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Design Innovation Paper: Design Innovation Paper

Optimizing Separate Exhaust Turbofans for Cruise Specific Fuel Consumption

[+] Author and Article Information
Syed J. Khalid

Gas Turbine Systems Solutions, LLC,
Palm Beach Gardens, FL 33418
e-mail: sjkhalid@hotmail.com

Contributed by the Turbomachinery Committee of ASME for publication in the JOURNAL OF ENGINEERING FOR GAS TURBINES AND POWER. Manuscript received May 9, 2017; final manuscript received July 12, 2017; published online August 16, 2017. Editor: David Wisler.

J. Eng. Gas Turbines Power 139(12), 125001 (Aug 16, 2017) (7 pages) Paper No: GTP-17-1162; doi: 10.1115/1.4037316 History: Received May 09, 2017; Revised July 12, 2017

Cruise specific fuel consumption (SFC) of turbofan engines is a key metric for increasing airline profitability and for reducing CO2 emissions. Although increasing design bypass ratio (BPR) of separate exhaust turbofan configurations improves cruise SFC, further improvements can be obtained with online control actuated variable geometry modulations of bypass nozzle throat area, core nozzle throat area, and compressor variable vanes (CVV/CVG). The scope of this paper is to show only the benefits possible, and the process used in determining those benefits, and not to suggest any particular control algorithm for searching the best combination of the control effectors. A parametric cycle study indicated that the effector modulations could increase the cruise BPR, core efficiency, transmission efficiency, propulsive efficiency, and ideal velocity ratio resulting in a cruise SFC improvement of as much as 2.6% depending upon the engine configuration. The changes in these metrics with control effector variations will be presented. Scheduling of CVV is already possible in legacy digital controls; perturbation to this schedule and modulation of nozzle areas should be explored in light of the low bandwidth requirements at steady-state cruise conditions.

Copyright © 2017 by ASME
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References

Figures

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Fig. 1

Separate exhaust, twin spool turbofan engine

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Fig. 2

Separate exhaust turbofan schematic showing station numbers and secondary flows

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Fig. 3

High level overview of design point calculation

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Fig. 4

Nozzle area variations, BPRdes = 8, FN = 10,000

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Fig. 5

Bypass nozzle and CVV variations, BPRdes = 8

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Fig. 6

Depiction of compressor variable geometry scheduling. Note: CVV/CVG angle is the angle IGVs subtend from the axial direction. The downstream few stators are ganged with the IGV's but their turning is less than that of IGV's.

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Fig. 7

Changes in core efficiency with bypass nozzle and CVV Variations: design BPR = 8 configuration

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Fig. 8

Effect of bypass nozzle and CVV/CVG variations on Transmission efficiency: design BPR = 8

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Fig. 9

Booster operating point excursion during bypass nozzle and CVV variations: design BPR = 8 configuration

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Fig. 10

HPC operating line during variations: design BPR = 8

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Fig. 11

Minimum excursion of LPT operating point during bypass nozzle and CVV variation: design BPR = 8

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Fig. 12

SFC improvement with bypass nozzle and CVV variations: design BPR = 12 configuration

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Fig. 13

Effect of bypass nozzle area opening on cruise SFC in the case of design BPR = 12 configuration

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Fig. 14

Design BPR = 12, bypass nozzle variation only at cruise

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Fig. 15

Design BPR = 12, effect of bypass nozzle area increase on transmission efficiency

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Fig. 16

Fan operating line migration with bypass nozzle area increase at cruise: design BPR = 12

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Fig. 17

Booster operating point marches toward stall line with bypass nozzle opening: design BPR = 12

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Fig. 18

Effect of bypass nozzle area variation on cruise HPT efficiency: design BPR = 12 configuration

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