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RESEARCH PAPERS

Mechanism of the Interaction of a Ramped Bleed Slot With the Primary Flow

[+] Author and Article Information
B. A. Leishman

 Rolls-Royce plc., Derby DE24 8BJ, UK

N. A. Cumpsty1

 Rolls-Royce plc., Derby DE24 8BJ, UK

1

Now at Department of Mechanical Engineering, Imperial College, London SW7 2AZ, UK.

J. Turbomach 129(4), 669-678 (Dec 21, 2006) (10 pages) doi:10.1115/1.2752193 History: Received December 07, 2006; Revised December 21, 2006

An experimental and computational study of the ramped bleed slot in a compressor cascade is presented. The geometry is a circumferential slot downstream of the stator blade trailing edge, with endwall ramps inside the blade passage, and the paper builds on work previously reported for different bleed off-take geometries (Leishman, 2007, ASME J. Turbomach., 129, pp. 645–658; Leishman, 2007, ASME J. Turbomach., 129, pp. 659–668). The strong interaction between any bleed slot and the primary flow through the cascade can be strong, thereby causing the levels of loss and blockage in the primary flow leaving the blade passage to be increased at some bleed flow rates. Radial flow into the bleed slot is highly nonuniform because the blade-to-blade pressure field causes flow to enter the bleed slot preferentially where the static pressure is high, and to spill out into the primary flow where the static pressure is low. The mechanism for the ramped bleed slot is different from that described in the earlier papers for other geometries. For the ramped bleed slot a static pressure field, with large variations of static pressure in the circumferential direction, is set up in the slot because the endwall flow entering the slot has higher stagnation pressure downstream of the pressure surface than downstream of the suction surface of the upstream blades. The flow entering the slot with high stagnation pressure is brought to rest in a stagnation point on the downstream surface of the slot, and the consequent variation in static pressure on the rear surface sets tangential and radial components of velocity which are a large fraction of the freestream velocity. As well as demonstrating the mechanism for the flow behavior, the paper presents results of experiments and calculations to demonstrate the behavior and gives guidance for the design of bleed slots by stressing the fundamental features of the flow.

Copyright © 2007 by American Society of Mechanical Engineers
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References

Figures

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Figure 1

Ramped bleed slot geometry

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Figure 2

Measured (exp) and calculated (CFD) bleed characteristics

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Figure 3

Measured contours of stagnation pressure loss (Yp) 0.25 axial chord downstream of the blade row (increment 0.1, minimum contour 0.1)

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Figure 4

Calculated contours of: (a) static pressure at 10% span; and (b) radial velocity at 10% span for 3.2% bleed rate

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Figure 5

Calculated contours of radial velocity within slot—(a) zero bleed; (b) 5.2% bleed; and (c) 9.5% bleed

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Figure 6

Dividing streamline stagnating on rear surface of bleed slot

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Figure 7

Calculated (4.2% bleed) contours of static pressure across the rear surface of the bleed slot

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Figure 8

Measured (exp) and calculated (CFD) pitchwise variation of static pressure (Cp) across the rear surface of the bleed slot for three bleed rates

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Figure 9

Calculated pitchwise variation of static pressure (Cp) across rear surface of bleed slot at 10%, 50%, and 90% slot depth, for three bleed rates

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Figure 10

Calculated (4.2% bleed) contours of tangential velocity (Vt) across the rear surface of the bleed slot

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Figure 11

Calculated pitchwise variation of tangential velocity (Vt) across rear surface of the bleed slot at 10%, 50%, and 90% depth, for three bleed rates

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Figure 12

Calculated (3.7% bleed) flow within bleed slot at 50% slot depth—(a) static pressure; (b) tangential velocity; and (c) axial velocity with vectors

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Figure 13

Calculated (3.2% bleed) spillage jet—(a) calculated streamlines; and (b) radial velocity contours at 10% slot depth

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Figure 14

Calculated pitchwise variation of static pressure (Cp) across rear surface of slot at 10% depth for three bleed rates and different inlet boundary layer thickness

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Figure 15

Dividing streamline stagnating on rear surface of slot with inlet boundary layer total pressure profile—(a) thin inlet boundary layer; and (b) thick inlet boundary layer

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Figure 16

Calculated pitchwise variations of tangential velocity (Vt) across rear surface of slot at 10% slot depth for different bleed rates and inlet boundary layer thickness

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