Abstract
Swirling flow inside draft tube at part load condition during cavitation inception induces large pressure fluctuations. For suppression of the draft tube surge, fins with air injection holes are employed on the draft tube wall. The protrusion of air injection holes obstructs the incoming flow and creates flow complexities near the holes. The present study transiently investigates the effect of protrusion of these holes on the performance and flow phenomena of the Francis turbine numerically. Reynolds averaged Navier-Stokes (RANS) equations are solved along with a two-phase mixture model using Scale Adaptive Simulation Shear Stress Transport (SAS-SST) model to solve the turbulent flows. The numerical methodology is then verified by an experiment based on the International Standard (IEC 60193) norms. The hole protrusion reduces the efficiency of the turbine and exhibits higher strength of the vortex rope which is minimized using fins without holes. The swirl number is reduced by about 34% for the draft tube without holes. The random pressure fluctuations caused by the hole’s intrusiveness are also suppressed when the holes are removed.
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