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TECHNICAL PAPERS

Influence of Sweep on Axial Flow Turbine Aerodynamics at Midspan

[+] Author and Article Information
Graham Pullan

Whittle Laboratory, Department of Engineering,  University of Cambridge, United Kingdomgp10006@cam.ac.uk

Neil W. Harvey

 Roll-Royce plc, Derby, United Kingdom

J. Turbomach 129(3), 591-598 (Jul 14, 2006) (8 pages) doi:10.1115/1.2472397 History: Received July 13, 2006; Revised July 14, 2006

Sweep, when the stacking axis of the blade is not perpendicular to the axisymmetric streamsurface in the meridional view, is often an unavoidable feature of turbine design. Although a high aspect ratio swept blade can be designed to achieve the same pressure distribution as an unswept design, this paper shows that the swept blade will inevitably have a higher profile loss. A modified Zweifel loading parameter, taking sweep into account, is first derived. If this loading coefficient is held constant, it is shown that sweep reduces the required pitch-to-chord ratio and thus increases the wetted area of the blades. Assuming fully turbulent boundary layers and a constant dissipation coefficient, the effect of sweep on profile loss is then estimated. A combination of increased blade area and a raised pressure surface velocity means that the profile loss rises with increasing sweep. The theory is then validated using experimental results from two linear cascade tests of highly loaded blade profiles of the type found in low-pressure aeroengine turbines: one cascade is unswept, the other has 45deg of sweep. The swept cascade is designed to perform the same duty with the same loading coefficient and pressure distribution as the unswept case. The measurements show that the simple method used to estimate the change in profile loss due to sweep is sufficiently accurate to be a useful aid in turbine design.

Copyright © 2007 by American Society of Mechanical Engineers
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Figures

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Figure 1

Definition of sweep

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Figure 2

45deg swept blade designed on a streamsurface section

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Figure 3

45deg swept blade, contours of pitchwise averaged Vs, Δcon=0.05V2

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Figure 4

Schematics of loading coefficient definition, without (left) and with sweep

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Figure 5

Area ratio—contours of (1+tan2αm2)∕(cos2λ+tan2αm2)

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Figure 6

“Vs factor”—contours of (1+sin3λcos3αm2)

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Figure 7

Loss ratio—contours of  ∣ζs∣λ∕∣ζs∣λ=0

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Figure 8

Measured and predicted midspan pressure distributions, datum blade

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Figure 9

Loss and area ratio, αm2=62.8deg

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Figure 10

Predicted pressure distributions, datum and swept blade

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Figure 11

Schematic of datum, unswept cascade, side view

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Figure 12

Schematic of swept cascade, side view

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Figure 13

Swept cascade inlet, meridional (left), and inlet plane views

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Figure 14

Photograph of the inlet contraction for the swept cascade

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Figure 15

Measured contours of (p01−p0)∕(p01−p2), inlet plane, Δcon=0.01, without (left) and with bleeds

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Figure 16

Suction-surface flow visualization, swept cascade (view perpendicular to suction surface at trailing edge)

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Figure 17

Measured midspan cp distributions, unswept blade, 1.7×105<Recm<3.3×105

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Figure 18

Measured midspan cpλ distributions, swept blade, 1.7×105<Recm<3.3×105

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Figure 19

Midspan pressure distributions for both blades at the same Recm

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Figure 20

Measured mixed-out profile loss at midspan

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