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Propagating disturbances as a prestall instability in axial compressors
Citation key eck_diss_2020_en
Author Mario Eck
Year 2020
DOI 10.14279/depositonce-10567
Location Berlin
Month September
Editor DepositOnce - Institutional Repository for Research Data and Publications of TU Berlin
How Published Creative Commons Attribution 4.0 International CC BY 4.0
School Technische Universit├Ąt Berlin
Abstract The present work deals with the experimental analysis of aerodynamic instabilities in axial compressors. A very effective measure for increasing engine efficiency and reducing noise emissions is to increase the bypass ratio. In modern aircraft engines, this measure is clearly reflected by increasing fan diameters and more compact core engines. This trend inevitably causes the relative gaps between the compressor blade tips and the adjacent side wall to grow. For this reason, a highly complex, unsteady flow phenomenon, the Rotating Instability (RI), is to be assumend to play an increasingly important role in the development of future axial compressors. It has been established by now that RI is a prestall phenomenon. Referring to this finding, the present thesis aims at clarifying the potential of RI as a stall precursor by systematically evaluating the characteristics of RI changing during the throttling process. In this context, this work makes a significant contribution in improving the fundamental understanding of the flow physics of RI. The experiments have been carried out at two complementary compressor test facilities. The flow field in a hub clearance gap of a compressor stator and the tip clearance gap of a compressor rotor has been investigated using pressure measurement techniques installed flush with the surface. By means of temporally and spatially highly resolved pressure and velocity patterns, a comprehensive picture of the propagation area of coherent structures undoubtedly to be attributed to RI could be obtained. In the course of the investigations a leading edge separation could be excluded as the physical cause for those structures. Rather, discrete disturbances are seen to form in the shear layer between the main and the clearance flow while propagating in circumferential direction. The circumferential count of disturbances corresponding to the mode order of RI, changes stochastically in time. However, it was found that the predominant modes have different probabilities, with the mode inducing largest pressure amplitudes occurring most frequently. In addition, this work shows results on the formation of RI in eccentric rotors, which are the first of their kind.
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