000225092 001__ 225092
000225092 005__ 20181203024533.0
000225092 0247_ $$2doi$$a10.1007/s00348-016-2257-y
000225092 022__ $$a0723-4864
000225092 02470 $$2ISI$$a000388613000005
000225092 037__ $$aARTICLE
000225092 245__ $$aLDV survey of cavitation and resonance effect on the precessing vortex rope dynamics in the draft tube of Francis turbines
000225092 260__ $$bSpringer Verlag$$c2016$$aNew York
000225092 269__ $$a2016
000225092 300__ $$a16
000225092 336__ $$aJournal Articles
000225092 520__ $$aThe 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.
000225092 700__ $$aFavrel, A.
000225092 700__ $$aMuller, A.
000225092 700__ $$aLandry, C.
000225092 700__ $$aYamamoto, K.
000225092 700__ $$g104417$$aAvellan, F.$$0241012
000225092 773__ $$j57$$tExperiments In Fluids$$k11$$q168
000225092 909C0 $$xU10309$$0252135$$pLMH
000225092 909CO $$pSTI$$particle$$ooai:infoscience.tind.io:225092
000225092 917Z8 $$x104417
000225092 937__ $$aEPFL-ARTICLE-225092
000225092 973__ $$rREVIEWED$$sPUBLISHED$$aEPFL
000225092 980__ $$aARTICLE