Experimental study of rocket plume expansion in the rocket-based combined-cycle engine under the ejector mode

In this paper, we utilized high-speed schlieren technology to investigate the impact of expansion degrees on the evolution of subsonic-supersonic mixing layers. Specifically, we explored characteristics of supersonic and subsonic flows with Mach numbers of 1.27 and 0.75. Our findings revealed that over-expansion of the supersonic flow facilitated transition, causing the generation position of Kelvin-Helmholtz vortexes to shift forward by 35.3% similar to 45.1%. Additionally, the growth rate of visual thickness was observed to increase to 2.7 similar to 3.5 times in the incipient stage of the mixing layer under these conditions. We employed root-mean-square analysis to observe the rapid increase of turbulence intensity and the expansion of the strong turbulent region in the incipient stage (within x/H = 0 similar to 1.5), from under-expansion to over-expansion conditions. Our results indicate that the turbulent region and intensity play a crucial role in the evolution of mixing layers. Spatial correlation analysis revealed larger turbulent structures under over-expansion effects, with structures at x/H = 1.0 being over 8.2% larger than those under weak under-expansion conditions. This turbulent mechanism explains why the over-expansion of rocket plumes can enhance mixing layers. Therefore, designing the rocket in the over-expansion state is an effective strategy for rocket-based combined-cycle engines under the ejector mode. Moreover, we noted that mixing layers and Kelvin-Helmholtz vortexes deflected to the low-pressure flow side, and pressure-unmatched conditions would cause flatten turbulent structures in subsonic-supersonic mixing layers.(c) 2023 Elsevier Masson SAS. All rights reserved.