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Research Papers

Spike and Modal Stall Inception in an Advanced Turbocharger Centrifugal Compressor

[+] Author and Article Information
Z. S. Spakovszky

Gas Turbine Laboratory, Massachusetts Institute of Technology, Cambridge, MA 02139

C. H. Roduner

 ABB Turbo Systems Ltd., Baden 5400, Switzerland

The surge line at part speed was unaltered between the compressor rig test and the turbocharger experiment and is thus not shown.

The closed loop test facility is usually operated at subatmospheric inlet conditions.

The slope of the static pressure rise characteristic is proportional to the damping of the natural flow oscillations of the compression system, which can grow into full scale instability, rotating stall, and surge.

The very low amplitude systematic and periodic oscillation is due to electronic noise.

Although incompressible flow is assumed in the dynamic model, the necessary mean flow calculation used as input to the model is compressible.

This suggests that the spatial flow field resolution of the model is limited to the fourth harmonic, indicating that, for the semi-actuator disk assumption to hold, the circumferential length scale of the flow nonuniformity must cover at least two blade pitches within half a wavelength.

The ducting and collector systems were designed using an acoustic flow model to avoid potential resonance phenomena.

J. Turbomach 131(3), 031012 (Apr 10, 2009) (9 pages) doi:10.1115/1.2988166 History: Received December 21, 2007; Revised March 22, 2008; Published April 10, 2009

In turbocharger applications, bleed air near the impeller exit is often used for secondary flow systems to seal bearing compartments and to balance the thrust load on the bearings. There is experimental evidence that the performance and operability of highly-loaded centrifugal compressor designs can be sensitive to the amount of bleed air. To investigate the underlying mechanisms and to assess the impact of bleed air on the compressor dynamic behavior, a research program was carried out on a preproduction, 5.0 pressure ratio, high-speed centrifugal compressor stage of advanced design. The investigations showed that bleed air can significantly reduce the stable flow range. Compressor rig experiments, using an array of unsteady pressure sensors and a bleed valve to simulate a typical turbocharger environment, suggest that the path into compression system instability is altered by the bleed flow. Without the bleed flow, the prestall behavior is dominated by short-wavelength disturbances, or so called “spikes,” in the vaneless space between the impeller and the vaned diffuser. Introducing bleed flow at the impeller exit reduces endwall blockage in the vaneless space and destabilizes the highly-loaded vaned diffuser. The impact is a 50% reduction in stable operating range. The altered diffuser characteristic reduces the compression system damping responsible for long-wavelength modal prestall behavior. A four-lobed backward traveling rotating stall wave is experimentally measured in agreement with calculations obtained from a previously developed dynamic compressor model. In addition, a self-contained endwall blockage control strategy was employed, successfully recovering 75% of the loss in surge-margin due to the bleed flow and yielding a one point increase in adiabatic compressor efficiency.

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Copyright © 2009 by American Society of Mechanical Engineers
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References

Figures

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Figure 5

Qualitative description of change in diffuser static pressure rise due to endwall leakage flow

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Figure 6

Static pressure rise characteristics of vaned diffuser subcomponents at 105% corrected speed

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Figure 7

Streamlines of swirling flow in vaneless space (adopted from Ref. 10)

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Figure 8

Unsteady pressure traces in the vaneless space at 100% corrected speed: no leakage (left) and endwall leakage (right)

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Figure 9

Unsteady pressure traces in the vaneless space at 105% corrected speed: no leakage (left) and endwall leakage (right)—operating points correspond to the crosses shown in Fig. 1

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Figure 10

Two paths into instability: spike and modal stall patterns in centrifugal compressor vaned diffusers

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Figure 11

Streamwise activity of stall precursors at 100% speed: no leakage (top) and endwall leakage (bottom)

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Figure 12

Unsteady centrifugal compressor model including the effects of endwall flow leakage

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Figure 13

Calculated long-wavelength perturbations: backward traveling four-lobed wave sets stability limit

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Figure 14

Effect of self-regulated endwall blockage control on centrifugal compressor performance and stability

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Figure 4

Static pressure rise characteristics of vaned diffuser subcomponents at 100% corrected speed

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Figure 3

Steady and unsteady diffuser instrumentation

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Figure 2

Schematic of compressor cross section indicating location of bleed flow near impeller exit

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Figure 1

Measured high-speed performance of data and redesigned compressor configurations

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