• 환경영향평가
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• 제16권1호
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• pp.15-26
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• 2007

Surface water is the main drinking water source in Korea. Algal bloom caused by phytoplankton in reservoir is common event in every summer season. To prevent or control the algal blooms, artificial circulation system has been adopted in many reservoirs, including Hoengseung reservoir. Total 7 hydraulic-gun-aerators were installed around the intake tower in Hoengseung reservoir since 2000. This study is to elucidate the effects of hydraulic-gun-aerators on phytoplankton bloom, pH, DO, temperature and evaluate the selective withdrawal and vertical distribution of phytoplankton by means of submersible fluorescence probe, which features high correlation with a standard ISO method (r=0.90, P<0.0001) for chlorophyll-a quantification.

• 한국대기환경학회지
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• 제25권4호
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• pp.304-315
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• 2009

Numerical simulations were carried out to understand the effect of Sea Surface Temperature (SST) spatial distribution on regional circulation. A three-dimensional non-hydrostatic atmospheric model RAMS, version 6.0, was applied to examine the impact of SST forcing on regional circulation. New Generation Sea Surface Temperature (NGSST) data were implemented to RAMS to compare the results of modeling with default SST data. Several numerical experiments have been undertaken to evaluate the effect of SST for initialization. First was the case with NGSST data (Case NG), second was the case with RAMS monthly data (Case RM) and third was the case with seasonally averaged RAMS monthly data (Case RS). Case NG showed accurate spatial distributions of SST but, the results of RM and RS were $3{\sim}4^{\circ}C$ lower than buoy observation data. By analyzing practical sea surface conditions, large difference in horizontal temperature and wind field for each run were revealed. Case RM and Case RS showed similar horizontal and vertical distributions of temperature and wind field but, Case NG estimated the intensity of sea breeze weakly and land breeze strongly. These differences were due to the difference of the temperature gradient caused by different spatial distributions of SST. Diurnal variations of temperature and wind speed for Case NG indicated great agreement with the observation data and statistics such as root mean squared error, index of agreement, regression were also better than Case RM and Case RS.

• 전력전자학회논문지
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• 제20권2호
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• pp.130-136
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• 2015

This study describes the structure and operation modes of vertical stabilization (VS) converter for international thermonuclear experimental reactor (ITER) and proposes a protection method. ITER VS converter supplies voltage (${\pm}1000V$)/current (${\pm}22.5kA$) to superconducting magnets for plasma current vertical stabilization. A four-quadrant operation must be achieved without zero-current discontinuous section. The operation mode of the VS converter is separated in 12-pulse mode, 6-pulse mode and circulation current mode according to the magnitude of the load current. Protection measures, such as bypass and discharge, are proposed for abnormal conditions, such as over current, over voltage, short circuit, and voltage sag. VS converter output voltage is controlled to satisfy voltage response time within 20 msec. Bypass operation is completed within 60 msec and discharge operation is performed successfully. The feasibility of the proposed control algorithm and protection measure is verified by assembling a real controller and implementing a power system including the VS converter in RTDS for a hardware-in-loop (HIL) facility.

• 한국환경과학회지
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• 제18권11호
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• pp.1271-1281
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• 2009

The heavy snowfall event over the eastern part of Seoul, Korea on Mar. 04, 2008 has been abruptly occurred after the frontal system with the heavy snowfall event had been past over the Korean peninsula on Mar. 03, 2008. Therefore, this heavy snowfall event couldn't be predicted well by any means of theoretical knowledges and models. After the cold front passed by, the cold air mass was flown over the peninsula immediately and became clear expectedly except the eastern part and southwestern part of peninsula with some large amount of snowfall. Even though the wide and intense massive cold anticyclone was expanded and enhanced by the lowest tropospheric baroclinicity over the Yellow Sea, but the intrusion and eastward movement of cold air to Seoul was too slow than normally predicted. Using the data of numerical model, satellite and radar images, three dimensional analysis Products(KLAPS : Korea Local Analysis and Prediction System) of the environmental conditions of this event such as temperature, equivalent potential temperature, wind, vertical circulation, divergence, moisture flux divergence and relative vorticity could be analyzed precisely. Through the analysis of this event, the formation and westward advection of lower cyclonic circulation with continuously horizontal movement of air into the eastern part of Seoul by the analyses of KLAPS fields have been affected by occurring the heavy snowfall event. As the predictability of abrupt snowfall event was very hard and dependent on not only the synoptic atmospheric circulation but also for mesoscale atmospheric circulation, the forecaster can be predicted well this event which may be occurred and developed within the very short time period using sequential satellite images and KLAPS products.

