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What is the FRANCIS TURBINE CAVITATION?
What is the cavitation problem?
The cavitation problem in hydraulic turbines is a well known problem produced by bubbles of vapour when the pressure of the liquid is lower than the vapour pressure. This produces erosion in the moving part of the turbines runner and shaft.
Swirling Helicoidal Vortex
However a new phenomenon appears when the turbine is subjected to variations of load due to changes of power demanded by the electric system and it is the development of a cavitation precessing vortex rope at the outlet of a Francis turbine runner operating at part load conditions.
This type of cavitation is a swirling helicoidal vortex ropelike composed of vapour bubbles, turning at a certain frequency.
It acts as an excitation source for the hydraulic system, leading to the propagation of pressure fluctuations in the hydraulic circuit which are greatly amplified in case of resonance.
In some cases this type of cavitation has destroyed a turbine in hours when the resonance between the natural frequency of the turbine system and the induced vortex rope cavitation is produced.
Hydroacoustic Vortex Rope Waves
Therefore, the assessment of the stability of hydropower plants operating at part load is crucial in order to ensure the safe extension of their operating range.
The problem is that the physical mechanisms driving the excitation source and its interaction with the hydraulic system remain unclear. The research work is to investigate the influence of the operating conditions on the dynamics of the cavitation precessing vortex rope and the excitation source it induces, as well as the interaction with the system.
This cavitation swirling flow leaving the Francis turbine when operating at low load induces pressure fluctuations in the range of 0.2 to 0.4 times the natural frequency of the system. The precession movement of the vortex rope is an excitation source for the whole hydraulic system. The frequency of the excitation may match with one of the eigenfrequency of the system leading to resonance phenomena.
In the case of the upper part load operation range, pressure surge fluctuations may occur in a higher frequency range between 2 and 4 times the runner frequency and may produce modulations at the vortex rope precession frequency.
The mechanism that induces this upper part load resonance is still unknown. Under certain operating conditions, the first hydro-acoustic natural frequency of the system can match the precession frequency, inducing a hydro-acoustic resonance.