Research Papers

Numerical Investigation of the Unsteady Interaction Within a Close-Coupled Centrifugal Compressor Used in an Aero Engine

[+] Author and Article Information
Benjamin Wilkosz

e-mail: wilkosz@ist.rwth-aachen.de

Peter Jeschke

e-mail: jeschke@ist.rwth-aachen.de
Institute of Jet Propulsion and Turbomachinery,
RWTH Aachen University,
Templegraben 55,
Aachen 52062, Germany

Caitlin Smythe

GE Aviation,
1000 Western Avenue,
Lynn, MA 01910
e-mail: caitlin.smythe@ge.com

Not shown here due to lack of space.

Pressure gradient between the blade PS and SS.

The validation of the diffuser centerline static pressure recovery can be seen in the Appendix. A comparison of the velocity field with unsteady particle image velocimetry data is shown by Findeisen [15].

This quantity specifies the local irreversible specific entropy production due to friction and heat dissipation and is used here to identify dominant loss mechanisms, which is more difficult when using the entropy in the highly 3D flow due to the transport character of this quantity. In order to apply the transport equation for turbulent flow the Boussinesq-approximation proposed by Moore et al. [39] is used.

1Corresponding author.

Contributed by the International Gas Turbine Institute (IGTI) of ASME for publication in the Journal of Turbomachinery. Manuscript received May 3, 2013; final manuscript received June 20, 2013; published online September 26, 2013. Editor: Ronald Bunker.

J. Turbomach 136(4), 041006 (Sep 26, 2013) (12 pages) Paper No: TURBO-13-1068; doi: 10.1115/1.4024892 History: Received May 03, 2013; Revised June 20, 2013

The present work forms part of a research project of the Institute of Jet Propulsion and Turbomachinery at the RWTH Aachen University in collaboration with GE Aviation. The subject is the detailed numerical analysis of the unsteady flow field, focusing on the interaction between the impeller and the passage diffuser of a close-coupled transonic centrifugal compressor used in an aero engine. The centrifugal compressor investigated is characterized by a close-coupled impeller and passage diffuser with a radial gap of only 3.6%. The close coupling tends to provide a high aerodynamic efficiency but simultaneously cause a high unsteady interaction between the impeller and the diffuser. These unsteady effects can have a significant impact on the performance of both components and present a challenge to state-of-the-art numerical methods. With increasing compressor efficiency, the more important it is to have an understanding of the detailed unsteady flow physics. Experimental data was obtained from a state-of-the art centrifugal compressor test rig located at the Institute of Jet Propulsion. Steady and unsteady pressure measurements within the impeller and diffuser are used to gain detailed information on the temporal, time-averaged, and spectral pressure distributions within the stage to validate the CFD. The work presented here shows the unsteady phenomena caused by the interaction and the location and propagation of these phenomena within the centrifugal stage. Within the impeller, the exducer is in first order excited by the blade passing frequency (BPF) of the diffuser, whereas in the diffuser both the BPF and the passage passing frequency (PPF), are present up until the end of the pipe-diffuser. Significant effects on the integral component performance could only be identified for the impeller. Special focus is paid to evaluate the diffuser upstream pressure field, since this is the major source of unsteadiness within the impeller. The performance of the rotor decreases due to the unsteady interaction. This effect is traced back to the unsteady tip-clearance flow, in which the time-averaged mass transport decreases, whereas the specific entropy production increases in a nonlinear way. Within the diffuser, local effects counteracting with respect to the integral performance are found. In front of the throat, there is less decay in the total pressure as a result of tangentially expanding pressure waves. Within the passage a decrease in flow uniformity in the unsteady flow is identified as the reason for the lower diffusion up until the throat and higher losses within the downstream diffuser passage.

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Fig. 1

Cross-section of the GE test rig at the RWTH-Aachen

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Fig. 2

Schematic view of the centrifugal stage

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Fig. 3

3D view of the CFD domain: complete GE centrifugal stage

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Fig. 4

Comparison of the stage, impeller, and diffuser performance for the RANS-URANS(TA) with the experimental data

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Fig. 5

Static pressure amplitude in the relative frame of the 1st BPF at 50% span

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Fig. 6

Diffuser upstream potential field from the diffuser in the circumferential direction

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Fig. 7

Impeller (a) shroud static pressure build-up, (b) blade loading, and (c) unsteady upstream diffuser potential field

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Fig. 8

Increase in specific entropy due to unsteadiness in the impeller

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Fig. 9

Spectral analysis for the pipe-diffuser

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Fig. 10

Meridional development of the total pressure loss and pressure recovery within the diffuser

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Fig. 11

Visualization of the irreversible specific entropy production

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Fig. 12

Visualization of the pressure waves in the circumferential direction

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Fig. 13

Correlation between the throat blockage and secondary flow (left) and visualization of the change in the flow diffusion within the pipe (right)

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Fig. 14

Diffuser centerline static pressure recovery at the ADP




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