Hybrid rocket combustion displays low frequency instability(LFI, 10~30Hz) at a certain condition. Vortex shedding in the post-chamber is suspected to cause the occurrence of Also POD analysis shows that dominant symmetric spatial modes in the stable combustion shift suddenly into asymmetric spatial pattern with the appearance of LFI. Especially, the appearance of mode 3 is a typical change of flow dynamics in unstable combustion representing a rotational fluid motions associated with vortex shedding.

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Visualization of the flow field inside the post chamber of hybrid rocket has been conducted with direct imaging technique. It has been suggested that the flow dynamics inside the post chamber of hybrid rocket strongly affects the occurrence of Low frequency instability (LFI). Conventional observations of pressure and temperature measurements are insufficient to investigate the flow dynamics, thereby the visualization was applied. Direct imaging was used and the simple inexpensive method showed its capability of capturing interesting flow dynamics at reduced accuracy. First of all, in-phase like behavior between the unsteady heat release approximated by luminosity fluctuation and pressure oscillation has been confirmed. The coupling mechanism between local heat release and pressure oscillation of non-acoustic instability would lie in the hydrodynamics. Further analysis was done on visualized flow field with Proper orthogonal decomposition. The difference due to the existence of LFI was repeatedly appeared in mode 3 analysis. As a result, it is suggested that mode 2 and 3 interaction generates a rotating motion. The frequency characteristic of the rotating motion is found to be at a higher frequency than the most dominant frequency in the vicinity of 20 Hz. It is suspected that this rotating motion is closely related to vortex generation and is seemed to share similar characteristics to the Benard-von Karman instability found in bluff-body combustions. Investigations will be extended to the previously found cases of instability occurred experiments to understand the change in flow dynamics due to LFI.