Effects of the vortex rope dynamic characteristics on the swirling flow in the draft tube cone

Abstract

As the Francis turbine operated at the off-design conditions, the hydraulic instability may occurs due to the vortex rope developed in the draft tube. The swirling flow produced at the blade trailing edge has been proved to have a direct effect on the vortex rope characteristics. Research on the swirling flow is meaningful for understanding the hydraulic instability mechanisms. In this paper, the different swirling flow characteristics at part load and overload conditions were studied by numerical method. The relations between the vortex rope dynamic characteristics and the swirling flows were deeply investigated. The results showed the swirl number, which was chosen to stand for the strength of the swirling flow, changed with the vortex rope evolution. The swirl number fluctuations and its frequency were closely related to the vortex rope precession and its diameter and length. The dominant frequencies of the swirling flow in spiral vortex rope flow field were consistent with the vortex rope. However, the swirl number frequency turned out to be the vortex rope harmonic frequencies in the columnar vortex rope condition. Besides, the propagation laws of the swirling flow in the draft tube cone were also different in the different vortex rope flow fields.