Research Papers

Ultrashort Nacelles for Low Fan Pressure Ratio Propulsors

[+] Author and Article Information
Andreas Peters

Gas Turbine Laboratory,
Department of Aeronautics and Astronautics,
Massachusetts Institute of Technology,
Cambridge, MA 02139
e-mail: andreaspeters5@alum.mit.edu

Zoltán S. Spakovszky

Gas Turbine Laboratory,
Department of Aeronautics and Astronautics,
Massachusetts Institute of Technology,
Cambridge, MA 02139

Wesley K. Lord, Becky Rose

Pratt & Whitney,
East Hartford, CT 06118

Contributed by the International Gas Turbine Institute (IGTI) of ASME for publication in the JOURNAL OF TURBOMACHINERY. Manuscript received July 7, 2014; final manuscript received July 22, 2014; published online September 10, 2014. Editor: Ronald Bunker.

J. Turbomach 137(2), 021001 (Sep 10, 2014) (14 pages) Paper No: TURBO-14-1120; doi: 10.1115/1.4028235 History: Received July 07, 2014; Revised July 22, 2014

As the propulsor fan pressure ratio (FPR) is decreased for improved fuel burn, reduced emissions and noise, the fan diameter grows and innovative nacelle concepts with short inlets are required to reduce their weight and drag. This paper addresses the uncharted inlet and nacelle design space for low-FPR propulsors where fan and nacelle are more closely coupled than in current turbofan engines. The paper presents an integrated fan–nacelle design framework, combining a spline-based inlet design tool with a fast and reliable body-force-based approach for the fan rotor and stator blade rows to capture the inlet–fan and fan–exhaust interactions and flow distortion at the fan face. The new capability enables parametric studies of characteristic inlet and nacelle design parameters with a short turn-around time. The interaction of the rotor with a region of high streamwise Mach number at the fan face is identified as the key mechanism limiting the design of short inlets. The local increase in Mach number is due to flow acceleration along the inlet internal surface coupled with a reduction in effective flow area. For a candidate short-inlet design with length over diameter ratio L/D = 0.19, the streamwise Mach number at the fan face near the shroud increases by up to 0.16 at cruise and by up to 0.36 at off-design conditions relative to a long-inlet propulsor with L/D = 0.5. As a consequence, the rotor locally operates close to choke resulting in fan efficiency penalties of up to 1.6% at cruise and 3.9% at off-design. For inlets with L/D < 0.25, the benefit from reduced nacelle drag is offset by the reduction in fan efficiency, resulting in propulsive efficiency penalties. Based on a parametric inlet study, the recommended inlet L/D is suggested to be between 0.25 and 0.4. The performance of a candidate short inlet with L/D = 0.25 was assessed using full-annulus unsteady Reynolds-averaged Navier–Stokes (RANS) simulations at critical design and off-design operating conditions. The candidate design maintains the propulsive efficiency of the baseline case and fuel burn benefits are conjectured due to reductions in nacelle weight and drag compared to an aircraft powered by the baseline propulsor.

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Fig. 1

Baseline propulsor with L/D = 0.5

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Fig. 2

Dividing streamlines between internal and external flows for a candidate short-inlet design with L/D = 0.19

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Fig. 3

Distributions of inlet area (top) and averaged Mach number (center) through the inlet and isentropic Mach number along bottom inlet (bottom) for L/D = 0.5, L/D = 0.25, and L/D = 0.19 propulsors at cruise

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Fig. 4

Overview of computational approach to quantify the incidence distortion mechanisms in the baseline propulsor

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Fig. 5

Superposition of the contributions from nonuniform inflow and pylon/bifurcation upstream influence to the stagnation pressure variation downstream of the rotor at midspan in the baseline propulsor with L/D = 0.5 at cruise

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Fig. 6

Example for top inlet and nacelle shape description based on supercritical airfoil

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Fig. 7

Parametric description of bottom inlet LE

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Fig. 8

Piecewise inlet and nacelle geometry definition using Bezier curves

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Fig. 9

Control volume definition for calculation of engine propulsive efficiency

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Fig. 10

Fan efficiency for no inlet swirl (black), 5 deg counter-swirl (light gray), and 5 deg co-swirl (dark gray) at the T/O rotation operating condition. The results from RANS and body force simulations are plotted as solid and dashed lines, respectively. For a single set of body force coefficients, the rotor performance is captured over the entire speedline.

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Fig. 11

Candidate short-inlet propulsor with L/D = 0.25 at the wing CLmax operating condition. Using force distributions obtained from steady, single-passage RANS simulations, the body force method is capable of capturing the interaction between the rotor and inlet flow.

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Fig. 12

Spanwise profile of rotor incidence corrected by midspan value for short-inlet design with L/D = 0.25 at wing CLmax

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Fig. 13

Candidate short-inlet propulsor with L/D = 0.25 at the wing CLmax operating condition (AoA = 29 deg). The results for the flow-through nacelle simulation show separated inlet flow (left). Due to the favorable pressure gradient generated by the rotor near the shroud, the inlet flow is attached in the powered nacelle case with body forces in the rotor blade row (right).

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Fig. 14

Top and bottom inlet and nacelle sections for the baseline L/D = 0.5 and the L/D = 0.25 and L/D = 0.19 candidate short-inlet designs

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Fig. 15

Mach number distribution (top) and axial Mach number distribution (bottom) for the baseline propulsor (left) and the candidate short-inlet designs with L/D = 0.25 (center) and L/D = 0.19 (right) at cruise

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Fig. 16

Mach number distribution (top) and axial Mach number distribution (bottom) for the baseline propulsor (left) and the candidate short-inlet designs with L/D = 0.25 (center) and L/D = 0.19 (right) at the wing CLmax operating condition

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Fig. 17

Time-averaged spanwise profiles of axial Mach number (left) and rotor incidence relative to midspan incidence (right) at the bottom fan face for the baseline (L/D = 0.5) and the candidate short-inlet propulsors (L/D = 0.25 and L/D = 0.19) at the wing CLmax condition

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Fig. 18

Hypothesized trend in propulsive efficiency with inlet L/D (bottom) as a result of the trade-offs between fan efficiency, nacelle drag, and inlet pressure recovery (top)



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