The present work deals with time-resolved investigation of the flow field during acoustic self-excitation by a lean premixed flame in a dump combustor with varying equivalence ratio at a constant air flow rate. Simultaneous measurements of pressure fluctuations, velocity fields using Time resolved Particle imaging velocimetry (TR-PIV) and CH* chemiluminescence were performed. The pressure, velocity and chemiluminescent intensity time traces were Fourier transformed to estimate the frequency and amplitudes. Conditions of maximum pressure amplitude correspond to the prevalence of intermittent bursts in pressure, velocity, and chemiluminescent intensity. Further, Proper orthogonal decomposition (POD) is applied to the chemiluminescent intensity and velocity fields. The POD mode shapes are able to capture the modes pertaining to both the acoustic and vortex mode of flame/flow oscillations. The burst oscillations are understood by examining the sequence of time-resolved velocity and chemiluminescent intensity during their growth and decay regimes. The growth of oscillations is promoted by the flame heat release fluctuations following the pattern of the large-scale vortex roll-up in the recirculation zone downstream of the dump plane, causing a tendency of acoustic excitation at the vortex mode. As the amplitude rises, the natural acoustic mode of the duct is simultaneously amplified, leading to small-scale vortices shed from the step corner at the acoustic time scale. These small-scale vortices adversely interact with the large-scale vortex controlling the heat release, resulting in its weakening and hence the decay of oscillations. This behavior was further observed in the spatially averaged vorticity along the shear layer. In addition to this, the time traces of the pressure and the velocity fluctuations at the shear layer and located half step height from the separation point were overlapped. The overlapped time traces showed a drift in the instantaneous phase during which the growth and decay of the oscillations were observed.

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