• Transactions of the Korean Society of Automotive Engineers
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• v.5 no.2
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• pp.136-148
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• 1997

The present study has been carried out to develop a computational procedure for the analysis of the off-design performance in centrifugal compressors with vaneless diffusers by integrating empirical loss models and analytical equations. Losses in centrifugal compressors stem from a number of sources and their exact calculation is not yet possible. This study investigates several modeling schemes and shows that a fairly good prediction can be achieved by a proper selection of the most important flow parameters resulting form a meanline one-dimensional analysis. The performance maps for compressors are calculated and compared with measured performance maps. The off-design performance characteristics in terms of the pressure ratio vs. mass flow produced have generally correct forms. However, no universal means have been found to predict accurately the onset of surge. The prediction method developed through this study can serve as a tool to ensure good matching between parts and it can assist the understanding of the operational characteristics of general purpose centrifugal compressors.

• Journal of the Korean Solar Energy Society
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• v.25 no.4
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• pp.101-110
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• 2005

An experimental study of heat transfer of submerged water jet impinging normally on a flat plate is presented. Heat transfer measurements obtained with Reverse cone type nozzle(Rcone) were compared to those obtained with Cone type nozzle(Cone) and Square edged type nozzle(Vert) of the same diameter(D=8mm) for different jet velocities in the range of $3{\sim}7m/s(Re_D=30000{\sim}70000)$ and various nozzle-to target spacings($H/D=2{\sim}10$). The local Nusselt number profiles exhibited a sharp drop for $r/D{\leq}0.5$ and 2nd, 3rd peaks revealed at r/D=2, 3 respectively, followed by a slower decrease there after. The peaks were weakened with increasing the nozzle-to target spacing and decreasing the jet velocity. The stagnation Nusselt number of the Reverse cone type nozzle was larger than those of the other two nozzles for H/D=2. 10, but Cone type nozzle had the highest value for $H/D=4{\sim}8$. Also average Nusselt number of the Reverse cone type nozzle was higher than those of the other two nozzles at $H/D=2{\sim}10$, except for $V_o=7ms$ of H/D=6.

• Journal of the Korean Society of Safety
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• v.19 no.1
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• pp.124-130
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• 2004

The structure of the nonpremixed methane-air counterflow flames in microgravity was investigated by axisymmetric simulation with Fire Dynamics Simulator (FDS) to evaluate the numerical method and to see the effects of strain rate and fuel concentration on the diffusion flame structure in microgravity. Results of FDS for the methane mole fractions, $X_m$=20, 50, and 80% in the fuel stream, and the global strain rates $a_g$=20, 50, and $90s^{-1}$ for each methane mole fraction were compared with those of OPPDIF, an one-dimensional flamelet code. There was good agreement in the temperature and axial velocity profiles between the axisymmetric and one-dimensional computations. It was shown that FDS is applicable to the counterflow flames in a wide range of strain rate and fuel concentration by predicting accurately the flame thickness, flame positions and stagnation points.

• Transactions of the Korean Society of Mechanical Engineers
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• v.18 no.4
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• pp.1019-1030
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• 1994

Natural convection heat transfer from a horizontal ice cylinder immersed in quiescent cold pure water was studied experimentally. The experiment was conducted for the ambient water temperatures ranging from $2.0^{\cric}C$ to $10.0^{\circ}C$. The flow fields around an ice cylinder and its melting shapes were visualized and local Nusselt numbers obtained. Especially, its attention was focused on the density maximum effects and stagnation point Nusselt number. From the visualized photographs of flow fields, three distinct flow patterns were observed with the ambient water temperature variation. The melting shapes of ice cylinder are various in shape with flow patterns. Steady state upflow was occured at the range of $2.0^{\circ}C \leq T_{\infty} \leq 4.6^{\circ}C$ and steady state downflow was occured at $T_{\infty} \geq 6.0^{\circ}C$. In the range of $4.7^{\circ}C < T_{\infty} < 6.0^{\circ}C$, three-dimensional unsteady state flow was observed. Especially, the melting shapes of ice cylinder have formed the several spiral flutes for the temperatures ranging from $5.5^{\circ}C$ to $5.8^{\circ}C$. For upflow regime, the maximum stagnation point Nusselt number exists at $T_{\infty} = 2.5^{\circ}C$ and as the ambient water temperature increases the Nusselt number decreases. At ambient water temperature of about $5.7^{\circ}C$, Nusselt number shows its minimum value.

• Transactions of the Korean Society of Mechanical Engineers
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• v.11 no.2
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• pp.345-353
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• 1987

An analysis is performed to study flow and heat transfer characteristics of mixed free and forced convection about a sphere. Nonsimilar boundary layer equations which are valid over the entire regime of mixed convection are derived in terms of the mixed convection parameter, Gr/Re$^{2}$, through a dimensional analysis. The transformed conservation equations are solved by a finite difference method for the whole range of mixed convection regime. Numerical results for fluids having the Prandtl number 0.7 and 7 are presented. As the mixed convection parameter increases, the local friction coefficient and local heat transfer coefficient increases as well. For small Prandtl number, the friction coefficient is larger, while for large Prandtl number, the heat transfer coefficient is larger. Natural convection effect on the forced flow is more sensitive for small Prandtl number fluid. Flow separation migrates rearward as an increase in the mixed convection parameter. For small Prandtl number, the buoyancy effect is relatively small so that the flow separation occurs earlier.

