• Transactions of the Korean hydrogen and new energy society
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• v.33 no.1
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• pp.95-104
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• 2022

Micro hydropower is a readily available renewable energy source that can be harvested utilizing hydrokinetic turbines from shallow water canals, irrigation and industrial channel flows, and run-off river stream flows. These sources generally have low head (<1 m) and low velocity which makes it difficult to harvest energy using conventional turbines. A horizontal-axis screw turbine was designed and numerically tested to extract power from such low-head water sources. The 3-bladed screw-type turbine is placed horizontally perpendicular to the incoming flow, partially submerged in a narrow water channel at no-head condition. The turbine hydraulic performances were studied using Computational Fluid Dynamics models. Turbine design parameters such as the shroud diameter, the hub-to-shroud ratios, and the submerged depths were obtained through a steady-state parametric study. The resulting turbine configuration was then tested by solving the unsteady multiphase free-surface equations mimicking an actual open channel flow scenario. The turbine performance in the shallow channel were studied for various Tip Speed Ratios (TSR). The highest power coefficient was obtained at a TSR of 0.3. The turbine was then scaled-up to test its performance on a real site condition at a head of 0.3 m. The highest power coefficient obtained was 0.18. Several losses were observed in the 3-bladed turbine design and to minimize losses, the number of blades were increased to five. The power coefficient improved by 236% for a 5-bladed screw turbine. The fluid losses were minimized by increasing the blade surface area submerged in water. The turbine performance was increased by 74.4% after dipping the turbine to a bottom wall clearance of 30 cm from 60 cm. The final output of the novel horizontal-axis screw turbine showed a 2.83 kW power output at a power coefficient of 0.63. The turbine is expected to produce 18,744 kWh/year of electricity. The design feasibility test of the turbine showed promising results to harvest energy from small hydropower sources.

• Korean Chemical Engineering Research
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• v.55 no.2
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• pp.201-213
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• 2017

Water is removed from crude oil containing water by using oil separator. This study aims to develop a three-dimensional (3D) Eulerian computational fluid dynamics (CFD) model to predict the separation efficiency of air-water-oil separator. In the incompressible, isothermal and unsteady-state CFD model, air is defined as continuous phase, and water and oil are given as dispersed phase. The momentum equation includes the drag force, lift force and resistance force of porous media. The standard k-${\varepsilon}$ model is used for turbulence flow. The exit pressures of water and oil play an important role in determining the liquid level of the oil separator. The exit pressures were identified to be 6.3 kPa and 5.1 kPa for water and oil, respectively, to keep a liquid level of 25 cm at a normal operating condition. The time evolution of volume fractions of air, water and oil was investigated. The settling velocities of water and oil along the longitudinal separator distance were analyzed, when the oil separator reached a steady-state. The oil separation efficiency obtained from the CFD model was 99.85%, which agreed well with experimental data. The relatively simple CFD model can be used for the modification of oil separator structure and finding optimal operating conditions.

• Journal of Korea Water Resources Association
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• v.42 no.7
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• pp.579-589
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• 2009

Hwang River in South Korea, has experienced channel adjustments due to dam construction. Hapcheon main dam and re-regulation dam. The reach below the re-regulation dam (45 km long) changed in flow regime, channel width, bed material distribution, vegetation expansion, and island formation after dam construction. The re-regulation dam dramatically reduced annual peak flow from 654.7 $m^3$/s to 126.3 $m^3$/s and trapped the annual 591 thousand $m^3$ of sediment load formerly delivered from the upper watershed since the completion of the dam in 1989. An analysis of a time series of aerial photographs taken in 1982, 1993, and 2004 showed that non-vegetated active channel width narrowed an average of 152 m (47% of 1982) and non-vegetated active channel area decreased an average of 6.6 km2 (44% of 1982) between 1982 and 2004, with most narrowing and decreasing occurring after dam construction. The effects of daily pulses of water from peak hydropower generation and sudden sluice gate operations are investigated downstream of Hapcheon Dam in South Korea. The study reach is 45 km long from the Hapcheon re-regulation Dam to the confluence with the Nakdong River. An analysis of a time series of aerial photographs taken in 1982, 1993, and 2004 showed that the non-vegetated active channel width narrowed an average of 152 m (47% reduction since 1982). The non-vegetated active channel area also decreased an average of 6.6 $km^2$ (44% reduction since 1982) between 1982 and 2004, with most changes occurring after dam construction. The average median bed material size increased from 1.07 mm in 1983 to 5.72 mm in 2003, and the bed slope of the reach decreased from 0.000943 in 1983 to 0.000847 in 2003. The riverbed vertical degradation is approximately 2.6 m for a distance of 20 km below the re-regulation dam. It is expected from the result of the unsteady sediment transport numerical model (GSTAR-1D) steady simulations that the thalweg elevation will reach a stable condition around 2020. The model also confirms the theoretical prediction that sediment transport rates from daily pulses and flood peaks are 21 % and 15 % higher than their respective averages.

• The Journal of Engineering Geology
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• v.5 no.3
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• pp.289-300
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• 1995

At the initial stage of the underground reservoir design one should thoroughly consider surface and subsurface hydrology, hydrogeologic characteristics of aquifer system, and the function of cut - off wall because it is linked to the effective management. In this study, three dimensional finite difference model was applied to analyse the function of Ian underground reservoir at Kyungbuk Province. The steady and unsteady state conditions after construction of the underground dam were simulated through the model, and from these results the groundwater budget and the safe yield were determined. The model simulation indicates the infiltration of irrigation water to be one of the major factors of seasonal fluctuation of groundwater level. The recharge rates of irrigation water were estimated as 4.3mm/d during May and June, and 1.7mm/d during July and Agust. Groundwater recharge from the watershed area estimated to about $0.04m^3/s$, almost consistent through the year. In 1984, groundwater discharge through the transverse section of the dam was $0.002m^3/s$ and the optimum yield for two momths(July and Aguest)was $254000m^3$, however, the discharge became $0.013m^3/s$ in1993, implying the failure of cut -off function. without appropaiate of the cut - off wall, optiumum yield during the irrigaton period would be $93, 000m^3$.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.36 no.2
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• pp.137-145
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• 2008

An experimental study was performed in order to investigate the influence of Reynolds number on the drag coefficient variations of an oscillating airfoil. A NACA 0012 airfoil was sinusoidally pitched at the quarter chord point with an oscillating amplitude of ${\pm}6^{\circ}$. The free-stream velocities were 1.98, 2.83 and 4.03 m/s and the corresponding chord Reynolds numbers were $2.3{\times}10^4$, $3.3{\times}10^4$ and $4.8{\times}10^4$, respectively. The drag coefficient was calculated from the ensemble average velocity measured by an X-type hot-wire probe(X-type, 55R51) in the near-wakes region. In the case of Re=$2.3{\times}10^4$, variation of drag coefficient shows a negative damping (counter-clockwise variation), which implies an unstable state which could be excited by aerodynamic force, whereas the drag coefficient represents the positive damping (clockwise variation) as the Reynolds number increases from Re=$3.3{\times}10^4$ to $4.8{\times}10^4$. Hence, the drag coefficient variations show significant differences between Re=$2.3{\times}10^4$ and $4.8{\times}10^4$이다.