Abstract:Variation of the bypass nozzle exit area enables optimization of the turbofan engine operating cycle over a wider range of operational conditions resulting in improved thrust and/or fuel consumption. Two mechanisms for varying the nozzle area have been investigated. The first uses an array of chevrons which when closed, form a full body of revolution and when warped/curved, increase the exit area while forming a serrated trailing edge. The second technique incorporates an axially translating section of the nacelle shroud and uses the change in the nozzle boat-tail radial location with the axial location as a means to vary the nozzle exit area. To analyse the effects on a typical rotor/stator stage, computational fluid dynamics simulations of the NASA Rotor 67, Stator 67A stage integrated into a custom-built nacelle were performed. Nozzles with 8, 12, and 16 chevrons were simulated to evaluate the impact of the variation in geometry upon the nacelle wake and local forces. Gross thrust of the nacelle and the turbulent kinetic energy (TKE) variation through the wake is compared. The chevron nozzle attains a nearly 2 per cent maximum thrust improvement over the translating nozzle technique. The chevron nozzle also has significantly lower (nearly 8 per cent) peak TKE levels in the jet plume.

Simulations of the NASA Rotor 67, Stator 67A stage integrated into a bespoke nacelle were performed using ANSYS CFX. The throat area of the nacelle nozzle was varied by use of warped chevrons. 8, 12 and 16 chevron nozzles were simulated to evaluate the impact of the variation in geometry upon the nacelle wake and local forces. The force produced from the nozzle and fan, is compared to the baseline case where the throat area is optimised for cruise conditions. The variation in gross thrust between cases is also analysed. The turbulence kinetic energy and total temperature variation through the wake is compared. A reduction in peak wake mixing massflow-averaged turbulent kinetic energy of 11.9% was attained. Surface force measurements of the rotor, stator and nozzle duct indicate a rising thrust loss with increasing nozzle throat area. However, measurement of the nozzle exit velocities and massflow rates enable subsequent estimation of the gross thrust which indicate a rise of 2.4% relative to the baseline is achievable.

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