Combined air and water injection in Francis turbine draft tube at part load

The flow instabilities and pressure pulsations at Part load (PL) are detrimental to the turbine's life and performance. The present work reports the combined water and air injection in the draft tube (DT) cone. The water is injected through the center of the hub cone using a nozzle with a diameter of 12.7 mm. The air is admitted through the DT periphery. The water injection percentage 0.5% to 2% of the turbine inlet flow rate, and the air is injected at 0.5% to 2.5%. The spectral analysis assesses the pressure amplitude and dominating frequency affected by the combined water and air jet injection. The effect of combined air and water jet injection on synchronous (plunging) and asynchronous pressure pulsations (rotating) is analysed at two different measurement planes of the draft tube cone.