A Study On Pressure Fluctuation Induced By Interblade Vortices In A Runner Of Francis Hydraulic Turbine

For a long period, vortex rope in draft tube and Karman vortex are believed to be the primary factor of stability violation. However, as huge Francis hydraulic turbines are widely used, other important factors, such as interblade vortices, are found to respond the stability violation.When Francis turbines are operated at off design conditions, not only vortex rope in the draft tube occurs, but also the vortex bunches, which are called as interblade vortices, generate and flow out from the blades. Divided by the design head, interblade vortices can be separated to be high head interblade vortices and low head interblade vortices. When the turbine is operated at higher head, the high head interblade vortices are produced by flow separation at pressure side of the blade near the crown. When the turbine is operated at lower head, the low head interblade vortices are produced by flow separation at suction side of the blade near the crown.From the previous view, interblade vortices distribute at different channels. They are difficult to unite and have little influence to the stabilization. But if interblade vortices develop to strong vortex band, and the tail is unstable, the stabilization will be damaged and the noise will occur.Experimental and numerical simulation methods are used to investigate the interblade vortices in this paper. The strain fluctuation on the blade of prototype and model Francis hydraulic turbine is measured by electric measure of strain-flake method on High Head Experimental Table II in Haerbin Big Electric Machine Factory. The experimental results show that the frequency of the strain fluctuation keeps constant when the mass flow changes. The CFD results of part load condition show that the interblade vortices occur at the suction side of the blades near the crown, because of the flow separation at the same locations. Compared with vortex in draft tube, the frequency of pressure fluctuation induced by interblade vortices is much higher. The experimental and CFD results prove that the frequency of pressure fluctuation induced by interblade vortices is equals to the rotational frequency.