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Research Papers: Gas Turbines: Structures and Dynamics

Experiments on an Axial Fan Stage: Time-Resolved Analysis of Rotating Instability Modes

[+] Author and Article Information
Benjamin Pardowitz

Engine Acoustics Department,
German Aerospace Center (DLR),
Institute of Propulsion Technology,
Müller-Breslau-Straße 8,
Berlin 10623, Germany
e-mail: Benjamin.Pardowitz@dlr.de

Ulf Tapken, Lars Neuhaus, Lars Enghardt

Engine Acoustics Department,
German Aerospace Center (DLR),
Institute of Propulsion Technology,
Müller-Breslau-Straße 8,
Berlin 10623, Germany

Contributed by the Structures and Dynamics Committee of ASME for publication in the JOURNAL OF ENGINEERING FOR GAS TURBINES AND POWER. Manuscript received August 1, 2014; final manuscript received September 5, 2014; published online December 9, 2014. Editor: David Wisler.

J. Eng. Gas Turbines Power 137(6), 062505 (Jun 01, 2015) (9 pages) Paper No: GTP-14-1455; doi: 10.1115/1.4028686 History: Received August 01, 2014; Revised September 05, 2014; Online December 09, 2014

Rotating instability (RI) occurs at off-design conditions in axial compressors, predominantly in rotor configurations with large tip clearances. Characteristic spectral signatures with side-by-side peaks below the blade passing frequency (BPF) are typically referred to RI located in the clearance region next to the leading edge (LE). Each peak can be assigned to a dominant circumferential mode. RI is the source of the clearance noise (CN) and an indicator for critical operating conditions. Earlier studies at an annular cascade pointed out that RI modes of different circumferential orders occur stochastically distributed in time and independently from each other, which is contradictory to existing explanations of RI. Purpose of the present study is to verify this generally with regard to axial rotor configurations. Experiments were conducted on a laboratory axial fan stage mainly using unsteady pressure measurements in a sensor ring near the rotor LE. A mode decomposition based on cross spectral matrices was used to analyze the spectral and modal RI patterns upstream of the rotor. Additionally, a time-resolved analysis based on a spatial discrete-Fourier-transform (DFT) was applied to clarify the temporal characteristics of the RI modes and their potential interrelations. The results and a comparison with the previous findings on the annular cascade corroborate a new hypothesis about the basic RI mechanism. This hypothesis implies that instability waves of different wavelengths are generated stochastically in a shear layer resulting from a backflow in the tip clearance region.

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References

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Figures

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Fig. 1

Inlet section of an axial fan stage equipped with sensor rings in the near-field and far-field used for unsteady pressure measurements

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Fig. 2

Performance map of the axial fan stage for different rotor speeds, blade numbers and tip clearances. Test points I–IV are chosen inside regions with observed RI (dashed segments).

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Fig. 3

Instationary pressure measurements in the rotor near-field at 3000 rpm (test point I), mean frequency spectrum (top) and decomposed circumferential mode amplitudes (bottom)

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Fig. 4

Spectral and modal characteristics measured in the near-field at 2000 rpm (test point II)

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Fig. 5

Spectral and modal characteristics measured in the near-field at 3000 rpm with a reduced rotor blade number of B = 12 (test point IV)

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Fig. 6

Spectral and modal characteristics measured in the near-field at 1000 rpm under mild stall conditions (test point III)

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Fig. 7

Time-resolved mode amplitudes at 3000 rpm (test point I) exemplified for an analysis of 40 (top) and 120 rotor revolutions (bottom)

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Fig. 8

Identification of modal triggers at 3000 rpm (test point I) using specific thresholds for amplitude (top) and phase (bottom) for the dominant RI mode of order m = 22

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Fig. 9

Averaged sequences obtained for test point I using the rotor-shaft-signal (top) or the dominant RI modes of order m = 21 (middle) or m = 22 (bottom) as trigger

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Fig. 10

CN observed at 3000 rpm (test point I). Spectral (top) and modal characteristics are determined both in the near-field (middle) as well as in the far-field (bottom).

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Fig. 11

CN observed at 2000 rpm (test point II) illustrated as in Fig. 10

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Fig. 12

Circumferential propagation velocities Um,φ of the RI modes in relation to the rotor tip speed Ur for various operating conditions (including the test points I–IV)

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Fig. 13

Characteristics measured at an annular cascade: circumferential modes (top), time-resolved mode amplitudes (middle), and a modal triggered sequence (bottom)

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Fig. 14

Characteristics measured at the rotor at 1000 rpm (test point III): circumferential modes (top), time-resolved mode amplitudes (middle), and a modal triggered sequence (bottom)

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