Acoustic Analysis of Hybrid Rocket Combustion Chambers

A method based on passive linear stability measurement has been developed to investigate combustion dynamics in hybrid rocket motors. A one-dimensional model is used to describe the acoustic motions inside the rocket chamber as a function of the motor axial coordinate. The equations of mass, momentum, and energy are solved for the average flow. A linearized analysis, which takes combustion response functions into account, provides pressure and velocity acoustic oscillations. When the boundary conditions at the nozzle inlet are enforced and the response functions are known, the acoustic modes and growth constant can be derived, whereas the coupled responses can be determined from the experimental measurement of the mode frequencies. If noise sources in the control volume are accounted for, then it is theoretically possible to determine the response functions at all frequencies simply by measuring the pressure fluctuations at the head end of the motor. Experimental data are analyzed and contrasted to the proposed one-dimensional model, showing a good agreement.