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978-3-8439-2172-5, Reihe Strömungsmechanik
Numerical simulation of pressure oscillations in large Francis turbines at partial and full load operating conditions and their effects on the runner structural behaviour and fatigue life
294 Seiten, Dissertation Technische Universität München (2014), Hardcover, D4
With the increase of the power density of hydraulic turbines and the extension of their operating range over the last decades, the fluid and mechanical dynamic effects in the machine became significantly more pronounced. Under severe operating conditions, the fluid flow pressure oscillations and the consequent dynamic mechanical stresses may lead to the fatigue failure of the turbine runner, with the occurrence of cracks. This has to be avoided in the early design phases, through the correct and accurate prediction of the transient fluid flow, dynamic structural motion, mechanical stresses and fatigue assessment. The numerical simulation method proposed here intends to supply an accurate tool to predict the transient flow phenomena and the dynamic mechanical stresses for the fatigue analysis.
The main part of the process concentrates on the CFD simulation of the transient fluid flow through the entire turbine. The numerical model reproduces the complete turbine geometry and counts with sophisticated hybrid turbulence models, as DES and SAS. The turbulence modelling showed up to be decisive for the proper turbine flow simulation. The transient pressure field history, provided by the CFD analysis, constitutes the input for the runner mechanical stress calculation. The structural simulation is carried out with the FE method for the transient solution of the runner motion for all time steps. The calculated dynamic mechanical stresses in the runner are used for the fatigue life prediction.
As example, a Francis turbine, with high specific speed and whose prototype is currently in operation, was simulated. Several operating points were chosen for the calculations, including full load, higher part load, part load and deep part load. These points were representative for different types of dynamic phenomena taking place during the machine operation, as rotor-stator interaction, draft tube instabilities, with the presence of rotating vortex rope, and runner channel vortices.
The numerical results of the transient fluid flow simulation were compared to experimental results from model tests and achieved very tight agreement, showing the high accuracy and advantages of the method. The accurate numerical simulation of the transient fluid flow through the hydraulic machine, the computational calculation of the runner structural response and the fatigue assessment offered the possibility to gain new knowledge about the dynamic behaviour of Francis turbines.