Numerical Studies On Pressure Fluctuation Characteristics Of High Head Francis Turbine

In recent years, with the increasing of unit capacity and installation size, as well as the improving of specific speed, the vibration of the Francis turbine is becoming more and more serious. One of the main reasons for the unsteady of the turbine is the pressure fluctuation caused by the interaction between the guide vane and runner. It can induce hydraulic vibration.leading to cracks of guide vanes and runner blades, which affect the stable operation of the unit. In this paper, the Francis-99 model turbine is taken as the research object, and the full flow path is calculated for the turbine. The pressure fluctuation and its propagation characteristics in the vaneless between the guide vane and the runner are studied to provide a reference for the stable operation of the turbine. The research work is divided into the following aspects:Firstly, Different guide vane opening conditions of Francis turbine are calculated to obtain the Hydraulic turbine operating parameters, and compared with the model experimental data. The results show that: The numerical values of the efficiency, head, torque and the mean pressure of each monitoring point are in good agreement with the experimental values. The calculated results of pressure fluctuation are basically consistent with the experimental results.Secondly, three different guide vane opening conditions of Francis turbine are calculated to analyze the pressure fluctuation characteristics of the whole turbine under different operation conditions, and the pressure pulsation of the vaneless region are studied thoroughly.The results show that: under the optimal operating conditions, the frequency of the pressure fluctuation in the vaneless zone between the guide vane and the runner which is caused by rotor-stator interaction effect, is mainly the passing frequency and its multiplier of the runner blade, and the amplitude of the pressure fluctuation is much larger than that of other locations.The pressure fluctuation in the vaneless zone propagates along the flow path of the turbine to the whole flow field. In the process of upstream and downstream propagation, the amplitude is decreasing. According to the analysis of the pressure fluctuation in the vaneless region under different working conditions, it is found that the main frequency of the pressure fluctuation in the vaneless region is not affected by the guide vane opening, but the farther away from the optimal condition, the more complex the frequency components of the pressure pulsation.Finally, numerical calculations are carried out on the same guide vane opening (small opening) of turbine under different flow conditions, and analyze the internal flow field distribution and pressure fluctuation characteristics of runner domain. The results show that under the two conditions, the runner domain has obvious the blade channel vortex, the flow pattern is poor, especially near the crown at the worst, and the farther from the crown, the better flow pattern; along with the increase of flow rate, the flow pattern of the runner is improved, the range of the blade channel vortex decreases and the impact is weakened. The pressure pulsation frequency of blade is 28fn, which is the passing frequency of the guide vane;the amplitude of pressure fluctuation of the blade surface pressure is higher than that of the suction side. The maximum of pressure pulsation amplitude appears at the blade inlet in the runner domain, and along the flow direction from the inlet to the outlet, the pressure pulsation amplitude decreases gradually; the pressure pulsation amplitude at inlet of the blade pressure surface from the crown to the shroud gradually decreases, but gradually increased in the middle of the blade and the outlet. The pressure distribution on the suction surface of the blade is basically the same as that of the pressure surface.