Upper part-load instability in a reduced-scale Francis turbine: an experimental study

• Published in: Experiments in fluids 2023-06, Vol.64 (6), Article 110
• Main Authors: Amini, Ali, Vagnoni, Elena, Favrel, Arthur, Yamaishi, Kazuhiko, Müller, Andres, Avellan, François
• Format: Article
• Language: English
• Subjects: Cavitation; Draft tubes; Engineering; Engineering Fluid Dynamics; Engineering Thermodynamics; Flow control; Flow stability; Fluid flow; Fluid- and Aerodynamics; Heat and Mass Transfer; High speed; Particle image velocimetry; Research Article; Skewed distributions; Turbines; Velocity; Velocity distribution; Vortices
• ISSN: 0723-4864; 1432-1114
• DOI: 10.1007/s00348-023-03649-0

Francis turbines with medium or high specific speeds may experience a particular type of instability in the upper part load in which the precessing vortex has an elliptical shape. The occurrence of the upper part-load instability (UPLI) is accompanied by large-amplitude pressure fluctuations at a distinct frequency between 2 and 4 times the runner rotational speed. This paper experimentally investigates UPLI for a reduced-scale Francis turbine. To investigate the causal factors of this instability, draft tube pressure measurements, particle image velocimetry, and high-speed flow visualizations have been performed at several operating points under cavitation and cavitation-free conditions . It is shown for the first time that for an operating point within the UPLI range, the vortex always features a circular section in cavitation-free conditions, which is preserved even after the initial appearance of cavitation. It is only below a certain Thoma number that the vortex section turns into an ellipse and shows an abrupt increase in pressure fluctuations. Analysis of the phase-averaged velocity fields reveals that a concentrated vortex with a large precession radius is a prerequisite for UPLI, while the instantaneous velocity fields clearly illustrate the asymmetric velocity distribution around the elliptical vortex. The existence of a breathing mode and the intermittent formation of two side vortices along the elliptical vortex rope are also evidenced by high-speed flow visualizations. These results provide a much deeper insight into the flow structures that favor the development of UPLI and help delimit its thresholds to higher precision, and thus, prevent its occurrence during turbine operations.