Flow characteristic investigation into inter-blade vortex for Francis turbine

The operating of hydroturbines is inclined towards the off-design conditions due to tremendous development and integration of renewable energy resources, which inevitably induces various types of cavitation flowing causes rapid degradation in performance. The inter-blade vortex can be interpreted as a peculiar cavitation flowing phenomenon developed in the blade channels and disappeared near the runner outlet at partial load conditions for Francis turbine, in combination with several adverse effects on pressure and velocity fields. However, there is limited investigation available on the flow characteristic and underlying mechanism of inter-blade vortex, as well as its influence on hydraulic performance for Francis turbine. This paper presented numerical and experimental investigations into cavitation two-phase fluid for a reduced scale model of Francis turbine. The numerical investigation was carried out by coupling the SST k-ω turbulent model and the Zwart cavitation model, and the experimental vortex structure was recorded by a high-speed camera through the transparent draft tube cone immediately downstream of the runner. The fluid structure of inter-blade vortex predicted by numerical simulation yielded a very good validation against the experimental visualization. At the given operating point, a periodic oscillation of vapor volume was obtained and accompanied by the precessing frequency of inter-blade vortex, which was about 0.9 times of the rotational frequency. The incipient of inter-blade vortex structure had been observed near the runner hub along the blade span-wise direction, under the action of centrifugal force, the main flow was susceptible to be offset towards runner shroud that forced the vortex structure to move towards the trailing edge of runner blade, as a consequence, the vortex filament left the runner outlet in the vicinity of runner shroud. The limited space between blade channels and large negative angle of attack, which caused remarkable flow separation and recirculation regions in the blade channels, were both responsible for the formation of inter-blade vortex. The pressure pulsation and spectrum analysis showed that the precessing frequency of inter-blade vortex was always captured in the vaneless space between the guide vane and the runner, the runner blade, as well as the draft tube, indicating that the frequency propagated to the upstream and downstream simultaneously. In the vaneless space, the maximum pressure amplitude was attained at the blade passage frequency induced by the rotor-stator interaction, but the frequency of inter-blade vortex was also powerful and cannot be neglected. In terms of the pressure minoring point near the runner inlet, both of the inter-blade vortex frequency and guide vane passage frequency were dominating the turbine. In the vicinity of trailing edge of runner blades, the guide vane passage frequency was completely dampened but the inter-blade vortex significantly promoted the pressure amplitude, which evidently indicated that the presence of inter-blade vortex had critical influence on the production of pressure fluctuation on the suction side adjacent to the trailing edge. The pressure oscillations in draft tube cone kept synchronized despite different positions, and the frequency of inter-blade vortex performed a dominant role on the excitation of pressure fluctuation. It was worth noting that severe backflow dominates on the central sections of draft tube cone and elbow owing to vital effects of inter-blade vortex on the internal flow fields of draft tube. The presented investigations provide greater insight into the complex fluid structure and better understanding of underlying mechanism of inter-blade vortex towards the Francis turbines.