Numerical Investigation of the Effects of Leakage Flow From Guide Vanes of Francis Turbines using Alternative Clearance Gap Method

Flow around the Guide vanes (GV) in Francis turbine differs with the shape of hydrofoils. The difference in the pressure of fluid travelling to pressure side and suction side of GV contributes to flow behavior. This study presents the numerical technique using alternative clearance gap method to predict the flow around GV and its consequent effect on turbine performance. GV profile has a significant effect on the performance of the turbine with sediment contained fluid flow. In this paper, symmetrical NACA ۰۰۱۲ and cambered NACA ۲۴۱۲, NACA ۴۴۱۲ hydrofoils are studied introducing ۰ mm, ۲ mm, and ۴ mm clearance gaps. Vortex filament can be seen when fluid leaves the clearance gap due to the leakage flow occurring through the gap. The intensity of vortex leaving clearance gap rises with an increase in the size of the clearance gap. However, in the case of asymmetrical GV profile, the velocity of fluid travelling along the vortex compared to that of symmetrical hydrofoil is lower. In case of low specific speed Francis turbines, this vortex is found to be a major reason to erode the runner surface due to high velocity of a sand particle travelling with them. With the alternative clearance gap approach, this paper compares the pressure pulsation downstream of GVs contributed by leakage flow for three NACA profiles, whose frequency is half of blade passing frequency.

Keywords:

Francis turbine , Sediment erosion , Leakage flow , RSI , Vortex , Blade passing frequency , Efficiency