• 수산해양기술연구
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• 제28권2호
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• pp.157-163
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• 1992

In this paper, the relationship between static pressure recovery and turbulent energy was presented in case of swirling flows into a conical diffuser. The distributions of turbulent energy in a diffuser sectional area were measured by a hot wire anemometer. The following conclusion can be drawn from the experiment. Diffuser loss is constituted by a dynamic pressure loss and total pressure loss. The static pressure recovery depends strongly on the total pressure loss. The static pressure recovery depends strongly on the total pressure loss, and the turbulent energy varies inversely as the static pressure recovery coefficient.

• 동력기계공학회지
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• 제2권1호
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• pp.31-38
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• 1998

A diffuser, an important equipment to change kinetic energy into pressure energy, has been studied for a long time. Though experimental and theoretical researches have been done, the understanding of energy transfer and detailed mechanism of energy dissipation is unclear. As far as numerical prediction of diffuser flows are concerned, various numerical studies have also been done. On the contrary, many turbulence models have constraint to the applicability of diffuser-like complex flows, because of anisotropy of turbulence near the wall and of local nonequilibrium induced by an adverse pressure gradient. The existing $k-{\epsilon}$ turbulence models have some problems in the case of being applied to complex turbulent flows. The purpose of this paper is to test the applicability of the nonlinear $k-{\epsilon}$ model concerning diffuser-like flows with expansion and streamline curvature. The results show that the nonlinear $k-{\epsilon}$ turbulence model predicted well the coefficient of pressure, velocity profiles and turbulent kinetic energy distributions, however the shear stress prediction was failed.

• 한국산업융합학회 논문집
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• 제25권4_2호
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• pp.637-644
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• 2022

In this study, the flow characteristics according to the shape of the vortex nozzle was studied by numerical analysis and the amount of microbubble generation was measured experimentally. The shape of the vortex nozzle is cylindrical, diffuser, and conical type. The axial fluid velocity in the induced tube gradually increased from the inlet to the outlet. In particular, the fluid velocity in the nozzle part increased rapidly. The velocity distribution of the fluid at the inlet of the induced tube showed that the flow rotates counterclockwise in the outer region and the inner center of the induced tube. At the outlet of the induced tube, the cylindrical and conical type showed rotational flow, and the diffuser type showed irregular turbulent flow. The dimensionless pressure ratio 𝜂 of the inner region of the induced tube was lower than that of the outer region. Also, 𝜂 near the outlet of the induced tube in cylindrical and conical type showed a similar tendency to the inlet area. At the outer region of inlet of induced tube, intense vorticity was observed on the wall and in lower region. At the inner region of inlet of induced tube, intense vorticity was observed on the inner wall of the induced tube and in the central region of the inlet of the induced tube. At the outlet of induced tube, in the case of the cylindrical and conical type, intense vorticity was observed near the inner wall, the diffuser type showed irregular strong vorticity inside the tube. The total number of bubbles measured was the most in the cylindrical type, and the microbubbles less than 50mm occurred the most in the conical type.

• 수산해양기술연구
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• 제28권2호
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• pp.151-156
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• 1992

The purpose of this paper is to investigate the relationship between static pressure recovery and velocity distributions in case of swirling flow into a conical diffuser. In this research, velocity distribution is measured by a multi-hole yaw-meter. The following conclusions can be drawn from the experiments. (1) The static pressure recovery depends strongly on the strength of a swirl. (2) A high pressure recovery coefficient is achieved by inserting a solid core into the diffuser center.

• International Journal of Fluid Machinery and Systems
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• 제4권1호
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• pp.47-56
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• 2011

Our previous experimental and numerical investigations of decelerated swirling flows in conical diffusers have demonstrated that water jet injection along the symmetry axis mitigates the pressure fluctuations associated with the precessing vortex rope. However, for swirling flows similar to Francis turbines operated at partial discharge, the jet becomes effective when the jet discharge is larger than 10% from the turbine discharge, leading to large volumetric losses when the jet is supplied from upstream the runner. As a result, we introduce the flow-feedback approach for supplying the jet by using a fraction of the discharge collected downstream the conical diffuser. Experimental investigations on mitigating the pressure fluctuations generated by the precessing vortex rope and investigations of pressure recovery coefficient on the cone wall with and without flow-feedback method are presented.

• Journal of Advanced Marine Engineering and Technology
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• 제22권5호
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• pp.595-608
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• 1998

A diffuser an important equipment to change kinetic energy into pressure energy has been studied for a long time. Though experimental and theoretical researches habe been done the understanding of energy transfer and detailed mechanism of energy dissipation is unclear. As far as numerical prediction of diffuser flows are concerned various numerical studies have also been done. On the contrary many turbulence models have constraint to the applicability of diffuser-like flows with expansion and streamline curvature. In order to obtain the reliability of k-$\varepsilon$ turbulence model modified combination turbulence models composed of the anisotropic k-$\varepsilon$model modified combination turbulence models composed of the anisotropic k-$\varepsilon$ model with Hanjalic-Launder's preferential normal strain and Pope's vortex stretching mechanism are proposed. The results of the present proposed models prove the fact that the coefficient of pressure and the shear stress are well predicted at the diffuser flow.

