Study on the flow characteristics and oscillating mechanism of a swirl flame

•POD was used to identify the pulsation frequency and the coherent structure of oscillation combustion and the results were compared with the experiment.•The PVC generated by swirler was captured and its periodic pulsation characteristics were showed.•The ISL and the OSL that appeared with the inner and outer recirculation zones were the locations at which the pulsation velocity was the strongest.•The results of Ruili integration combined with the pulsation characteristics of the CRZ were used to reveal the swirling oscillation combustion mechanism. The larger eddy simulation (LES) method was used to simulate the combustion oscillation using a partially premixed combustion model and coupling with a user-defined function. The simulated results were post-processed by proper orthogonal decomposition (POD) and compared with the experimental results. The results showed that the asymmetry of the upper and lower vortex structures in the inner recirculation zone represented the destruction of the vortex structures and reflected the occurrence of a processing vortex core (PVC). The PVC structure produced by the swirler was more complex than a cyclone. The inner shear layer (ISL) and the outer shear layer (OSL) appeared in the inner and outer recirculation zones respectively, which represented the region with the strongest velocity pulsation; the velocity pulsation of axial centerline could be used to characterize the pulsation energy when the combustion oscillation occurred. First, necking occurred in the inner recirculation zone, and then it adhered to the center of the swirler outlet. The ISL and OSL had strong shearing actions that generating pulsating energy, then they gradually developed downstream and merged into the inner recirculation zone. The different rotation directions of the inner and outer recirculation zones increased the instability of the boundary of the inner recirculation zone. It could also increase the breakdown of the asymmetric vortex structures and exacerbated the occurrence of the PVC. In addition, the phase difference between the heat release rate and pressure pulsation in different regions was different, which leaded to inconsistent driving characteristics of the flame oscillation.