Numerical Study Of The Impacts Of Geometry Parameters Of Convergent-divergent Nozzle With Circle To Rectangular Transition On Flow And Infrared Stealth Characteristics

With the development of infrared detection technology, the requirement of aircraft infraredstealth was gradual ly increasing, as well as the requirement of the exhaust infrared suppression. Inthis paper, based on the designing technology of round to rectangular2D nozzle with low infraredsignature, the engine exhaust nozzle features were researched by Numerical simulation.In this paper, with the thrust to weight ratio of10, an advanced engine AL-31F being object ofstudy, circle to rectangular transition was designed in base of the axisymmetric one, focusing on thekey designing parameters: length of expansion, expansion angle, the ratio of export width and heightas well as the surface of transition section, the aerodynamic characteristics and infrared radiationcharacteristics of nozzle were investigated, and what were further studied was the impacts of theshape of round to rectangular section on the changes of bypass ratio.Using FLUENT CFD software, numerical analysis was done to convergent-divergent nozzlemodels with circle to rectangular transition (convergent section is circle to rectangular transition, theexpanding section is using rectangular) with five different kinds of expansion length and expansionangle and four different kinds of transitional shapes. In the process from the subsonic flow to thesupersonic flow, the paper got the characteristics of the air flow of the different axial cross-section andthe flow parameters change of axial direction.It was found that inside the nozzle, with the axial distance increasing, the intensity of secondaryflow and the rotation of the fluid partical were enhanced; the influx blending was gradually enhanced.With the length of the nozzle increased, the air flow stability was enhanced, the thrustcoefficient increased, while temperature fluctuation of tail jet exit was weakened, the length of thehigh-temperature region was larger, the infrared radiation was stronger.The impacts of nozzle expansion angle on the tail jet pneumatic and infrared signature is weaker.In this study, the impacts of changes of four kinds of surface of round to rectangular transition on thedistribution of the flow parameters of the surface of the nozzle throat is greater.After the axisymmetric nozzle was changed into round to rectangular nozzle, engine bypass ratioincreased slightly. This paper studies the influence of surface changes of transitional section on bypassratio, it is found that the first-emergency and later-slow surface type could reduce bypass ratio, whilethe bypass ratio of round to rectangular nozzle using first-slow and later-emergency surface type isslightly larger. Nozzle infrared radiation characteristics were less affected by the different kinds of transitional section surfaces.