Review on the aerodynamic issues of the exhaust system for scramjet and turbine based combined cycle engine

A review of research progress in the design of the exhaust system for the scramjet and turbine based combined cycle (TBCC) engine is presented. Firstly, the technical challenges encountered in designing the exhaust system for a hypersonic propulsion system are highlighted and discussed, and the performance parameter definition as well as the theoretical thrust prediction for the exhaust system is introduced. The review of scramjet nozzle focuses on three aspects: 1) the design method of the single expansion ramp nozzle (SERN) for the integration of the airframe with the propulsion system, in which the design method developments of the two-dimensional (2D) SERN, SERN with lateral expansion and three-dimensional (3D) SERN with shape transition are all summarized; 2) the unique flow phenomena of the scramjet nozzle, including the nonuniform inflow and chemical nonequilibrium flow in SERN; 3) the coupling and interaction of the internal flow with the external freestream. Besides, the design and flow researches of the TBCC exhaust system is also reviewed for three parts: 1) variable geometry design for wide flight range, in which both a 2D and 3D exhaust system are described; 2) the overexpanded flow separation mechanism and its control at low flight Mach number; 3) mode transition from low-speed flowpath (LSF) to high-speed flowpath (HSF) for over-under exhaust system. Through the above summary and analysis, the current status, bottlenecks, and development trend of the exhaust system for an airbreathing hypersonic propulsion system can be further clarified.