Flame dynamics and combustion instability induced by flow-flame interactions in a centrally-staged combustor

This study mainly investigates the flame and flow characteristics under combustion instability in a centrally-staged swirl spray combustor, using 5 kHz CH* chemiluminescence (CL) and 15 Hz particle image velocimetry (PIV). The inlet air total temperature and total pressure are up to 600 K and 1 MPa, respectively. A dynamic pressure sensor with an acquisition frequency of 20 kHz is located downstream of the combustor casing. The pressure signal shows large-amplitude limit cycle oscillations when the fuel split ratio of the pilot stage decrease. The phase-averaged CH* CL images indicate that the periodic ignition/extinction of the main stage flame triggers combustion instability. A proper orthogonal decomposition (POD) analysis based on the reacting flow field further explains this phenomenon. The first two POD modes containing three coherent vortex pairs are associated with the extinction and ignition of the main stage flame, respectively. The vortex pairs located at the central shear layer (CSL) generated by Kelvin-Helmholtz (K-H) instability affect the primary recirculation zone (PRZ). The change in the size of the PRZ influences the main stage flame dynamics. In a cycle of oscillation, ignition happens when the size of PRZ increases and the coupling between the main and pilot stages enhances. Extinction takes place when the opposite happens. The flow-flame interaction mechanism demonstrated in this study provides strong experimental support for understanding combustion instability in aero-engine combustors.