Numerical analyses of pressure fluctuations induced by interblade vortices in a model Francis turbine

Interblade vortices can greatly influence the stable operations of Francis turbines. As visible interblade vortices are essentially cavitating flows, i.e., the ones to cause interblade vortex cavitations, an unsteady simulation with a method using the RNG k – ɛ turbulence model and the Zwart-Gerber-Belamri (ZGB) cavitation model is carried out to predict the pressure fluctuations induced. Modifications of the turbulence viscosity are made to improve the resolutions. The interblade vortices of two different appearances are observed from the numerical results, namely, the columnar and streamwise vortices, as is consistent with the experimental results. The pressure fluctuations of different frequencies are found to be induced by the interblade vortices on incipient and developed interblade vortex lines, respectively, on the Hill diagram of the model runner's parameters. From the centrifugal Rayleigh instability criterion, it follows that the columnar interblade vortices are stable and the streamwise interblade vortices are unstable in the model Francis turbine.