3D numerical analysis of pulsating water jet in the draft tube cone of hydraulic machinery

The fixed blade turbines, e.g. Francis, operating at part load, present a high level of swirl flow at the inlet of draft tube cone. When swirling flow is decelerating, it becomes unstable, giving rise at spiral vortex (or vortex rope). Vortex rope is the main cause for the occurrence of pressure fluctuations in draft tube of hydraulic turbines operating at part load. Different techniques have, been studied in order to mitigate the vortex rope, with not so big success. The water injection method developed in our laboratory from Politehnica University Timisoara has shown that a 10%-12% from the main flow is necessary in order to mitigate the pressure fluctuations. However, distinct low-frequency pressure oscillations are still exist. These plunging oscillations (low-frequency), are dangerous due to the waves traveling along to hydraulic passage. The paper continue our work in the pulsating water jet injection along the draft tube axis, in order to mitigate the vortex rope and the associated low-frequency oscillations. Nevertheless, the great calling of this control method is to eliminate the vortex rope by fragmenting the vortex sheet. The energy loss coefficient and kinetic to potential conversion ratio distributions are plotted along to the draft tube cone in order to evaluate the performances. In addition, the unsteady part of the pressure signal characterize by Direct Fourier Transform will be analysed in the case with and without pulsating jet method. The last part of the results will be focused on the pressure signal decomposition in order to show how the pulsating water jet practically changes the ability of the decelerated swirling flows to generate both rotating (asynchronous) and plunging (synchronous) fluctuations.