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research-article

Reverse thrust aerodynamics of variable pitch fans

[+] Author and Article Information
Tim S. Williams

Whittle Laboratory 1 JJ Thomson Ave Cambridge, Cambridgeshire CB3 0DY United Kingdom tsw30@cam.ac.uk

Cesare A. Hall

Whittle Laboratory 1 JJ Thomson Avenue Cambridge, CB3 0DY United Kingdom 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 7, 2019; final manuscript received March 8, 2019; published online xx xx, xxxx. Assoc. Editor: Kenneth Hall.

ASME doi:10.1115/1.4043139 History: Received January 07, 2019; Accepted March 08, 2019

Abstract

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.

The 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.

Copyright © 2019 by ASME
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