<p style="text-align: justify">Hydraulic turbines are now subjected to operating points different from the design point. The flow field inside the draft tube of a Francis turbine depends on the operating point. At off-design operations, flow instabilities such as vortex breakdown, reverse flows, and stagnant regions are observed. These flow instabilities are detrimental to the turbine life and also result in hydraulic losses and pressure pulsations in the draft. The study of turbine flow field can be helpful in devising measure which may help to alleviate the effects of these flow instabilities.</p><p style="text-align: justify">The present work shows the numerical flow field investigations by employing a vortex identification algorithm. The algorithm works by analyzing the local velocity gradients in the flow field. The algorithm helps in the identification of the local vortex center, shear layers, and pure rotational zones. The analysis has been performed at various locations in the draft tube by extracting the velocity fields at different time steps of the simulation.&nbsp;The analysis has been performed on two operating regions of interest. The first operating point involves a fully developed rotating vortex rope (RVR) in the draft tube which precesses at a frequency of 0.28 times of the runner rotation. This vortical structure is responsible for severe pressure pulsations and involves a shear layer between the stagnant region and the main flow. The present algorithm is able to identify those regions along with an eccentric local rotation center.&nbsp;The second operating regime shows characteristics of deep part load with central solid body rotation in the draft tube flow field. The results show highly swirling flows with very low axial velocity. The flow is confined primarily near the walls. The analysis shows that the extent of the stagnation region at deep part load is more and no inner shear layer is present as compared to the other part-load operations.</p>

Flow Field Investigation in Draft Tube of Francis Turbine at Off-Design Operation Using a Vortex Identification Algorithm

Paper Proceedings: https://asmedigitalcollection.asme.org/FEDSM/FEDSM2021/volume/85284

ASME 2021 Fluids Engineering Division Summer Meeting (FEDSM2021)

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