The Identification of a Vortex in the Draft-Tube Cone of a Model Francis Turbine

In this study we used the laser-Doppler anemometry (LDA) method to identify the initial phase of the vortex-structure development in the draft-tube cone of a model Francis turbine. The velocity profile of the fluid flow through the turbine's hydraulic passage and the kinematics of the vortex flow behind the turbine runner were investigated. The results of the measurements were compared with the results of the numerical modeling of the stationary flow inside the turbine's draft tube. The differences between the measured and calculated values of the local velocities on the measuring cross-section were considerable in the area of the initial vortex structure. These differences can be combined with the inappropriate modeling of the coherent vortex structures. The similarity of the kinematics of the coherent vortex structures under non-cavitating operating conditions and the structures of the cavitating vortex in the draft tube at the initial stage of cavitation were established. The article also introduces an analysis of specific low-frequency velocity field oscillations, which indicate the kinematics of the cavitating vortex.