Two-stage ignition dynamics of n -dodecane sprays under varying ambient oxygen concentrations: High-speed planar laser-induced fluorescence diagnostics and large-eddy simulations

Two-stage ignition of diesel sprays plays an important role in flame stabilization, combustion efficiency, and pollutant formation. In this study, high-speed planar laser-induced fluorescence (PLIF) diagnostics were combined with large-eddy simulations (LES) to examine the effect of ambient oxygen concentration on the auto-ignition of n-dodecane sprays in a constant-volume combustion chamber. The PLIF results show that oxygen enrichment promotes low-temperature ignition. As the ambient oxygen concentration increases from 13% to 21%, CH2O formation occurs earlier, and its onset location shifts upstream by approximately 4 mm toward the nozzle exit. Oxygen enrichment is also associated with an earlier evolution toward high-temperature ignition. This trend is reflected in the rapid consumption of CH2O and the appearance of strong polycyclic aromatic hydrocarbon-related signals at the spray front. The LES results further show that elevated oxygen concentration accelerates H2O2 formation and promotes its accumulation in fuel-rich mixtures. Meanwhile, higher oxygen availability increases the peak flame temperature and broadens the CO-production region, which allows the reaction front to penetrate further into the fuel core. LES-based kinetic analysis shows that fuel conversion is dominated by low-temperature chemistry during the early injection phase. As ignition progresses, increasing oxygen concentration gradually shifts the dominant pathways toward high-temperature thermal decomposition of n-dodecane.