Combustion Instability Involving Vortex Shedding in Hybrid Rocket Motors

Hybrid rocket motors usually have an aft-mixing chamber in order to improve combustion efficiency. The presence of a sudden expansion at the exit of the fuel port determines the formation of vortices, whose vigourous burning may drive acoustic waves in the chamber. The shedding of vortices itself is then affected by the flow fluctuations, producing a well-known feedback loop. A reduced-order model, introduced by Matveev and Culick in 2003, is here used to analyze this phenomenon. It is assumed that vortex burning is localized in space and time, and a kicked oscillator model is utilized. A one-dimensional model proposed by the authors is used to determine the values of the eingenacoustic modes and corresponding damping coefficients. Numerical results are compared to experimental data.