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Research Papers: Flows in Complex Systems

Prediction of the Nonuniform Tip Clearance Effect on the Axial Compressor Flow Field

[+] Author and Article Information
Young-Seok Kang

Advanced Propulsion Group, Korea Aerospace Research Institute, Eo-Eun Dong, Yu-Seong Gu, Daejon 305-333, Republic of Koreaelectra@kari.re.kr

Shin-Hyoung Kang

Department of Mechanical and Aerospace Engineering, Seoul National University, Sin-Rim Dong, Gwan-Ak Gu, Seoul 151-742, South Koreakangsh@snu.ac.kr

J. Fluids Eng 132(5), 051110 (May 14, 2010) (9 pages) doi:10.1115/1.4001553 History: Received August 12, 2009; Revised April 05, 2010; Published May 14, 2010; Online May 14, 2010

It is well-known that nonuniform tip clearance in an axial compressor induces pressure and velocity perturbations along the circumferential direction. This study develops a numerical modeling to predict perturbed flows in an axial compressor with a nonuniform tip clearance and presents a mechanism of the flow redistribution in the axial compressor at design and off-design conditions. The modeling results are compared with CFD results (2006, “Prediction of the Fluid Induced Instability Force of an Axial Compressor,” ASME FEDSM 2006, Miami, FL) not only to validate the present modeling, but also to investigate more detailed flow fields. In an axial compressor, nonuniform tip clearance varies local flow passage area and resultant axial velocity along the circumferential direction. There are small axial velocity differences between maximum and minimum clearances near the design condition, while large pressure differences are investigated according to local locations. However, contribution of the main flow region overrides the tip clearance effect as the flow coefficient deviates from the design condition. Moreover, the flow field redistribution becomes noticeably strong when the off-design effects are incorporated. In case of high flow coefficients, the low relative flow angle near the minimum clearance regions results in a large negative incidence angle and forms a large flow recirculation region and a corresponding large amount of loss occurs near the blade pressure surface. It further promotes strong flow field perturbations at the off-design conditions. The integration of these pressure and blade loading perturbations with a control volume analysis leads to the well-known Alford’s force. Alford’s force is always negative near the design condition; however, it reverses its sign to positive at the high flow coefficients. At the high flow coefficients, tip leakage flow effects lessen, while increased off-design effects amplify blade loading perturbations and a steep increase in Alford’s force. This study enables that nonuniform flow field, and the resultant Alford’s force, which may result in an unstable rotor-dynamic behavior, can be easily evaluated and assessed during the compressor, fan, or blower design process.

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Copyright © 2010 by American Society of Mechanical Engineers
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Figures

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Figure 1

Control volume of a rotor row with a nonuniform tip clearance distribution

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Figure 2

Schematic of an offset rotor model

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Figure 3

Total pressure loss coefficient from CFD and loss model

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Figure 4

Computational grid of the statically offset rotor row

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Figure 5

Distributions of normalized (a) axial velocity, (b) circumferential velocity, and (c) pressure perturbations at the blade inlet and outlet

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Figure 6

Pressure rise coefficients at different tip clearance heights

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Figure 7

Distributions of pressure coefficient at the (a) blade inlet, (b) mid-chord, and (c) blade outlet

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Figure 8

Alford’s force distributions from CFD and modeling

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Figure 9

Schematics of nonuniform tip clearance effects on pressure rise (a) without and (b) with off-design effects

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