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Research Papers

Investigation of Pre-Stall Behavior in an Axial Compressor Rotor—Part I: Unsteadiness of Tip Clearance Flow

[+] Author and Article Information
Yanhui Wu1

Qingpeng Li, Jiangtao Tian, Wuli Chu

School of Power and Energy,  Northwestern Polytechnical University, Xi’an, Shanxi 710072, People’s Republic of China

1

Corresponding author.

J. Turbomach 134(5), 051027 (May 24, 2012) (12 pages) doi:10.1115/1.4004752 History: Received June 21, 2011; Accepted July 24, 2011; Published May 24, 2012; Online May 24, 2012

To investigate the pre-stall behavior of an axial flow compressor rotor, which was experimentally observed with spike-type stall inception, systematic experimental and whole-passage simulations were laid out to analyze the internal flow fields in the test rotor. In this part, emphases were put on the analyses of experimental results and the predicted results from steady simulations and unsteady simulations, which converged to equilibrium solutions with nearly periodic fluctuations of efficiency. The objective was to uncover the unsteady behavior of tip clearance flow and its associated flow mechanism at near-stall conditions. To validate the steady simulation results, the predicted total characteristics and spanwise distributions of aerodynamic parameters were first compared with the measured steady data, and a good agreement was achieved. Then, the numerically obtained unsteady flow fields during one period of efficiency fluctuations were analyzed in detail. The instantaneous flow structure near casing showed that tip secondary vortex (TSV), which appeared in the previous unsteady single-passage simulations, did exist in tip flow fields of whole-passage simulations. The cyclical motion of this vortex was the main source of the nearly periodic variation of efficiency. The simulated active period of TSV increased when the mass flow rate decreased. The simulated frequency of TSV at flow condition very close to the measured stall point equaled the frequency of the characteristic hump identified from the instantaneous casing pressure measurements. This coincidence implied that the occurrence of this hump was most probably a result of the movement of TSV. Further flow field analyses indicated that the interaction of the low-energy leakage fluid from adjacent passages with the broken-down tip leakage vortex (TLV) was the flow mechanism for the formation of TSV. Once TSV appeared in tip flow fields, its rearward movement would lead to a periodic variation in near-tip blade loading, which in turn altered the strength of TLV and TSV, accordingly, the low-energy regions associated with the breakdown of TLV and the motion of TSV, thus establishing a self-sustained unsteady flow oscillation in tip flow fields.

Copyright © 2012 by American Society of Mechanical Engineers
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References

Figures

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Figure 1

Cross-sectional diagram of the test rig

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Figure 2

Typical stalling pattern of the test rotor

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Figure 3

The arrangement of pressure transducers

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Figure 4

db1 transformations for original signals

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Figure 5

The power density aiming to the low-frequency-band signals of d5, d4 at the last stable point from the measurements

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Figure 6

Computational mesh

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Figure 7

The comparison of efficiency histories between two operating conditions

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Figure 8

The comparison of predicted and experimental total performances

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Figure 9

The comparisons of predicted and experimental elemental performances

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Figure 10

Instantaneous leakage streamlines during one period of efficiency fluctuation

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Figure 11

Instantaneous velocity vectors on a plane nearly vertical to the vortex core of TSV at t = 20/50 T

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Figure 12

Instantaneous negative axial velocity distributions during one period of efficiency fluctuation

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Figure 13

Instantaneous static pressure distributions during one period of efficiency fluctuation

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Figure 14

Typical streamlines rolled into TSV at three representative instants during one unsteady cycle

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Figure 15

Instantaneous near-tip loading variations during one unsteady cycle of TSV

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