NUMERICAL SIMULATION OF VORTEX ROPE AND ITS MITIGATION IN FRANCIS TURBINE

Abstract

When Francis turbine operates at discharge lower than the best efficiency discharge, a vortex rope forms due to the coexistence of decelerated swirling flow and high speed flow around it causes a shear layer between the two. A scaled model of the Francis turbine operating at Tokke power plant in Norway known as Francis-99 is numerically simulated to investigate the off-design operation. Here 70% bep discharge is taken at inlet to capture the formation of vortex rope using SST k-ω as a turbulent axisymmetric swirling flow model. The accuracy of simulated swirling flow field with vortex rope is validated against the data of pressure measurements given on the NTNU website. This vortex rope rotates in the direction of swirl, at a frequency about one third of runner rotation frequency. It creates additional hydraulic losses with a proportionate depreciation of the kinetic to potential energy transformation efficiency. Therefore, in order to minimize the hydraulic losses for diffusers with swirl, a novel flow control method is introduced, which mitigates the vortex rope by injecting a water jet along the centerline at the runner crown tip. In the current study, the discharge through the control jet is varied by changing the velocity of the jet, keeping the diameter of jet pipe to be constant. The discharge of jet is taken according to the specific percent of inlet discharge. The impact of discharge variation on the mitigation of vortex rope is noted.