Effect of water jet area on mitigation of vortex rope in a Francis turbine

Abstract

The rotating vortex rope (RVR) is normally formed at part load operating conditions, which develops flow instability in the draft tube (DT) cone of a Francis turbine. The detrimental effects of RVR reduce the life and performance of the turbine. The present work mitigates RVR through axial water jet injection in the DT cone. This paper extends the author's previous work on water injection with a 50% reduction in the water jet area through two nozzles with a diameter of 12.7 mm. The effect of area reduction is analyzed by spectral analysis like power spectrum density and spatial harmonic decomposition of the pressure field measured at the wall of the cone. The effect of water jet injection on synchronous and asynchronous pressure pulsations is analyzed at both measurement planes of the cone. The improvement in pressure recovery in the cone and turbine efficiency due to water jet injection is also analyzed. The water is injected at 0.5%–2% of the turbine inlet flow at 22.10° guide vane opening.