Damping or gain? -The critical roles aluminum plays in the SRM stability

The aluminum combustion significantly affects the stability of solid rocket motors (SRM). In this study, the influence of aluminum combustion on the damping of SRM was investigated using the pulse decay method. Initially, the accuracy of the numerical method was validated using a theoretical model, followed by an analysis of the grid and monitoring locations for their independence. Subsequently, the flow field of SRM using propellants with and without aluminum were compared, and the oscillation attenuation coefficients for both cases were obtained. Finally, the damping of the motor system was computed for various aluminum particle sizes and concentrations. It is shown that aluminum combustion increases the temperature of the combustion chamber from 2970 K to 3500 K. This increase results in heightened gas-phase and nozzle damping, leading to faster oscillation attenuation. The aluminum oxide particles formed by aluminum combustion (30 mu m) introduce particle damping, which far exceeds the unstable gain caused by heat release rate (HRR) fluctuations by about 13 times. Therefore, aluminum continues to play a crucial damping role in the instability of SRM. When the frequency is fixed, there exists an optimal particle size that maximizes the damping effect of aluminum, and the particle damping increases as the aluminum content rises. Hence, for a specific motor model, selecting the appropriate diameter and aluminum content of particles can effectively suppress the occurrence of instability phenomena.