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Research Papers: Flows in Complex Systems

Application of Large Gurney Flaps on Low Reynolds Number Fan Blades

[+] Author and Article Information
David Greenblatt

Faculty of Mechanical Engineering, Technion – Israel Institute of Technology, Haifa 32000, Israel

J. Fluids Eng 133(2), 021102 (Feb 04, 2011) (8 pages) doi:10.1115/1.4003301 History: Received June 06, 2010; Revised December 19, 2010; Published February 04, 2011; Online February 04, 2011

On the basis of a semi-empirical model, large Gurney flaps of 10%, 20%, and 30% of the fan blade chord length were tested in a specially designed ventilation fan facility. At the highest volumetric flowrates tested, the flapped blades all produced higher pressures than the baseline nonflapped case. When proper accounting was made of fan rotational speed, all flapped blades produced consistently higher dimensionless pressures, with the 30% flap producing the highest pressures at large volumetric flowrates. Based on the assumption that sound power varies with the sixth power of fan rotation speed, it was shown that the sound pressure level could be reduced by nearly 4 dB. All flapped configurations produced higher mechanical efficiency than the baseline case but the mass of the flap relative to that of the blade emerged as an important parameter. A 10% flap, whose mass was negligible relative to the blade, produced the largest increase of 18% in static efficiency. Further research will focus on testing the flaps over the entire operational range, as well as on redesigning stiffer and lighter Gurney flaps. The introduction of three-dimensionality such as spanwise spaced holes, slits, or serrations that have previously been used to reduce airfoil drag will also be considered.

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Copyright © 2011 by American Society of Mechanical Engineers
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Figures

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Figure 1

(a) Schematic showing the physical effect of the Gurney flap on the trailing-edge region (Liebeck (10)); (b) vortex sheet model used by thin airfoil theory to describe the physical effect (modified from the model of Liu and Montefort (9))

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Figure 2

Schematic showing the effect of Reynolds number on two-dimensional aerodynamic efficiency

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Figure 3

Calculated lift enhancement and aerodynamic efficiency changes as a function of Gurney flap height

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Figure 4

(a) Fan root and tip sections (left and right, respectively) showing all of the relevant blade dimensions. (b) Fan testing facility showing the test fan, contraction, and auxiliary blower. The metal screen downstream of the blades and the bell-mouth entrance contraction are not shown. (c) Two photographs of the experimental setup. Left: view of the facility from the contraction side, showing the blades, optical grating/photovoltaic diode, and screen. Right: view of the facility from the blower side, showing the pressure-port locations (manometers not shown) and variable transformer.

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Figure 6

Fan pressure developed as a function of volumetric flowrate. Data are shown for the baseline case as well as the two 10% Gurney flaps (see Fig. 5). All lines are fourth-order polynomials through the data points.

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Figure 7

Fan pressure versus volumetric flowrate for baseline data and four variations of the Gurney flaps tested (see Fig. 5)

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Figure 8

(a) Fan dimensionless pressure versus dimensionless flowrate for the baseline and Gurney flap cases (design condition). (b) Fan dimensionless pressure versus dimensionless flowrate for the baseline and Gurney flap cases (off-design condition).

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Figure 9

Fan static efficiency as a function of dimensionless flowrate at the design condition, showing the effect of the Gurney flaps

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Figure 5

(a) Schematic of the 6.5% camber curved fan blade section, with a 10% thin Gurney flap mounted at the trailing-edge. (b) Different Gurney flap configurations tested, all referenced to the tip chord length CT: (i) 10% thin flap, (ii) 10% thick flap, (iii) 20% thin flap, (iv) 20% composite flap, and (v) 30% thick flap.

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