Unstable Part-Load Operation of a Model Francis Turbine : Evaluation of Disturbance Magnitude

The low frequency pressure oscillations observed at the draft tube wall of a Francis turbine operating at part load may be split into two uniform components: a rotating pressure with a constant area-averaged value, and a synchronous pulse involving the whole draft tube section. The synchronous pulse is sometimes used to evaluate the magnitude of disturbances generated by part-load operation of the turbine. A more global way to evaluate the magnitude of these disturbances is to estimate the emission of acoustic power at the spiral case inlet. In this paper, both methods are used on model test data from a high specific speed turbine. A good correlation is found between results from the two approaches.