Chemical speciation of premixed ethylbenzene flames at the soot onset limit at various (ϕ,Tϕ,T) pairs

This work investigates the similarities/differences of one-dimensional, laminar, atmospheric pressure premixed ethylbenzene flames at their soot onset threshold (ϕcriticalϕcritical). The goals of this investigation are to contrast the entire temperature profiles of the flames at various critical equivalence ratios, ϕcriticalϕcritical, and to report on hydrocarbon specie profiles. Liquid ethylbenzene was pre-vaporized in nitrogen, blended with an oxygen–nitrogen mixture and, upon ignition, premixed flat flames were stabilized over a burner at atmospheric pressure. The C/O ratio was regulated and simultaneously the temperature profile was adjusted by varying the heat lost from the flame to the burner, to obtain visually similar flames, all at their apparent soot onset limit. Temperature profiles of five such flames were determined with thermocouples. The results indicate that as the axial temperature profiles in the flames rose, ϕcriticalϕcritical ((C/O)critical) also increased. Sampling was performed at various heights along the axes of three of these flames (ϕc1=1.68ϕc1=1.68, ϕc2=1.74ϕc2=1.74 and ϕc3=1.83ϕc3=1.83) to monitor changes in chemical speciation. The mole fractions of CO increased in the order of increasing equivalence ratio and hence increasing flame temperature. The CO2 mole fractions of the three flames were similar. However, the mole fractions of light hydrocarbons and polycyclic aromatic hydrocarbons (PAH), which are suspected soot precursors, decreased in the order of increasing equivalence ratio and hence increasing flame temperature. Whereas direct measurements of particulate matter in the flame were not made in this work, these observations on soot precursors along with theoretical flame emissivity considerations suggest that particulate loadings at the sooting limit also decreased in the order of increasing equivalence ratio and hence increasing flame temperature.