In order to improve their competence in CFD, University of Trieste and Turboinštitut joined in the ACCUSIM (Accurate Simulations in Hydro-Machinery and Marine Propellers) EU project. One of the main goals of the ACCUSIM project is accurate prediction of cavitation in water turbines, pumps and marine propellers. The results for a Francis turbine, obtained in the first two years of the project, will be presented at the conference.
The consequences of cavitation in water turbines are flow instabilities, excessive vibrations, erosion of material surfaces and reduced efficiency. Accurate prediction of cavitation is more and more important because turbines, due to commercial reasons, often operate at regimes far from their best efficiency point. Besides, to cut manufacturing costs, the dimensions of turbines are reduced and to get the same power the rotating speed is increased. The consequence of higher speed is increased likelihood of cavitation.
Four main forms of cavitation, which can arise in Francis turbines, are: leading edge cavitation, travelling bubble cavitation, draft tube vortex swirl and inter-blade vortex cavitation. To obtain different forms of cavitation, flow at two operating regimes, part load and overload, was simulated. At first, steady-state simulations at overload were performed for different values of Thoma number. The SST turbulence model with curvature correction and Kato-Launder limiter was used. Three forms of cavitation were obtained: leading edge cavitation and cavitation near the trailing edge (travelling bubble cavitation), both on the suction side of the blades, and cavitating vortex core behind the hub at the centre of the draft tube.
The second operating regime was part load. To predict rotating vortex rope transient simulation with the SAS SST model was performed at the part load operating regime. Helical shape of the rotating vortex rope was well reproduced. Also, the inter-blade vortex cavitation located in the channels between the runner blades was obtained.
For both operating regimes, numerical results were compared to the experimental ones. Blades’ inlets are not visible on photos, because they were hidden behind the runner band. Therefore, the extent of the inlet cavitation cannot be compared to the observation on the test rig. The shape and extent of numerically predicted cavitation at the suction side of the blades near the trailing edges, and the shape and size of the vortex core behind the hub were in good agreement with the observation on the test rig. The level of predicted efficiency at overload was about 1.2% lower than the measured one. Taking into account that this operating regime was far from the best efficiency point, it can be concluded that the efficiency level was very well predicted. The effect of cavitation on the machine efficiency was well reproduced, with a slightly premature drop of the efficiency.
Dragica Jošt, Aljaž Škerlavaj, Mitja Morgut, Roberto Renato Stopar, Enrico Nobile
Kolektor Turboinštitut, Ljubljana, Slovenia & University of Trieste, Trieste, Italy