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Department of Energy and
Process Engineering
Spring 2017
Air Injection in Francis
Turbines
Supervisor:
Michel J. Cervantes
Livia I. Pitorac
Background
The introduction of renewable energy such as
wind and solar involves a significant increase
of the load variations as well as start/stop
cycles for hydraulic turbines to regulate the
grid frequency. As a turbine away from the best
efficiency point, vortex breakdown may occur
in the draft tube, a diffuser found immediately
after the runner. Vortex breakdown leads to the
formation of a rotating vortex rope (RVR) when
a turbine is operated below the best efficiency
point for single regulated turbines.
A RVR creates large axial and radial pressure
pulsations and thus limits the operating range
of the turbine and leads to wear. Mitigation of the
RVR has been researched for many years. Air
injection is among the most popular methods
to decrease the pressure oscillations. Water
injection and fins have also been investigated.
However, none of these methods have been
distinctly successful.
MASTER THESIS
This picture shows the 3D iso pressure surface during
part load, without (left) and with (right) jet injection (Resiga
2006,23rd IAHR Symp., 192)
RVR formation during
part load operation
RVR formation during
full load operation