• 한국유체기계학회 논문집
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• 제9권1호
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• pp.9-18
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• 2006

In the range of very low specific speed ($n_s<0.25$, non-dimensional), the performance of a centrifugal pump is much different from that of a centrifugal pump of normal ns and the efficiency of the pump drops rapidly with the decrease of $n_s$. In order to examine the reason of unstable performance characteristics of the very low $n_s$- centrifugal pump, the internal flow of the pump with a semi-open impeller is measured by a PTV(Particle Tracking Velocimetry) system. The purpose of this study is to make clear the internal flow characteristics and to obtain basic knowledge of the pump performance. The results show that the leakage flow through tip clearance give a strong effect on the flow pattern of impeller passage. A large vortex in the impeller passage and a strong reverse flow at impeller outlet are formed in the range of small flow rates, and the vortex and the reverse flow together reduce the absolute tangential velocity at the impeller outlet and cause the performance instability.

• International Journal of Fluid Machinery and Systems
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• 제6권3호
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• pp.152-159
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• 2013

A two-dimensional Darrieus-type hydroturbine system, installed with a wear for flow streams such as small rivers and waterways, has been developed for hydropower utilization of extra-low head less than 2m. There are several problems such as flow rate change and flowing wastes to be solved for its practical use in natural flow streams. In the present study, at first, a design guideline in the case of overflow or bypass flow is shown by using simple flow model. Next, in order to avoid the unexpected obstacles flowing into the hydroturbine, an installation of waste screening system is examined. It is confirmed that the screen is effective with some amount of bypass flow rate, however the output power is remarkably deteriorated.

• Ecology and Resilient Infrastructure
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• 제6권3호
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• pp.145-153
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• 2019

본 연구는 저수로 변화를 통한 하천 복원이 수생 서식처에 미치는 영향을 파악하기 위하여 원주천을 대상으로 물리서식처분석을 수행하였다. 대상 어종은 원주천에 서식하는 우점종인 피라미를 대상으로 하였다. 흐름 분석은 2차원 모형인 River2D 모형을 사용하였으며, 서식처 분석은 서식처 적합도 곡선을 이용하여 서식처의 양과 질을 산정하는 서식처 적합도 모형을 사용하였다. 먼저 저수로 복원을 위하여 자연 하도에 가까운 형태로 복원을 하고 저수로 폭을 확대 및 축소하여 설계하였다. 그 결과 저수로 폭의 변화를 확대하였을 때 대상 어종의 가중가용면적은 각각 약 72% 향상되었으며, 저수로 폭을 축소하였을 때는 서식처가 약 25% 악화되는 것으로 나타났다.

• International Journal of Fluid Machinery and Systems
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• 제6권3호
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• pp.160-169
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• 2013

Two-stage turbocharging is an important way to raise engine power density, to realize energy saving and emission reducing. At present, turbine matching of two-stage turbocharger is based on MAP of turbine. The matching method does not take the effect of turbines' interaction into consideration, assuming that flow at high pressure turbine outlet and low pressure turbine inlet is uniform. Actually, there is swirl flow at outlet of high pressure turbine, and the swirl flow will influence performance of low pressure turbine which influencing performance of engine further. Three-dimension models of turbines with two-stage turbocharger were built in this paper. Based on the turbine models, mechanism of swirl flow at high pressure turbine outlet influencing low pressure turbine performance was studied and a two-stage radial counter-rotation turbine system was raised. Mechanisms of the influence of counter-rotation turbine system acting on low-pressure turbine were studied using simulation method. The research result proved that in condition of small turbine flow rate corresponding to engine low-speed working condition, counter-rotation turbine system can effectively decrease the influence of swirl flow at high pressure turbine outlet imposing on low pressure turbine and increases efficiency of the low-pressure turbine, furthermore increases the low-speed performance of the engine.

• 한국추진공학회:학술대회논문집
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• 한국추진공학회 2008년 영문 학술대회
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• pp.775-780
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• 2008

This paper reports on the internal flow of a centrifugal mini pump working at the low flow rate operating conditions. The RNG $\kappa-\varepsilon$ turbulence model was employed to simulate the three-dimensional turbulent flow in the pump. To examine and certify the simulation results, a transparent acrylic centrifugal mini pump model which is suitable for PIV measurement has been developed. The tongue region and the passages region between blades were investigated using PIV. In order to eliminate the effect of refraction on the area closed to the wall and increase the measurement accuracy, the fluorescent particles were scatted into the working fluid with the tracing particles. It is found from the calculation and PIV measurement results that there is a large area of recirculation flow near the tongue at low flow rate operating conditions. The computationally predicted water head using the $\kappa-\varepsilon$ turbulence model at low flow rate operating conditions are in very good agreement with the experimentally measured water head and the mean velocity distributions at investigation area obtained by PIV and calculation showed a satisfactory agreement as well. Meanwhile, the results of PIV measurements show that the flow status in one passage is different to another. And for capturing the internal flow detail information, the $\kappa-\varepsilon$ turbulence model is not very suitable.

