• 에너지공학
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• 제10권4호
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• pp.349-356
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• 2001

물과 증기의 직접접촉에 의한 응축 열전달은 움직이는 증기/물 경계면에서 열 및 물질 전달이 이루어지는 현상으로서, 매우 큰 열전달계수를 수반하는 특징이 있기 때문에 이를 응용한 설계가 산업계에서 광범위하게 이루어지고 있다. 본 연구에서는 단일 수평 배관을 통해 과냉각수가 있는 응축탱크로 안정된 증기제트가 방출될 때, 증기제트 형상을 예측하는 간단한 응축해석모델을 제시하였다. 해석모델은 축대칭 좌표계에서 질량, 운동량 및 에너지 방정식과 증기/물 경계면에서 의 응축 특성을 고려한 열평형 방정식을 사용하여 유도하였다. 증기/물 경계에서의 매우 큰 열전달율은 기존의 실험을 근거로 한 유효열전도계수에 의해 반영되었다. 해석결과는 실험결과와 비교하였고, 제시된 해석 모델은 실험에서 관찰된 바와 같이 증기 질량유속과 수조 온도가 증가할수록 증기제트 크기(반경 및 길이)가 증가하는 경향을 보였다.

• Asian-Australasian Journal of Animal Sciences
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• 제20권6호
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• pp.894-899
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• 2007

An experiment was conducted using a [6, 6-$^2H$]glucose isotope dilution method to determine the effects of plantain (Plantago lanceolata L.) on plasma glucose metabolism in sheep taken from a thermoneutral environment and exposed to a hot environment. The sheep were fed either mixed hay (MH) of orchardgrass (Dactylis glomerata L.) and reed canarygrass (Phalaris arundinacea L.) at a 60:40 ratio or MH and plantain (PL) at a 9:1 ratio in a crossover design for each 23-day period. In both dietary treatments the metabolizable energy (ME) and crude protein intake were designed to be isoenergetic and isoproteinous at around maintenance level. The sheep were taken from a thermoneutral environment ($20^{\circ}C$, 70% RH) and exposed to a hot environment ($28-30^{\circ}C$, 70% RH) for 5 days. The isotope dilution method using a single injection of [6, 6-$^2H$]glucose was performed on the $18^{th}$ day of the thermoneutral environment and on the $5^{th}$ day of heat exposure. Plasma glucose pool size was numerically lower (p = 0.26) during heat exposure on both dietary treatments, and numerically higher (p = 0.13) on the MH diet irrespective of environmental temperature. Plasma NEFA concentration (p = 0.01) and glucose turnover rate (p = 0.03) were decreased during heat exposure, but remained similar between diets. It could be concluded that, although no positive impact of plantain on glucose metabolism was found under the present experimental conditions (plantain constituted only 10% of basal diet), plantain herb is an alternative to MH for rearing sheep in both thermoneutral and hot environments.

• 대기
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• 제23권4호
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• pp.377-387
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• 2013

Projections of changes in the low latitude atmospheric circulation under global warming are investigated using the results of the CMIP5 ensemble mean. For this purpose, 30-yr periods for the present day (1971~2000) and the end of the $21^{st}$ century (2071~2100) according to the RCP emission scenarios are compared. The wintertime subtropical jet is projected to strengthen on the upper side of the jet due to increase in meridional temperature gradient induced by warming in the tropical upper-troposphere and cooling in the stratosphere except for the RCP2.6. It is also found that a strengthening of the upper side of the wintertime subtropical jet in the RCP2.6 due to tropical upper-tropospheric warmings. Model-based projection shows a weakening of the mean intensity of the Hadley cell, an upward shift of cell, and poleward shift of the Hadley circulation for the winter cell in both hemispheres. A weakening of the Walker circulation, which is one of the most robust atmospheric responses to global warming, is also projected. These results are consistent with findings in the previous studies based on CMIP3 data sets. A weakening of the Walker circulation is accompanied with decrease (increase) in precipitation over the Indo-Pacific warm pool region (the equatorial central and east Pacific). In addition, model simulation shows a decrease in precipitation over subtropical regions where the descending branch of the winter Hadley cell in both hemispheres is strengthened.

