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chapter 5 turbulent diffusion flames - FedOA

chapter 5 turbulent diffusion flames - FedOA

Under these fundamental

Under these fundamental steps variables such as: temperature, pressure and fuel composition have non negligible effects on both qualitative and quantitative aspects in terms of combustion generated aerosols. 12

1.1 POLYCYCLIC AROMATIC HYDROCARBONS FORMATION IN COMBUSTION The key to understanding the process of PAH generation involves as start point the formation of the first aromatic ring. Significant advances have been made on this fundamental step in our knowledge of both the reactions sequences and the species involved. Similar to the general scheme suggested by Homann and Wagner [2] and Crittenden and Long [3], Bockhorn et al. [4] in the 1983, suggested a reaction sequence beginning with diacetylene and C2H forming a branched hydrocarbon radical followed by acetylene attack and ring closure leading to a phenylacetylene radical. This mechanism was supported by experimental results obtained studying flat premixed low pressure flames of propane, acetylene or benzene. Some years later Frenklach and Warnatz [5] made a decisive step towards a quantitative understanding of benzene formation by comparing concentration profiles predicted with a detailed kinetic model against experimental profiles of Bockhorn et al. [4] measured in the low pressure sooting acetylene flame above mentioned. The determination of net rates allowed assessment of the contribution of different benzene formation pathways included in the model. Conclusions from net rate analysis are of course dependent on the kinetics parameters used, and subsequent revisions of poorly known rate constants may have a significant impact on the computed contributions of the affected reaction pathways. Frenklach and Warnatz included four pathways leading to the first aromatic ring, all of them based on the cyclization of unsaturated aliphatic radicals: n-C6H5 → phenyl; i-C8H5 → C6H4C2H; n-C8H5 → C6H4C2H; n-C6H7 → benzene + H. Similar conclusions were reached by Colket [6] studying the pyrolysis of acetylene and vinylacetylene in a shock tube. A systematic screening of benzene formation pathways followed by the determination of the corresponding rate constants was performed by Westmoreland et al. [7] based on the concept of 13

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