Analysis on acoustic longitudinal oscillation frequency in combustion chamber with sudden change sections

Combustion chamber with case bonded propellant, often used in solid rocket motors, can be simplified into an ″H″ type sudden change section configuration with tube grain in the middle and cavities connected at both head end and aft end. In the paper, a physical model of ″H″ acoustic cavities was established; then the longitudinal frequency was calculated by theoretic and finite element model (FEM) methods. Symmetrical and unsymmetrical ″H″ acoustic cavities were used separately to investigate the influences of geometrical parameters on the oscillation frequency. The results indicate that the ratio of the length of grain to combustion chamber, the ratio of inner-outer grain radius and grain position have cooperative effects on the frequency. Considering the reasonable volumetric loading fraction of the grain, it is concluded that oscillation frequency, which is smaller than that in cylindrical cavity at beginning, increases continuously with burning surface regression.