Aluminum Combustion Driven Instabilities in Solid Rocket Motors

Abstract

This work describes an unreported instability found out by numerical simulations on a solid rocket motor. A simple motor with a cylindrical port is considered and predicted to be stable by single-phase computational fluid dynamics computations. When aluminum combustion is modeled, the motor experiences a strong instability on the first longitudinal acoustic mode. However, no vortex shedding is observed, meaning that it is a genuine combustion instability. A detailed numerical study leads to the conclusion that the instability is thermoacoustic and results from a coupling between chamber acoustics and aluminum combustion heat release. A parametric study points out the importance of aluminum distributed combustion, particularly the thickness of the combustion zone and the aluminum heat of reaction. Theoretical assessment of this instability is also obtained by revisiting the acoustic balance theory. An additional thermoacoustic stability integral is derived and appears to be a driving term. This term also helps to shed light on this instability and explains some of the computational fluid dynamics results. In particular, the underlying mechanism is found to be primarily caused by the acoustic boundary layer which creates high acoustic velocities that enhance heat release from burning aluminum particles.