Experimental and Numerical Investigations on the Mode Transition Process of an Exhaust System for an Over-under Turbine Based Combined Cycle

Abstract In this paper, the mode transition process of an over-under exhaust system for a turbine based combined cycle (TBCC) is investigated experimentally. The method of characteristics (MOC) and truncation method are employed to design the single expansion ramp nozzle (SERN) at the cruise condition, and the final configuration of the exhaust system is determined by resolving the turbine nozzle performance. The crank-link mechanisms are applied to convert the rotation modes of the splitter and cowl to the linear motion modes in the experimental model. The flowfield structure and pressure distribution along the symmetric plane are measured in the experiment. Furthermore, the computational fluid dynamics (CFD) approach is also adopted to simulate the flowfield of the exhaust system for comparison. The results denote that although the flowfield is greatly complicated as the interaction of the exhaust system plume with the external freestream, the internal flowfield of the exhaust system is independent of the external flow. The flowfield of the entire exhaust system is occupied by the turbine nozzle flow at the initial condition, while it is dominated by the ramjet nozzle flow at the end of the mode transition. At the afterburner and throttle stages of the turbine, the shear layer deflects toward the turbine nozzle flowfield, and the influence of the ramjet nozzle jet on the turbine nozzle flowfield is increased. The internal flow of the turbine nozzle is not influenced by the ramjet nozzle plume at the afterburner stage, while it is greatly affected by the ramjet nozzle plume at the throttle stage. Instead, the shear layer deflects toward the ramjet nozzle flowfield at the transitional stage, and the effect of the turbine nozzle flow on the ramjet nozzle flowfield is increased. Furthermore, the calculated flowfield structure and pressure distributions agree well with the experimental results, and the numerical method can be capable of capturing the flowfield feature of the exhaust system.