Numerical investigation of alleviation of undesirable effect of inter-blade vortex with air admission for a low-head Francis turbine

Abstract

In order to compensate for the stochastic nature of the power grid due to the tremendous development and the integration of renewable energy resources and meet its other requirements, the hydraulic turbines are forced to operate more frequently under partial load conditions with singular and misaligned flows inevitably excited by the inter-blade vortex. This paper presents numerical investigations of the unsteady characteristics of the inter-blade vortex for a low-head model Francis turbine. The SST k - ω turbulent model is used to close the unsteady Reynolds-averaged Navier-Stokes (RANS) equation. The flow structure of the inter blade vortex predicted by the numerical simulation is confirmed by experimental visualizations. It is shown that the total vortex volume in the runner sees a quasi-periodical oscillation, with significant flow separations created on the suction side of the runner blade. A counter measure by using the air admission into the water from the head cover is implemented to alleviate the undesirable effect of the inter-blade vortex. The analyses show that the development of the inter-blade vortex is significantly mitigated by the injecting air that controls and changes the spatial distribution of streamlines. Furthermore, the flow aeration with a suitable air flow rate can reduce the energy dissipation caused by the inter-blade vortex and plays a critical role in preventing the excessive amplitudes of the pressure fluctuation on the suction side of the runner blade. This investigation provides an insight into the flow mechanism underlying the inter-blade vortex and offers a reference to alleviate and mitigate the adverse consequences of the inter-blade vortex for the Francis turbine.