Abstract
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.
Highlights
Francis turbines operating in part-load conditions develop a precessing vortex rope within the draft tube with a frequency corresponding to 0.2 to 0.4 of the runner rotational frequency
To obtain the velocity fields in the draft tube, Particle Image Velocimetry (PIV) method has been implemented by means of a 200-mJ double-pulsed laser, a CCD camera and fluorescent particles, which allow for measurements in the presence of cavitation
The vortex rope cross section takes an elliptical shape and emits high-amplitudes noise at frequencies higher than ffrrrrrrrr
Summary
Francis turbines operating in part-load conditions develop a precessing vortex rope within the draft tube with a frequency (ffrrrrrrrr) corresponding to 0.2 to 0.4 of the runner rotational frequency. In the higher part load operation range, i.e. 70 to 85 % of the nominal discharge, this phenomenon might be accompanied by large-amplitude pressure fluctuations with a distinct frequency peak (ffuuuuuuuuuu) in the range of 2–4 times the runner rotational frequency [1, 2, 3] The occurrence of this instability leads to severe vibrations in the hydraulic circuit, which affect the performance of the machine. In addition to the self-rotation of the elliptical cross section, Nicolet et al [3] bring up the possibility of a breathing pattern in the evolution of the cavitation volume with time and the draft tube pressure
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