Particulate damping in solid propellant combustion instability
The effect of aluminum powder in damping high-frequency solid-propellant instability has been investigated in a vortex burner developed previously. The combustor was composed of a main chamber having a shallow center-perforated grain and a hot gas generator. The generator fed combustion gases tangentially into the main chamber causing transverse mode combustion instability. Aluminum powder was added either to the main propellant or to the gas generator charge. Measurement of the amplitude of the pressure oscillations indicated the effectiveness of the metal acting 1) as an ingredient at the solid surface and in the gas phase and 2) in the gas phase only. In the absence of aluminum the combustor was unstable, exhibiting an oscillation frequency of ^3800 cps with a peak-to-peak amplitude of 55%. The addition of fine aluminum powder to the propellant in the main chamber was sufficient to damp out the high-frequency instability. Addition of aluminum to the gas generator propellant only was also effective in eliminating instability provided that an equivalent concentration of metal particles was added. It was concluded that the addition of aluminum powder to composite solid propellants suppresses instability by acting as an attenuator of sound in the gas phase rather than altering the driving or response of the propellants. Viscous damping is inferred to be the principal damping mechanism. Addition of the aluminum produced a nonacoustic disturbance which appeared to be eliminated at higher operating pressures.