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Research Papers

An Investigation of Stall Inception in Centrifugal Compressor Vaned Diffuser1

[+] Author and Article Information
Jonathan N. Everitt

e-mail: jeveritt@mit.edu

Zoltán S. Spakovszky

e-mail: zolti@mit.edu
Gas Turbine Laboratory,
Department of Aeronautics and Astronautics,
Massachusetts Institute of Technology,
Cambridge, MA 02193

This definition differs from the “momentum averaged” flow angle used by Filipenco et al. [26] where the inlet flow angle was averaged over the entire span to characterize experimentally measured diffuser inlet conditions by one parameter. Here, mixed out averaging provides a physically meaningful method to describe an equivalent, spanwise distribution of pitchwise mixed flow at inlet to the diffuser.

To allow convergence to a user-defined mass flow, information must be artificially inserted onto the exit boundary. This creates pressure waves that propagate back into the diffuser domain which must be avoided in stability analyses.

Pure divergence from initial state (equivalent to negative stiffness).

Static stability is necessary but not sufficient: dynamic instability (negative damping) leads to exponentially growing oscillations and is often reached first [3].

The form, location and timescale of the perturbation was varied in parametric studies, and it was determined the final response was largely insensitive when applied on the shroud side [24].

The vortical structures have been identified through animation of the unsteady flow field; the oscillation frequency agrees with the convective flow velocity and the spacing of the vortical structures.

Winner of the “Best Paper Award” Turbomachinery Committee.

Contributed by the International Gas Turbine Institute (IGTI) of ASME for publication in the JOURNAL OF TURBOMACHINERY. Manuscript received July 21, 2011; final manuscript received August 31, 2011; published online November 5, 2012. Editor: David Wisler.

J. Turbomach 135(1), 011025 (Nov 05, 2012) (10 pages) Paper No: TURBO-11-1156; doi: 10.1115/1.4006533 History: Received July 21, 2011; Revised August 31, 2011

In compression systems, the stable operating range is limited by rotating stall and/or surge. Two distinct types of stall precursors can be observed prior to full scale instability: the development of long-wavelength modal waves or a short-wavelength, three-dimensional flow breakdown (so-called “spike” stall inception). The cause of the latter is not well understood; in axial machines it has been suggested that rotor blade-tip leakage flow plays an important role, but spikes have recently been observed in shrouded vaned diffusers of centrifugal compressors where these leakage flows are not present, suggesting an alternative mechanism may be at play. This paper investigates the onset of instability in a shrouded vaned diffuser from a highly loaded turbocharger centrifugal compressor and discusses the mechanisms thought to be responsible for the development of short-wavelength stall precursors. The approach combines unsteady 3D RANS simulations of an isolated vaned diffuser with previously obtained experimental results. The unsteady flow field simulation begins at the impeller exit radius, where flow is specified by a spanwise profile of flow angle and stagnation properties, derived from single-passage stage calculations but with flow pitchwise mixed. Through comparison with performance data from previous experiments and unsteady full-wheel simulations, it is shown that the diffuser is accurately matched to the impeller and the relevant flow features are well captured. Numerical forced response experiments are carried out to determine the diffuser dynamic behavior and point of instability onset. The unsteady simulations demonstrate the growth of short-wavelength precursors; the flow coefficient at which these occur, the rotation rate and circumferential extent agree with experimental measurements. Although the computational setup and domain limitations do not allow simulation of the fully developed spike nor full-scale instability, the model is sufficient to capture the onset of instability and allows the postulation of the following necessary conditions: (i) flow separation at the diffuser vane leading edge near the shroud endwall; (ii) radially reversed flow allowing vorticity shed from the leading edge to convect back into the vaneless space; and (iii) recirculation and accumulation of low stagnation pressure fluid in the vaneless space, increasing diffuser inlet blockage and leading to instability. Similarity exists with axial machines, where blade-tip leakage sets up endwall flow in the circumferential direction leading to flow breakdown and the inception of rotating stall. Rather than the tip leakage flows, the cause for circumferential endwall flow in the vaned diffuser is the combination of high swirl and the highly nonuniform spanwise flow profile at the impeller exit.

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References

Figures

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Fig. 1

Comparison of flow angles near stall at different operating speeds. Inset shows stage speedlines from single passage, mixing plane CFD simulations.

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Fig. 2

Conceptual approach

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Fig. 3

Computational domain and vaneless space fast-response pressure sensor array location. For clarity, mesh is shown only for one passage.

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Fig. 4

Diffuser static pressure rise coefficients at 78% and 100% design corrected speeds

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Fig. 5

Subcomponent characteristics for 100% design corrected speed

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Fig. 6

Subcomponent characteristics for 78% design corrected speed

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Fig. 7

Diffuser flow fields at midspan at 100% design speed, near stall

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Fig. 8

Mach number contours and velocity vectors at 90% span and near vane leading edge for operating points close to stall at 100% speed. The stagnation streamline is indicated.

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Fig. 9

Time snapshot of counter-rotating vortical structures 1, 2 and 3 convecting in vaneless space at 90% span. Vorticity shed from leading edge (A) merges with passing vortical structure 2 at a later time.

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Fig. 10

Flow near shroud recirculates in the vaneless space over a larger spanwise extent as diffuser inlet corrected flow is reduced; combined with separation from the vane leading edge, conditions on left allow short wavelength stall inception

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Fig. 11

Computed static pressure traces in the vaneless space at 100% design corrected speed for the operating point at 90%m·c,PDPR. The small black box represents duration and spatial extent of the initial forcing function.

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Fig. 12

Variation in diffuser passage inlet corrected flow as low stagnation pressure vortical structures pass by

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Fig. 13

Mach number contours and velocity vectors at 90% span and near vane leading edge for an operating point close to stall at 78% speed. The stagnation streamline is indicated.

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Fig. 14

Computed static pressure traces in vaneless space at 78% design corrected speed for the PDPR operating point. The small black box represents duration and spatial extent of the initial forcing function.

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Fig. 15

Experimentally measured static pressure traces in the vaneless space at 100% design corrected speed, adapted from Ref. [10]. The short-wavelength precursor is identified via the dotted line.

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Fig. 16

Computed static pressure traces in the vaneless space using full wheel body force simulation at 75% design speed, adopted from Ref. [10]. Backward traveling modal waves are observed prior to stall.

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Fig. 17

Suggested mechanism for short-wavelength stall inception in vaned diffusers

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