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Stability of the Circular Cascade
Citation key ulbricht_diss_2002_en
Author Iris Ulbricht
Year 2002
DOI 10.14279/depositonce-412
Location Berlin
Month June
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 Universit├Ąt Berlin
Abstract For an experimental investigation of unsteady flow patterns in compressor blade rows an axial compressor stator test rig was designed and built. Generation of rotating structures was of particular interest. Measurements were performed mainly with piezoresistive pressure transducers. All signal analysis was performed in the frequency domain. Additionally flow visualisations were performed for some operation points to get an impression of the three-dimensional steady flow-field in the circular compressor cascade. A number of rotating flow structures, which are well known from compressor rotors in actual machines, were created successfully. The phenomena were measured and analysed using the method described above. Instability phenomena showed up dependant on Reynolds number and incidence and were restricted to a limited incidence range. The measured pressure fluctuations contained components caused by flow instabilities and by resonance with the annular duct, the settling chamber, and the tube connections. Both components were observed propagating axially as well as in circumferential direction. For high incidences so-called rotating instabilities known from compressor rotors with large tip clearances could be observed. These are usually understood as a rotating source for pressure fluctuations. Thus the measured signal is frequency-modulated, as opposed to the common rotating stall signal. Rotating stall is usually understood as a rotating blockage, so the measured signal contains only one frequency. At very large incidences spinning modes appeared. Spinning modes are formed by a combination of unsteady aerodynamics and acoustics. Aerodynamic periodic pressure fluctuations, e. g. caused by periodic boundary layer separation, excite a circumferential eigenmode of the duct. The system s acoustic answer may influence the aerodynamic source. If there is resonance, the amplitude of the pressure fluctuations increases dramatically, resulting in both noise and mechanical stresses. A static observer will see an apparently rotating signal whose rotational speed is faster than the speed of sound. Both rotating instabilities and spinning modes potentially cause problems in actual machines that may lead to severe blade damage. This thesis describes the first observation and investigation of so-called rotating instability in a static system. The rotating instabilities observed are accompanied by a separation of the hub boundary layer. The separation line was visualised for the steady flow field and shows up for the blade row investigated at about ┬╝ of cord length in front of the blades leading edges. Using oil flow visualisations of the steady flow field, a transport mechanism in circumferential direction could be shown. Also for the first time rotating instabilities that occurred in a single stage laboratory compressor s stator were investigated. Spinning modes have been observed in compressor and ventilator stators earlier. This thesis describes for the first time the occurrence of spinning modes in an isolated compressor stator.
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