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Research Papers

Characterization and Impact of Secondary Flows in a Discrete Passage Centrifugal Compressor Diffuser

[+] Author and Article Information
David W. Erickson

GE Aviation,
Lynn, MA 01910
e-mail: david1.erickson@ge.com

Choon S. Tan

Department of Aeronautics and Astronautics,
Massachusetts Institute of Technology,
Cambridge, MA 02139
e-mail: choon@mit.edu

Michael Macrorie

GE Aviation,
Lynn, MA 01910
e-mail: michael.macrorie@ge.com

1Corresponding author.

Contributed by the International Gas Turbine Institute (IGTI) of ASME for publication in the Journal of Turbomachinery. Manuscript received December 9, 2018; final manuscript received January 7, 2019; published online February 26, 2019. Assoc. Editor: Kenneth Hall.

J. Turbomach 141(7), 071009 (Feb 26, 2019) (17 pages) Paper No: TURBO-18-1352; doi: 10.1115/1.4042646 History: Received December 09, 2018; Accepted January 08, 2019

Truncating the exit of a discrete passage centrifugal compressor diffuser is observed to enhance a research compressor's stall line. By interrogating the experimental data along with a set of well-designed Reynolds-Averaged Navier–Stokes computations, this improvement is traced to the reduced impact of secondary flows on the truncated diffuser's boundary layer growth. The secondary flow system is characterized by counter-rotating streamwise vortex pairs that persist throughout the diffuser passage. The vortices originate from two sources: flow nonuniformity at the impeller exit and separation off the leading edge cusps unique to a discrete passage diffuser. The latter detrimentally impacts the diffuser pressure rise capability by accumulating high loss flow along the diffuser wall near the plane of symmetry between the vortices. This contributes to a large passage separation in the baseline diffuser. Using reduced-order modeling, the impact of the vortices on the boundary layer growth is shown to scale inversely with the diffuser aspect ratio, and thus, the separation extent is reduced for the truncated diffuser. Because the diffuser incidence angle influences the strength and location of the vortices, this mechanism can affect the slope of the compressor's pressure rise characteristic and impact its stall line. Stall onset for the baseline diffuser configuration is initiated when the vortex location and the corresponding passage separation transition from pressure to suction side with increased cusp incidence. Conversely, because the extent of the passage separation in the truncated diffuser is diminished, the switch in separation side does not immediately initiate instability.

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References

Zachau, U., 2007, “Experimental Investigation on the Diffuser Flow of a Centrifugal Compressor Stage With Pipe Diffuser,” Ph.D. thesis, RWTH Aachen University, Aachen, Germany.
Zachau, U., Buescher, C., Niehuis, R., Hoenen, H., Wisler, D. C., and Moussa, Z. M., 2008, “Experimental Investigation of a Centrifugal Compressor Stage With Focus on the Flow in the Pipe Diffuser Supported by Particle Image Velocimetry (PIV) Measurements,” Proceedings of ASME Turbo Expo 2008: Power for Land, Sea, and Air, Berlin, Germany, June 9–13.
Kunte, R., Schwarz, P., Wilkosz, B., Jeschke, P., and Smythe, C., 2011, “Experimental and Numerical Investigation of Tip Clearance and Bleed Effects in a Centrifugal Compressor Stage With Pipe Diffuser,” Proceedings of ASME Turbo Expo 2011, Vancouver, Canada.
Kunte, R., Jeschke, P., and Smythe, C., 2013, “Experimental Investigation of a Truncated Pipe Diffuser With a Tandem Deswirler in a Centrifugal Compressor Stage,” ASME J. Turbomach., 135, 031019. [CrossRef]
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Cumpsty, N. A., 1989, Compressor Aerodynamics, John Wiley & Sons, Inc., New York, NY.
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Wilkosz, B., 2014, “Aerodynamic Losses in an Aero Engine Centrifugal Compressor With a Close-Coupled Pipe-Diffuser and a Radial-Axial Deswirler,” Ph.D. thesis, RWTH Aachen University, Aachen, Germany.
Erickson, D., 2017, “Characterization of Performance-Limiting Flow Mechanisms in a Centrifugal Compressor Stage,” M.S. thesis, Massachusetts Institute of Technology, Cambridge, MA.
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Greitzer, E. M., Tan, C. S., and Graf, M. B., 2004, Internal Flow: Concepts and Applications, Cambridge University Press, Cambridge, UK.
Drela, M., and Giles, M. B., 1987, “Viscous-Inviscid Analysis of Transonic and Low Reynolds Number Airfoils,” AIAA J., 25, pp. 1347–1355. [CrossRef]
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Figures

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Fig. 6

CFD model of compressor stage featuring baseline diffuser

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Fig. 5

Overall compressor pressure ratio versus inlet corrected mass flow based on experimental measurements, highlighting key operating points (note: key operating points averaged from multiple tests).

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Fig. 4

Types and locations of permanent instrumentation utilized in research

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Fig. 3

Cutaway views of diffuser inlet geometry featuring leading edge cusps

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Fig. 2

Cutaway views of experimental centrifugal compressor, featuring baseline and truncated diffusers. Stations and coordinate systems also defined.

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Fig. 1

Experimental centrifugal compressor in baseline configuration

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Fig. 7

Diffuser static pressure recovery coefficient at 100% Nc25 versus position along diffuser centerline, forward wall (note: experimental points averaged from multiple tests). Experimental and simulated throat pressure recoveries match, so flow angles also match.

