Abstract: The effect of main carbon chain length on soot formation and evolution of fatty acid methyl esters (FAMEs) diffusion flames were studied in this work. Laminar coflow diffusion flames of methyl butyrate, methyl octanoate and methyl decanoate were selected as target flames. Laser induced incandescence (LII) and thermophoretic sampling particles diagnostics (TSPD) were adopted as experimental methods to analyze the soot volume fraction and soot particle morphology evolution in tested flames. Transmission electron microscopy (TEM) analysis was applied to obtain morphology information of soot particles sampled by TSPD. The results show that as the length of main carbon chain in FAMEs increases, soot volume fraction along the centreline of laminar diffusion flames increases too. Soot formation is also enhanced with earlier soot inception process and faster surface growth and aggregation. Chemical kinetic simulations of FAMEs decomposition and oxidation are employed to verify and illustrate the experimental results. It can be observed that FAMEs with longer main carbon chain are decomposed at lower temperature and intermediate species such as C2H2 and C2H4 are produced more in the decomposition and oxidation process, which enhances the formation of polycyclic aromatic hydrocarbon. Sensitivity analysis of benzene production suggests that the first aromatic ring in methyl butyrate decomposition mainly forms in C4+C2 pathway. While in methyl octanoate and methyl decanoate decomposition, benzene is produced in both C4+C2 and C3+C3 pathway and C3+C3 pathway takes the lead.

Key words: coflow laminar diffusion flames; laser induced incandescence (LII); thermophoretic sampling particles diagnostics (TSPD); chemical kinetic simulation; fatty acid methyl esters (FAMEs); soot particle; carbon chain length