A Two-Equation Soot Model Applied to the Oxidation of Aliphatic and Aromatic Fuels in Shock Tubes J. Marquetand *,1 , U. Riedel 2 1 Interdisciplinary Center for Scientific Computing (IWR), Heidelberg University, Germany 2 Institute of Combustion Technology, German Aerospace Center (DLR), Stuttgart, Germany Abstract The formation of soot particles is described by a two-equation model based on previous work [1]. The two equations model the change rate of the soot concentration and the volume fraction. Both are solved fully coupled to the rate equations of the gas-phase species. In addition to nucleation from propargyl - already considered in [1] - phenyl is contributing to the formation of soot particles. The model is tested for soot formation under shock-tube conditions with aliphatic and aromatic fuels like n-heptane and toluene. The modified description of the nucleation process improves the model prediction for the formation of soot from aromatic fuels. Introduction The reduction of the soot particle emission is the most important aim in current soot formation research as soot is known to cause environmental and health problems, and it also can cause material damages in gas turbines, for example. On the other hand, soot plays an important role in the heat transfer of burners and furnaces. There are numerous interacting reactions and pro- cesses happening while a particle is formed from species in the gas phase and starts to grow. At the onset, there is the formation of the first aromatic ring from species in the gas phase, such as acetylene and propargyl [2] which result from the decomposition of the fuel molecules. This ring formation is regarded as one rate-limiting step in the evolution of soot particles [3]. The first aromatic ring gives rise to the build-up of polyaromatic hydrocarbons (PAHs): The number of rings increases by reactions with gas-phase species, mainly the ones mentioned above [2]. Further growth of the PAHs via joining and ongoing formation of aromatic rings fi- nally leads to the nucleation of so-called primary soot particles [2]. The size of these particles increases when species from the gas phase, mainly acetylene [4], react with the surface. Another effect leading to larger soot particles is the coagulation of two particles [2]. This does not affect the total volume of the soot particles at all, but decreases the number of particles in the system, thus reduces the soot concentration. Instead of increasing the volume of the particles oxidation decreases the size of the particles or even completely oxidizes them. This process is assumed to strongly depend on OH - . Implementing a detailed description of all these soot formation processes in models of multidimensional com- bustion systems is still prohibitive, due to the enourmous computational effort. Therefore, a reduced soot model is needed. Here, an existing two-equation model [5] is used, which is ex- tended to give better results for aromatic fuels. This mod- ified model was tested for soot formation under shock tube conditions for n-hepatne and toluene oxidation. Model description Gas-phase kinetics In this work a recently-developed kinetical model [5] is used for the simulation of the chemical reactions. It is based on previous work [6, 7], which was extended by several PAH formation and growth pathways up to seven aromatic rings [8]. The kinetical model describes processes in the gas- phase relevant for soot formation like the closure of the first aromatic ring from small hydrocarbons and the growth and formation of PAHs. For calculations with the two-equation model, up- dates to the hydrocarbon reactions from C 1 to C 4 were made following literature recommendations for the rate coefficients [9]. Detailed Soot Model The transition of the gas-phase species to particulate matter and the growth of the soot particles is formulated as in [7] with modifications resulting from the additional PAHs [8] mentioned above. The soot precursors and the particles are treated as polymer species permitting the formulation of elemen- tary reactions. There are four polymer species represent- ing the soot precursors, the soot particles and activated variants of both possessing radical sites. The equations of the polymer species are solved with a discrete compartment method approach [10] which combines aspects of sectional and moment methods for population balance equations. In this work a single com- partment is used ranging over the whole particle-size axis, thereby neglecting size-dependent kinetic effects. Within this compartment the conservation equations for * Corresponding author: jens.marquetand@iwr.uni-heidelberg.de Proceedings of the European Combustion Meeting 2009