• 에너지공학
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• 제2권3호
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• pp.251-257
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• 1993

유한체적법(Finite Volume Method)을 이용하여 블럭이 부착된 수평 유로의 강제 대류 열전달 현상을 해석하였다. 블럭의 갯수, Reynolds 수 그리고 블럭 간의 간격을 변화시키면서 해석하였고 블럭이 부착된 수평판은 전도벽면 (conducting plate)으로 해석하였다. 블럭내에서는 1/2 체적과 전체적에서 열원이 있다고 가정하였으며, 유동은 2차원 정상상태 비압축성 유동이라고 하였다. 전체적에서 열원이 있는 경우보다 1/2 체적에서 열원이 있는 경우가 전체 온도값이 높게 나타났으며, Reynolds 수와 블럭 사이의 간격이 커질수록 온도값이 낮아진다. 수평판이 단열되어 있는 경우보다 전도벽면으로 해석한 경우가 온도값이 낮게 나타나며 수평판이 단옅되어 있다고 가정한 경우에는 최고온도 값의 위치가 단열면 근처이나 전도벽면의 경우에는 블럭내의 중심에서 우측으로 치우쳐 있다. 그리고, 블럭의 1/2 체적에서 열원이 있는 경우의 최고 온도 위치는 블럭의 전체적에서 열원이 있는 경우의 위치보다 더 상단에 치우침을 알 수 있다. Nusselt 수는 블럭 윗면의 좌측 부분에서 다른 부분보다 높은 수치를 나타내고 있으며 Reynolds 수가 커질수록 Nusselt 수의 값이 증가한다. 그리고, 블럭내의 최고 온도값은 이러한 무차원 변수들과의 상관 함수를 이용하여 예측하였다.

• 대기
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• 제29권5호
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• pp.585-597
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• 2019

This study evaluates the performance of Weather Research and Forecasting (WRF) model in simulating temperature over the COordinated Regional climate Downscaling EXperiment-East Asia (CORDEX-EA) Phase 2 domain for the reference period (1981~2005), and assesses the changes in temperature and its extremes in the mid-21st century (2026~2050) under global warming based on Representative Concentration Pathway (RCP) scenarios. MPI-ESM-LR forced by two RCP scenarios (RCP2.6 and RCP8.5) is used as initial and lateral boundary conditions. Overall, WRF can capture the observed features of temperature distribution reflecting local topographic characteristic, despite some disagreement between the observed and simulated patterns. Basically, WRF shows a systematic cold bias in daily mean, minimum and maximum temperature over the entire domain. According to the future projections, summer and winter mean temperatures over East Asia will significantly increase in the mid-21st century. The mean temperature rise is expected to be greater in winter than in summer. In accordance with these results, summer (winter) is projected to begin earlier (later) in the future compared to the historical period. Furthermore, a rise in extreme temperatures shows a tendency to be greater in the future. The averages of daily minimum and maximum temperatures above 90 percentiles are likely to be intensified in the high-latitude, while hot days and hot nights tend to be more frequent in the low-latitude in the mid-21st century. Especially, East Asia would be suffered from strong increases in nocturnal temperature under future global warming.

• 한국신재생에너지학회:학술대회논문집
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• 한국신재생에너지학회 2009년도 춘계학술대회 논문집
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• pp.373-373
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• 2009

Relative humidity, membrane conductivity and water activity are critical parameters of polymer electrolyte membrane fuel cells (PEMFC) for high performance and reliability. These parameters are closely related with temperature. Moreover, the ideal values of these parameters are not always identical along the channels. Therefore, the cooling channel design and its operating condition should be well optimized along the all location of the channels. In the present study, we have performed a numerical investigation on the effects of cooling channels on performance of a PEMFC. Three-dimensional Navier-Stokes equations are solved with the energy equation including heat generated by the electrochemical reactions in the fuel cell. The present numerical model includes the gas diffusion layers (GDL) and serpentine channels for both anode and cathode gas flows, as well as cooling channels. To accurately predict the water transport across the membrane, the distribution of water content in the membrane is calculated by solving a nonlinear differential equation with a nonlinear coefficient, i.e., the water diffusivity which is a function of water content as well as temperature. Main emphasis is placed on the heat transfer between the solid bipolar plate and coolant flow. The present results show that local current density is affected by cooling channels due to the change of the oxygen concentration and the membrane conductivity as well as the water content. It is also found that the relative humidity is influenced by the generated water and the gas temperature and thus it affects the distribution of fuel concentration and the conductivity of the membrane, ultimately fuel cell performance. Unit-cell experiments are also carried out to validate the numerical models. The performance curves between the models and experiments show reasonable results.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2001년도 추계학술대회논문집B
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• pp.499-504
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• 2001

