experiments were carried out to investigate the effects of nanoparticle deposition on heater surfaces on flow critical heat flux (CHF) and boiling heat transfer. It is known that CHF enhancement by nanoparticles results from porous structures that are very similar to layers of Chalk River unidentified deposit formed on nuclear fuel rod surfaces during the reactor operation period. Although previous studies have investigated the surface effects through surface modifications, most studies are limited to pool boiling conditions, and therefore, the effects of porous surfaces on flow boiling heat transfer are still unclear. In addition, there have been only few reports on suppression of wetting for decoupled approaches of reasoning. In this study, bare and $Al_2O_3$ nanoparticle-coated surfaces were prepared for the study experiments. The CHF of each surface was measured with different mass fluxes of $1,600kg/m^2s$, $1,800kg/m^2s$, $2,100kg/m^2s$, $2,400kg/m^2s$, and $2,600kg/m^2s$. The nanoparticle-coated tube showed CHF enhancement up to 17% at a mass flux of $2,400kg/m^2s$ compared with the bare tube. The factors for CHF enhancement are related to the enhanced rewetting process derived from capillary action through porous structures built-up by nanoparticles while suppressing relative wettability effects between two sample surfaces as a highly wettable R-123 refrigerant was used as a working fluid.

• Journal of Radiation Protection and Research
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• 제43권4호
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• pp.160-169
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• 2018

Background: This study aims to develop an integrated optical system that can simultaneously or selectively measure the signals obtained from radioluminescence (RL), thermoluminescence (TL), and optically stimulated luminescence (OSL), which are luminescence phenomena of materials stimulated by radioactivity, heat, and light, respectively. The luminescence mechanism of various materials could be investigated using the glow curves of the luminescence materials. Materials and Methods: RL/TL/OSL integrated measuring system was equipped with a X-ray tube (50 kV, $200{\mu}A$) as an ionizing radiation source to irradiate the sample. The sample substrate was used as a heating source and was also designed to optically stimulate the sample material using various light sources, such as high luminous blue light emitting diode (LED) or laser. The system measured the luminescence intensity versus the amount of irradiation/stimulation on the sample for the purpose of measuring RL, TL and OSL sequentially or by selectively combining them. Optical filters were combined to minimize the interference of the stimulation light in the OSL signal. A long-pass filter (420 nm) was used for 470 nm LED, an ultraviolet-pass filter (260-390 nm) was used for detecting the luminescence of the sample by PM tube. Results and Discussion: The reliability of the system was evaluated using the RL/OSL characteristics of $Al_2O_3:C$ and the RL/TL characteristics of LiF:Mg,Cu,Si, which were used as dosimetry materials. The RL/OSL characteristics of $Al_2O_3:C$ showed relatively linear dose-response characteristics. The glow curve of LiF:Mg,Cu,Si also showed typical RL/OSL characteristics. Conclusion: The reliability of the proposed system was verified by sequentially measuring the RL characteristics of radiation as well as the TL and OSL characteristics by concurrent thermal and optical stimulations. In this study, we developed an integrated measurement system that measures the glow curves of RL/TL/OSL using universal USB-DAQs and the control program.

• 융합정보논문지
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• 제9권8호
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• pp.155-163
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• 2019

수평원형관에서 나노입자인 산화알미늄과 기본유체인 물의 혼합인 나노유체에 대한 층류 혼합대류열전달현상을 유한체적법의 수치적 방법으로 규명하였다. 나노유체에 대하여 2상 혼합모델을 적용하였으며, 나노입자의 물성은 온도와 체적농도의 함수를 사용하였다. 수치해석에 적용한 모든 모델의 타당성 검증을 위하여 Kim등의 실험결과와 비교하였으며 좋은 결과를 얻었다. 벽면을 일정한 열유속으로 가열하므로 나노유체는 벽면부근에서 형성된 부력에 의하여 2차유동이 생성된다. Richardson수와 나노입자의 농도가 증가할수록 강한 2차유동이 형성되어 열전달을 향상시키게 된다. 또한 Richardson수와 나노입자의 농도가 증가하면 대류열전달계수와 전단응력도 증가한다. 이런 연구들은 열교환기의 성능향상을 위하여 나노유체를 적용하는데 기본자료로 활용이 가능하다. 이번 연구를 기반으로 향후 2중관형열교환기등 다양한 열교환기에 적용할 예정이다.

• Nuclear Engineering and Technology
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• 제53권6호
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• pp.1970-1978
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• 2021

The aim of this study was to optimize the target, moderator, and collimator (TMC) in a neutron beam generator for the accelerator-based BNCT (A-BNCT) system. The optimization employed the Monte Carlo Neutron and Photon (MCNP) simulation. The optimal geometry for the target was decided as the one with the highest neutron flux among nominates, which were called as angled, rib, and tube in this study. The moderator was optimized in terms of consisting material to produce appropriate neutron energy distribution for the treatment. The optimization of the collimator, which wrapped around the target, was carried out by deciding the material to effectively prevent the leakage radiations. As results, characteristic of the neutron beam from the optimized TMC was compared to the recommendation by the International Atomic Energy Agent (IAEA). The tube type target showed the highest neutron flux among nominates. The optimal material for the moderator and collimator were combination of Fluental (Al₂0₃+AlF₃) with ⁶⁰Ni filter and lead, respectively. The optimized TMC satisfied the IAEA recommendations such as the minimum production rate of epithermal neutrons from thermal neutrons: that was 2.5 times higher. The results can be used as source terms for shielding designs of treatment rooms.