Further investigation on the pressure fluctuations caused by cavitation vortex rope in an elbow draft tube

TLDR: In this article, the authors demonstrate the experimental results of the pressure fluctuations with irregular nature, which are induced by the vortex rope in an elbow draft tube having a longer inlet once, and analyze the fluctuating wall-pressures measured at both inlet cone and elbow sections of the draft tube.

Abstract: Although the studies on pressure surge in elbow draft tubes of Francis turbines have been carried out by many researchers, pressure fluctuations associated with the unstable behavior of the cavitated vortex rope in the tube are still unclarified, and further understanding of their occurrence is required. In this paper, the author demonstrate the experimental results of the pressure fluctuations with irregular nature, which are induced by the vortex rope in an elbow draft tube having a longer inlet once. The spectrum method was used to analyze the fluctuating wall-pressures measured at both inlet cone and elbow sections of the draft tube. A high speed video system was also employed for observing the behavior of the cavitated vortex rope. From the experiment, three types of pressure fluctuations with different dominant frequencies were found out, and the mechanism of draft-tube-surging was explained reasonable based on the relationships among those pressure fluctuations.