Low Frequency Instability in Hybrid Rocket Combustion

Hybrid rocket combustion frequently displays a sudden amplification of combustion pressures leading into low frequency instability (LFI) with peak frequency of 10~20Hz. A series of experimental tests was designed to examine the triggering mechanism of LFI. To this end, a couple of parameters was selected and the sensitivity of each parameter to instability was evaluated including volume ratios between main and post chambers, oxidizer mass flow rates, and solid fuel types. The results showed that the initiation of LFI was related with the flow modifications caused by vortex shedding and volume ratios between main and post chambers. Additional analysis was done to understand the critical role of vortex shedding on the initiation of LFI in the post chamber. Also, the CH* luminosity images and POD (Proper Orthogonal Decomposition) analysis show the lumped flow structures with time-varying intensity in unstable combustion. A periodic change in the local heat release appears to generate large-scale flow rotations, and the large-scale flow rotations are likely to be associated with the periodic flame extinction at the frequency of about 20 Hz. Also, the flame extinction seems to take a crucial role in the occurrence of LFI