• 한국정밀공학회지
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• 제30권2호
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• pp.223-230
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• 2013

When a new nuclear fuel is developed, an irradiation test needs to be carried out in the research reactor to analyze the performance of the new nuclear fuel. In order to check the performance of a nuclear fuel during the irradiation test in the test loop of a research reactor, sensors need to be attached in and out of the fuel rod and connect them with instrumentation cables to the measuring device located outside of the reactor pool. In particular, to check the temporary temperature change at the center of a nuclear fuel during the irradiation test, a thermocouple should be instrumented at the center of the fuel rod. Therefore, a hole needs to be made at the center of fuel pellet to put in the thermocouple. However, because the hardness and the density of a sintered $UO_2$ pellet are very high, it is difficult to make a small fine hole on a sintered $UO_2$ pellet using a simple drilling machine even though we use a diamond drill bit made by electro deposition. In this study, an automated drilling machine using a CVD diamond drill has been developed to make a fine hole in a fuel pellet without changing tools or breakage of workpiece. A sintered alumina ($Al_2O_3$) block which has a higher hardness than a sintered $UO_2$ pellet is used as a test specimen. Then, it is verified that a precise hole can be drilled off without breakage of the drill bit in a short time.

• 대한기계학회논문집A
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• 제34권7호
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• pp.935-941
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• 2010

하나로는 한국에 설치되어 있는 열출력 30MW의 개방 수조형 연구로 이다. 이는 발전로와 달리 원자로에서 발생하는 열을 이용하여 전기를 생산하는 것 대신에 원자로의 노심 온도를 유지하기 위하여 냉각탑을 통해 대기로 이 열을 냉각한다. 냉각탑 월간 점검 중에 냉각탑 4번의 냉각팬 감속기가 기준을 상회하는 고진동을 기록하였다. 본 연구의 목적은 고진동의 원인을 찾아 정상적으로 수리하기 위함이다. 연구 방법은 FFT 스펙트럼 기법을 적용하여 고진동의 원인을 분석하였다. 그 결과 고진동 주파수는 피니언 기어의 고유 진동수의 두 배인 354Hz이었다. 피니언 기어를 점검한 결과 이빨 표면이 깨져 있었다. 깨진 피니언 기어를 제거하고 새것으로 교체한 후에는 감속기는 정상적으로 작동하였다.

• 전자공학회논문지D
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• 제36D권6호
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• pp.44-51
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• 1999

High density DRAM의 cell capacitor로 촉망 받고 있는 고유전체 BST박막 커패시터의 저 전계(<0.2MV/cm) 영역에서의 전기전도 현상을 분석하였다. 저 전계 영역에서 Pt/BST/Pt구조의 MIM 커패시터에 일정 전계를 인가한 후 전류를 측정하는 I(t)방법을 이용하여 유전완화전류와 누설전류를 분리해내어 박막의 측정온도 변화, 전계의 크기, 인가방향 변화, 후속 열처리에 따른 BST 박막의 전기전도 기구를 분석하였다. 그 결과, 유전완화전류는 Hoppiong process에 의한 BST박막내부의 trap된 전자들의 이동에 의한 전하재배치로 설명되어지며, 누설전류도 박막내의 trap에 의한 poole-Frenkel process에 의한 것임을 알 수 있었다. 그리고 각 전류성분에 기억하고 있는 trap이 BST박막내의 산호 결핍임을 추정하였다.

• 한국지구과학회지
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• 제36권7호
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• pp.611-616
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• 2015

엘니뇨와 남방진동(엔소)은 변동 주기가 2-8년으로 넓게 걸쳐있으며 그 진폭과 주기 또한 천천히 변하는데 이런 특징을 각각 엔소 불규칙성과 엔소 변조라 한다. 이 연구는 비선형 대기 변동성을 나타나는 Lorenz-63 모형과 간단한 충전 진동자 모형을 결합함으로써 비선형 저차 기후모델을 개발하였다. 이 모델은 동태평양의 해수면 온도 변동의 중심주기, 넓은 주기성, 강도의 수십 년 변동 등과 같은 관측에서 보이는 엔소 특징을 잘 재현하였다. 이것은 대기 카오스 강제력이 엔소의 불규칙성과 변조를 이끌 수 있음을 보여준다. 덧붙여 모델은 서태평양 온난역의 대류활동이 강해지면 라니냐 발생 가능성이 높아지는 것을 제시하였다. 이 모델은 간단하면서도 적도 태평양의 대기-해양 비선형 상호작용을 잘 모사하고 있기에 향후 장기 기후변화 연구에 활동될 것으로 기대된다.

