Study On Cavitation Characteristics Of Francis Turbine Under Sand - Laden Flow

Most rivers in China contain a lot of sediment, and mixed-flow water turbine running in those rivers is subjected to the abrasion from sediment, especially for the edges close to the gaps on the blades.At the same time, the sediment in rivers also make it easy to produce cavitation,which that generate cavitation erosion.Cavitation erosion Combined with abrasion generated by sediment has more serious impact on the blades of turbine, greatly reduce the efficiency and stability of turbine.In order to improve the ability to resist cavitation and abrasion of turbine, it’s very important to explore the mechanism of cavitation and abrasion, as well as their combined effect on turbine. However, the factors affecting on the work of cavitation, abrasion and their coefficient is complicated, not only the flow field parameters like velocity and pressure, but also material characteristics like size and density of silt.Further more, researches focused on this domain were most carried out on experimental study, lack of related numerical simulation.Based on the above problems, this article simulated the internal flows of mixed-flow water turbine in rising and silt water these two different situations,then analyzed their cavitation performance,respectively. The main research content is summarized as follows:1) established the models of casing, guide vane, stay vane, draft tube of the mixed-flow water turbine by UG modeling software, then used tetrahedron, hexahedron non-structural mesh methods to mesh above parts of turbine.2) explored the cavitation characteristic of mixed-flow water turbine in the situation of rinsing. Simulated cavitation of different working conditions like inlet flow, cavitation coefficient, opening angle of guide vane using SST k-ω turbulence model and transport cavitation model (Zwert-Gerber-Belamri cavitation model) which based on the mean cavitation flow theory.3) explored the combined effect generated by cavitation and abrasion of mixed-flow water turbine in the situation of sand water. Obtained the mechanism how abrasion produced by silt of different size and density and cavitation effect together on the turbine using Euler model and cavitation model. In the results of rinsing situation, with the increasing of mass flow in the inlet, the cavitation region at the back of the runner near the shroud outlet first increased and then decreased, the posterior lobe channel vortex runner zone decreased and then increased, but did not cause leaf channel vortex cavitation. With the reduce of cavitation coefficient, runner cavitation region increased gradually. With the decrease of guide vane opening, the cavitation area of the blade gradually reduced and the leaf channel vortex cavitation emerged when in the small opening. When the flow less than the optimum efficiency point flow, the spiral vortex occurs at the draft tube inlet and when the flow above the optimum efficiency point flow, the bubbly vortex appears at the draft tube inlet. The turbine efficiency in the cavitation condition is lower than non-cavitation conditions. In the results of sand water situation, sediment area at the back of the runner blade inlet numerous extended to the front of the runner blade outlet when the sediment grain size and concentration of sedimen increased. On the influence of cavitation to the sediment wear, sediment concentration zone in the cavitation condition would be greater than the calculated non-cavitation condition; on the influence of sand to the cavitation, when the particle sediment and concentrations of sediment increased, it would promote runner cavitation occur firstly, then inhibit runner cavitation; on the influence of sand to draft tube vortex, the spiral vortex occur when the particle sediment increased; On the influence of sand to efficiency, the torque of the runner became smaller with the increase of sediment concentration. The results of the above flow characteristics within the turbine to improve the efficiency and stability of Francis turbine has a certain theoretical reference significance and practical value.