Within the present paper the development and interaction of secondary flow structures at different hub clearance gaps at high blade loadings are investigated. Three-dimensional numerical computations were carried out at three levels of hub clearance, ranging from zero to 3% of blade chord. Experimental results at 1% and 3% chord hub clearance were obtained to asses the validity of the numerical predictions. The experimentally and numerically obtained results provide a detailed picture of the development, the interaction of secondary flow structures and the influence of clearance flow for high blade loading. Therefore the time averaged flow features obtained from the experimental test rig and from numerical simulations are compared. In addition this investigation tends to explain the phenomenon of rotating instability for this compressor stator at a specific operating point and different hub clearance gaps by analyzing the secondary flow structures in the leading edge region at the hub.
The unsteady endwall pressure distribution is investigated by means of highly resolved pressure measurements and numerical simulations in an axial compressor stator exhibiting the phenomenon of rotating instability. The endwall flow has been observed to play an important role in the inception of rotating instability. The statistical properties of the measured pressure fluctuations at the hub endwall of the axial compressor stator are compared for a series of blade loadings. The results allow conclusions about the location of regions being closely connected to the unsteady dynamics related to the rotating instability phenomenon. Furthermore the operating points with detected rotating instability are compared with unsteady numerical results.
An experimental study was conducted to investigate the influence of Mach number and aerodynamic loading on Rotating Instability (RI). Investigations were performed on an annular compressor cascade with radial clearance at the hub. In addition to steady flow field measurements, time resolved measurements of static wall-pressure at the hub, close to the tip clearance, were performed. For a specific range of incidence angles, RI could be detected at all flow speeds (Ma = 0.2–0.7). Shortly before the inception of RI, a sudden increase in pressure loss coefficient and outflow angle was detected by steady flow field measurements. The time resolved measurements showed an abruptly broadband increase of amplitude in the frequency range, in which RI occurs at higher loadings. RI propagates with 25–33% of the circumferential component of the upstream flow speed. Furthermore it is possible to scale the power spectral density function of the unsteady data with Strouhal number.
Within the present paper a detailed experimental investigation is presented. The influence of blade loading on the development and interaction of secondary flow structures within an annular compressor stator cascade is examined. Experimental results at 3% chord hub clearance were obtained at four different blade loadings. Included are blade and endwall flow visualization, time resolved measurements of the static pressure on the endwall and radial-circumferential hot-wire traverse measurements within the passage as well as five-hole probe traverse measurements at the inlet and the outlet of the passage. The experimentally obtained results give detailed insight on the effect of the incidence on the development and interaction of the clearance vortex, horse-shoe vortex and the passage vortex. Furthermore it will be shown that a vortex breakdown of the clearance vortex occurs at higher loadings.
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