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Simulation of Pressure Fluctuations under Part-load Operating Condition in a Francis Turbine

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Tutor: WangLeQin
School: Zhejiang University
Course: Chemical Process Equipment
Keywords: Francis turbine,Large Eddy Simulation (LES)¡¢Pressure pulsation,Computational Flu
CLC: TV734.1
Type: Master's thesis
Year:  2013
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Abstract:
With the development of the energy industry, China has gradually entered a period of large capacity, gaint units, high voltage and a high degree of information technology. As the expansion of the power system capacity and the increase of the voltage level, hydropower units of more than1million kilowatts have become possible. However, the operation stability is one of most critical technical problems in the design of one-million-kilowatt hydropower units compared to the traditional ones. And. On the one hand, the pulsation energy is generally raised with an increase of the water energy in large-capacity units. And on the other, the structural support stiffness decreases with the increasing of the unit size which will cause a reduction of the natural frequency and furthermore will raise the possibility of resonance. So, higher requirements of the three key performance indicators including energy, cavitation, stability are put forward in the design of the one-million-kilowatt Francis turbine units. A three-dimensional full-flow simulation of the Francis turbine was completed using Computational Fluid Dynamics (CFD) software in this paper, and the pressure pulsation in the model turbine under part-load operating conditions was analyzed. Simulation results have provided a reference for the design and optimization of the one-million-kilowatt Francis turbine units. The main contents are as follows:(1) Simulation method validation. A three-dimensional simulation of unsteady flow in a model Francis turbine is completed using LES turbulence model and the pressure pulsation amplitude and frequency under two different operating conditions are obtained. The validity and practicability of the simulation method used in this paper are illustrated with the experiment results for this model turbine.(2) Analysis of the pressure pulsation in runner and guide vanes. Based on the three-dimensional whole flow passage simulation results of the unsteady flow in the model turbine, pressure pulsation distribution and its changing law in the vanes, guide vanes and runner zones are predicted and analyzed. Furthermore, properties including the cause and transfer law for the pressure pulsation, the generation and shape of the channel vortex and the flow field characteristics under part-load operating conditions were studied.(3) Analysis of the pressure pulsation in the draft tube. Due to the operational features of the Francis turbine,6monitoring face and36points were spotted in the draft tube during the simulation process. Pressure pulsation amplitude and frequency in the draft tube under part-load operating conditions were obtained and the distributing law was analyzed as well as the changing rules. According to the properties of the vortex rope, it is indicated that the periodic swirl of the vortex rope in the draft tube is the main reason for the low-frequency pressure pulsation under part-load operating conditions.
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