Unsteady flow and fluid-structure interactions in steam turbine control valves – measures for the stabilization of spherical control valves

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With a steady growth of the share of renewable energy in the overall power supply, a flexible operation of conventional power plants becomes increasingly important. In the future, steam turbines will be expected to provide balancing power, which requires a fast and robust regulation of the turbine. In general, short-term load changes can be realized using control valves, for which the extended time at part load conditions can represent a major challenge. Depending on the valve design, unsteady flow conditions at throttled conditions can result in fluid-structure interactions with severe vibrations, possibly leading to a mechanical failure. Besides more complex designs, the spherical valve as a particularly vibration endangered geometry is still installed especially in smaller steam turbines. To ensure a secure and reliable part load operation, good knowledge of the flow topologies as well as the accurate prediction of flow instabilities are therefore essential. For that purpose, experimental studies are important but available test data especially for an elastic setup is rare for steam turbine control valves.Based on the outcome of the previous DFG project, new geometric measures (flow-straightening, swirl- and turbulence generation) for a targeted manipulation of the flow field and suppression of flow instabilities in control valves will be investigated. Supported by the results, a more stable operation of valves at part load is intended, also considering possible upgrades for existing valves.Different valve configurations will be investigated experimentally on a scaled control valve, capturing not only the time-resolved pressure field but also the dynamics of the valve. With the examination of acoustic excitations, its interaction with the flow field is another major interest. The experimental studies will be accompanied by transient 3D CFD simulations, which will provide a detailed insight into the spatial flow field after its validation against test data. Besides the stiff valve, the elastic configuration will be assessed using coupled flow and structural simulations enabling a better understanding of the excitation mechanisms and the impact of the investigated geometric variations.In the medium-term, the analysis of different valve modifications and acoustic interactions of the flow with acoustic resonances for a spherical control valve design is intended to increase the flexibility of steam turbines and simultaneously improve efficiency and flow stability at part load operation.