Effect of main stage swirl intensity on spray fields in a centrally staged spray combustor

To satisfy low-emission requirements for aero-engines, the Lean Premixed Pre-vaporized (LPP) centrally staged combustor is widely adopted. However, the coupling between the main stage air and the pilot spray remains unclear. This paper investigates the influence of main stage swirl intensity (S_m = 0.5, 0.7, 0.9) on the pilot spray structure using Planar Mie Scattering (P-Mie). Results reveal a nonlinear regulatory effect of swirl intensity on spray morphology and mixing. As S_m increases from 0.5 to 0.9, the Primary Recirculation Zone (PRZ) strengthens. The differential enhancement between the radial pressure gradient and centrifugal forces accounts for the increased radial spreading angle and decreased spray cone angle. Notably, at S_m = 0.7, a stable high-concentration core persists downstream,resulting in a high Spatial Mixing Inhomogeneity (SMI) value. Dynamic analysis reveals that the spray field is dominated by the Precessing Vortex Core (PVC), characterized by spatially localized high-order modes. High swirl intensity is detrimental to maintaining stability along the flow path. A unique mode coupling appears at S_m = 0.7: the axial pulsation and PVC helical modes maintain a double-peak distribution with consistent frequencies. This coupling results in the highest coherent structure energy and slowest downstream decay, demonstrating superior structural robustness. The findings clarify how the main stage swirl number regulates PVC mode coupling to alter fuel mixing and stability, providing key insights for the design of centrally staged combustors.