• 한국전산유체공학회지
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• 제20권4호
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• pp.14-20
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• 2015

Without electirc power to cool down the hot reactor core, passive systems utilizing natural circulation are becoming a big specialty of recent neculear systems after the severe accident in Fukusima. When we consider the natural circulation in a pool, thermal mixing phenomena may start from single phase circulation and can continue to two phase condition. Since the CUPID code, which has been developed for two-phase flow analysis, can deal with the phase transition phenomena, the CUPID would be pertinent to natural convection problems in single- and two-phase conditions. Thus, the CUPID should be validated against single- and two-phase natural circulation phenomena. For the first step of the validation process, this study is focused on the validation of single-phase natural circulation. Moreover, the CUPID code solves the fluid properties by the relationship to pressure and temperature from the steam table considering non-condensable gas effects, so that the effects from variable properties are included. Simple square thermal cavity problems are tested for laminar and turbulent conditions against numerical and experimental data. Throughout the investigation, it is found that the variable properties can affect the flow field in laminar condition, but the effect becomes weak in turbulence condition, and the CUPID code implementing steam table is capable of analyzing single phase natural circualtion phenomena.

• 대한용접접합학회:학술대회논문집
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• 대한용접접합학회 2009년 추계학술발표대회
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• pp.55-55
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• 2009

FGB(Flexible Glasswool Backing) Submerged Arc Welding has been one of the main welding processes for one side butt welding in shipbuilding industries, which can efficiently improve the welding productivity by the addition of a supplementary filler metal into the molten weld pool. As recent ships have become larger in size, the application of high tensile and higher grade of steels has been continuously increased. Single pass FGB SA welding process accompanies such a high heat input when welding thick plates that the mechanical properties of weld metal can be dramatically degraded. This study has been performed in order to obtain high toughness and tensile properties of high heat input FGB SA welds, and to evaluate the effect of alloy elements on their mechanical properties. To complete welding 25mm-thick EH36 grade steel plate by single pass, 1.2mm diameter and 1.0mm long cut wires has been distributed in the groove before welding, and three different test coupons have been made using C-1.5%Mn, C-1.8%Mn-0.5%Mo, and C-1.4%Mn-1.7%Ni cut wires to investigate the influence of nickel(Ni) and molybdenum(Mo) on the mechanical properties of welds. Test results showed that the addition of Ni and Mo effectively promotes the formation of Acicular Ferrite(AF), while significantly reducing the amount of Grain Boundary Ferrite(GBF) in weld metal microstructures, which resulted in a beneficial effect on low temperature impact toughness and strength.

• 터널과지하공간
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• 제8권2호
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• pp.107-117
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• 1998

In the case of a fire disaster in a uni-directional road tunnel, it is important to determine the critical ventilation velocity to prevent the backflow travelling toward the tunnel exit where vehicles are stopped. The critical ventilation velocity is horizontal velocity to prevent hot smoke from moving toward the tunnel exit. According to Froude modelling, the model tunnel whcih was 300mm in diameter and 21 m in length was made of acryl tubes. Inner section of acryl tubes was clothed with polycarbonate. 1/20 scaled model vehicles were installed to simulate the situation that vehicles are stopped in the tunnel exit. Methanol in a pool type burner was burned in the middle of tunnel to simulate a fire hazard. In this study, the basis of determining the critical ventilation velocity is the ventilation flow rate that is able to maintain the allowable CO concentration in the tunnel section. We assumed that the allowable CO concentration was backflow dispersion index. Futhermore, We intended to find out CO distribution and temperature distribution according as we changed ventilation velocity. The results of this study were that no backflow happened when ventilation velocity was 0.52 m/s in the case of 5.75 kW. If we adapt these results of a fire disaster releasing 10MW heat capacity in real tunnel which is 400m in length, no backflow happens when ventilation velocity is 2.31m/s. After we figured out dimensionless heat release rate and dimensionless ventilation velocity of model test and those of real test to verify experimental correctness, we tried to find out correlation between experimental results of model tunnel and those of real tunnel.