Active flow control of the vortex rope and pressure pulsations in a swirl generator

Abstract

The vortex rope and pressure pulsations caused by a radial pressure gradient in the conical diffuser of a swirl generator is controlled using continuous slot jets with different momentum fluxes and angles injected from the runner crown. The swirl apparatus is designed to generate flows similar to those in the different operating conditions of a Francis turbine. The study is done with numerical modelling using the hybrid URANS-LES (Unsteady Reynolds-Averaged Navier-Stokes-Large Eddy Simulation) method with the rotor-stator interaction. The comprehensive studies of Javadi and Nilsson [Time-accurate numerical simulations of swirling flow with rotor-stator interaction. Flow, Turbulence and Combustion, Vol. 95,, and Javadi, Bosioc, Nilsson, Muntean and Susan-Resiga [Experimental and numerical investigation of the precessing helical vortex in a conical diffuser, with rotor-stator interaction. ASME Journal of Fluids Engineering, doi:10.1115/1.4033416] are considered as the bench mark, and the capabilities of the technique is studied in the present work with the validated numerical results presented in those studies. The pressure pulsations caused by the pressure gradient generated by the swirl, present at off-design conditions, are cumbersome for hydropower structures. The investigation shows that the pressure pulsation, velocity fluctuations and the size of the vortex rope decrease when the jet is injected from the runner crown. The flow rate of the jet is less than 3% of the flow rate of the swirl generator. The momentum flux, angle of injection of the jet and the position of the slot are important factors for the effectiveness of the flow control technique. ARTICLE HISTORY

Experimental Study of Transient Flow Regimes in a Model Hydroturbine Draft Tube

“…Recent studies of the PVC effect [7][8][9] have focused on developing methods for controlling and suppressing PVC in hydroelectric units, wherein data on the influence of vortex phenomena on the transient turbine are poorly presented in the literature. Transient regimes can occur in regions with a PVC, so it is of considerable interest to study the mechanisms of PVC formation in various transient regimes.…”

A Novel Passive Method to Control the Swirling Flow with Vortex Rope from the Conical Diffuser of Hydraulic Turbines with Fixed Blades

“…These active control methods act to increase the axial flux of the moment of momentum to stabilize the flow instead of to diminish the swirl level below the critical level (to decrease the tangential flux of the moment of momentum) like the passive control methods mentioned above. Javadi and Nilsson numerically examined a new idea about the water injection method [29], and considered a swirl component superposed over an axial jet in order to improve its effects. Their investigations have shown that the pressure pulsations, velocity fluctuations and the size of the vortex rope decrease when a jet with a flow rate less than 3% is injected from the runner crown.…”

Hydrodynamic structure of laminar flows with oppositely-swirled coaxial layers

“…The effective development of these technologies is impossible without knowledge of the hydrodynamics of such flows. Such flows also arise in cyclones and draft tubes behind Francis hydraulic turbines in non-optimal operation modes [10][11][12][13][14].…”