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Research Papers: Gas Turbines: Aircraft Engine

Aerodynamic Design of Separate-Jet Exhausts for Future Civil Aero-engines—Part II: Design Space Exploration, Surrogate Modeling, and Optimization

[+] Author and Article Information
Ioannis Goulos

Propulsion Engineering Centre,
Cranfield University,
Bedfordshire MK430AL, UK
e-mail: i.goulos@cranfield.ac.uk

John Otter

Propulsion Engineering Centre,
Cranfield University,
Bedfordshire MK430AL, UK
e-mail: j.j.otter@cranfield.ac.uk

Tomasz Stankowski

Propulsion Engineering Centre,
Cranfield University,
Bedfordshire MK430AL, UK
e-mail: t.stankowski@cranfield.ac.uk

David MacManus

Propulsion Engineering Centre,
Cranfield University,
Bedfordshire MK430AL, UK
e-mail: D.G.Macmanus@cranfield.ac.uk

Nicholas Grech

Installation Aerodynamics,
Rolls-Royce plc,
Trent Hall 2.2, SinA-17,
Derby DE24 8BJ, UK
e-mail: Nicholas.Grech@Rolls-Royce.com

Christopher Sheaf

Installation Aerodynamics,
Rolls-Royce plc,
Trent Hall 2.2, SinA-17,
Derby DE24 8BJ, UK
e-mail: Christopher.Sheaf@Rolls-Royce.com

Contributed by the Aircraft Engine Committee of ASME for publication in the JOURNAL OF ENGINEERING FOR GAS TURBINES AND POWER. Manuscript received November 22, 2015; final manuscript received December 18, 2015; published online March 15, 2016. Editor: David Wisler.

J. Eng. Gas Turbines Power 138(8), 081202 (Mar 15, 2016) (12 pages) Paper No: GTP-15-1539; doi: 10.1115/1.4032652 History: Received November 22, 2015; Revised December 18, 2015

The aerodynamic performance of the bypass exhaust system is key to the success of future civil turbofan engines. This is due to current design trends in civil aviation dictating continuous improvement in propulsive efficiency by reducing specific thrust and increasing bypass ratio (BPR). This paper aims to develop an integrated framework targeting the automatic design optimization of separate-jet exhaust systems for future aero-engine architectures. The core method of the proposed approach is based on a standalone exhaust design tool comprising modules for cycle analysis, geometry parameterization, mesh generation, and Reynolds-averaged Navier–Stokes (RANS) flow solution. A comprehensive optimization strategy has been structured comprising design space exploration (DSE), response surface modeling (RSM) algorithms, as well as state-of-the-art global/genetic optimization methods. The overall framework has been deployed to optimize the aerodynamic design of two civil aero-engines with separate-jet exhausts, representative of current and future engine architectures, respectively. A set of optimum exhaust designs have been obtained for each investigated engine and subsequently compared against their reciprocal baselines established using the current industry practice in terms of exhaust design. The obtained results indicate that the optimization could lead to designs with significant increase in net propulsive force, compared to their respective notional baselines. It is shown that the developed approach is implicitly able to identify and mitigate undesirable flow-features that may compromise the aerodynamic performance of the exhaust system. The proposed method enables the aerodynamic design of optimum separate-jet exhaust systems for a user-specified engine cycle, using only a limited set of standard nozzle design variables. Furthermore, it enables to quantify, correlate, and understand the aerodynamic behavior of any separate-jet exhaust system for any specified engine architecture. Hence, the overall framework constitutes an enabling technology toward the design of optimally configured exhaust systems, consequently leading to increased overall engine thrust and reduced specific fuel consumption (SFC).

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Figures

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Fig. 1

Notionally defined axisymmetric geometry for a very-high-BPR turbofan engine with separate-jet exhausts

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Fig. 2

Upper-level overview of the developed software architecture

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Fig. 3

Geometric design approach employed in GEMINI

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Fig. 4

Developed framework for DSE and optimization

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Fig. 5

Two-dimensional axisymmetric geometries of investigated engine architectures: (a) design representative of future engine architectures (E1) and (b) design representative of current engine architectures (E2)

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Fig. 6

Mach number contours for the baseline exhaust system designs at DP midcruise conditions: (a) design representative of future engine architectures (E1) and (b) design representative of current engine architectures (E2)

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Fig. 7

Design space definition: (a) bypass duct outer line position ybpout=Rbpout/Lductin, (b) ybpin=Rbpin/Lductin, (c) nozzle CP to exit area ratio Aratio=ACP/Aexit and length ratio κlenin=LinNozzle/h2, (d) outer line slope at the CP θCPout, (e) CP inner/outer curvature radius ratio κCPin/out=RcurveCP,in/out/h2, (f) core cowl length lcrcowl=Lcrcowl/Rfan, (g) zone 3 vent exit position lz3exit=Lz3exit/Lcrcowl, (h) zone 3 exit Mach number Mz3exit, and (i) core cowl angle θcrcowl and outer line angle θnozzleout

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Fig. 8

Linear correlation estimation between design variables and performance metrics: (a) future E1 engine and (b) current E2 engine

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Fig. 9

LOO cross-validation applied to the structured surrogate models for the E1 future engine model: (a) bypass nozzle discharge coefficient CDBypass and (b) overall thrust coefficient CVOverall

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Fig. 10

LOO cross-validation applied to the structured surrogate models for the E2 current engine model: (a) bypass nozzle discharge coefficient CDBypass and (b) overall thrust coefficient CVOverall

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Fig. 11

Evolutionary computation for the optimization of CVOverall—convergence process

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Fig. 12

Comparison between the baseline and optimum exhaust designs for the future E1 engine architecture: (a) baseline exhaust design and (b) exhaust design optimized for CVOverall

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Fig. 13

Comparison between the baseline and optimum exhaust designs for the current E2 engine architecture: (a) baseline exhaust design and (b) exhaust design optimized for CVOverall

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