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Research Papers: Gas Turbines: Turbomachinery

Stall Inception, Evolution and Control in a Low Speed Axial Fan With Variable Pitch in Motion

[+] Author and Article Information
Stefano Bianchi, Alessandro Corsini, Luca Mazzucco, Lucilla Monteleone

Dipartimento di Ingegneria Meccanica e Aerospaziale,  Sapienza University of Rome, Via Eudossiana, 18, I-00184 Rome, Italy

Anthony G. Sheard

 Fläkt Woods Limited, Axial Way, Colchester, CO4 5AR UK

J. Eng. Gas Turbines Power 134(4), 042602 (Jan 30, 2012) (10 pages) doi:10.1115/1.4004726 History: Received May 16, 2011; Accepted May 20, 2011; Published January 30, 2012; Online January 30, 2012

Obtaining the right pitch in turbomachinery blading is crucial to efficient and successful operations. Engineers adjust the rotor’s pitch angle to control the production or absorption of power. Even for low speed fans this is a promising tool. This paper focuses on the inception and the evolution of the flow instabilities in the tip region which drive the stall onset in low speed axial fans. The authors conducted an experimental study to investigate the inception patterns of rotating stall evolution at different rotor blade stagger-angle settings with the aim of speculating on stable operating margin. The authors drove the fan to stall at the design stagger-angle setting and then operated the variable pitch mechanism in order to recover the unstable operation. They measured pressure fluctuations in the tip region of the low-speed axial-flow fan fitted with a variable pitch in motion mechanism, with flush mounted probes. The authors studied the flow mechanisms for spike and modal stall inceptions in this low-speed axial-flow fan which showed relatively small tip clearance. The authors cross-correlated the pressure fluctuations and analyzed the cross-spectra in order to clarify blade pitch, end wall flow, and tip-leakage flow influences on stall inception during the transient at the rotor blades’ different stagger-angle settings. The authors observed a rotating instability near the maximum pressure-rise point at both design and low stagger-angle settings. The stall inception patterns were a spike type at the design stagger-angle setting as a result of the interaction between the incoming flow, tip-leakage flow and end wall backflow.

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Copyright © 2012 by American Society of Mechanical Engineers
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Figures

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Figure 1

Test rig setup scheme (not to scale)

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Figure 2

Total pressure rise and efficiency maps

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Figure 3

Sectional view of the impeller with the microphones mounted flush with the casing wall

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Figure 4

Pressure time history for the two different probes flush mounted at the blade tip region

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Figure 5

Detail of time sequence during stall inception

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Figure 6

Cross correlation spectral amplitude of the two pressure signal during (a) the fan normal operations, (b) the stall inception and (c) during the stall event

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Figure 7

Pressure time history for the two different probes flush mounted at the blade tip region

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Figure 8

Detail of the time sequence during the stall recovery

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Figure 9

Spectral energy density of wall pressure: (a) overall evolution, and (b) stall recovery transient

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Figure 10

Fan Performance Chart after the stall recovery by VPIM blades

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