• 대한환경공학회지
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• 제34권11호
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• pp.772-779
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• 2012

본 연구에서는 원뿔형으로 개발된 미세기포형 산기관의 성능향상을 위하여 운용적 측면에서 실험과 전산모사적 방법을 이용하였다. 산기관 잠김깊이의 변화에 대해 성능실험이 용이한 실증 규모의 타워형 생물반응기에 산기관을 장착하여, 잠김깊이가 표준산소전달계수($K_{L}a_{20}$) 및 표준산소전달효율(SOTE) 등 산소전달 성능에 미치는 영향을 실험적으로 연구하였다. 또한, 2상 유동에 관한 유체역학적 전산모사를 이용하여 산기관 수와 잠김깊이에 대한 유동현상을 파악함으로써 산소전달 성능변화에 대한 원인을 규명하였다. 산소전달 성능실험결과, 산기관의 잠김깊이를 6 m에서 12 m로 증가시킴에 따라 표준산소 전달계수는 7% 증가하였으나 표준산소전달효율은 39~72% (5.6 %/m)로 대폭 상승하였다. 유동 해석결과, 산기관의 수가 증가함에 따라 공기 체적분율 및 공기와 시험수의 유속 모두 증가하였으며, 공기와 시험수의 유속 경향은 유사하게 나타났다. 잠김깊이가 증가함에 따라서 공기 체적분율, 공기 및 시험수의 유속은 조금씩 감소하는 경향을 나타냈다. 선회 병류유동은 기포류의 확산과 상승속도를 결정하는 주요 인자로서 선회강도가 과도하게 큰 경우에는 기포 체류시간과 체류량이 감소되므로 산소전달 성능을 저하를 예측할 수 있었다.

• KIEE International Transactions on Electrophysics and Applications
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• 제4C권4호
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• pp.137-141
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• 2004

In this paper, a thermopneumatic PMDS (polydimethlysiloxane) micropump with nozzle/diffuser elements is presented. The micropump is composed of nozzle/diffuser elements as dynamic valves, an actuator consisting of a circular PDMS diaphragm and a Cr/Au heater on a glass substrate. Four PDMS layers are used for fabrication of an actuator chamber, actuator diaphragm by a spin coating process, spacer layer, and nozzle/diffuser by the SU-8 molding process. The radius and thickness of the actuator diaphragm is 2 mm and 30 ${\mu}{\textrm}{m}$, respectively. The length and the conical angle of the nozzle/diffuser elements are 3.5 mm and 20$^{\circ}$, respectively. The actuator diaphragm is driven by the air cavity pressure variation caused by ohmic heating and natural cooling. The flow rate of the micropump in the frequency domain is measured for various duty cycles of the square wave input voltage. When the square wave input voltage of 5 V DC is applied to the heater, the maximum flow rate of the micropump is 44.6 ${mu}ell$/min at 100 Hz with a duty ratio of 80% under the zero pressure difference.

• International Journal of Fluid Machinery and Systems
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• 제2권4호
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• pp.346-352
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• 2009

The objective of the present work is to improve numerical predictions of unsteady turbulent swirling flows in the draft tubes of hydraulic power plants. We present Large Eddy Simulation (LES) results on a simplified draft tube consisting of a straight conical diffuser. The basis of LES is to solve the large scales of motion, which contain most of the energy, while the small scales are modeled. LES strategy is here preferred to the average equations strategies (RANS models) because it resolves directly the most energetic part of the turbulent flow. LES is now recognized as a powerful tool to simulate real applications in several engineering fields which are more and more frequently found. However, the cost of large-eddy simulations of wall bounded flows is still expensive. Bypass methods are investigated to perform high-Reynolds-number LES at a reasonable cost. In this study, computations at a Reynolds number about 2 $10^5$ are presented. This study presents the result of a new near-wall model for turbulent boundary layer taking into account the streamwise pressure gradient (adverse or favorable). Validations are made based on simple channel flow, without any pressure gradient and on the data base ERCOFTAC. The experiments carried out by Clausen et al. [1] reproduce the essential features of the complex flow and are used to develop and test closure models for such flows.

• 대한기계학회논문집B
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• 제23권11호
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• pp.1399-1409
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• 1999

It is known that previous models are unsatisfactory in predicting adverse pressure gradient turbulent flows. In the present paper, a revised low Reynolds number $k-{\varepsilon}$ model is proposed. In this model, a newly developed term is added lo the dissipation rate equation. In order to reflect appropriate effects for an adverse pressure gradient. The added tenn is derived by considering the distribution of mean velocity and turbulent properties in the turbulent flow with, adverse pressure gradient. The new $k-{\varepsilon}$ model was applied to calculations of flat plate flow with adverse pressure gradient, conical diffuser flow and backward facing step flow. It was found that the three numerical results showed better agreement than other models compared with DNS results and experimental ones.