Dynamic characteristics and stability evaluation of cavitation-induced flow instability in a conical diffuser

It is known that cavitation surge, which is a self-excited vibration caused by cavitation volume fluctuation, may occurs in a draft tube of a water turbine when it is operated under overload. In this study, a cavitation surge was generated by a swirling flow into a Venturi tube, and its dynamic characteristics and stability were evaluated using a one-dimensional model and a transfer matrix. In addition, the variation of the cavitation volume was calculated using the binarization process with a high speed camera and compared with CFD. As a result, it is possible to evaluate the period of the cavitation volume fluctuation by the binarization process, but it is difficult to evaluate its volume quantitatively. The magnitudes of the cavitation compliance and the mass flow gain factor decrease with the increase of the cavitation coefficient. The magnitudes of the mass flow gain factor also tend to decrease with decreasing swirl number. The phase of the mass flow gain factor tends to delay as the cavitation coefficient increases. It is found that the phase of the mass flow gain factor and the cavitation compliance are also important parameters for the stability.