PAH formation behavior of laminar counterflow nonpremixed flames have been investigated experimentally under various conditions. Experimental conditions such as stoichiometric mixture fraction (Z_st), molar stoichiometry (Fuel/O₂/N₂), and the bulk velocity gradient are widely modulated. Regarding the effect of Z_st, it is confirmed that PAH formation is basically suppressed with increasing Z_st. However, quantitative dependence of formation of each compound on Z_st variation is different. This may due to the difference of dominative reaction pathway. Concerning the effect of variation of the molar stoichiometry, it is found that formation of PAH is suppressed under higher dilution levels in the molar stoichiometry. With regard to the effect of variation of the bulk velocity gradient, enhanced PAH formation occurs under higher velocity gradient conditions. This trend is suitably explained based on the laminar diffusion flamelet. In addition, for C₃H₈ flames, it is found that formation behavior of naphthalene and acenaphthylene, which are regarded as key compounds for general formation mechanism of PAH and soot, are different against variation of the flame structure. From those results, the preferable condition of diffusion flame for higher efficiency of fuel consumption with lower PAH formation is considered based on the laminar diffusion flamelet concept.

It has been found in previous studies that a fuel jet in a diffusion flame branches in Y shape under the influence of sound waves propagating transversely across the jet. To explore this effect, the authors have investigated the behavior of a methane jet in a diffusion flame under the effect of acoustic excitation. Shadowgraphy is adopted for the visualization. It is found that the jet injected from a nozzle meanders before it diverges into two branches. The estimated meandering frequencies of the jet correspond to those of sound waves. The behavior of the jet depends on jet velocity, sound frequency, and diameter of the nozzle. For the injecting nozzle of 3 mm in diameter, the greatest influence appears at 10 m/s in velocity and 1500 Hz in frequency. The decrease in velocity and the increase in frequency cause the decrease in the branch angle of the jet. Distinguishable influence is observed under the condition of Re (Reynolds number) ≤2300 and St (Strouhal number) ≤0.9. The range in St increases as the diameter of the nozzle increases, whereas the range in Re changes insignificantly.