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Research Papers

Aerodynamics of a Letterbox Trailing Edge: Effects of Blowing Rate, Reynolds Number, and External Turbulence on Aerodynamic Losses and Pressure Distribution

[+] Author and Article Information
N. J. Fiala

EERC, University of North Dakota, Grand Forks, ND 58202

J. D. Johnson

 United States Air Force, 518 Combat Sustainment Squadron, Hill AFB, UT 84056

F. E. Ames

Department of Mechanical Engineering, University of North Dakota, Grand Forks, ND 58202

J. Turbomach 132(4), 041011 (May 04, 2010) (11 pages) doi:10.1115/1.3195035 History: Received May 25, 2009; Revised May 26, 2009; Published May 04, 2010; Online May 04, 2010

A letterbox trailing edge configuration is formed by adding flow partitions to a gill slot or pressure side cutback. Letterbox partitions are a common trailing edge configuration for vanes and blades, and the aerodynamics of these configurations are consequently of interest. Exit surveys detailing total pressure loss, turning angle, and secondary velocities have been acquired for a vane with letterbox partitions in a large-scale low speed cascade facility. These measurements are compared with exit surveys of both the base (solid) and gill slot vane configurations. Exit surveys have been taken over a four to one range in chord Reynolds numbers (500,000, 1,000,000, and 2,000,000) based on exit conditions and for low (0.7%), grid (8.5%), and aerocombustor (13.5%) turbulence conditions with varying blowing rate (50%, 100%, 150%, and 200% design flow). Exit loss, angle, and secondary velocity measurements were acquired in the facility using a five-hole cone probe at a measuring station representing an axial chord spacing of 0.25 from the vane trailing edge plane. Differences between losses with the base vane, gill slot vane, and letterbox vane for a given turbulence condition and Reynolds number are compared providing evidence of coolant ejection losses, and losses due to the separation off the exit slot lip and partitions. Additionally, differences in the level of losses, distribution of losses, and secondary flow vectors are presented for the different turbulence conditions at the different Reynolds numbers. The letterbox configuration has been found to have slightly reduced losses at a given flow rate compared with the gill slot. However, the letterbox requires an increased pressure drop for the same ejection flow. The present paper together with a related paper (2008, “Letterbox Trailing Edge Heat Transfer—Effects of Blowing Rate, Reynolds Number, and External Turbulence on Heat Transfer and Film Cooling Effectiveness,” ASME, Paper No. GT2008-50474), which documents letterbox heat transfer, is intended to provide designers with aerodynamic loss and heat transfer information needed for design evaluation and comparison with competing trailing edge designs.

Copyright © 2010 by American Society of Mechanical Engineers
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Figures

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Figure 1

Schematic of large-scale incompressible flow vane cascade wind tunnel

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Figure 2

Schematic of 11 times scale cascade test section

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Figure 3

Schematic of cross section for letterbox pressure vane

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Figure 4

Surface pressure distribution comparison for base, gill slot, and letterbox vanes, ReC=2,000,000

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Figure 5

Pressure surface discharge static pressure comparison for letterbox and gill slot vanes

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Figure 6

Five-hole cone probe sensitivity for yaw angle, static pressure, and total pressure recovery, ReC=2,000,000

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Figure 7

Exit survey loss contours and secondary flows, ReC=1,000,000: (a) base vane, low turbulence; (b) gill slot vane, full flow, low turbulence; (c) letterbox vane, full flow, low turbulence; (d) gill slot vane, full flow, grid turbulence; (e) letterbox vane, full flow, grid turbulence; (f) base vane, aerocombustor turbulence; (g) letterbox vane, full flow, aerocombustor turbulence; and (h) letterbox vane, 200% design flow, aerocombustor turbulence

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Figure 8

Midspan total pressure loss coefficient Ω as a function of cross-passage distance for letterbox vane for varying design flow, AC, and ReC=1,000,000

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Figure 9

Cross passage averaged total pressure loss coefficient Ω as a function of cross span distance for letterbox vane, varying turbulence intensity, design flow, and ReC=1,000,000

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Figure 10

Cross passage averaged total pressure loss coefficient Ω as a function of cross span distance, letterbox vane, design flow, AC, and ReC=500,000, 1,000,000, and 2,000,000

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Figure 11

Cross passage averaged total pressure loss coefficient Ω as a function of cross span distance for letterbox vane, varying design flow, AC, and ReC=1,000,000

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Figure 12

Cross passage averaged turning angle β as a function of cross span distance for letterbox vane, comparing turbulence levels, design flow, and ReC=1,000,000

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Figure 13

Cross passage averaged turning angle β as a function of cross span distance for letterbox vane, design flow, AC, and ReC=500,000, 1,000,000, and 2,000,000

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Figure 14

Full passage averaged total pressure loss coefficient Ω as a function of exit chord Reynolds number and turbulence condition, letterbox and gill slot vanes, and design flow

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Figure 15

Full passage averaged total pressure loss coefficient Ω as a function of percent design flow and turbulence condition, letterbox and gill slot vanes, and ReC=1,000,000

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