• 유체기계공업학회:학술대회논문집
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• 유체기계공업학회 2002년도 유체기계 연구개발 발표회 논문집
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• pp.152-157
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• 2002

A cross-flow fan is widely used on many industrial fields: a blower for the general industry, mining industry, automobile and home appliances. The design point of the cross-flow fan is generally chosen by based on the region within low static pressure and high flow rate. It relatively makes high dynamic pressure at low speed because a working fluid passes through an impeller blade twice. However, it has low static pressure efficiency between $30\%$ and $40\%$ because of relative high impact loss. Recently, in the air-conditioning systems, the operating behaviors at the off-design points are highly regarded to broaden the application area for various air-cooling loads. Especially, at the low flow rate, there exists a rapid pressure head reduction, a noise increase and an irregular flow against a rearguider as a scroll of centrifugal fan. Numerical analyses are carried out for cross-flow fan including the impeller, the rearguider and the stabilizer. Numerical domains are discretized by hexahedral cells. Three-dimensional, unsteady governing equations are solved using FVM, SIMPLE algorithm, sliding grid system and standard k-$\epsilon$ turbulence model.

• 대한기계학회논문집B
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• 제29권7호
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• pp.773-783
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• 2005

Internal flow measurement of very low specific-speed semi-open impellers has been carried out by PIV in order to understand better the internal flow patterns that are responsible fur the unique performance of these centrifugal pumps operating in the range of very low specific speed. Two types of impellers, one equipped with six radial blades (Impeller A) and the other with four conventional backward-swept blades (Impeller B), are tested in a centrifugal pump operating at a non-dimensional specific-speed of $n_s=0.24$. Complex flow patterns captured by PIV are discussed in conjunction with the overall pump performance measured separately. It is revealed that Impeller A achieves higher effective head than Impeller B even though the flow patterns in Impeller A are more complex, exhibiting secondary flows and reverse flows in the impeller passage. It is shown that both the localized strong outward flow at the pressure side of each blade outlet and the strong outward through-flow along the suction side of each blade are responsible for the better head performance of Impeller A.

• 대한토목학회논문집
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• 제34권4호
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• pp.1125-1138
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• 2014

본 연구는 광동댐 유역을 대상으로 RCPs (Representative Concentration Pathways) 기후변화 시나리오의 Arc-SWAT 적용으로 평균유출량과 저유량 계열을 구축하고 경계핵함수(Boundary Kernel)를 이용하여 비매개변수적 갈수빈도 해석을 수행하였다. 분석결과, RCPs 시나리오 하에서 가까운 미래의 유출량 감소로 인한 가뭄발생빈도가 증가하였으며, RCP8.5에서 저유량 계열의 변동폭이 크게 나타났다. Median flow의 갈수량 빈도해석결과 가까운 미래(2030s)의 30년 빈도 갈수량의 경우 Historic 기간에 비하여 증가(RCP4.5: +22.4%, RCP8.5: +40.4%)하였으나, 먼 미래(2080s)에는 갈수량 감소(RCP4.5: -4.7%, RCP8.5: -52.9%)로 인한 가뭄발생빈도가 커지는 것으로 분석되었다. 또한 Quantile 25% flow 저유량 계열의 경우 먼 미래에 빈도별 갈수량이 감소(RCP4.5: -20.8% ~ -60.0%, RCP8.5: -30.4% ~ -96.0%)하여 극심한 가뭄의 발생빈도가 커질 것으로 분석되었다. RCPs 시나리오 적용에 따른 비매개변수적 갈수빈도 해석 결과는 한반도 중권역별 수자원개발계획 수립과 기후변화 대응책 마련을 위한 기초자료로 활용이 가능할 것이다.

• 한국대기환경학회지
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• 제13권1호
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• pp.65-77
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• 1997

A theoretical and numerical investigation on the boundary-layer flow over a two- or three-dimensional hill is presented. The numerical model is based on the finite volume method with boundary-fitted coordinates. The k-$\varepsilon$ turbulence model with modified wall function and the low-Reynolds-number model are employed. The hypothesis of Reynolds number independency for the atmospheric boundary-layer flow over aerodynamically rough terrain is confirmed by the numerical simulation. Comparisons of the mean velocity profiles and surface pressure distributions between the numerical predictions and the wind-tunnel experiments on the flow over a hill show good agreement. The linear theory provides generally good prediction of speed-up characteristics for the gentle-sloped hills. The flow separation occurs in the hill slope of 0.5 and the measured reattachment points are compared with the numerical prediction. It is found that the k- $\varepsilon$ turbulence model is reasonably accurate in predicting the attached flow, while the low- Reynolds-number model is more suitable to simulate the separated flows.ows.

• International Journal of Fluid Machinery and Systems
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• 제5권1호
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• pp.30-37
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• 2012

A two dimensional Darrieus-type turbine has been proposed for the hydropower utilization of extra-low head less than 2m. In a practical use of Darrieus-type hydro-turbine, head and flow rate may be varied temporally and seasonally. Considering that the cost advantage is required for the low head hydro turbine system, the Darrieus turbine should be operated with high efficiency in the wider range of flow rate possibly by using an additional device with simpler mechanism. In the present paper, an adjustment of inlet nozzle section by lowering the inlet nozzle height is proposed to obtain the preferable inlet velocity in low flow rate conditions. Effects of resulting spanwise partial inlet flow are investigated. Finally, an effective modification of inlet nozzle height over flow rate variation is shown.