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Research Papers

Stall Warning by Blade Pressure Signature Analysis

[+] Author and Article Information
Anna Young

e-mail: amy21@cam.ac.uk

Ivor Day

e-mail: ijd1000@cam.ac.uk

Graham Pullan

e-mail: gp10006@cam.ac.uk
Whittle Laboratory,
University of Cambridge,
Cambridge CB3 0DY, UK

Contributed by the International Gas Turbine Institute (IGTI) of ASME for publication in the JOURNAL OF TURBOMACHINERY. Manuscript received August 12, 2011; final manuscript received August 22, 2011; published online October 30, 2012. Editor: David Wisler.

J. Turbomach. 135(1), 011033 (Oct 30, 2012) (10 pages) Paper No: TURBO-11-1182; doi: 10.1115/1.4006426 History: Received August 12, 2011; Revised August 22, 2011

At low mass flow rates, axial compressors suffer from flow instabilities leading to stall and surge. The inception process of these instabilities has been widely researched in the past---primarily with the aim of predicting or averting stall onset. In recent times, attention has shifted to conditions well before stall and has focused on the level of irregularity in the blade passing signature in the rotor tip region. In general, the irregularity increases in intensity as the flow rate through the compressor is reduced. Attempts have been made to develop stall warning/avoidance procedures based on the level of flow irregularity, but little effort has been made to characterize the irregularity itself, or to understand its underlying cause. Work on this project has revealed for the first time that the increase in irregularity in the blade passing signature is highly dependent on both tip-clearance size and eccentricity. In a compressor with small, uniform, tip-clearance, the increase in blade passing irregularity that accompanies a reduction in flow rate will be modest. If the tip-clearance is enlarged, however, there will be a sharp rise in irregularity at all circumferential locations. In a compressor with eccentric tip-clearance, the increase in irregularity will only occur in the part of the annulus where the tip-clearance is largest, regardless of the average clearance level. In this paper, some attention is also given to the question of whether the irregularity observed in the prestall flow field is due to random turbulence or to some form of coherent flow structure. Detailed flow measurements reveal that the latter is the case. From these findings, it is clear that a stall warning system based on blade passing signature irregularity would be difficult to implement in an aero-engine where tip-clearance size and eccentricity change during each flight cycle and over the life of the compressor.

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References

Figures

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Fig. 2

Hypothetical sketch of irregularity against flow coefficient (for one pressure transducer only)

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Fig. 3

Irregularity against flow coefficient for the datum concentric compressor

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Fig. 4

Near-stall irregularity against circumferential position for the datum concentric compressor (tip-clearance 1.7% span)

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Fig. 5

Near-stall irregularity against circumferential position with different average tip-clearances (concentric)

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Fig. 6

Irregularity against flow coefficient for the datum eccentric compressor

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Fig. 7

Near-stall irregularity against circumferential position for datum concentric and eccentric compressors

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Fig. 8

Comparison of near-stall irregularity levels with two levels of average tip-clearance (75% eccentricity)

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Fig. 9

Total-to-static pressure rise characteristics for a compressor with four levels of concentric tip-clearance

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Fig. 10

Effect of tip-clearance asymmetry on local stability

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Fig. 11

Measuring positions for detailed casing pressure measurements

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Fig. 12

Fourier transforms from pressure transducers in different circumferential locations (3.3% tip-clearance, 75% eccentricity)

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Fig. 13

Irregularities in casing static pressure at different circumferential positions (3.3% tip-clearance, 75% eccentricity). NB: data sets were not recorded simultaneously, so disturbance tracking is not possible in this figure.

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Fig. 14

Propagation of “blue holes” in the large tip-clearance of the compressor (3.3% tip-clearance, 75% eccentricity). Note: the variation in spacing between the blue holes remains the same as they propagate from one measuring position to the next.

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Fig. 15

Irregularity against stage number for the high-speed compressor (84% speed, 3.5% bleed)

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Fig. 16

Irregularity against mass flow rate for a high-speed compressor at varying bleed rates (84%speed, stage 5 only)

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Fig. 17

Changes in alarm point over the life of an engine

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