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UNSTEADY FLOW IN TURBOMACHINERY
Art der Abschlussarbeit
Habilitation
Autoren
- Mailach, Ronald
Betreuer
- Prof. Dr.-Ing. Konrad Vogeler
Abstract
The main source of unsteadiness in the stable operating range of turbomachinery is the aerodynamical interaction of the rotor and stator blade rows. The blades and vanes, moving relatively to each other, aerodynamically interact because of the viscous wakes and the potential effects of the blades. In addition, the wakes and potential effects superimpose with other flow patterns, for instance the tip clearance vortices and other secondary flow phenomena. Furthermore in transonic compressors the interaction of wakes and shocks plays an important role. As a result, the real flow field is highly periodically unsteady and very complex, especially in multistage turbomachinery. Although this fact has received increasing attention within recent years, blade row interactions effects are not yet typically addressed in current design systems of turbomachinery (Van Zante et al., 2002b, Gorell et al., 2003). Also Howell et al. (2002) pointed out the requirement of the ability of modern design methods to predict unsteady flow features. With increasing aerodynamic loading of the blades and higher Mach numbers the consideration of rotor-stator-interactions gains in importance. It is therefore one of the challenges of the present and future design of compressors and turbines to include beneficial unsteady effects to improve the engine parameters. This requires a detailed physical understanding of the unsteady flow field and the resulting effects on the performance and flow stability.rnThe structure of the periodical flow field pattern and the strength of the periodical unsteadiness are influenced by various geometrical and aerodynamical parameters. Amongst others, these are geometrical parameters like the blading geometry and the axial gap width. Further important parameters are the Reynolds number and the Mach number, the free-stream turbulence, the profile pressure distribution, the boundary layer characteristics, the blade loading and the wake properties. rnDifferent aspects of the aerodynamic blade row interactions are important for the further improvement of turbomachines concerning efficiency, noise reduction while retaining a reliable engine operation. It is observed in several studies, that the profile losses and hence the compressor performance of embedded stages are altered compared to isolated blade rows due to blade row interaction effects. The knowledge of the unsteady blade boundary layer development is especially important for a possible reduction of turbomachinery blade losses. Furthermore the boundary layer state substantially influences the heat transfer between the blade surface and the fluid, which is an important aspect for turbine blades. Due to aerodynamic blade row interactions the pressure distribution on the blades considerably changes in time. For this reason unsteady aerodynamic blade forces and moments are generated. Critical blade vibrations are excited if the frequency of the aerodynamic stimulation matches the natural frequencies of the blades. This can lead to a reduction of fatigue life or even a destruction of the blading. Furthermore blade row interactions are an important source of noise in turbomachines. For these reasons the effects of the periodical unsteady flow field have to be considered during the design process of turbomachines. rnIn this manuscript experimental investigations on different aspects of blade row interactions in the Dresden Low-Speed Axial Research Compressor (LSRC) will be discussed in comparison to the existing physical understanding of these effects, known from literature. This includes results on the periodical unsteady flow field within the axial gaps between the blade rows as well as the flow field within the rotor blade passages. Subsequently, the unsteady behaviour of the blade boundary layer will be considered. Further aspects are the influence of wakes and potential effects on the unsteady profile pressures and the excited blade pressure forces. Eventually, selected results concerning the influence of blade row clocking on the unsteady profile pressures will be shown. This way a comprehensive picture of the different features of blade row interaction in the Dresden LSRC is provided.
Schlagwörter
Turbomachinery
Berichtsjahr
2009
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