Soot and PAH Formation Characteristics of Methanol-Gasoline Belnds in Laminar Coflow Diffusion Flames

Particulate matter emissions are becoming a big issue for GDI engines as the emission regulations being more stringent. Methanol has been considered to be an important alternative fuel to reduce soot emissions. To understand the effect of methanol addition on soot and polycyclic aromatic hydrocarbons (PAHs) formation, the 2-D distributions of soot volume fraction and different size PAHs relative concentrations in methanol/gasoline laminar diffusion flames were measured by TC-LII and PLIF techniques. The effect of methanol was investigated under the conditions of the same carbon flow and the same flame height. The methanol volume fraction was set as M0/20/40/60/80. The results showed that the natural luminescent flame lift-off height and soot lift-off height increases consistently with the increasing methanol content due to the increase of outlet velocity of fuel vapor. Methanol addition is able to inhibit the soot and PAHs formations significantly, which may be largely due to the dilution of aromatics and methanol molecular structure. The effect of methanol on reducing soot is weakened with the increasing methanol ratio. Under the same flame height condition, the peak soot volume fraction in the M20, M40, M60, and M80 flames reduces by 35.6%, 58.7%, 74.9% and 88.5%. The PAHs concentration of four different scales decrease with the increasing methanol content, and the largest aromatic ring (450 nm) decrease the most. As the number of rings of aromatics increases, it peaks at a higher height, and its highly concentrated area gradually moves from the center of the flame to the two wings of the flame. The initial height of soot formation increases with the increasing methanol ratio, which increases more significantly under the same flame height condition than that under the same carbon flow condition.