Mixing enhancement of high-bypass turbofan exhausts via contouring of fan nozzle

Abstract

The purpose of this paper is to consider experimentally and computationally the use of axial flow to enhance the mixing of the jets from high-bypass turbofan nozzles. Mixing enhancement is caused by flow separation in the convergent-divergent secondary (fan) nozzle operated at overexpanded conditions. The experiments were conducted on a scaleddown version of a typical turbofan engine exhaust. Here the exit-to-throat area ratio of the secondary nozzle is conveniently adjusted via the axial position of the secondary nozzle relative to the primary nozzle exit. Mean velocity surveys of the jet plume were taken at Mach numbers in the range 0.6-1.0 representative of modern turbofan engines. The degree of jet mixing enhancement depends primarily on the nozzle area ratio. The effect of nozzle pressure ratio, which ranged from 1.3 to 1.9, is relatively minor. The high-velocity region of the jet is reduced by about 3 fan diameters and the primary potential core region practically eliminated at nozzle area ratios at or above 1.4. Selected jets were also investigated computationally. The computations capture the salient physics of flow separation and reproduce well the experimentally observed reduction of the potential core region in the vicinity of the nozzle. Further downstream, however, the computations do not capture accurately the experimental trends.