High-speed characterization of spray ignition and combustion-coupled dynamics via PLIF, X-ray PCI, and broadband imaging

This study investigates spark ignition dynamics in a liquid-fueled model combustor under ambient inlet conditions. Two hollow-cone nozzles with different inlet geometries are compared using simultaneous high-speed X-ray phase-contrast imaging (PCI) and broadband imaging to capture spray and flame behavior before and after stabilization. The spray dispersion and hollow-cone lobe structures are quantified via fuel planar laser-induced fluorescence (PLIF). Lagrangian particle tracking is performed to extract droplet size distributions, velocities, and evaporation rates, providing a micro-scale description of the spray-combustion dynamics near the nozzle. While reduced evaporation due to droplet-droplet interactions has been evidenced in simplified arrays and computational models, direct experimental quantification within sprays remains limited. The results demonstrate that dense-spray shielding significantly suppresses bulk evaporation, with combusting spray effective evaporation rates decreasing by ≈ 30 % ( 0.45 to 0.30 -- 0.32 mm 2 /s ) across the combined operational envelope of both evaluated nozzles. Theoretical models of the spray geometry further indicate that droplet-droplet interactions reduce the effective evaporation rate by at least 45% relative to isolated droplets. However, across this same combined operational envelope, the empirical ignition probability increased from 0.01 to 0.37 suggesting that successful ignition is primarily governed by localized vaporization in the immediate vicinity of the spark. Furthermore, combustion-induced thermal expansion is found to accelerate droplets and alter their trajectories, whereas the early-stage flame kernel trajectory remains essentially identical regardless of ignition success.