New insight in Francis turbine cavitation vortex rope: role of the runner outlet flow swirl number

At part load operation, Francis turbines experience the development of a cavitation vortex rope in the draft tube, whose precession acts as a pressure excitation source. In case of resonance, the resulting pressure pulsations lead to unacceptable torque and power fluctuations on the prototype machin...

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Bibliographic Details
Published in:Journal of hydraulic research Vol. 56; no. 3; pp. 367 - 379
Main Authors: Favrel, Arthur, Gomes Pereira Junior, Joao, Landry, Christian, Müller, Andres, Nicolet, Christophe, Avellan, François
Format: Journal Article
Language:English
Published: Madrid Taylor & Francis 04.05.2018
Taylor & Francis Ltd
Subjects:
Cavitation
Cavitation flow
Cavitation number
Computational fluid dynamics
Empirical analysis
Fluid flow
Francis turbine
hydro-acoustic resonance
Mathematical models
Modelling
Outlet flow
Parameters
part load
Precession
Pressure
Prototypes
Resonance
Resonant frequencies
Resonant frequency
Rope
Scale (ratio)
Stability
swirl number
Systems stability
Torque
Turbine engines
Turbines
vortex rope
Vortices
ISSN:0022-1686, 1814-2079
Online Access:Get full text
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Summary:At part load operation, Francis turbines experience the development of a cavitation vortex rope in the draft tube, whose precession acts as a pressure excitation source. In case of resonance, the resulting pressure pulsations lead to unacceptable torque and power fluctuations on the prototype machine, putting at risk the system stability. However, the accurate prediction of resonance conditions at the prototype scale remains challenging since it requires a proper hydro-acoustic modelling of the draft tube cavitation flow. Furthermore, both the head and discharge values have an impact on the precession frequency of the vortex and the natural frequency of the system. The present paper demonstrates for the first time that the influence of both parameters on the frequencies of interest can be represented by a single parameter, the swirl number. Its analytical expression is derived as a function of the operating parameters of the machine. It is used to establish empirical laws enabling the determination of both frequencies and finally the operating parameters in resonance conditions on the complete part load operating range at the model scale. The methodology presented in this paper represents a decisive step towards the prediction of resonances on the complete part load operating range of the prototype.
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ISSN:0022-1686
1814-2079
DOI:10.1080/00221686.2017.1356758