A new type of cavitation (damage) has been recently discovered from the Three Gorges turbines, that suggests a complicated inception mechanism involving the boundary-layer K-mode instability triggering cavitation nucleation. As one of the elements, the level of free-stream turbulence increased by wrongly designed guide-plate on the Three Gorges turbines could be a primary concern in the sense of receptivity and the transient growth of K-mode instability. The investigation focuses on the flow analysis of the on-coming flow influenced by the guide plate in terms of flow structure(s) and turbulence level variation. Firstly, CFD has been performed to obtain the main flow features in the whole turbine passage with more detailed analysis on the free-stream pressure-fluctuations near the lower surface of the guide vanes. The unsteady flow characteristics, especially the pressure fluctuations of low-frequency spectrum have been studied that significantly alternate the turbulence intensities and spectrum in the free-stream flow. The results verify that the addition of the guide-plate increases the free-stream turbulence, particularly contributing to the occurrence of a strongest component of extremely low-frequency pressure-fluctuation (i.e. the 0.336 Hz for the case 2 with opening of 16°). These low-frequency fluctuations readily transmit throughout the entire flow passage of the turbine. An extremely large-scale and united unsteady vortex structure that occupies the whole flow passage of a Francis turbine, has been identified for the first time. That is, a vortex-ring structure triggered by the guide-plate, through the connection of the vortices in the stay-vane and guide-vane channels plus those in the runner channels, further interacts with the helical vortex-rope in the draft tube at part-load conditions (e.g. 16° and 30°), forming such an united giant vortex structure. This extremely large-scale vortex structure is thus responsible for the components of extremely low-frequencies (i.e., 0.336 Hz for 16°, 0.15 Hz for 30°) which have been identified from two part-load conditions. These convincing results have proved how this wrongly designed guide-plate increases the free-stream turbulence by inducing extra unsteadiness with gust-like low-frequency and explained that the flow upstream the cone of draft-tube could affect the cone flow significantly, especially under part-load operation conditions.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:618985 |
| Date | January 2014 |
| Creators | Chen, Ting |
| Publisher | University of Warwick |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://wrap.warwick.ac.uk/63284/ |
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