I-13, 1 Modeling of Size Distribution Functions and Chemical Structure of Combustion-Formed Particles M. Sirignano 1 , J. Kent 2 , A. D'Anna 1 1. Dipartimento di Ingegneria Chimica, Università di Napoli “Federico II”, Napoli, Italy 2. School of Aerospace, Mechanical & Mechatronic Engineering, University of Sydney, Sydney, Australia 1. Introduction The formation of soot in combustion is a complex process involving gas-phase chemical kinetics, heterogeneous reactions on the particle surface and particle dynamics. Modeling of these processes in combustion environments has received great attention in recent years [1-5]. Models, today, are able to approximately simulate the concentration and size distributions of particles in many combustion systems including premixed and diffusion flames [6-13]. In recent years the use of new and sophisticated diagnostics has increased our knowledge of particle inception, growth and coagulation. Polycyclic aromatic hydrocarbons (PAHs), formed close to the flame front and grow in the post-flame regions, reach molecular masses of 1000-3000u typical of incipient particles of few nanometers. Thereafter they grow and coagulate determining the size distribution functions of the particles. These results have stimulated modeling activity in the area of particle inception. Models hypothesize that PAHs of some sizes begin to stick to each other during collision, thus forming PAH dimmers, trimers and so on. Pyrene was considered the first aromatic species in the PAH series able to form stable dimers at flame temperatures and the dimers were considered as the first soot nuclei. Surface addition mainly by acetylene was responsible for soot loading [2,5]. Models which combine dimerization of PAHs with the formation of ring-ring aromatic species through a purely-chemical mechanism have recently been proposed [8-12]. The growing molecular species acquire the properties of a condensed phase. Surface reactions with gas-phase species, mainly acetylene and PAHs, and coagulation of the particles determine their final concentration and size distribution. Surface growth is based on chemical analogy of gas-phase aromatic chemistry. In the discrete-sectional method, used here, the ensemble of aromatic compounds with molecular mass higher than the largest aromatic compounds in the gas-phase is divided into classes of different molecular mass and all reactions are treated in the form of gas-phase chemistry using compound properties such as mass, numbers of carbon and hydrogen atoms averaged within each section. The molecular mass distribution of the species is obtained from the calculation and is not hypothesized a priori. Particle evolution is followed by combining the laws of reacting flows with the population balance for suspended particles. Next generation models need to predict not only particle concentrations and size distribution functions but also their morphology and chemistry in order to better predict particle radiative properties and health effects. The present paper presents a first approach to follow chemical evolution of the particles formed in premixed flames by simulating both molecular growth and H/C variation of the particles along the flame axis. A previously developed detailed kinetic model of particle formation has been extended to include a double discretization of the particle phase: in addition to the division into sections for the carbon atoms, the scheme also provides different values of H/C for each carbon number. The kinetic mechanism has been tested to simulate a