Analysis and manipulation of the separation zone in an overexpanded combined exhaust nozzle

Abstract

The combined exhaust nozzle is an important component of the over-under turbine-based combined cycle engine. During the subsonic and transonic flight Mach number, the ramjet duct suffers severe overexpansion, resulting in complex separation phenomena and poor performance. In the present paper, the fluidic injection is applied to improve the severely overexpanded nozzle's performance based on the idea of separation-zone manipulation. The separation patterns in a ramjet duct are numerically investigated, and the position of the separation point is found to be critical to performance. Separation that originates near the throat produces a longer pressure plateau and raises the nozzle's thrust, while cowl-side separation leads to a better thrust angle compared to ramp-side separation. Based on this phenomenon, the fluidic injection is introduced to manipulate the separation zone to occur at proper locations. The results show that the fluidic injection with a secondary pressure ratio of 2.0 can successfully induce a favorable separation pattern over a large range of working conditions. The average thrust coefficient is increased by 7.3%, and the thrust angle is reduced by 34.8%. The separation-zone manipulation concept is also verified using a 3D nozzle model which shows a similar benefit.