Research Papers

On the Influence of a Hubside Exducer Cavity and Bleed Air in a Close-Coupled Centrifugal Compressor Stage

[+] Author and Article Information
Peter Kaluza

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

Christian Landgraf, Philipp Schwarz, Peter Jeschke

Institute of Jet Propulsion and Turbomachinery,
RWTH Aachen,
Templergraben 55,
Aachen 52062, Germany

Caitlin Smythe

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

1Corresponding author.

Contributed by the International Gas Turbine Institute (IGTI) of ASME for publication in the JOURNAL OF TURBOMACHINERY. Manuscript received September 8, 2016; final manuscript received December 7, 2016; published online March 7, 2017. Editor: Kenneth Hall.

J. Turbomach 139(7), 071011 (Mar 07, 2017) (9 pages) Paper No: TURBO-16-1232; doi: 10.1115/1.4035606 History: Received September 08, 2016; Revised December 07, 2016

In aero-engine applications, centrifugal compressors are often close-coupled with their respective diffusers to increase efficiency at the expense of a reduced operating range. The aim of this paper is to show that state-of-the art steady-state computational fluid dynamics (CFD) simulations can model a hubside cavity between an impeller and a close-coupled diffuser and to enhance the understanding of how the cavity affects performance. The investigated cavity is located at the impeller trailing edge, and bleed air is extracted through it. Due to geometrical limitations, the mixing plane is located in the cavity region. Therefore, the previous analyses used only a cut (“simple”) model of the cavity. With the new, “full” cavity model, the region inside the cavity right after the impeller trailing edge is not neglected anymore. The numerical setup is validated using the experimental data gathered on a state-of-the art centrifugal compressor test-rig. For the total pressure field in front of the diffuser throat, a clear improvement is achieved. The results presented reveal a drop in stage efficiency by 0.5%-points caused by a new loss mechanism at the impeller trailing edge. On the hubside, the fundamentally different interaction of the cavity with the coreflow increases the losses in the downstream components resulting in the mentioned stage efficiency drop. Finally, varying bleed air extraction is investigated with both cavity models. Only the full cavity (FC) model captures the changes measured in the experiment.

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Fig. 1

Test rig cross view with instrumentation

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Fig. 2

Schematic of the truncated diffuser

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Fig. 3

CFD domain with the full cavity model

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Fig. 4

Geometry of the aftbleed cavity: (a) overview, (b) detail: full cavity model, and (c) detail: simple cavity model

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Fig. 5

Circumferential averaged flow field for OP2 with nominal bleed extraction

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Fig. 6

Trailing edge flow visualization in the rotating frame: (a) simple cavity and (b) full cavity

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Fig. 7

Vortices and region of recirculation in the diffuser with nominal bleed extraction

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Fig. 8

Isentropic stage efficiency: comparison of both cavity models to experiment

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Fig. 9

Comparison of the total pressure contour on the pitot plane for OP2: (a) experiment, (b) CFD: FC, and (c) CFD: SC

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Fig. 10

Pressure rise at the diffuser shroud for OP2 with nominal bleed

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Fig. 11

Diffuser performance comparison

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Fig. 14

Circumferential averaged streamlines and entropy of the coreflow (cr ≥ 10 m/s) in the VS and PVS

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Fig. 13

Stage total pressure ratio with nominal and deactivated aftbleed extraction versus reduced diffuser inlet massflow

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Fig. 12

Differences between the simple and the full cavity model (Δ = SC − FC) in circumferential averaged flow profiles at the pipe inlet




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