Abstract
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.
© 2023 Turbomachinery Society of Japan, Korean Fluid Machinery Association, Chinese Society of Engineering Thermophysics, IAHR