A validation of a 3D CFD model for predicting local subcooling of moderator in the vicinity of calandria tubes in a CANDU reactor is performed. The small scale moderator experiments performed at Sheridan Park Experimental Laboratory(SPEL) in Ontario, Canada[1] is used for the validation. Also a comparison is made between previous CFD analyses based on 2DMOTH and PHOENICS, and the current model analysis for the same SPEL experiment. For the current model, a set of grid structures for the same geometry as the experimental test section is generated and the momentum, heat and continuity equations are solved by CFX-4.3, a CFD code developed by AEA technology. The matrix of calandria tubes is simplified by the porous media approach. The standard $k-\varepsilon$ turbulence model associated with logarithmic wall treatment and SIMPLEC algorithm on the body fitted grid are used and buoyancy effects are accounted for by the Boussinesq approximation. For the test conditions simulated in this study, the flow pattern identified is a buoyancy-dominated flow, which is generated by the interaction between the dominant buoyancy force by heating and inertial momentum forces by the inlet jets. As a result, the current CFD moderator analysis model predicts the moderator temperature reasonably, and the maximum error against the experimental data is kept at less than $2.0^{\circ}C$ over the whole domain. The simulated velocity field matches with the visualization of SPEL experiments quite well.

• 대한기계학회논문집B
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• 제25권5호
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• pp.722-730
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• 2001

The local heat transfer of an axisymmetric submerged air jet impinging on a heated flat plate is investigated experimentally with the variation of mesh-screen solidity. The screen installed in front of the nozzle exit modifies the flow structure and local heat transfer characteristics. The mean velocity and turbulence intensity profiles of streamwise velocity component are measured using a hot-wire anemometry. The temperature distribution on the heated flat surface is measured with thermocouples. The smoke-wire flow visualization technique was employed to understand the near-field flow structure qualitatively for different mesh screens. Large-scale toroidal vortices and high turbulence intensity enhance the heat transfer rate in the stagnation region. For a higher solidity, turbulence intensity become higher which increases the local heat transfer at small nozzle-to-plate spacings such as L/D<6. The local and average Nusselt numbers of impinging jet from the $\sigma$(sub)s=0.83 screen at L/D=2 are about 5.6∼7.5% and 7.1% larger than those for the case of no screen, respectively. For the nozzle-to-plate spacings larger than 6, however, the turbulence intensities for all tested screens approach to an asymptotic curve and the mean velocity along the jet centerline decreases monotonically. As the nozzle-to-plat spacing increases for high solidity screens, the heat transfer rate decreases due to the reduction in turbulence intensity and jet momentum.

• 한국대기환경학회지
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• 제28권6호
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• pp.615-631
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• 2012

This study provides an evaluation for capability of Integrated Climate and Air quality Modeling System (ICAMS) on future regional scale climate projection. Temperature and precipitation are compared between ground-level observation data and results of regional models (MM5) for the past 30 years over the Korean peninsula. The ICAMS successfully simulates the local-scale spatial/seasonal variation of the temperature and precipitation. The probability distribution of simulated daily mean and minimum temperature agree well with the observed patterns and trends, although mean temperature shows a little cold bias about $1^{\circ}C$ compared to observations. It seems that a systematic cold bias is mostly due to an underestimation of maximum temperature. In the case of precipitation, the rainfall in winter and light rainfall are remarkably simulated well, but summer precipitation is underestimated in the heavy rainfall phenomena of exceeding 20 mm/day. The ICAMS shows a tendency to overestimate the number of washout days about 7%. Those results of this study indicate that the performance of ICAMS is reasonable regarding to air quality predication over the Korean peninsula.