• 한국기계가공학회지
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• 제15권5호
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• pp.8-17
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• 2016

While greenhouses have been utilized as a sustainable alternative to traditional soil farming, they are often powered by diesel boilers that necessitate vast amounts of non-renewable energy and emit toxic fumes. Thus, geothermal heat pumps have been proposed as a more energy-efficient substitution for diesel boilers. Currently, most horticultural facilities in the United States use shallow geothermal systems, and are often equipped with horizontal underground heat exchangers as well as heat pump equipment. These shallow geothermal systems require a large drilling site and heat pump to function, which results in high maintenance costs. The heat pump itself consumes a large amount of power, which degrades system performance. Conversely, high temperatures can be attained within a single borehole in deep geothermal vertical closing systems without using a heat pump. This setup can dramatically reduce the power consumption and improve system performance. In this study, we have modeled a circulation simulator after the circulation systems in deep geothermal facilities to analyze a 2000-meter borehole in Naju-Sanpo-myeon. The simulator is operated by manipulating various putative parameters affecting system performance to analyze the system's coefficient of performance.

• 한국수소및신에너지학회논문집
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• 제24권2호
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• pp.128-135
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• 2013

The nuclear fuel cycle plant is composed of various subsystems such as a fuel storage and delivery system (SDS), a tokamak exhaust processing system, a hydrogen isotope separation system, and a tritium plant analytical system. Korea is sharing in the construction of the International Thermonuclear Experimental Reactor (ITER) fuel cycle plant with the EU, Japan, and the US, and is responsible for the development and supply of the SDS. Hydrogen isotopes are the main fuel for nuclear fusion reactors. Metal hydrides offer a safe and convenient method for hydrogen isotope storage. The storage of hydrogen isotopes is carried out by absorption and desorption in a metal hydride bed. These reactions require heat removal and supply respectively. Accordingly, the rapid storage and delivery of hydrogen isotopes are enabled by a rapid cooling and heating of the metal hydride bed. In this study, we designed and manufactured a vertical-type hydrogen isotope storage bed, which is used to enhance the cooling performance. We present the experimental details of the cooling performances of the bed using various cooling parameters. We also present the modeling results to estimate the heat transport phenomena. We compared the cooling performance of the bed by testing different cooling modes, such as an isolation mode, a natural convection mode, and an outer jacket helium circulation mode. We found that helium circulation mode is the most effective which was confirmed in our model calculations. Thus we can expect a more efficient bed design by employing a forced helium circulation method for new beds.

• 한국안전학회지
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• 제35권3호
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• pp.96-104
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• 2020

The numerical simulations were conducted to investigate the thermal-fluid phenomena occurred inside the experimental apparatus during a PCCS, used to remove heat released in accidents from a containment of light water nuclear power plant, operation. Numerical simulations of the flow and heat transfer caused by wall condensation inside the containment simulation vessel (CSV), which equipped with 18 vertical heat exchanger tubes, were conducted using the commercial computational fluid dynamics (CFD) software ANSYS-CFX. Shear stress transport (SST) and the wall condensation model were used for turbulence closure and wall condensation, respectively. The simulation using the actual size of the apparatus. However, rather than simulating the whole experimental apparatus in consideration of the experimental cases, calculation resources, and calculation time, the simulation model was prepared only in CSV. Selective simulation was conducted to verify the effects of non-condensable gas(NC gas) concentration, CSV internal pressure, and wall sub-cooling conditions. First, as a result of the internal flow of CSV, it was observed that downward flow due to condensation occurred surface of the vertical tube and upward flow occurred in the distant place. Natural convection occurred actively around the heat exchanger tube. Due to this rising and falling internal flow, natural circulation occurred actively around the heat exchanger tubes. Next, in order to check the performance of built-in condensation model using according to the non-condensable gas concentration, CSV internal flow and wall sub-cooling, the heat flux values were compared with the experimental results. On average, the results were underestimated with and error of about 25%. In addition, the influence of CSV internal pressure and wall sub-cooling was small, but when the condensate was highly generated due to the low non-condensable gas concentration, the error was large compared to the experimental values. This is considered to be due to the nature of the condensation model of the CFX code. However, in spite of the limitations of CFD, it is valid to use the built-in condensation model of CFD for PCCS performance prediction from a conservative perspective.

