Influence of swirling flow parameters on frequency response of a simplified draft-tube in presence of cavitation

This paper focuses on investigating the influence of swirling flow parameters on frequency response of a simplified draft-tube under different cavitation numbers by using experimental approaches. The experiments are conducted for three swirl configurations featuring different swirl intensities conforming with the flow field in the draft-tube during high-load operation of a hydro-turbine. For this purpose, three stationary-blades swirl generators are used to generate axisymmetric swirling flows in a Venturi-tube with a diffuser resembling the draft-tube cone of a hydraulic turbine; and for each swirl configuration, the experiments are conducted over a wide range of cavitation numbers and flowrate values. Experiments include transient pressure measurements at different locations and high-speed visualization of cavitating flow in the Venturi. The results of transient pressure measurements are analyzed to identify the frequency spectrum of pressure fluctuations in the draft-tube. The first natural frequency of the draft-tube is identified for a wide range of operating conditions; and several correlations are established for its relationship with the flow parameters (cavitation number and swirl number). It is observed that the operating flowrate does not affect the natural frequency of the draft-tube. An unstable mode of cavitating flow is highlighted and a transition from stable to unstable modes is identified. Investigating the transition mechanism from the stable to the unstable modes of cavity led to identifying an excitation force for the onset of cavitation instability in the draft-tube. High-speed visualization of cavitation in the draft-tube is performed to capture the mean cavity volume and its fluctuations in time domain. The 1st natural frequency of the draft-tube is further predicted by analyzing the fluctuations of cavity volume. Furthermore, a 1D hydroacoustic model is also used to predict the local wave speed and the 1st natural frequency of the draft-tube. The obtained values of natural frequency from cavity visualization and 1D modelling are compared with the results of pressure measurements at the end.