Fuel stratification effects on thermoacoustic instability for swirl spray flame in an aero-engine centrally-staged combustor

The centrally-staged combustor features a flexible radial fuel stratification to achieve well-organized energy utilization but still faces serious thermoacoustic instability. In this study, we conducted experiments with four stratification ratios (SRs) under the realistic stratified swirl spray flame configuration. The stratification effect on thermoacoustic instability is investigated using the 20 kHz simultaneous PIV and CH₂O-PLIF optical diagnostic. The combustor is prone to severe thermoacoustic oscillation at lower SR, in which the pressure fluctuation substantially follows the heat release dynamic with a short time delay. At higher SR, the relative phase between them becomes irregular accompanied by intermittent oscillations. Two types of flame dynamics are found. The heat release region features an axial mode for lower SR, while it presents the transverse oscillation which destroys the thermoacoustic coupling for higher SR. Further, a coupling metric quantifying the similarity between the vortex probability distribution and flame surface density is proposed to measure the vortex-flame coupling. The highest value for the lowest SR is attributed to the strong flame-flow interaction downstream of the primary recirculation zone, which contributes to the unsteady heat release. Finally, it is found that the local flame stabilization is similar to the counterflow flame.