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Numerical investigation of the effects of leakage flow from guide vanes of Francis tur-bines using alternative clearance gap method
Author(s): Saroj Gautam, Hari Prasad Neopane, Biraj Singh Thapa, Sailesh Chitrakar, Baoshan Zhu
Keywords: Blade Passing Frequency, Clearance Gap, Guide Vanes, Leakage Flow, Vortex
Guide vane profile has significant effect on the performance of turbine with sediment contained fluid flow. In this paper symmetrical NACA 0012 and cambered NACA 2412, NACA4412 hydrofoils are studied introducing 0 mm, 2 mm and 4 mm clearance gap. Vortex filament can be seen when the fluid leaves clearance gap due to leakage flow occurring through the gap. The intensity of vortex leaving clearance gap rises with increase in size of clearance gap. However, in case of asymmetrical guide vane profile, 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 the major reason to erode the runner surface due to high velocity of sand particle travelling along with them. With the alternative clearance gap approach, this paper compares pressure pulsation downstream of guide vanes contributed by the leakage flow for three NACA profiles, whose frequency is half of the blade passing frequency.