Experimental investigation on the characteristics and alleviation of the upper part load pressure fluctuation in a Francis turbine

The Francis turbine is a renowned reaction hydraulic turbine known for its remarkable ability to convert the energy of flowing water into mechanical energy. Operating near the best efficiency point, the Francis turbine experiences the upper part load pressure fluctuation, which is characterized by an abrupt high amplitude increment and decay and can put the stability of a power plant at risk. This study experimentally investigated the amplitude-frequency characteristics of this special pressure fluctuation. The findings indicate that the most powerful amplitude reaches up to 15.35% of the tested water head in the vaneless space, and the characteristic frequency ranges from 1.6 to 2.3 times the rotating frequency. Notably, the study has found that the vortex rope in the draft tube exhibits a slender and spring-like within the upper part load region, and it looks like a spring stretching and compressing during the evolution of vortex rope, implying energy accumulation and release. Furthermore, a technical approach to alleviate high-amplitude pressure fluctuation in upper part load region is proposed that controls the velocity distributions at the runner outlet. A comparison of experimental test evidence shows that an impressive result is obtained with a reduction of the maximum amplitude from 10.75% to 3.52% in spiral casing, and from 15.35% to 3.03% in the vaneless space. In addition, the first-order frequencies at the spiral casing are primarily reduced, while the draft tube shows local increment, and the runner blade passage frequency becomes dominant in the vaneless space.