Impact, analysis, and mitigation of flow instabilities in draft tube of Francis turbines

Hydropower is a clean, renewable, and highly efficient energy source and accounts for about 16% of global electricity production. Apart from baseload generation, hydraulic turbines are more and more often subjected to less efficient off-design operating conditions to handle the fluctuating electricity demands and stabilize the power grid. These sub-optimal operations result in flow instabilities such as vortex breakdown and inter-blade vortices, which reduce the overall hydraulic efficiency and produce severe pressure pulsations. This review summarizes various forms of flow instabilities and the theories developed to explain the phenomena. The impact of these instabilities on the turbine operation is discussed. Mathematical models developed to describe the swirling flows downstream of a Francis runner are also presented. Parameters that influence the characteristics of the flow instabilities are outlined. The review concludes with a discussion of various methods developed to alleviate the adverse effects of swirling flows on Francis turbines.