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Fig. 8

Overall compressor performance versus inlet corrected mass flow. Stability line of truncated diffuser configuration is improved relative to baseline diffuser configuration. CFD solutions overestimate pressure ratio and efficiency, but capture overarching trends. (a) Compressor stagnation pressure ratio versus corrected mass flow. (b) Compressor polytropic efficiency versus corrected mass flow.

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Fig. 9

Diffuser static pressure recovery and stagnation pressure loss coefficient versus impeller exit flow angle at 100% Nc25 (note: key experimental operating points averaged from multiple tests). Experimental data indicate no difference between diffuser configurations, while CFD shows differences that follow piecewise behavior (regimes of behavior denoted 1, 2, and 3) corresponding to changes in separation side and extent. (a) Baseline diffuser. (b) Truncated diffuser.

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Fig. 10

Distributions of vorticity (left), transverse velocity gradient (middle), and stagnation pressure loss coefficient (right) at various crossflow planes throughout diffuser passage. Counter-rotating streamwise vortex pairs compress the boundary layer in the transverse direction, accumulating low-momentum end wall flow between the vortices and contributing to downstream separation.

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Fig. 13

Normalized circulation around the vortex regions versus the impeller exit flow angle. Pressure and suction side incidence vortices rapidly increase in strength below and above the threshold flow angle, respectively. Background vortex strength is unaffected by the flow angle at X3, but increases slowly with a reduced flow angle at X8. (a) X3 cut plane (downstream of the suction side leading edge). (b) X8 cut plane (downstream of the pressure side leading edge).

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Fig. 14

Streamlines through background vortices for the baseline diffuser 100E CFD simulation. Background vortices result from the convection of streamwise vorticity originating at the impeller exit.

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Fig. 11

Distribution of vorticity in diffuser passage direction at 100% Nc25 for baseline diffuser. The presence and strength of counter-rotating vortices on diffuser pressure and suction sides depends on the operating point or the incidence angle.

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Fig. 12

Diffuser secondary flow structures

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Fig. 15

Convection of the circumferential fluid line through the diffuser. Streamline curvature skews the fluid line, altering the length in the streamwise direction. Streamwise vorticity is similarly impacted by vortex line stretching and skewing. Therefore, the strength of background vortices is amplified with a reduced incidence angle.

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Fig. 16

Streamlines through incidence vortices for the baseline diffuser. Pressure side incidence vortices shown at a low flow angle (80C) and suction side incidence vortices shown at a high flow angle (100S′). Incidence vortices originate with separation off leading edge cusps and associated introduction of the boundary layer vorticity into the main flow stream.

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Fig. 17

3D effect of transverse velocity gradient, ∂w/∂z, on the boundary layer growth rate. Illustration shows ∂w/∂z < 0, which increases the boundary layer growth rate relative to pure 2D boundary layer due to the accumulation of high loss flow from sides.

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Fig. 18

Transverse velocity gradient in the z-direction. Pressure and suction side boundary layer compression between vortices indicated by ∂Vz/∂z < 0.

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Fig. 19

Velocity and velocity gradient fields associated with the infinitesimal 2D vortex of strength Γ = ωxdA

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Fig. 20

Measured and estimated strength of the transverse velocity gradient at the diffuser midplane versus the impeller exit flow angle. The suction side experiences a negative transverse velocity gradient at high flow angles and the pressure side at low flow angles. Dominant contribution due to incidence vortices. (a) X3 cut plane (downstream of the suction side leading edge). (b) X8 cut plane (downstream of the pressure side leading edge).

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Fig. 22

Boundary layer displacement thickness versus throughflow position for the baseline diffuser from CFD calculation and 2D boundary layer model calculations with and without vortex source terms. Excluding vortex source terms reduces displacement thicknesses, changes separation side at 100E, and delays separation at 100S′. (a) 100E and (b) 100S′.

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Fig. 21

CFD-calculated boundary layer growth rates and vortex source terms versus throughflow position for the baseline diffuser. The growth rate closely correlates with vortex source terms on the pressure side at 100E and the suction side at 100S′.

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Fig. 27

Impeller pressure rise coefficient versus exit flow coefficient. Characteristic slopes do not depend on speed below 80% Nc25, but become steeper above 80% Nc25. This is due to the high sensitivity of impeller efficiency to positive incidence experienced at low speed.

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Fig. 23

Boundary layer shape factor versus throughflow position for the baseline diffuser from CFD calculation as well as 2D boundary layer model calculations with and without vortex source terms. Excluding vortex source terms changes the separation side at 100E and delays separation at 100S′. (a) 100E and (b) 100S′.

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Fig. 24

Mass blockage versus throughflow position for the baseline diffuser passage (downstream of throat) from CFD calculation as well as 2D boundary layer model calculations with and without vortex source terms. Excluding vortex source terms reduces blockage. (a) 100E and (b) 100S′.

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Fig. 25

Static pressure recovery coefficient versus throughflow position for the baseline diffuser from CFD calculation as well as 2D boundary layer model calculations with and without vortex source terms. Excluding vortex source terms increases pressure recovery coefficient. (a) 100E and (b) 100S′.

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Fig. 26

Illustration of diffusion system, impeller, and overall compressor pressure rise coefficients versus impeller exit flow coefficient, demonstrating the combined effects of impeller and diffusion system on overall compressor stability. With the baseline diffuser, compressor stability dominated by transition in the diffuser separation side. With the truncated diffuser, compressor stability determined by a combination of impeller and diffuser pressure rise characteristics. Decreased slope of impeller characteristic with increased speed enables increased flow range with the truncated diffuser, but not with the baseline diffuser.

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