Correlating Sediment Erosion in Rotary-Stationary Gaps of Francis Turbines with Complex Flow Patterns

Abstract

The presence of a gap between guide vanes and top-bottom covers and rotating-stationary geometries induces the secondary flows in Francis turbines. The secondary flow developed in the clearance gap of guide vanes induces the leakage vortex that travels towards turbine downstream affecting the runner. Likewise, secondary flows from the gap between rotor-stator component enter the upper and lower labyrinth region. Francis turbines when operated with the sediment-laden water, sediment contained flows affect these gaps thus increasing the size of the gap and increasing the leakage flow. This work examines the secondary flows developing at these locations of Francis turbine and consequent sediment erosion effects. A reference Francis turbine of Bhilangana III Hydropower Plant (HPP), India with a specific speed (Ns=85.4) severely affected by sediment erosion problem was selected for this study. All the components of the turbine were modelled and a reference numerical model was developed. This numerical model was validated with the numerical uncertainty measurement and with the experimental results. Erosion at guide vanes were due to the development of leakage flow inside the guide vane clearance gaps. At the runner inlet, erosion was mainly due to leakage vortex from clearance gap and leakage flow from rotor-stator gaps.