Peripheral air jet injection at part load operation of a low head Francis turbine

The rotating vortex rope in the flow field generates low-frequency, high-amplitude pulsations at part load (PL) operation of a Francis turbine. These high amplitude pressure pulsations are responsible for power swing, cyclic fatigue, heavy structural vibrations, draft tube surge, and mechanical failure of turbine components. The fluid injection methods are generally employed to arrest or mitigate these low frequencies pressure pulsations. In the present work, the air is injected at a rate of 0.5% to 2.5% of the turbine flow rate at PL. The draft tube (DT) cone is modified to accommodate the air injection mechanism. Spectral analysis and spatial harmonic decomposition (SHD) are performed on pressure data for detailed investigation. The SHD is performed to get the synchronous (plunging component) and asynchronous fluctuations (rotating component) of pressure pulsations in the DT cone. The air injection in the DT cone successfully arrested the low-frequency pulsations of high amplitudes.