Effect of serpentine nozzle on bypass ratio of turbofan engine exhaust

Serpentine nozzles have been widely used in various types of aircrafts to strengthen survivability for their predominant performance in suppressing infrared radiation signature (IRS) emitted by engine exhausts. The existence of exhaust mixer from the engine turbine, including double ducts (bypass and core flow), the lobed mixers and the tail cone, could lead to complex flow features interacting with serpentine nozzle in turbofan engine. For turbofan engine with axisymmetric nozzle, when axisymmetric nozzle is replaced by serpentine nozzle, the variation of bypass ratio of engine exhaust will affect the operating situation of engine, which is harmful to engine. Thus, the aim of this paper is to study the effect of double serpentine convergent nozzle on bypass ratio of turbofan engine exhaust as compared to that of axisymmetric convergent nozzle in the two types of bypass ratio cases, and to search for the reasonable ways to march the bypass ratio of turbofan engine exhaust between double serpentine convergent nozzle and axisymmetric convergent nozzle. The basic flow characteristics of exhaust with double serpentine convergent nozzle are investigated. The mechanism that the variation of bypass ratio of exhaust with double serpentine convergent nozzle compared with axisymmetric convergent nozzle is illustrated. The solving method of the variation of exhaust bypass ratio is presented. The results show that the pressure gradient in the direction normal to streamlines induced and developed by bended duct at the double serpentine convergent nozzle inlet will change the flowing abilities through bypass and core flow apparently. The bypass ratio of exhaust with serpentine nozzle increased 8.1% highest compared with that of exhaust with axisymmetric nozzle, when the mass fluxes are consistent under the same boundary conditions. The paper designs an axisymmetric convergent section connecting exhaust mixer and double serpentine convergent nozzle respectively. The bypass ratio of exhaust with redesigned serpentine nozzle changed 1.2% mostly compared with that of exhaust with axisymmetric nozzle. The bypass ratio of engine exhaust with redesigned serpentine nozzle is almost coincident with that of engine exhaust with axisymmetric nozzle.