Measurements of pressure effects on PAH distribution and 2D soot volume fraction diagnostics in a laminar non-premixed coflow flame

The soot formation process has been investigated at pressures up to 16 bar using a non-premixed laminar coflow flame with nitrogen-diluted ethylene. 2D diffuse line-of-sight attenuation (2D LOSA) and planar laser-induced incandescence (PLII) were used to measure soot volume fraction (SVF). The peak SVF increased exponentially with increasing pressure and the spatial distribution of soot volume fraction changed substantially. At pressures below 6 bar, the two techniques agreed well. At pressures above 6 bar, the techniques began to disagree, with 2D LOSA showing higher peak SVF values at a location lower in the wings of the flame compared to PLII. Errors in the LOSA measurements due to the molecular absorption of PAHs were assessed by performing measurements with bandpass filters centered at 435 nm and at 647 nm. Furthermore, the evolution of polycyclic aromatic hydrocarbons (PAH) in the flame was studied using planar laser-induced fluorescence (PLIF) with the excitation laser set at 282.85 nm and compared to LOSA measurements. Fluorescence signals were captured using bandpass filters (350 nm, 400 nm, 450 nm, and 510 nm) corresponding to increasing PAH size. The peak concentration of PAHs moved closer to the burner nozzle as pressure increased. Absorption by PAH were unable to explain discrepancies between LOSA measurements and PLII measurements. Using the Rayleigh-Debye-Gans approximation for polydisperse fractal aggregates (RDG-PFA), the differences between LOSA and PLII measurements were analyzed, and it was found that LOSA is more sensitive to the soot primary particle diameter due to changes in the scattering to absorption ratio (ρ_sa). The effect of gate duration on SVF imaging with PLII is also reported.