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.

在喷射器模式下，具有复杂背景波的超音速混合层是由火箭射流和夹带空气在基于火箭的联合循环（RBCC）发动机的混合段中形成的。为了在密闭空间中实现一次流和二次流的混合增强，在本研究中设计了一个收敛-发散混合管道。对于RBCC喷射器模式的典型飞行状态，在无背压条件下，数值模拟了混合器几何形状（包括收缩率，混合器的喉部位置和会聚角）对会聚-扩散混合管中流动结构和混合特性的影响调查。还介绍了这种混合器配置中混合增强机制的详细讨论。结果表明，适当增大收缩比可以显着增强本底激波与超声速混合层在汇流-扩散混合管道中的相互作用，从而有效地促进了一次流动和二次流动之间的快速充分混合。此外，缩短从混合器入口到喉部位置的长度可以在一定程度上提高混合层的生长速率和在汇流-扩散混合管道的上游区域中的两个流的混合效率。然而，在下游远场区域，喉部位置的变化对超声速混合层中标量混合过程的影响很小。此外，结果发现，增大会聚角对增强会聚-混合管的混合性能没有明显影响。相反，它可能导致总压力恢复急剧下降。进一步的分析表明，在压缩角产生的斜向冲击和在发散段中形成的分离冲击与超音速混合层相互作用，这有助于会聚-扩散混合管道中的混合增强。