In order to solve the problem of the low-frequency pressure pulsation of large Francis turbine draft tube caused by spiral vortex rope at part load operation, a simple and effective method which can weaken the vortex rope, called hydraulic disturbance, was introduced. Using the model test method, a structure for hydraulic disturbance was designed on a model turbine, and pressure fluctuation tests with hydraulic disturbance and without were carried on. The pressure pulsation amplitude and frequency characteristics at different part loads were analyzed, and the mitigation effect of pressure fluctuation with hydraulic disturbance method was studied. Studies have shown that without hydraulic disturbance there is a vortex rope in the draft tube, whose frequency is about 1/5 rotating frequency, which causes strong low frequency pressure fluctuation near the wall of draft tube, and this low frequency pressure fluctuation is the main reason for the unsteady operation of the turbine. While with hydraulic disturbance the amplitude of the low frequency pressure fluctuation is significantly reduced, but the frequency of pressure fluctuation is not changed obviously. A new way of reducing pressure fluctuation in the draft tube is presented, by which the operation stability of Francis turbine can be improved.
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