LDV survey of cavitation and resonance effect on the precessing vortex rope dynamics in the draft tube of Francis turbines

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• 作者：A. Favrel ; A. Müller ; C. Landry ; K. Yamamoto ; F. Avellan
• 刊名：Experiments in Fluids
• 出版年：2016
• 出版时间：November 2016
• 年：2016
• 卷：57
• 期：11
• 全文大小：3,445 KB
• 刊物类别：Engineering
• 刊物主题：Engineering Fluid Dynamics
  Fluids
  Industrial Chemistry and Chemical Engineering
  Measurement Science and Instrumentation
  Thermodynamics
  Theoretical and Applied Mechanics
  
• 出版者：Springer Berlin / Heidelberg
• ISSN：1432-1114
• 卷排序：57

文摘

The large-scale penetration of the electrical grid by intermittent renewable energy sources requires a continuous operating range extension of hydropower plants. This causes the formation of unfavourable flow patterns in the draft tube of turbines and pump-turbines. At partial load operation, a precessing cavitation vortex rope is formed at the Francis turbine runner outlet, acting as an excitation source for the hydraulic system. In case of resonance, the resulting high-amplitude pressure pulsations can put at risk the stability of the machine and of the electrical grid to which it is connected. It is therefore crucial to understand and accurately simulate the underlying physical mechanisms in such conditions. However, the exact impact of cavitation and hydro-acoustic resonance on the flow velocity fluctuations in the draft tube remains to be established. The flow discharge pulsations expected to occur in the draft tube in resonance conditions have for instance never been verified experimentally. In this study, two-component Laser Doppler Velocimetry is used to investigate the axial and tangential velocity fluctuations at the runner outlet of a reduced scale physical model of a Francis turbine. The investigation is performed for a discharge equal to 64 % of the nominal value and three different pressure levels in the draft tube, including resonance and cavitation-free conditions. Based on the convective pressure fluctuations induced by the vortex precession, the periodical velocity fluctuations over one typical precession period are recovered by phase averaging. The impact of cavitation and hydro-acoustic resonance on both axial and tangential velocity fluctuations in terms of amplitude and phase shift is highlighted for the first time. It is shown that the occurrence of resonance does not have significant effects on the draft tube velocity fields, suggesting that the synchronous axial velocity fluctuations are surprisingly negligible compared to the velocity fluctuations induced by the vortex precession.