• Journal of Agricultural Extension & Community Development
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• v.19 no.2
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• pp.497-509
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• 2012

19세기 말, 일본(1877년)과 중국(1892년)은 서양으로부터 비슷한 시기에 포도주를 들여왔지만 그 발전과정에는 많은 차이가 있다. 일본은 소규모의 포도주기업 위주로 지속적인 발전을 해온 반면, 중국은 소수 대규모로 발전해 오다 중간에 정치경제적인 이유로 정체되었는데 최근에 와서 많은 발전을 이룩하고 있다. 이 때문에 양국 간 포도주산업과 지역의 발전측면에서 보면, 두 국가 간의 이러한 차이의 비교는 양국 간의 포도주산업과 지역의 발전에 많은 시사점을 제공한다. 특히, 현재 중국농민의 권익과 농민전업조직의 필요성이 갈수록 중시되는 상황에서 일본 카추누마(胜沼)지역의 지방정부, 포도주기업, 농가 간 유기적인 포도주 생산과 지역발전을 위한 합작방식은 향후 중국 옌타이(烟台)지역의 포도주산업의 발전뿐만 아니라 농촌지역 내 협력조직체제의 구축과 발전에도 많은 시사점을 주고 있다.

• Electronics and Telecommunications Trends
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• v.25 no.4
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• pp.130-141
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• 2010

2000년대 이후 국내 통신시장이 인수합병을 통해 주요 사업자로 재편되면서 유무선과 차세대 신규 서비스를 중심으로 통신사업자간 경쟁이 치열하게 전개되고 있다. 통신사업자간 경쟁은 개별 서비스간 경쟁을 넘어 에코시스템의 경쟁으로 부상하고 있으며 이에 따라 요금경쟁, 결합서비스 제공경쟁, 신규 서비스 설비경쟁을 통한 고객지향적인 비즈니스 전략이 기업의 핵심전략으로 부각되고 있다. 이러한 시장에서의 경쟁은 사업자의 재무상황에 영향을 미치고 있다. 특히 국내 통신시장이 대부분 성숙기에 진입, 재무적 측면에서 매출이 정체되는 상황에서 마케팅 비용이 급증하는 현상이 나타나고 있다. 이에 본 고에서는 국내 주요 유무선 통신사업자의 재무추이를 분석하고 우리나라 통신시장의 지속적인 성장을 위한 시사점을 제시하였다.

• Nuclear industry
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• v.16 no.3 s.157
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• pp.68-81
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• 1996

구미 선진국을 필두로 원자력 개발이 세계적으로 정체되고 있는 가운데, 세계의 원자력발전량은 94년 실적으로 2조1,296억kWh로서 세계 총 발전량의 약 17$\%$를 공급하였다. 미국은 지금도 세계 원자력 설비용량의 30$\%$를 점하고 있고, 프랑스$\cdot$일본$\cdot$옛 소련이 그 뒤를 잇고 있다. 급속한 경제발전과 에너지부족에 대응하기 위하여 아시아 여러나라를 중심으로 원자력 개발의욕이 높은 점과 전 지구적인 문제가 된 환경문제를 고려하면, 원자력은 세계의 에너지문제를 말할 때 앞으로도 중요한 주제가 될 것이다. 핵 비확산과 안전성의 확보, 폐기물 처리$\cdot$처분이라는 공통의 과제를 해결하기 위해서는 지금이야말로 폭?은 관점에서 국제적 수준의 대응이 요구되고 있다.

• Proceedings of the Korea Water Resources Association Conference
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• 2004.05b
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• pp.1371-1375
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• 2004

하천은 대부분이 단일하천이 아닌 몇 개의 지류가 본류와 만나는 복잡한 하천망으로 구성되어 있다. 이때 지류와 본류가 만나는 합류부는 단일하천내 통상의 단면에 비하여 수위, 유속등의 수리학적 특성이 복잡한 현상을 보이게 되려 상류로부터 유입되는 본류와 지류의 유량비, 합류 각도 등에 따라 합류부에서의 수리학적 특성은 매우 민감하게 변화된다. 이러한 변화는 본류 및 지류에 의한 배수효과도 발생된다. 합류부의 배수효과는 합류점뿐만 아니라 합류점 상류의 본류와 지류의 수위 및 유속변화를 유발하여 홍수피해를 가중시키게 된다. 따라서 본류와 지류가 만나는 합류부의 정확한 수리특성의 파악은 안정적인 하천기능을 유지하기 위한 매우 중요한 요소라 할 수 있다. 이를 위해서 합류후 정체구간의 수리 특성분석과 그에 따른 통수능의 저하가 본류에 미치는 배수효과를 분석하였다.

• Journal of the Korea Academia-Industrial cooperation Society
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• v.22 no.3
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• pp.58-64
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• 2021

As the number of registered vehicles increases, traffic congestion will worsen worse, which may act as an inhibitory factor for urban social and economic development. Through accurate traffic flow prediction, various AI techniques have been used to prevent traffic congestion. This paper uses the data from a VDS (Vehicle Detection System) as input variables. This study predicted traffic flow in five levels (free flow, somewhat delayed, delayed, somewhat congested, and congested), rather than predicting traffic flow in two levels (free flow and congested). The Catboost model, which is a machine-learning algorithm, was used in this study. This model predicts traffic flow in five levels and compares and analyzes the accuracy of the prediction with other algorithms. In addition, the preprocessed model that went through RandomizedSerachCv and One-Hot Encoding was compared with the naive one. As a result, the Catboost model without any hyper-parameter showed the highest accuracy of 93%. Overall, the Catboost model analyzes and predicts a large number of categorical traffic data better than any other machine learning and deep learning models, and the initial set parameters are optimized for Catboost.