Measurement of the velocity-coupled admittance in a solid propellant rocket motor

The experiment attempted to measure the velocity-coupled admittance of solid propellant by a direct measurement of oscillating velocities. The velocity-coupled admittance is a critical parameter for combustion stability of solid propellant rocket motors. Since the velocity-coupled admittance in the present research was defined as the complex ratio of the oscillatory mass-flow velocity from the burning surface of a solid propellant to the oscillatory cross-flow velocity above the surface, these oscillatory velocities were the primary quantities to be investigated to determine the velocity- coupled admittance. The oscillatory velocities were measured simultaneously inside the rectangular chamber with an experimental method based on the magnetic flowmeter, a device that can measure directly the velocity of the ionized gas. The oscillatory flows were created by a rotating gear over the sonic nozzle of the chamber. The modulation was generated at both the resonant and nonresonant mode frequencies for comparison of the modulation effects on the velocity coupling. The experiment result showed that the oscillatory velocity profiles had distinctive characteristics for both the resonant and the nonresonant mode modulation. The dependency of the velocity-coupled admittance on the spatial position was observed in the resonant mode modulation.; A numerical analysis was formulated to simulate the flowfield of the oscillating velocities. The governing equations for the oscillatory flowfield were derived from a boundary layer-type equation for a two-dimensional laminar compressible flow with wall transpiration. The numerical results showed that the oscillatory flowfield was dependent on the modulation frequency and that the thickness of the acoustic boundary layer with wall transpiration increased far more than that of the classical Stokes layer. Numerical results were also compared with the experimental data obtained from the actual hot gas tests. The predicted oscillatory velocities in the resonant mode showed a qualitative agreement with the measured data.