Diesel soot formation

Numerical simulations of soot formation were performed for n-dodecane spray using the transported probability density function (TPDF) method. Liquid n-dodecane was injected with 1500 bar fuel pressure into a constant-volume vessel with an ambient temperature, oxygen volume fraction and density of 900 K, 15% and 22.8 kg/m 3 , respectively. The interaction by exchange with the mean (IEM) model was employed to close the micro-mixing term. The unsteady Reynolds-averaged Navier-Stokes (RANS) equations coupled with the realizable k-ϵ turbulence model were used to provide turbulence information to the TPDF solver. A 53-species reduced n-dodecane chemical mechanism was employed to evaluate the reaction rates. Soot formation was modelled with an acetylene-based two-equation model which accounts for simultaneous soot particle inception, surface growth, coagulation and oxidation by O 2 and OH. The modelling results for ignition delay, lift-off length, flame length evolution and distribution of soot volume fraction () are compared with the corresponding experimental data. Good predictions of the temporal evolution and spatial extent of the soot volume fraction have been observed. The findings suggest that the transported probability density function approach for soot modelling is a promising framework.

Oxidation of 20 hydrocarbons (from C10 to C42) representative of soluble organic fraction of Diesel soots were investigated by mixing the hydrocarbon (solid at ambient temperature) with a 0.55 wt%Pt/Al 2 O 3 catalyst (Pt particle size below 1.2 nm). Oxidation rates were characterized by the temperature of half-conversion (T 50) and by the quantity of oxygen consumed during the reaction, which allows to determine the amount of the solid hydrocarbon (initially 100 mg of a mixture of 0.133 mmol HC with 2 g of catalyst) actually oxidized during heating in 1%O 2 /He. A preliminary study carried out with two selected hydrocarbons showed that turnover frequencies (TOF) are little dependent on the Pt loading. The hydrocarbons should be vaporized before them to react with the Pt catalyst. Ideally, temperatures of light-off and of vaporization should coincide for the optimal transformation into CO 2. The molecular structure of the hydrocarbon (number of aromatic rings, H/C ratio, condensed structures, etc.) is a key-parameter for both oxidation and volatility, which explains why a good correlation was observed between T 50 and boiling temperatures T b. A comparison with light-off tests performed in a stream of gaseous hydrocarbon (vaporized upstream the catalyst) showed that oxidation rates depend on the same structural parameters, except when the hydrocarbon is too volatile (i.e. naphthalene) or, on the contrary, not sufficiently volatile (i.e. n-alkanes in C20-C34). In the first case, a large HC fraction is desorbed without being oxidized while in the second case, oxidation rate is limited by the vaporization.