Suppression of vortex rope formation and pressure fluctuation using anti-swirl fins in a Francis turbine model at part load condition with cavitation inception point

Abstract

Vortex rope formation at part load (PL) with cavitation inception causes pressure fluctuations inside the draft tube (DT) of a Francis turbine which may fail the turbine due to resonance and erosion. The pressure surge can be minimized by using anti-swirl fins which ensure safe turbine operation. The present study examines the effect of fin sizes and locations on the internal flow characteristics of the Francis turbine and predicts its adverse effect on the pressure surge. Three cases are investigated in which internal flow physics are compared among DTs with longer fins, shorter fins, and no fins. At the cavitation inception point under PL conditions, the characteristics are thoroughly studied numerically using ANSYS-CFX with structured and unstructured grids. Cavitation and PL conditions are designated by Thoma number 0.266 and guide vane angle 16°. Numerical methodology is then verified by an experiment based on International Standard (IEC 60193). The vortex rope occurrence is suppressed using fins on the DT periphery and longer fins that are extended up to the elbow exhibit the lowest strength of the vortex rope. Maximum pressure recovery inside the DT is achieved using longer fins. The swirl intensity is remarkably reduced by about 94% with longer fins. The pressure peak of low frequency is about 60% suppressed using longer fins. The PL and cavitation-induced instabilities and vibrations are significantly mitigated by longer fins, while shorter fins also moderately minimize it. Hence, energy production is preferable with longer fins because of the safe and stable turbine function.