The flow in the off-design operation of a Francis turbine may lead to the formation of spiral vortex breakdowns in the draft tube, a diffuser installed after the runner. The spiral vortex breakdown, also named a vortex rope, may induce several low-frequency fluctuations leading to structural vibrations and a reduction in the overall efficiency of the turbine. In the present study, synchronized particle image velocimetry, pressure, and turbine flow parameter (Q, H, α, and T) measurements have been carried out in the draft tube cone of a high head model Francis turbine. The transient operating condition from the part load to the best efficiency point was selected to investigate the mitigation of the vortex rope in the draft tube cone. The experiments were performed 20 times to assess the significance of the results. A precession frequency of 1.61 Hz [i.e., 0.29 times the runner rotational frequency (Rheingans frequency)] is observed in the draft tube cone. The frequency is captured in both pressure and velo...

中文翻译：

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在逆压力梯度下缓解高雷诺数下螺旋涡流击穿的实验研究

混流式水轮机非设计运行中的流动可能导致在引流管中形成螺旋涡流破坏，引流管是安装在转轮之后的扩散器。螺旋涡流击穿，也称为涡绳，可能会引起若干低频波动，从而导致结构振动和涡轮机整体效率的降低。在本研究中，同步粒子图像测速、压力和涡轮流动参数（Q、H、α 和 T）测量已在高水头模型弗朗西斯涡轮的尾水管锥体中进行。选择从部分负载到最佳效率点的瞬态运行条件来研究尾水管锥体中涡流绳的缓解。实验进行了 20 次以评估结果的显着性。进动频率为 1。在引流管锥体中观察到 61 Hz [即转轮旋转频率（Rheingans 频率）的 0.29 倍]。频率在压力和速度中被捕获...