Design and optimization of cavity implement in a solid rocket ramjet under different flight conditions

Alterations in flight conditions modify flow characteristics in a solid rocket ramjet (SRRJ) engine, leading to variations in cavity effectiveness. To develop a cavity design with the potential for robust performance across a wide-range flight condition, numerical simulations are performed at Mach 3 and 6 to investigate the influence mechanism of the cavity on gas-solid two-phase mixing and combustion. A large-scale cavity design enhancing the performance of the SRRJ engine under different flight conditions is identified through structural optimization, including position, quantity, scale, length-to-depth ratio, and concentration ratio. The results indicate: (1) Position and length-to-depth ratio of the cavity exhibit a more pronounced impact on engine performance at Mach 6. The concentration ratio of the large-scale cavity dominates at Mach 3. Forward migration of the small-scale cavity leads to performance reduction, which the increase in cavity quantity mitigates. The large-scale cavity exhibits superior gas-solid two-phase mixing and combustion organization performance, proving greater suitability for wide-range operation. (2) Decreasing the length-to-depth ratio or increasing the concentration ratio enhances particle-oxygen mixing and combustion, but cavity depth should be constrained in an optimal range to prevent reductions in oxygen utilization and combustion intensity. (3) Through structural optimization of the cavity, the SRRJ engine achieves performance improvements up to 12% and 7.4% at Mach 3 and Mach 6, respectively, compared to the benchmark.