Modification of a Diesel Oil Surrogate Model for 3D CFD Applied to Conventional and HCCI Combustion
Paper in proceedings, 2008
This paper describes an analysis of the Diesel Oil Surrogate (DOS) model used at Chalmers University (Sweden), including 70 species participating in 310 reactions, and subsequent improvements prompted by the model’s systematic tendency to under-predict the combustion intensity in simulations of kinetically-driven combustion modes, e.g. Homogeneous Charged Compression Ignition (HCCI).
Key bases of the model are the properties of a model Diesel fuel with the molecular formula C14H28. In the vapor phase, a global reaction decomposes the starting fuel, C14H28, into its constituent components; n-heptane (C7H16) and toluene (C7H8). This global reaction was modified to yield a higher n-heptane:toluene ratio, due to the importance of preserving an n-heptane-like cetane number. Three different (composite) versions of this global decomposition reaction were investigated separately and compared:
1. 3C14H28 + O2 => 4C7H16 + 2C7H8 +2H2O
2. 2C14H28 => 3C7H16 + C7H8
3. 3C14H28 + 3.5O2 => 5C7H16 + 0.5C7H8 + 3.5CO2
Reaction 1 is the original formulation and reactions 2 and 3 are novel alternatives. The effects of modifying the activation energies (E), collision frequency factors (A) and oxygen concentration exponents (AEO) of these reactions was examined when integrated into the 3D KIVA-3V CFD code, while leaving the elementary reactions unchanged. The results were compared with data obtained from measurements of a High Speed Direct Injected (HSDI) engine, operating in both conventional and HCCI combustion modes. Finally, the global reaction version and the rate constants giving the best agreement for the examined regimes were identified.