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Research Papers

Influence of the Swirling Flow in the Side Cavities of a High-Pressure Centrifugal Compressor on the Characteristics of Excited Acoustic Modes

[+] Author and Article Information
N. Petry

e-mail: nico.petry@siemens.com

S. König

e-mail: koenig.sven@siemens.com
Energy Sector Oil and Gas Division,
Siemens AG,
Duisburg 47053, Germany

F.-K. Benra

Institute of Energy and Environmental
Engineering—Turbomachinery,
University of Duisburg-Essen,
Duisburg 47048, Germany
e-mail: friedrich.benra@uni-due.de

Previously accomplished tests revealed that neither the resonance amplitude of acoustic modes nor the determined swirl-factor is significantly influenced by the acceleration of the compressor.

The maximum deviation of a measured pressure with respect to the circumferential mean value is below one percent [27].

In case of resonance the gas in the side cavities is excited to fluctuations with increased amplitudes. The forced response of the gas does not mandatory have to exactly coincide with the shape of the acoustic eigenmode, but is expected to be similar. Therefore, the term “excitation of an acoustic eigenmode” is used.

Referred to as harmonic spectrograms in Ref. [5].

Due to their high eigenfrequencies these high order modes are only excited by high engine order pressure fluctuations in the rotational speed range of the compressor.

In case of equal acoustic eigenfrequencies this would lead to a smaller Mach number as well (see Eq. (6) for mac>0).

1Corresponding author.

Contributed by the International Gas Turbine Institute (IGTI) of ASME for publication in the Journal of Turbomachinery. Manuscript received July 2, 2012; final manuscript received July 16, 2012; published online March 25, 2013. Editor: David Wisler.

J. Turbomach 135(3), 031024 (Mar 25, 2013) (11 pages) Paper No: TURBO-12-1112; doi: 10.1115/1.4007544 History: Received July 02, 2012; Revised July 16, 2012

Previous experimental investigations revealed the existence of acoustic modes in the side cavities of a high-pressure centrifugal compressor. These modes were excited by pressure patterns which resulted from rotor/stator-interactions (often referred to as Tyler/Sofrin-modes). The acoustic modes were significantly influenced by the prevailing flow in the side cavities. The flow field in such rotor/stator-cavities is characterized by a high circumferential velocity component. The circumferential velocity of the flow and the phase velocity of the acoustic eigenmode superimpose each other, so that the frequencies of the acoustic eigenmodes with respect to the stator frame of reference follow from the sum of both velocities. In the previous study the circumferential velocity was estimated based on existing literature and the phase velocities of the acoustic modes were calculated via an acoustic modal analysis. Based on these results the rotational speeds of the compressor, where acoustic modes were excited in resonance, were determined. The present paper is based on these results and focuses on the influence of the swirling flow and the coupling of the excited acoustic modes between the two side cavities. Such a coupling has been predicted in previous numerical studies but no experimental evidence was available at that time. In this study the circumferential velocities of the flow are determined by measuring the actual radial pressure distribution in the side cavities and assuming radial equilibrium. The determined values are directly used for the prediction of the rotational speeds at resonance. The values for the rotational speeds at resonance predicted that way are compared to the resonance speeds found in the experiments. Further on, simultaneously measured pressure fluctuations in the shroud and hub side cavities with respect to the rotor frame of reference give evidence about the coupling of the acoustic modes between the two side cavities in case of resonance. If the experimentally determined swirling flow velocity is accounted for in the prediction of acoustic resonances, the calculated rotational speeds of resonance are in good agreement with the experimental findings in most cases. Neglecting the flow in the cavities, however, leads to large deviations between calculated and experimentally determined rotational speeds. Varying the operating point of the compressor results in changes of the circumferential velocities in the side cavities and, therefore, in changes of the rotational speeds of resonance. Contrary to the acoustic modes calculated via a finite element analysis by the authors of this paper in previous studies the excited acoustic modes in the experiments are mostly not coupled between the two side cavities, but are localized to one of both cavities. This finding is assumed to be caused by the flow field in the compressor.

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References

Figures

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Fig. 1

Picture of the test rig compressor

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Fig. 2

Fast-responding pressure transducers and pressure taps in the side cavities of the compressor

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Fig. 3

Exemplarily plotted radial pressure and swirl-factor distributions for the OPs choke and design

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Fig. 4

Classification of the flow in the side cavities according to Daily and Nece [33], diagram according to Will [32]

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Fig. 5

Local acoustic eigenmode (mac = 18). (a) Pressure distribution in an axial section. (b) Pressure distribution on the impeller coverdisk.

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Fig. 6

EO-spectrogram for run-up29, shroud cavity (measuring data of sensor p-R-S1)

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Fig. 7

EO-spectrogram for run-up34, shroud cavity (measuring data of sensor p-R-S1)

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Fig. 8

EO-spectrogram for run-up29, hub cavity (measuring data of sensor p-R-H1)

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Fig. 9

EO-, phase and coherence diagram (f*R = 50)

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