CFD INVESTIGATION OF ENGINE EXHAUST SWIRLING FLOW IN HIGH ASPECT RATIO RECTANGULAR DUCTS

摘要

Increasing requirements for performance, survivability, and costs associated with helicopter engine / airframe integration demands updated design and analysis tools to model a helicopter engine exhaust system. Typical helicopter exhaust systems are comprised of a primary nozzle, which functions as a low pressure ratio ejector, and secondary nozzles. Helicopter exhaust flow characteristics also include residual exhaust swirl which varies as a function of engine power. A CFD model was generated for a complex helicopter engine exhaust primary nozzle and validated with full scale model test data. CFD analyses were accomplished for several different flow rates and residual swirl angles. Exhaust nozzle exit plane total pressure characteristics compared closely with measured test data. At the nozzle exit planes, non-uniform total pressure profiles were observed for both the analysis and measured data due to nozzle exhaust flow migrating to either the top or bottom corners of the duct. Larger pressure gradients were observed with the higher swirl angles. Features of the nozzle flowfield included the formation of two vortices at the nozzle exit plane, localized at the corners and independent of swirl angle. CFD analyses showed that the exhaust nozzle inlet boundary layer vorticity was entrained into the bulk of the flow due to boundary layer separation at the duct transition. The successful modeling and validation of the complex primary nozzle flow characteristics will lead to a complete helicopter engine exhaust system modeling tool. The design and analysis tool will be used to optimize the engine exhaust performance and/or IR suppressor system for existing and future helicopter development programs.