Low frequency instability in laboratory-scale hybrid rocket motors

Hybrid rocket combustion frequently displays a sudden amplification of combustion pressures leading into low frequency instability (LFI) with peak frequency of 10–20 Hz. A series of experimental tests was designed to examine the triggering mechanism of LFI, which occurred at a certain combustion condition. 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 to the flow modifications caused by vortex shedding and volume ratios between main and post chambers. Once LFI was initiated at a certain chamber configuration, the variation of oxidizer mass flow rates and the use of different solid fuel did not alter the triggering mechanism of LFI. Additional attention was focused to understand the critical role of vortex shedding on the initiation of LFI in the post chamber. The results confirmed that pressure oscillations by the thermal lag of solid fuel could be suddenly amplified, which leads to LFI in the case of resonating with unknown sources of pressure oscillations associated with vortex shedding in the post chamber. However, the details of triggering mechanism and the coupling of vortex shedding with additional pressure perturbations still remain unresolved.