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Research Papers

A 3D Compressible Flow Model for Weak Rotating Waves in Vaneless Diffusers—Part I: The Model and Mach Number Effects

[+] Author and Article Information
Hua Chen

Turbo Technologies, Honeywell UK Ltd, Stanley Green Trading Estate, Cheadle Hume, Cheshire SK8 6QS, UKhua.chen@honeywell.com

Feng Shen, Xiao-Cheng Zhu, Zhao-Hui Du

 Shanghai Jiaotong University, 800 Dongchuan Road, Min Hang, Shanghai 200240, China

J. Turbomach 134(4), 041010 (Jul 21, 2011) (9 pages) doi:10.1115/1.4003655 History: Received September 04, 2010; Revised December 31, 2010; Published July 21, 2011; Online July 21, 2011

A three-dimensional compressible flow model is presented to study the occurrence of weak rotating waves in vaneless diffusers of centrifugal compressors. The diffuser considered has two parallel walls, and the undisturbed flow is assumed to be circumferentially uniform, isentropic, and to have no axial velocity. Linearized 3D compressible Euler equations were casted on a rotating coordinate system traveling at the same angular speed as the wave cells. A uniform static pressure at the outlet of the diffuser was imposed. Complex functions of the solutions to the equations were obtained by a second-order finite difference method and the singular value decomposition technique. The influences of the inlet Mach number of undisturbed flow, inlet spanwise distribution of undisturbed radial velocity, and diffuser radius ratio on the rotating waves were studied and results show that (1) the critical flow angle and rotating wave speed are both affected by the Mach number. However, the angle only increases slightly with the Mach number while the wave speed increases rapidly with the Mach number; (2) inlet distribution has minor influences on diffuser instability but the wave speed increases with the inlet distortion; (3) diffuser instability increases rapidly and the wave speed decreases quickly with the diffuser radius ratio; and (4) backward traveling rotating wave may occur if diffuser is sufficiently long and the inlet Mach number is sufficiently small.

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Copyright © 2012 by American Society of Mechanical Engineers
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Figures

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Figure 1

(a) Diffuser model in the z-r plane and (b) diffuser model in the r-θ plane

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Figure 2

Computational grid

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Figure 3

Disturbances of the first radial mode, away from resonance (11×26 grid, M=0.1, and bz=0.15; others same as Fig. 3a of Ref. 17: Rf=2.0, f=0.1, Vrm=0.095, and n=1)

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Figure 4

Disturbances of the first radial mode, resonance state (11×26 grid, M=0.1, and bz=0.15; others same as Fig. 3b of Ref. 17: Rf=2.0, f=0.1, Vrm=0.095, and n=1)

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Figure 5

Density disturbances of the first radial mode, varying Mach number with Vrm=0.131, f=0.1, Rf=2.0, bz=0.15, n=1, and β=90 deg

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Figure 6

Effects of inlet Mach number on resonant waves, conditions same as in Fig. 4

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Figure 7

Uniform and nonuniform axial distributions of inlet radial velocity of undisturbed flow

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Figure 8

Effects of inlet Mach number on the real part of axial velocity disturbance under the nonuniform inlet condition of Fig. 7

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Figure 9

Resonance under linear inlet distribution, β=90 deg and n=1

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