Flow-related mechanisms of draft tube surges observed at partial load of a francis turbine.

Abstract

Theoretical and experimental studies were carried out to clarify flow-related mechanisms of pressure surges, which corresponded to violent pressure fluctuations observed in the elbow type draft tube of a Francis turbine at partial load. From analysis of pressure fluctuations, it is shown that a synchronous surge is regarded as the resonant condition of the gas volume in a cavitated spiral vortex core to the oscillation of pressure recovery caused by precession or rotation of the core in the bend inlet. The theory indicates that a rotating pressure surge occurs in a conical draft tube. This is proved by the experiments.