Equivalence ratio variation and combustion instability in hybrid rocket

In this paper, a series of combustion tests using PMMA/O2 was conducted to examine the initiating characteristics of low frequency instability (LFI) in relation to the changes in both flow and combustion conditions. By selecting parameters such as the oxidizer mass flow rate, fuel grain length, and mixing ratio of mixture oxidizer composed of oxygen and nitrogen; combustion tests were conducted to vary the equivalence ratio in the combustion. This study further validated instability characteristics in connection with coupling behavior between the high frequency pressure and heat release oscillations in 500 Hz band. Based on the results, a combustion stability map was plotted and it shows that the initiation of LFI is contingent upon the satisfaction of specific conditions of flow and combustion. The results also confirmed that the initiation of LFI is highly linked to an establishment of a positive coupling between high frequency pressure and heat release oscillations. In addition, equivalence ratio was found to be a critical factor in the transition of coupling status between the two high frequency oscillations. For a given flow condition, the sole increase in equivalence ratio above a certain threshold resulted in the phase shift between the two oscillations leading to a positive coupling or vice versa.