58 / 94
Background
Increased demand for flexibility in the power
supply has lead to an increase in operation
outside the best efficiency point. Part load
operation causes a dramatical 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
that occurs 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 important 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, and 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
Department of Energy and
Process Engineering
2013-2017
Pressure Pulsations in
Francis Turbines
Supervisor:
Ole Gunnar Dahlhaug
Co-supervisor:
Torbjørn K. Nielsen
Peter Joachim Gogstad
Frequency analysis of a pressure pulsations in a Francis
draft tube
58