Efficacy of ancillary fluid injection technique for mitigation of vortex rope in hydraulic turbines: A review

Abstract

Penetration of variable renewable energy into the electric grid requires parallel employment of power generation sources that can quickly balance the fluctuation in frequency of electric grid. Hydro power, being one of the most flexible power supplies, thus takes on an instrumental role in balancing the grid. However, in hydraulic turbines such flexibility is achieved at the expense of departure from turbine operation at the best efficiency point (BEP). Turbine operation at off-design conditions like part load (PL) or high load (HL) results in significant flow instability in the draft tube. Of particular prominence is the vortex breakdown phenomenon which manifests as a precessing vortex rope in the draft tube. The vortex rope causes dynamic pressure fluctuations in the system, additive to the compromise in efficiency. If resonant with the system natural frequency, these pressure fluctuations may affect the mechanical torque delivered by the turbine and cause a phenomenon called power swing. As such it is of paramount importance to circumvent the effects of the rope. Amidst many techniques developed by various investigators for mitigation of the rope, particularly recent the ancillary fluid injection technique exhibits promise. Ancillary fluid injection involves riddance of the stagnation zone that leads to rope formation, through transfer of momentum from a fluid injected at an appropriate location and velocity. This is usually accomplished using water. Admission of air for flow aeration is another method. An experimental as well as numerical perspective of these techniques as developed by various researchers is laid, to assess the benefit obtained alongside the entailing compromise.