Investigation on the fluid and structural dynamic behavior of steam turbine inlet valves at part load operation

In this thesis the aerodynamic and structural dynamic behavior of high pressure turbine inlet valves used for large power stations is investigated. Emphasis is put on the investigation of the flow field and the flow induced vibrations at part load operation. At this operational range a large pressure difference exists between the inlet and the outlet of the valve causing an extreme acceleration of the flow in the valve gap between the valve plug and the valve seat. The high speed flow being discharged from the gap forms a jet in the valve diffuser. There the kinetic energy of the jet is dissipated by various flow instabilities causing pressure fluctuations. These pressure fluctuations act on the valve plug and hence cause intense vibrations. Due to the increasing use of renewable energy, the operational mode of steam turbine installations is changed from base load operation to intermediate operation with highly variable power output and frequent startup operations. As long time part load operation was not previously considered, there is a demand for research. The study on the aero and structural dynamic phenomena is carried out with numerical models. Prestudies on validation cases show that classical two-equation turbulence models are not capable to model flow instabilities and the generation of pressure fluctuations properly. A Hybrid turbulence model combining the advantages of the RANS and the SAS-method is used instead. The CFD simulations reveal that wall jet separations existing at certain operation conditions have a significant effect on the flow topology in the valve diffuser and increase the level of the dynamic lateral forces acting on the valve plug. The analysis of the CFD study shows, that the wall jet separation is related to the failure of the Coandă effect in underexpanded wall jets. In case of the flow topology with the attached wall jet CFD simulations predict that a significant amount of the dynamic force is related to an acoustic mode acting on the valve plug. The acoustic mode is excited by a shear layer in the valve diffuser. Pertubations of the shear layer being caused by vortex-generating devices or by vibrations of the plug reduce the excitation of the mode and the level of the dynamic axial forces. If the jet is detached, the oscillation of oblique shocks and the partial unsteady reattachment of the jet cause intense pressure fluctuations and hence high dynamic forces acting on the valve plug. From an aerodynamic point of view, the flow topology with the attached jet is more favorable than the detached flow topology. Structural dynamic simulations are preformed to analyse the impact of the flow topology on the vibrations of the valve plug. One one way as well as two way coupled calculations are conducted. The structural dynamic analysis predict that the detached flow topology causes higher structural vibrations than the attached flow topology.