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Background
Frequency analysis of a pressure pulsations in a Francis draft
tube
Increased demand for flexibility in the power supply has lead to an increase in operation
outside best efficiency point. Part load operation causes dramatic changes in the flow regime
of the draft tube. For Francis turbines operating at part load, there is a mismatch between
swirl generated by the guide vanes and the momentum extracted by the turbine runner.
The vortex breakdown than occurring in the draft tube is recognized as the main cause of
severe flow instabilities and pressure fluctuations. The consequences are known to be heavy
vibrations and noise, which may cause high fatigue load and ultimately lead to mechanical
failure. Mitigation of pressure pulsations is considered an imported task since it will increase
the life time of the turbine.
Different methods for mitigating the vortex rope and the pressure fluctuations such as air
injection, water injection, runner shaft extension have been investigated. The aim of this
thesis is to further investigate these methods to find the best way of mitigating pressure
pulsations.
PhD Thesis
Waterpower Laboratory
Pressure Pulsations in Francis
Turbines
By Peter Joachim Gogstad
Supervisor: Ole Gunnar Dahlhaug
and Torbjørn K. Nielsen