Numerical simulation of turbulent propane-air combustion with nonhomogeneous reactants

High-resolution two-dimensional numerical simulations have been performed for premixed turbulent propane-air flames propagating into regions of nonhomogeneous 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, part-load operating condition. Salient findings are as follows: (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) For the conditions simulated, differences in global heat release and flame area (length) between homogeneous and nonhomogeneous reactants having the same overall stoichiometry are small. (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 steady laminar diffusion flamelet concepts is problematic. (5) Of the chemical species considered, HCO mass fraction or the product of CH₂O and OH mass fractions correlates best with local heat-release rate [1]. (6) 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. Copyright (C) 2000 The Combustion Institute.