Vortex rope instabilities in a model of conical draft tube

Abstract

Abstract. We report on experimental studies of the formation of vortex ropes in a laboratory simplified model of hydroturbine draft tube. Work is focused on the observation of various flow patterns at the different rotational speed of turbine runner at fixed flow rate. The measurements involve high-speed visualization and pressure pulsations recordings. Draft tube wall pressure pulsations are registered by pressure transducer for different flow regimes. Vortex rope precession frequency were calculated using FFT transform. The experiments showed interesting features of precessing vortex rope like twin spiral and formation of vortex ring.

Investigation of the Pressure Fluctuation Alleviation in a Hydraulic Turbine by Runner Modification

“…Because of operating flexibility of hydraulic turbines, it is often operated at off-design conditions, which leads to the existence of circumferential velocity, sometimes intensively, at outlet of the runner. This phenomenon means that water will shift to draft tube wall when it flows out from the runner cone, so that in the center of draft tube there is now a dead water region [3,4] with the characteristics of low pressure and low axial velocity, which induces cavitation which we call the "vortex rope" [5] or "cavitation vortex rope" [6]. This dead water region will have a circumferential velocity because its external water has an intensive circumferential velocity, rotating at a certain speed in draft tube conical part.…”