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Study of the vortex-induced pressure excitation source in a Francis turbine draft tube by particle image velocimetry

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Abstract

Francis turbines operating at part-load experience the development of a precessing cavitation vortex rope at the runner outlet, which acts as an excitation source for the hydraulic system. In case of resonance, the resulting pressure pulsations seriously compromise the stability of the machine and of the electrical grid to which it is connected. As such off-design conditions are increasingly required for the integration of unsteady renewable energy sources into the existing power system, an accurate assessment of the hydropower plant stability is crucial. However, the physical mechanisms driving this excitation source remain largely unclear. It is for instance essential to establish the link between the draft tube flow characteristics and the intensity of the excitation source. In this study, a two-component particle image velocimetry system is used to investigate the flow field at the runner outlet of a reduced-scale physical model of a Francis turbine. The discharge value is varied from 55 to 81 % of the value at the best efficiency point. A particular set-up is designed to guarantee a proper optical access across the complex geometry of the draft tube elbow. Based on phase-averaged velocity fields, the evolution of the vortex parameters with the discharge, such as the trajectory and the circulation, is determined for the first time. It is shown that the rise in the excitation source intensity is induced by an enlargement of the vortex trajectory and a simultaneous increase in the precession frequency, as well as the vortex circulation. Below a certain value of discharge, the structure of the vortex abruptly changes and loses its coherence, leading to a drastic reduction in the intensity of the induced excitation source.

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Acknowledgments

The research leading to the results published in this paper is part of the HYPERBOLE Research Project, granted by the European Commission (ERC/FP7-ENERGY-2013-1-Grant 608532). The authors would also like to thank BC Hydro for making available the reduced-scale model, in particular Danny Burggraeve and Jacob Iosfin. Moreover, the authors would like to acknowledge the commitment of the Laboratory for Hydraulic Machines’ technical staff, especially Georges Crittin, Maxime Raton, Victor Rivas, Alain Renaud, and Vincent Berruex.

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Authors and Affiliations

  1. Laboratory for Hydraulic Machines, Ecole Polytechnique Fédérale de Lausanne, Av. de Cour 33 bis, 1007, Lausanne, Switzerland

    A. Favrel, A. Müller, C. Landry, K. Yamamoto & F. Avellan

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  1. A. Favrel
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  2. A. Müller
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  3. C. Landry
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  4. K. Yamamoto
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  5. F. Avellan
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Correspondence to A. Favrel.

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Favrel, A., Müller, A., Landry, C. et al. Study of the vortex-induced pressure excitation source in a Francis turbine draft tube by particle image velocimetry. Exp Fluids 56, 215 (2015). https://doi.org/10.1007/s00348-015-2085-5

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  • DOI: https://doi.org/10.1007/s00348-015-2085-5

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