Low Frequency Instability in Labscale Hybrid Rocket Motors

Abstract

Hybrid rocket combustions frequently displayed a sudden amplification of combustion pressures developing into low frequency instability (LFI) with peak frequency of 10~20Hz. A series of experimental test was aimed to study the initiation mechanism of LFI, which was suddenly amplified at a certain condition during the combustion. To this end, a couple of parameters was selected and the sensitivity of each parameter to instability was evaluated including volume ratio between main and post chamber, oxidizer mass flow rate, and solid fuel type. Results showed that the initiation of LFI was primarily related with the flow modifications caused by volume ratio between main and post chamber. Once LFI was initiated at a certain chamber configuration, however, the variation of oxidizer mass flow rate and solid fuel did not alter the triggering mechanism of LFI. Additional study was also made to understand the critical role of vortex shedding on the initiation of LFI in the post chamber. And it confirmed that pressure oscillations by thermal lag of solid fuel would be suddenly amplified leading to LFI in case of resonating with unknown source of pressure oscillations associated with vortex shedding in the post chamber. However, details of triggering mechanism and coupling of vortex shedding with additional pressure perturbations were still remained unresolved.