Abstract
To realize a multienergy complementary system involving hydropower and other energy sources, hydraulic turbines frequently run under partial flow conditions in which a unique flow phenomenon, the channel vortex, occurs in the runner, causing fatigue failure and even cavitation to the turbine blade. Cavitation severely shortens the service life of the unit and terribly limits the output of the turbine under partial flow conditions. In this paper, a numerical model of a Francis turbine was created with tetrahedral grids; the large eddy simulation (LES) method based on the WALE subgrid scale model and the Schnerr–Sauer cavitation model was adopted to carry out numerical simulation of the Francis turbine; and a vortex identification method based on the Q criterion was used to capture and analyze the channel vortex. The calculation results showed that a negative impact angle at the inlet of the runner occurred when the turbine ran under partial flow conditions, leading to three different types of channel vortexes in the blade channel. Also, different channel vortexes caused cavitation on different positions on the runner, and the volume change of cavitation showed periodic properties.
Highlights
The fluid structure of the hydraulic turbine is so complicated under partial flow conditions that it often leads to unstable operation problems such as channel vortexes, flow-induced vibration of the blades, cavitation, and cavitation erosion inside the turbine [6,7]
Certain studies have shown that when a turbine runs under partial flow conditions, outflow and backflow occur in the blade channel, and channel vortexes are formed; turbulent backflow causes unstable channel vortexes and even serious vibration and cavitation in the blade passage [9]
The large eddy simulation (LES) method based on the Walladapting Local Eddy-viscosity model (WALE) subgrid scale stress model was used with the Schnerr–Sauer cavitation model to study the flow characteristics in the flow passage of a Francis turbine under partial flow conditions
Summary
The total global installed hydropower capacity had reached 1308 GW in. 2019, and the generating capacity had reached 4306 TWh, according to the 2020 Hydropower. The fluid structure of the hydraulic turbine is so complicated under partial flow conditions that it often leads to unstable operation problems such as channel vortexes, flow-induced vibration of the blades, cavitation, and cavitation erosion inside the turbine [6,7]. These problems reduce the efficiency of the turbine and damage its flow components [8]. Channel vortexes do not necessarily lead to cavitation, but a close relationship exists between these two phenomena [10,11]
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