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Experimentelle Untersuchungen zum Stabilitätsverhalten ebener Diffusoren im Unterschall
Citation key kuehle_diss_2002_en
Author Ralph Kühle
Year 2002
DOI 10.14279/depositonce-573
Location Berlin
Month November
Editor DepositOnce - Institutional Repository for Research Data and Publications of TU Berlin
How Published In Copyright: http://rightsstatements.org/vocab/InC/1.0/
School Technische Universität Berlin
Abstract The current investigations deal with the stability behaviour of separated flow in plane diffusers in the subsonic region. Nowadays diffusers play an important part both in process engineering facilities and in turbomachinery. They are used to transform kinetic energy of the system into pressure energy, keeping any losses of energy to a minimum. Research focused on the influence of the components downstream of the diffuser, since diffusers are nearly always used in complex flow systems, where they have a substantial influence on the overall quality of the system. The investigations showed a distinct dependence of the flow and separation characteristics, both on the inlet boundary conditions (Reynolds-/Mach-number) and the outlet boundary conditions, i.e. on the geometry of the components downstream of the diffuser. Here, the outlet boundary condition proved to be the dominant influence in respect of the diffuser's separation behaviour. An instabilities proved to be primarily dependant on the separation characteristics in the diffuser, which were in turn determined by the outlet boundary condition. They could be roughly subdivided into two types. One of them showed a change in separation from a large-scale reverse flow to smaller local separation regions and the other one a spontaneous spatial shift of the large-scale reverse flow within the diffuser. Any influence on the overall system caused by a change in the blockage was observed only in the former separation type, whereas a spatial shift within the diffuser had no effect whatsoever on the overall system. The experimental investigations were carried out using steady and unsteady measurement techniques as well as visualisation methods. A special method was developed to assess the prevailing flow conditions. This method is based on data reduction of the steady and unsteady pressure signals and their correlation with the visualisations. This produced the identification of typical flow characteristics whilst significantly reducing the data quantity. The evaluated probability density allowed both, conclusions on the flow characteristics and comparisons of the fluctuation values of the flow. Consequently, the individual representations of the unsteady pressure signals, the evaluation of the fluctuation value and the visualisation could be summarised and hence noticeably simplified.
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