Research Papers

Reverse Thrust Aerodynamics of Variable Pitch Fans

[+] Author and Article Information
Tim S. Williams

Whittle Laboratory,
University of Cambridge,
1 JJ Thomson Avenue, Cambridge CB3 0DY, UK
e-mail: tsw30@cam.ac.uk

Cesare A. Hall

Whittle Laboratory,
University of Cambridge,
1 JJ Thomson Avenue, Cambridge CB3 0DY, UK
e-mail: cah1003@cam.ac.uk

1Corresponding author.

Contributed by the International Gas Turbine Institute (IGTI) of ASME for publication in the Journal of Turbomachinery. Manuscript received January 07, 2019; final manuscript received March 08, 2019; published online March 28, 2019. Assoc. Editor: Kenneth Hall.

J. Turbomach 141(8), 081008 (Mar 28, 2019) (9 pages) Paper No: TURBO-19-1005; doi: 10.1115/1.4043139 History: Received January 07, 2019; Accepted March 08, 2019

Variable pitch fans are of interest for future low-pressure ratio fan systems since they provide improved operability relative to fixed pitch fans. If they can also be re-pitched such that they generate sufficient reverse thrust they could eliminate the engine drag and weight penalty associated with bypass duct thrust reversers. This paper sets out to understand the details of the 3D fan stage flow field in reverse thrust operation. This study uses the Advanced Ducted Propulsor variable pitch fan test case, which has a design fan pressure ratio of 1.29. Comparison with spanwise probe measurements show that the computational approach is valid for examining the variation of loss and work in the rotor in forward thrust. The method is then extended to a reverse thrust configuration using an extended domain and appropriate boundary conditions. Computations, run at two rotor stagger settings, show that the spanwise variation in relative flow angle onto the rotor aligns poorly to the rotor inlet metal angle. This leads to two dominant rotor loss sources: one at the tip associated with positive incidence and the second caused by negative incidence at lower span fractions. The second loss is reduced by opening the rotor stagger setting, and the first increases with rotor suction surface Mach number. The higher mass flow at more open rotor settings provide higher gross thrust, up to 49% of the forward take-off value, but is limited by the increased loss at high speed.

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Fig. 1

Diagram of fan blade repitching for reverse thrust

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Fig. 2

Global reverse thrust flow field sketched from laser Doppler velocimetry [7] for the Advanced Ducted Propulsor 17 in. fan.

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Fig. 3

NASA Advanced Ducted Propulsor test rig [9]

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Fig. 4

ADP computational domain for forward operation. Mixing planes are drawn as dashed lines.

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Fig. 5

Comparison of measured and computed inlet boundary layer profiles

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Fig. 6

Measured and computed bypass characteristics

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Fig. 7

Measured and computed station F2 radial profiles of total pressure and temperature ratios at 88% ΩTO

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Fig. 8

Reverse thrust blade-to-blade mesh topology at the hub (left) and casing (right), repitching through feather

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Fig. 9

Boundary conditions for reverse thrust calculation, free-stream boundaries not to scale: (a) free-stream cylinder domain and (b) ducted outlet domain

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Fig. 10

Rotor outlet profiles for calculations with equal rotor mass flow rate for the nozzle and free-stream cylinder domains at −92 deg stagger and 88% ΩTO: (a) normalized mass flow per unit area and (b) total and static pressure ratio

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Fig. 11

Fan stage reverse total pressure ratio and isentropic efficiency characteristics

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Fig. 12

Overview of the reverse thrust flow field identifying key flow features. The top diagram indicates location of the blade-to-blade cuts: (a) bypass stator, (b) core stator, (c) rotor 40% span, and (d) rotor 90% span.

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Fig. 13

Absolute and relative flow angles from bypass stator outlet to rotor inlet, at −86 deg stagger and 88% ΩTO

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Fig. 14

Rotor outlet station 2 divided for loss integration into sections: (i) free-stream, (ii) pressure surface separation, (iii) wake, and (iv) casing

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Fig. 15

Loss breakdown on the reverse working line at (a) −92 deg and (b) −86 deg stagger

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Fig. 16

Estimate of gross thrust, and work input normalized by 100% speed take-off values



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