Analysis of the effects of expansion ratio on the structure and infrared radiation characteristics of engine under-expanded plume under different altitudes

The exhaust plume ejected from a rocket engine exhibits high temperature and pressure, leading to intense infrared radiation signals. Variations in expansion ratio and ambient pressure influence the plume’s flow field structure, which in turn affects these signals. This study investigates the effects of the expansion ratio, which ranges from under-expanded (2.78) to the optimal state (5.65), and the flight altitude, varying from 0 km to 30 km, on the flow field and infrared radiation (IR) characteristics of the exhaust plume. The exhaust plume flow field is numerically solved using the Computational Fluid Dynamics (CFD) method, while its radiative transfer is modeled using the Backward Monte Carlo method. Additionally, the gas radiative properties in the radiative transfer process are computed using the line-by-line method. The simulation results show that plume diffusion increases significantly with altitude, but it is less sensitive to changes in expansion ratios at a given altitude. At the lower altitudes, such as 0 km and 5 km, the difference in total radiation intensity due to varying expansion ratios is minimal. However, as altitude increases, the disparity in total radiation intensity among different expansion ratios grows noticeably. At an altitude of 30 km, the radiation increment caused by the expansion ratio exceeds threefold compared to that at 0 km. This is attributed to the smaller expansion ratio intensifying the afterburning reactions and promoting the formation of strongly radiative species, such as H₂O and CO₂.