Combustion Instability and LES Analysis in Hybrid Rocket Motor

Combustion tests were conducted in a hybrid rocket motor with a PMMA/CrOX combination to study the possible interaction between acoustic perturbations due to a vortex shedding and an unsteady heat release on the non-linear combustion. Pressure measurement data showed that a sudden pressure drop of about 10 psi occurred during the nominal combustion only when disk 4 was used. This peculiar discontinuity in pressure signal is thought to be described by a negative DC-shift and it is conjectured that the perturbation generated by disk 4 located in the pre-chamber may be out of phase with the that of the shedding vortex produced in the post chamber. In order to understand the dynamic role of vortex shedding process, a numerical study with LES technique was performed in a duct flow with wall blowing. The turbulent structures were investigated in the context of the formation of roller-like vortices, kinematic modification of near-wall coherent structures, and the existence of periodic oscillatory flow having an instantaneous large negative velocity. Even though it is expected that the excursion to negative velocity may significantly influence on the temperature fluctuation near the wall surface, its mechanism is not fully understood yet and it should be further pursued in the future study.