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

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.