Transient dynamics of the precessing vortex core in an intermittently shape-transitioning swirl flame

Abstract

The transient dynamics of the precessing vortex core (PVC), a globally unstable mode, in an intermittently shape-transitioning swirl flame is investigated by means of simultaneous OH planar laser induced fluorescence (PLIF) and stereoscopic particle image velocimetry (S-PIV) at a repetition rate of 10 kHz. A time-frequency analysis finds that the PVC is present in steadily lifted flames and at non-reacting conditions, whereas it is suppressed when the flame is steadily attached. While the flame shape spontaneously transitions between these two states, the onset of the growth/decay of the PVC amplitude occurs prior to the flame lift-off/reattachment. The transient formation of the PVC before flame lift-off is quantitatively elucidated by a transient linear stability analysis (TLSA), which well predicts the instability frequency and absolute growth rate. The TLSA reveals that the onset of the flow transition from convective to absolute instabilities, corresponding to the growth of the PVC, is attributed to continuous suppression of the convective perturbation and viscous diffusion, and afterwards the absolute instability is sustained only due to the weakened convection. The suppression of the convection and viscous diffusion terms is mainly due to the changes in the transient mean velocity and density fields within the inner shear layer, which are likely triggered in the first place by unsteady heat loss of the flame to the dump plane of the combustor.