Vortex rope at part load
Flexible power generation, more wind power, and less electricity demand often force hydraulic turbines to operate at part load conditions (below 90% of BEP) for long time. The hydraulic turbines experience dynamic instability. Unstable flow conditions are mainly responsible for the high-amplitude low frequency pressure pulsations in the turbine, which constantly affect the dynamic stability. The instabilities at the part load conditions are well known, and significant efforts have been applied to investigate the consequences. The majority of numerical studies have been performed to investigate the part load flow instabilities associated with the vortex rope. Still it is challenging to obtain reliable information about the formation of vortex rope in the turbines. Figure below shows how flow condition changes in the draft tube with time. A numerical simulation was performed on a high head model Francis turbine at part load (~68% load). Hexahedral mesh in the entire turbine was created, which included 35 million nodes (y+<1). The time-step size was 0.5 degree of runner rotation. The simulation was performed for total 20 revolutions of the runner. The figure shows the flow condition during last 10 revolutions of the runner.
Vortex rope at part load operating condition in a high head Francis turbine. (Chirag Trivedi, Sep 1, 2016)