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

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

Favrel, A.
•
Muller, A.
•
Landry, C.
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2016
Experiments In Fluids

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.

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Type
research article
DOI
10.1007/s00348-016-2257-y
Web of Science ID

WOS:000388613000005

Author(s)
Favrel, A.
Muller, A.
Landry, C.
Yamamoto, K.
Avellan, F.  
Date Issued

2016

Publisher

Springer Verlag

Published in
Experiments In Fluids
Volume

57

Issue

11

Start page

168

Editorial or Peer reviewed

REVIEWED

Written at

EPFL

EPFL units
LMH  
Available on Infoscience
January 24, 2017
Use this identifier to reference this record
https://infoscience.epfl.ch/handle/20.500.14299/133686
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