Effect of aromatic fuels and premixing on aromatic species and soot distributions in laminar, co-flow flames at atmospheric pressure

The aim of the present study is to investigate the effects of aromatic fuel structure on the aromatic species and the soot volume fraction in laminar, co-flow flames at atmospheric pressure. Both non-premixed and partially-premixed flames (jet equivalence ratios of 24 and 6) are studied. The four fuels consist of binary mixtures of n-dodecane with one of four aromatic species: n-propylbenzene, toluene, m-xylene, and 1,3,5-trimethylbenzene. For all flame conditions, the total carbon flow rate of the fuel mixture is kept constant. Furthermore, in each binary mixture, the carbon mole fraction from the n-dodecane and the aromatic component is kept constant. Laser-induced incandescence (LII) and laser extinction are used to obtain two-dimensional soot volume fraction in the flames. Laser-induced fluorescence (LIF) is used to obtain the two-dimensional aromatic species distribution in the flames. The experimental results indicate that the 1,3,5-trimethylbenzene/n-dodecane flame has the highest peak soot volume fraction amongst the four fuels; the other three fuel mixtures, within measurement uncertainty, have similar peak soot volume fraction. In addition, premixing changes the spatial distribution of polycyclic aromatic hydrocarbons and soot in the flames. In the non-premixed cases, the peak soot volume fraction is located in an annular region near the flame sheet, whereas the soot field is more uniform across the cross-section of the flame at a jet equivalence ratio of 6. A published aromatic fuel chemical mechanism is used to understand the differences in the pathways to soot precursors among the aromatic fuels. The combined 2D LIF and LII result provide a unique dataset to validate soot and chemical numerical models for the four aromatic fuels: n-propylbenzene, toluene, m-xylene, and 1,3,5-trimethylbenzene.