Reorganization of flow field due to load rejection driven self-mitigation of high load vortex breakdown in a Francis turbine

This paper reports the findings of an investigation on self-mitigation of the vortex breakdown phenomenon in a high-head model Francis turbine draft tube diffuser during the transition from high load to design operating conditions. The transient operating sequence is achieved by closing the flow regulating guide vanes assuming a linearly proportional decrease in flow rate. Scale-adaptive simulation shear stress transport turbulence model is used to ensure that the large-scale structures of the unsteady flow are resolved delivering a higher accuracy compared to complete averaging. The simulation is validated through a comparison between numerical and experimental axial velocity profiles on a radial line in the draft tube near its inlet. At high load, the numerical results agree satisfactorily with experiment, excepting slightly increased deviation in the central region due to the presence of vortex breakdown. However, at best efficiency point, a close agreement between numerical and experimental velocity profiles is seen in the central region as well. At high load, the vortex core is swollen, has sharp twists, encloses zones of flow stagnation and intermittent flow reversal, and is wrapped by a well-sped outflow through strong shear layers. Commencement of the transient sequence results in a gradual reorganization of the velocity field, leading to purge of major part of the vortex breakdown, like flow reversals and stagnation, within 50% of the time of load rejection. Onward, the flow is gradually restored to a streamlined, defect-free form. A comprehensive analysis and visualization of the evolving flow field is disseminated by this article.