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Research Papers: Flows in Complex Systems

Turbulence and Secondary Flows in an Axial Flow Fan With Variable Pitch Blades

[+] Author and Article Information
Jesús Manuel Fernández Oro

Área de Mecánica de Fluidos, Universidad de Oviedo, Campus de Viesques, 33271 Gijón (Asturias), Spainjesusfo@uniovi.es

Rafael Ballesteros-Tajadura, Eduardo Blanco Marigorta, Katia María Argüelles Díaz, Carlos Santolaria Morros

Área de Mecánica de Fluidos, Universidad de Oviedo, Campus de Viesques, 33271 Gijón (Asturias), Spain

J. Fluids Eng 130(4), 041101 (Apr 01, 2008) (11 pages) doi:10.1115/1.2903523 History: Received July 24, 2007; Revised January 04, 2008; Published April 01, 2008

This paper analyzes the structure of turbulence and secondary flows at the exit of an axial flow fan with variable pitch blades. The influence of changing the blades’ pitch angle over the turbulent structures is assessed by means of turbulence intensity values and integral length scales, obtained by using hot-wire anemometry for several test conditions. Since total unsteadiness is composed of both periodic and random unsteadiness, it is necessary to filter deterministic unsteadiness from the raw velocity traces in order to obtain turbulence data. Consequently, coherent flow structures were decoupled and thus, levels of turbulence—rms values of random fluctuations—were determined using a filtering procedure that removes all the contributions stemming from the rotational frequency, the blade passing frequency, and its harmonics. The results, shown in terms of phase-averaged distributions in the relative frame of reference, revealed valuable information about the transport of the turbulent structures in the unsteady, deterministic flow patterns. The anisotropic turbulence generated at the shear layers of the blade wakes was identified as a major mechanism of turbulence generation, and significant links between the blade pitch angle and the wake turbulent intensity were established. In addition, the autocorrelation analysis of random fluctuations was also used to estimate integral length scales—larger eddy sizes—of turbulence, providing useful data for computational fluid dynamics applications based on large eddy simulation algorithms. Finally, contours of radial vorticity and helicity gave a detailed picture of the vortical characteristics of the flow patterns, and the definition of secondary flow as the deviation of the streamwise component from the inviscid kinematics was introduced to determine the efficiency of the blade design in the energy exchange of the rotor.

Copyright © 2008 by American Society of Mechanical Engineers
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References

Figures

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Figure 2

Rotor blade characteristics (design)

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Figure 3

Fan performance curves

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Figure 4

Sketch of the triple hot-wire probe. Measurement chain.

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Figure 5

Power spectra and time evolutions of different velocity components of an individual trace measured downstream of the rotor. u* corresponds to (a) instantaneous, u*=u; (b) ensemble averaged, u*=Ũ; (c) fluctuation, u*=u′; and (d) filtered, u*=⟨u′⟩.

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Figure 6

Turbulence level and integral length scale of streamwise velocity of the inlet flow

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Figure 7

Simplified sketch of the contribution of large and small eddies in the random fluctuations of the velocity traces for fully developed turbulence (adapted from Ref. 18)

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Figure 8

Diagram of total unsteadiness rotor downstream. Influence of both blade pitch angle and operating conditions.

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Figure 9

Blade-to-blade distribution of turbulence intensity in the relative frame of reference. Influence of the operating conditions in the design geometry.

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Figure 10

Blade-to-blade distribution of turbulence intensity in the relative frame of reference. Influence of the blade pitch angle at off-design conditions.

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Figure 11

Radial distribution of integral length scales at the rotor exit. Influence of operating conditions and blade pitch angle.

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Figure 12

Contours of radial vorticity at the rotor exit. Design conditions.

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Figure 13

Blade-to-blade distributions of radial vorticity at midspan. Influence of operating conditions and blade pitch angle.

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Figure 14

Nondimensional helicity distributions at the rotor exit. Influence of operating conditions.

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Figure 15

Nondimensional helicity distributions at the rotor exit. Influence of blade pitch angle.

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Figure 16

Definition of SFs

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Figure 17

Secondary velocity vectors

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Figure 18

SF patterns at off-design conditions

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