An experimental study on the upper part-load elliptical vortex instability in a Francis turbine

This paper presents preliminary results of an experimental study on the occurrence and development of the upper part-load instability in a reduced-scale Francis turbine. The study includes draft tube pressure measurements, high-speed flow visualization, and particle image velocimetry. Our results reveal that for an operating point within the range of the upper part-load instability (70 to 85 % of the nominal discharge), the vortex rope has a circular cross section in non-cavitating conditions, which is preserved even after the appearance of cavitation within the vortex core. It is only below a certain cavitation number that the vortex cross section turns into an ellipse, which is associated with an abrupt increase in the pressure fluctuations with a distinct peak in the frequency domain. A further decrease in the cavitation number results in a constant decrease in the activated frequency while the amplitude of these oscillations experience a rise followed by a quick drop. Phase-averaged velocity fields show that the occurrence and development of cavitation within the vortex rope result in a more diffused distribution of the angular momentum. The instantaneous velocity fields, on the other hand, reveal that the elliptical vortex has various states with either diffused or concentrated velocity distributions, which makes the use of the averaged velocity field for this point less relevant.