The numerical and experimental investigation of erosion induced leakage flow through guide vanes of Francis turbine

Abstract

In Guide Vanes (GV) of Francis turbines, a portion of the pressure head of water converts into velocity head. This causes high acceleration of the flow in GV before reaching the runner. Furthermore, GVs are accompanied with a small clearance gap at both ends to adjust the opening angle based on various operating conditions. In the case of sediment affected power plants, the hard fine particles mixed in water erode the connecting ends due to horse-shoe vortices. This erosion together with the head cover deflection due to water pressure increases the size of the gap. Due to the adjacent pressure and suction sides in GV, the flow passes through the gap from high pressure side to low pressure or suction side. This leakage flow disturbs the main flow in the suction side, which can be observed in the form of a vortex filament. Depending upon the GV profile and opening angle, the vortex can have different characteristics. This study uses numerical and experimental techniques to study the potential effects of the leakage flow in overall performances of the turbine. The experiment is done to measure the velocity field around GV using Particle Image Velocimetry (PIV) technique on a GV cascade rig. The GV in this rig corresponds to 1:1 scale model of 4.1 MW Francis turbine, with the chord length of 142 mm and span height of 97 mm. Similarly, 14 pressure sensors are placed around the GV cover plate to measure the GV loading. The velocity and pressure fields are compared with the results of CFD. In this study, two GV profiles (NACA0012 and NACA4412) and 7 opening angles (-5°, --3°, -1°, 0°, 1°, 3°, 5° compared to the design point) of the GV are studied. Results show that at Best Efficiency Point (BEP) and small opening or closing, the pressure difference between the adjacent sides of GV and consequently, the leakage flow and the intensity of the vortex filament in NACA4412 is less than in NACA0012. However, at high opening angle or during full load, the direction of the leakage flow in NACA4412 is in opposite direction due to small or negative GV loading compared to BEP. It is shown how these vortices affect the runner performances and how the particles erode the runner inlet as a consequence of these vortices.