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Numerical simulation of turbulent propane-air combustion with non-homogeneous reactants : initial results

High-resolution two-dimensional numerical simulations have been initiated for premixed turbulent propane-air flames propagating into regions of non-homogeneous reactant stoichiometry. Simulations include complex chemical kinetics, realistic molecular transport, and fully resolved hydrodynamics (no turbulence model). Aerothermochemical conditions (pressure, temperature, stoichiometry, and turbulence velocity scale) approach those in an automotive gasoline direct-injection (GDI) engine at a low-speed, light-load operating condition. Initial results suggest that: 1) There is no leakage of the primary fuel (propane) behind an initial thin premixed heat-release zone. This ‘primary premixed flame’ can be described using a monotonic progress variable and laminar premixed flamelet concepts. 2) Following an initial transient, global heat release with non-homogeneous reactants is lower than with homogeneous reactants for the same overall reactant stoichiometry. Flame area (length) is greater with non-homogeneous reactants. 3) Beyond three-to-four flame thicknesses behind the primary flame, practically all hydrocarbon fuel has broken down into CO and H2. 4) The rate of heat release in the ‘secondary reaction zone’ behind the primary premixed flame is governed by turbulent mixing and the kinetics of CO2 production. Mixture-fraction-conditioned secondary heat release, CO, and CO2 production rates are qualitatively similar to results from a first-order conditional-moment-closure (CMC) model; CMC gives poor results for H2, H2O, and radical species. Description of the secondary heat release using simple laminar diffusion flamelet concepts is problematic. 5) Computational considerations demand modifications to chemical mechanisms involving C3H7 and CH3CO. Specific changes are proposed to strike a satisfactory balance between accuracy and computational efficiency over a broad range of reactant stoichiometry.