• 대한지리학회지
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• 제48권1호
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• pp.37-55
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• 2013

기존의 푄 관련 선행연구에서는 크게 영서 지방에서 푄(Foehn)이 두드러지게 나타난다고 하였다. 그러나 영서 지방의 어디에서 두드러지게 나타나며 어떠한 분포양상을 보이는지에 대해서는 연구된 바가 없다. 본 연구는 영서 지방 중 홍천 지역을 대상으로 푄 발생 시의 일최고기온분포도 작성을 통해 국지적인 규모에서 푄의 빈도와 강도에 어떠한 차이가 나는지를 파악하고자 하였다. 연구대상기간은 2003년부터 2012년까지 봄에서 초여름(3~6월)이다. 기온과 해발고도를 변수로 한 공동크리깅(CoKriging)기법을 사용하였을 경우 푄 발생일의 일최고기온분포도 작성에 있어서 보다 높은 정확도를 나타내었다. 일최고기온분포도의 작성을 통해 푄은 홍천 전 지역에서 나타나는 것이 아니며 일부 지역에서만 나타나는 것을 확인하였다. 특히 홍천강 하류 지역에서 푄이 빈번하며 강도 또한 강함을 파악하였다. 홍천 지역 주민들의 푄 인식 정도에 대하여도 조사해 보았다. 봄철에 고온 건조함을 느끼지만 그것이 푄에 의한 것인지 인식은 하지 못하고 있었다. 본 연구의 국지기후 규모에서 푄을 분석하는 절차와 기법은 지역기후 이해와 연구분야에 기여 할 수 있을 것이며 특히 근래에 봄철부터 여름철에 두드러지게 나타나는 고온, 여름철 폭염, 봄철 농작물 생육 등과 관련하여 응용하여 적용할 수 있을 것이라 생각한다.

• Journal of Magnetics
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• 제21권3호
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• pp.476-489
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• 2016

In the present study, squeezing flow of micropolar nanofluid between parallel infinite disks in the presence of magnetic field perpendicular to plane of the disks is taken into account. The constitutive equations that govern the flow configuration are converted into nonlinear ordinary differential with the help of suitable similarity transforms. HAM package BVPh2.0 has been employed to solve the nonlinear system of ordinary differential equations. Effects of different emerging parameters like micropolar parameter K, squeezed Reynolds number R, Hartmann number M, Brownian motion parameter Nb, thermophoresis parameter Nt, Lewis number Le for dimensionless velocities, temperature distribution and concentration profile are also discussed graphically. In the presence of strong and weak interaction (i.e. n = 0 and n = 0.5), numerical values of skin friction coefficient, wall stress coefficient, local Nusselt number and local Sherwood number are presented in tabulated form. To check the validity and reliability of the developed algorithm BVPh2.0 a numerical investigation is also a part of this study.

• Nuclear Engineering and Technology
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• 제52권2호
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• pp.258-270
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• 2020

A new task of using the Jacobian-Free-Newton-Krylov (JFNK) method for the PWR core transient simulations involving neutronics, thermal hydraulics and mechanics is conducted. For the transient scenario of PWR, normally the Picard iteration of the coupled coarse-mesh nodal equations and parallel channel TH equations is performed to get the transient solution. In order to solve the coupled equations faster and more stable, the Newton Krylov (NK) method based on the explicit matrix was studied. However, the NK method is hard to be extended to the cases with more physics phenomenon coupled, thus the JFNK based iteration scheme is developed for the nodal method and parallel-channel TH method. The local gap conductance is sensitive to the gap width and will influence the temperature distribution in the fuel rod significantly. To further consider the local gap conductance during the transient scenario, a 1D mechanics model is coupled into the JFNK scheme to account for the fuel thermal expansion effect. To improve the efficiency, the physics-based precondition and scaling technique are developed for the JFNK iteration. Numerical tests show good convergence behavior of the iterations and demonstrate the influence of the fuel thermal expansion effect during the rod ejection problems.

• 대한기계학회논문집B
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• 제25권12호
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• pp.1869-1878
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• 2001

Experiments are carried out to investigate the effect of pulsations on the flow and heat transfer characteristics of an axisymmetric impinging jet on a flat plate heated by using a gold coated aim. Vertex motion in the impinging jet is visualized using a fog generator, and a thermochromatic liquid crystal (TLC) technique is used to measure the time averaged local temperature distributions on the impingement plate. In addition, the quantitative data for mean velocity and turbulence intensity are obtained employing hot-wire anemometer. Parameters such as pulsating frequency (f = 0, 10 and 20 Hz) and the nozzle-to-palate spacing (H/D = 2, 10) are considered at the jet Reynolds number of 20,000. Consequently, the significant changes of flow structure and local Nusselt number distribution due to pulsations are observed. In the case of H/D = 2, the enhanced heat transfer coefficient exceeding 30 % is observed at the stagnation point. At the high H/D, heat transfer rate increases with pulsation frequency.