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Research Papers

A 3D Compressible Flow Model for Weak Rotating Waves in Vaneless Diffusers—Part II: Detailed Results

[+] Author and Article Information
Feng Sheng

 Shanghai Jiaotong University, 800 Dongchuan Road, Min Hang, Shanghai 200240, Chinashenfeng1981@live.cn

Hua Chen

 Honeywell Turbo Technologies Ltd., Stanley Green Trading Estate, Cheadle Hume, Cheshire SK8 6QS, UK

Xiao-cheng Zhu, Zhao-hui Du

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

J. Turbomach 134(4), 041011 (Jul 21, 2011) (7 pages) doi:10.1115/1.4003654 History: Received September 04, 2010; Revised December 29, 2010; Published July 21, 2011; Online July 21, 2011

A 3D compressible flow model was presented in Part I of the paper to study the occurrence of weak rotating waves in vaneless diffusers of centrifugal compressors. In this paper, detailed results on the influences of flow and diffuser geometry parameters, including inlet Mach number, inlet distortion, wave number, diffuser outlet-to-inlet radius ratio, diffuser width to inlet radius ratio, and impeller backswept angle, on the rotating waves are presented. It was found that inlet spanwise distortion of radial velocity has little effects on diffuser stability, but rotating wave speed increases with the distortion. The speed also increases with inlet Mach number, so does diffuser instability. Impeller backswept improves diffuser stability and this effect increases with diffuser radius ratio. Multiple resonances were found when impeller backswept is coupled to inlet distortion of radial velocity. These resonances may have similar stabilities but with different wave speeds, suggesting that two rotating waves with different rotating speeds may occur at the same time. Diffuser width was found to have little effects on stability and on wave speed if the same maximum and same minimum values of inlet distortion of radial velocity are kept, but have some effects if the values are not kept. A comparison was also made between the present model predictions and results in open literatures, and good agreement with the experimental results than previous 2D models was achieved.

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Figures

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Figure 1

Axial distributions of mean radial velocity at inlet: (a) linear distributions and (b) conic distributions

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Figure 2

Resonant f and Vrm under linear inlet distribution, β=90 deg, n=1, Rf=1.5, 1.8, and 2.2

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Figure 3

Resonant f and Vrm under conic 1 inlet distribution, β=90 deg, n=1, Rf=1.5, 1.8, and 2.2

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Figure 4

Resonant f and Vrm under conic 2 inlet distribution, β=90 deg, n=1, Rf=1.5, 1.8, and 2.2

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Figure 5

Resonant f and Vr of different wave numbers, uniform inlet and β=90 deg: (a) inlet Mach number=0.1, (b) inlet Mach number=0.5, and (c) inlet Mach number=0.9

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Figure 6

Resonant f and Vrm of different wave numbers, conic 2 distribution, δ=15 deg, β=90 deg, Rf=2.2

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Figure 7

Definition of impeller backswept angle β

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Figure 8

Effects of backswept angle β on critical value of Vr, uniform inlet condition, and n=1

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Figure 9

Effects of backswept angle β on critical value of f, uniform inlet condition, and n=1

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Figure 10

Multiple resonance points, Conic 2 inlet distortion, δ=15 deg, n=1. R1, R2, and R3 refer to three (if there are) resonances in the multiple resonant cases: (a) Rf=1.5, (b) Rf=1.8, and (c) Rf=2.2

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Figure 11

Two ways to obtain inlet distribution for bz=0.30 from one for bz=0.15: (a) keeping the same δ angle and (b) keeping the maximum and minimum values

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Figure 12

Effects of diffuser width bz, β=90 deg, n=1: (a) critical values of Vrm and (b) critical values of f

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Figure 13

Nondimensional radial velocities in a vaneless diffuser from Ref. 3, filled symbols for no stall condition, and open symbols for after stall condition

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Figure 14

Comparison with experimental results of critical angle, n=1 and β=90 deg used in present model: (a) M=0.1 used for present model, (b) M=0.5 used for present model, and (c) M=0.9 used for present model

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