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chapter 5 turbulent diffusion flames - FedOA

chapter 5 turbulent diffusion flames - FedOA

PAH growth, comprised of

PAH growth, comprised of replicating-type growth of PAH beyond the prescribed size; and (III) spherical particle formation and growth of the resulting particles. In step (III), PAH formed in (II) may coagulate forming larger compounds which can grow by surface reactions. Nucleation describes the growth of planar PAH via the HACA sequence beginning with one-ring species and preceding up to an infinite size using the technique of chemical lumping. The aromatic Ai, containing i fused rings, formed in the nucleation process are then allowed to coagulate, that is, all the Ai collide with each other forming dimers; the dimers in turn, collide with Ai forming trimers or with other dimers forming tetramers; and so on. PAH beginning with the dimers were assumed to be soot particles and it was allowed to add and lose mass by surface reactions. The chemical mechanism adopted for this heterogeneous process is based on the H- abstraction/C2H2-addition reaction sequence and rate constants were estimated based on analogous gas phase reactions of one-ring aromatics: benzene and phenyl. Differently from Frenklach and Wang, the PAH and soot formation in combustion was also described by D’Anna and Violi [17] coupled the HACA pathway with the use of resonantly stabilised free radicals (RSFR mechanism) as described above. D’Anna and Violi [17] obtained good agreement with experimental data in laminar premixed flames of both aliphatic and aromatic fuels. Moreover the kinetic scheme includes growth of species by fuel pyrolysis, particulate formation, growth, aggregation and oxidation. PAH formation includes reaction pathways leading to the formation of nanometric-sized particles and their coagulation to larger soot particles. A discrete-sectional approach [35] is used for the gas-to-particle process; the ensemble of compounds with molecular mass higher than the largest aromatic compound in the gas-phase is divided into classes of different molecular mass and all reactions are treated in the manner of gas phase chemistry using compound properties such as mass and the numbers of carbon and hydrogen atoms averaged within each section. Particle size distributions are obtained by solution of the transport equation for each section. A brief summary of the scheme is given here. 24

Gas Phase The gas-phase formation and growth of aromatic compounds bridges the main oxidation chemistry and particulate formation. The gas-phase kinetic mechanism to model hydrocarbon oxidation and pyrolysis is built onto the GRI mechanism for C1 and C2 species and Miller and Melius suggestions for benzene formation [9]. The formation of naphthalene, the first compound in the PAH series, is modeled through two routes: the first, usually known as the HACA mechanism [34] is also used here to model the formation of multi-ring structures such as phenanthrene, pyrene and higher order PAHs up to coronene. The second is the combination of resonantly stabilized radicals. Two different reaction sequences are included for the formation of naphthalene: the combination of two cyclopentadienyl radicals and the combination of benzyl and propargyl radicals. The cyclopentadienyl and indenyl radical combination is also used for the modeling of phenanthrene. Details of the reaction pathways and the choice of the reaction constant used are reported in previous papers [17] where the complete kinetic scheme for the gas-phase is listed. There are 70 species in the gas phase and about 380 gas-phase reactions. Growth of aromatics and particle inception Acetylene and aromatics, from benzene to coronene, are the building blocks for the growth process which lead to the formation of high-molecular-mass, molecular particles. The mechanism consists of the sequential addition of either acetylene or aromatic molecules to aromatic radicals. Classes of compounds termed sections, each covering a mass range are utilized. Sections are characterized by their average molecular mass and by their number of carbon and hydrogen atoms. The chemical reactions are treated in the same way as for gas phase chemistry. The aromatic growth and oxidation mechanism is schematized as follows: Rx1 Ai => Ri + H Rx2 Ai + H => Ri + H2 Rx3 Ai + OH => Ri + H2O 25

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