Numerical investigation on flow and mixing characteristics inside a converging-diverging mixing duct of rocket-based combined-cycle engine in ejector mode

The supersonic mixing layers with complex background waves are formed by the rocket jet and the entrained air in the mixing section of rocket-based combined-cycle (RBCC) engine during ejector mode. To achieve mixing enhancement of primary and secondary flows in confined space, a converging-diverging mixing duct was designed in this study. For a typical flight regime of RBCC ejector mode, the effects of the mixer geometry including the contraction ratio, the throat position of mixer and the converging angle on the flow structures and mixing characteristics in the converging-diverging mixing duct under no backpressure condition were numerically investigated. A detailed discussion on the mechanism of mixing enhancement in this mixer configuration was also presented. The results indicate that appropriately increasing the contraction ratio can significantly strengthen the interactions of background shock waves with supersonic mixing layer in the converging-diverging mixing duct, which effectively promotes the rapid and sufficient mixing between primary flow and secondary flow. Besides, shortening the length from the mixer inlet to the throat position can enhance the growth rate of mixing layer and the mixing efficiency of two flows in the upstream region of converging-diverging mixing duct to some extent. However, in the downstream far-field region, the variations of throat position have little impact on the scalar mixing process in supersonic mixing layer. Moreover, it is found that increasing the converging angle has no obvious effect on intensifying the mixing performance of the converging-diverging mixing duct. On the contrary, it may lead to a dramatic decrease in total pressure recovery. Further analysis reveals that the oblique shocks generated at the compression corner and the separation shocks formed in the divergent section interact with the supersonic mixing layer, which contributes to mixing enhancement in the converging-diverging mixing duct. (C) 2020 Elsevier Masson SAS. All rights reserved.