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RISS 인기검색어
- 검색키워드
- 저자 : Mehrdad Raisee (검색결과 4 건)
- 무료
- 기관 내 무료
- 유료
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A Novel Guide Vane System Design to Mitigate Rotating Vortex Rope in High Head Francis Model Turbine
Jesline Joy,Mehrdad Raisee,Michel J. Cervantes 한국유체기계학회 2022 International journal of fluid machinery and syste Vol.15 No.2
Guide vanes are a mechanical system used to direct flow in the desired direction. At lower operating conditions, implementing a guide vane system in the draft tube of a hydro-turbine can decrease the excess swirl in the flow and, thus, reduce pressure fluctuations. The present study discusses a numerical methodology to design an effective guide vane system in the draft tube of a high head Francis model turbine. The numerical method is computationally efficient and thus, saves excess computational time and data storage required for parametric analysis of the guide vane system. The aim is to mitigate the ‘rotating’ vortex rope with minimum additional losses in the turbine. The factors considered for the guide vane system design are a) number of guide vanes, b) chord length, c) span, d) inlet-outlet angles of the guide vanes, and e) their position in the draft tube. The parametric study comprises a) ideal guide vane design and b) realistic guide vane design study. The ideal guide vane design study was with the standalone draft tube domain. The realistic guide vane design study used the passage domains of the distributor, runner, and complete draft tube. From the ideal guide vane design study, a guide vane system with two or three guide vanes of chord 86% of runner radius and leading-edge span of 30% of runner radius effectively mitigates the rotating vortex rope. The system with three guide vanes is the most efficient when placed at some distance below the runner exit with mitigation above 95%
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Shahab Shiraghaee,Joel Sundström,Mehrdad Raisee,Michel J. Cervantes 한국유체기계학회 2023 International journal of fluid machinery and syste Vol.16 No.2
The swirling flow under the part-load (PL) operation of hydraulic turbines causes the formation of a rotating vortex rope (RVR) inside the draft tube. The RVR precession induces pressure fluctuations and periodic loadings that can jeopardize the hydraulic machine's structure and performance. The behavior of these fluctuations is closely related to the characteristics of the RVR, which can vary for different PL conditions. A series of experiments have been performed to characterize the periodic behavior and stability of the RVR under various PL operating conditions. For this purpose, wall pressure measurements were conducted at two sections along the draft tube axis of a scaled-down model of the U9 Kaplan turbine. Spectral analyses were performed, and the periodic stability of the RVR was investigated based on the statistical analyses of the amplitude and frequency of the pressure oscillations. The results showed that the amplitude of the pressure oscillations increases with the decrease of the discharge while the stability of the RVR decreases. Upstream, the behavior and stability of the RVR asynchronous mode pressure oscillations are highly dependent on the flow conditions at the draft tube entrance. However, due to the downstream RVR dissipation, the effects imposed by the elbow become the dominant factor in the RVR behavior close to the elbow. The statistical method proposed in this study provided an in-depth view of the RVR instantaneous behavior, allowing a more accurate characterization of the phenomenon than only studying its spectral content.
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Reduced Numerical Modeling of a High Head Francis Turbine Draft Tube at Part Load
Jesline Joy,Michel J. Cervantes,Mehrdad Raisee 한국유체기계학회 2021 International journal of fluid machinery and syste Vol.14 No.1
In the present study, a reduced model of the Francis-99 model turbine was investigated numerically at part load operating condition. The reduced model consists of a standalone draft tube domain of the Francis-99 model turbine. Numerical studies performed in the past on nearly complete hydro-turbine models (inclusive of the spiral casing, distributor domains, runner, and draft tube) reportedly consist of a large number of computational grids. This may increase the computational costs and data storage required to perform numerical analysis, which could be a setback for future research on new design concepts and optimization study of the draft tube domain. The reduced model was developed by mapping the phase averaged axial, radial, and tangential velocity profiles from the runner exit to the inlet of the standalone draft tube domain. Additionally, turbulent kinetic energy (k) and turbulent eddy dissipation (ε) variables were also considered for better flow prediction inside the draft tube domain. Two methods for mapping inlet boundary conditions were considered in the present study. In the first method, the entire planar profile of the runner-draft tube interface was considered. In the second method, the variables along a radial profile at the runner exit were considered with an axis-symmetric flow assumption over the entire draft tube inlet plane. The numerical results obtained from the Francis-99 reduced model turbine were validated against the numerical model of the NVKS Francis-99 model turbine (with available structured mesh) that was also analysed using the passage flow numerical technique and available experimental results. The results were found to be in reasonable agreement, with each other. The present study could be useful for the future mitigation study of rotating vortex rope by modifying the draft tube domain.
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Experimental determination of void fraction in surface aeration using image processing technique
Amir Mohammadpour,Mohammad Ali Akhavan-Behabadi,Masoud Ebrahimzadeh,Pedram Hanafizadeh,Mehrdad Raisee 대한기계학회 2015 JOURNAL OF MECHANICAL SCIENCE AND TECHNOLOGY Vol.29 No.6
In this paper, a new method for determination of void fraction in surface aeration process is presented and discussed. The proposedmethod is based on the image processing technique. The experimental setup has been designed to create various surface aeration conditionsin the water. Void fraction has been calculated for the wide range of water height, impeller immersion depth and rotational speed. Experiments have been performed in an open cubic tank with side length of 60 cm, equipped with one Rushton disk turbine. Moreover,the void fraction has been measured with level gauge method. The results showed that the image processing technique provides moreaccurate results than the level gauge measurements for void fraction calculation in surface aeration especially in low void fraction aeration. In addition, the experimental data revealed that increase in impeller immersion depth and rotational speed increase void fraction andoxygen transfer rate in surface aeration process.
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