On the Dynamics of Instability Mitigation by Actuating Swirler Motion in a Lean Premixed Turbulent Combustor

In this paper, we present a novel initial attempt on analysis of the mitigation mechanism of instability by rotating the otherwise static swirler in a lean premixed, swirl stabilized, labscale combustor. It has been reported in our previous work that increasing the swirler rotation rate mitigates the self-excited thermoacoustic instability in a model lab-scale combustor, over a range of conditions. Here, it is found that for a given period of observation, instead of a continuous and gradual decrease in the time localized pressure amplitude from the fully unstable state towards the fully mitigated state, the fraction of the time during which instability is present is reduced. With increasing swirler rotation rates, the instability becomes more intermittent with progressive reduction in frequency of their occurrence. High speed PIV results are also presented along with simultaneous pressure signals which support this claim. Such an intermittent route to instability mitigation could be attributed to the background turbulent flow field and is reminiscent of the intermittent opposite transition (implemented by changing the Reynolds number) from a fully chaotic state to a fully unstable state as recently discovered in Nair, Thampi and Sujith [1]. An attempt is made to model the behavior of pressure oscillations using the well established mean-field Kuramoto model. The variation of the order parameter r, which is the parameter for the measurement of synchronization between the oscillators provides critical insights on the transition from the unstable, intermittent to stable states.