Particle Image Velocimetry Investigation of the Leakage Flow Through Clearance Gaps in Cambered Hydrofoils

Abstract

In Francis turbines, which are normally designed at a reaction ratio of 0.5, the available pressure energy in the fluid is converted into 50% kinetic energy before entering the runner. This causes high acceleration of the flow in guide vanes (GVs), which adds to the unsteadiness and losses in the turbine. In sediment-affected power plants, the hard sand particles erode and gradually increase the clearance gap between the GV and facing plates, which causes more disturbances in downstream turbine components. This study focuses on investigating the flow through the clearance gap of the GV with cambered hydrofoil shapes by using particle image velocimetry (PIV) technique. The measurements are carried out in one GV cascade rig, which produces similar velocity fields around a GV, as compared to the real turbine. The investigation is done in two cases of cambered GV National Advisory Committee for Aeronautics (NACA) profiles, and the comparison of the velocity and pressure distribution around the hydrofoil is done with the results in symmetric profile studied earlier. It is seen that the pressure distribution around the hydrofoil affects the velocity field, leakage flow, and characteristics of the vortex filament developed inside the cascade. NACA4412, which has flatter suction side (SS) than NACA2412 and NACA0012, is seen to have smaller pressure difference between the two adjacent sides of the vane. The flow inside the clearance gap of NACA2412 enforces change in the flow angle, which forms a vortex filament with a rotational component. This vortex along with improper stagnation angle could have greater consequences in the erosion of the runner inlet (RIn) and more losses of the turbine.