• 한국해안·해양공학회논문집
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• 제27권2호
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• pp.79-93
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• 2015

Gulf Stream 인근해역에서 해양 중규모 와동의 삼차원 구조분석을 HYCOM 수치모델을 사용하여 수행하였다. 시계방향 및 반시계 방향으로 회전하는 와동들의 구조를$1/12^{\circ}$와 $1/48^{\circ}$ 두개의 모델 해상도를 사용하여 비교하였다. 라그랑지안 모수인 Finite Size Lyapunov Exponent (FSLE) 와 Okubo-Weiss parameter(OW) 분포를 분석한 결과 표층의 와동구조가 수심 깊은 곳까지 영향을 미치는 것으로 나타났는데, 이는 와동에 의한 수평방향 해수운동이 수직방향 해수운동보다 크기 때문인 것으로 해석되었다. 고해상도 모델의 경우 와동근처에서 10 km 미만의 미세난류구조 들이 많이 발견되었으며, 이러한 미세난류구조들은 고해상도 모델의 와동근처에서 해수의 움직임을 저해상도 모델보다 불규칙하게 만드는 것으로 나타났다. 이러한 미세난류구조에 의한 해수의 불규칙한 움직임은 분산계수 (dispersion coefficient)에도 영향을 미치는데, 수평 분산계수의 경우 해수운동이 자유로운 고해상도 모델이 저해상도 모델보다 그 값이 더 크게 나타났다. 수직 분산계수의 값은 저해상도 모델에서 더 크게 나왔는데, 이는 와동의 경사진 궤도를 따라 움직이는 저해상도 모델의 해수운동이 수직 분산계수값을 증가시키기 때문인 것으로 들어났다. 상대 수직 분산계수의 경우 이러한 궤도의 영향이 줄어들기 때문에 해수의 수직운동을 측정하는데 있어 절대 수직 분산계수 보다 더 적합한 것으로 판명되었다.

• 한국해양학회지:바다
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• 제18권4호
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• pp.266-276
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• 2013

바다-육지-대기로 이루어진 기후 시스템에서 가장 큰 탄소의 저장고는 바다이다. 바다가 대기로부터 탄소를 흡수하는 주요 수단은 생물 활동에 의한 것으로서, 광합성에 의해 유기 물질로 동화된 탄소가 해저로 침강하고 분해되는 과정에서 깊은 바다물은 탄소를 축적하게 된다. 이러한 탄소 수송 작용을 생물 펌프라 부르며, 해수면 탄소 농도를 낮춤으로써 대기 중 이산화탄소 분압을 낮은 상태로 유지해주는 중요한 기작이다. 생물 펌프에 의해 해저에 축적된 탄소는 해양 순환에 의해 해수면에 돌아오고, 해양-대기 기체 교환에 의해 대기로 배출된다. 바다가 대기와 소통하는 이산화탄소의 양은 과거 빙하기-간빙기 기후 변동과 관련하여 과거 수십만년동안 대기 중 이산화탄소 분압변화에 주도적인 역할을 하여 온 것으로 알려져 있다. 본 논문에서는 바다에서 일어나는 탄소 순환을 간단하게 소개하고, 해양 순환의 변화가 어떻게 탄소 순환을 변형시키고, 대기 중 이산화탄소에 영향을 미치는지를 기후 변동의 관점에서 살펴보고자 한다.

• 한국환경과학회지
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• 제16권9호
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• pp.1051-1061
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• 2007

The impact of midlatitude synoptic system (upper-level trough) on typhoon intensity change was investigated by analyzing the spatial and temporal characteristics of vertical wind shear (VWS), relative eddy momentum flux convergence (REFC), and potential vorticity (PV). These variables were computed over the radial mean $300{\sim}1,000km$ from the typhoon center by using GDAPS (Global Data Assimilation and Prediction System) data provided by the Korea Meteorological Administration (KMA). The selected cases in this study are typhoons Rusa (0215) and Maemi (0314), causing much damage in life and property in Korea. Results show that the threshold value of VWS indicating typhoon intensity change (typhoon to severe tropical storm) is approximately 15 m/s and of REFC ranges 6 to 6.5 $ms^{-1}day^{-1}$ in both cases, respectively. During the period with the intensity of typhoon class, PVs with 3 to 3.5 PVU are present in 360K surface-PV field in the cases. In addition, there is a time-lag of 24 hours between central pressure of typhoon and minimum value of VWS, meaning that the midlatitude upper-level trough interacts with the edge of typhoon with a horizontal distance less than 2,000 km between trough and typhoon. That is, strong midlatitude upper-level divergence above the edge of the typhoon provides a good condition for strengthening the vertical circulation associated with the typhoons. In particular, when the distance between typhoon and midlatitude upper-level trough is less than 1,000 km, the typhoons tend to weaken to STS (Severe Tropical Storm). It might be mentioned that midlatitude synoptic system affects the intensity change of typhoons Rusa (0215) and Maemi (0314) while they moves northward. Thus, these variables are useful for diagnosing the intensity change of typhoon approaching to the Korean peninsula.