Transition of combustion instability in hybrid rocket by swirl injection

A series of experimental tests was conducted to investigate the effect of swirl injection on the combustion instability in hybrid rocket using combustion visualization technique. As a result, swirl injection seems to modify two main physical processes responsible for the initiation of LFI. First one is the transition of coupling status between two fluctuations of combustion pressure (p') and heat release (q') of 500 Hz band. The transition of p' and q' to negative coupling seems to be a crucial modification for stabilizing the combustion. Another noticeable effect of swirl injection is the gradual increase in the peak frequency of pressure oscillation from its initial value of around 20 Hz–50 Hz band, which is caused by the thermal lag, as the swirl intensity increases. Also, POD (Proper Orthogonal Decomposition) analysis was done to understand flow response to the swirl injection near the fuel surface. The analysis of temporal behavior of mode 1 and 2 suggested that the swirl injection with proper intensity not only suppress the generation of p' of 500 Hz band but induces a shift of low frequency peaks of 20 Hz band by adjusting the turbulent structures in boundary layer. And the shift of low frequency peaks seems to be another contributing factor for combustion stabilization.