Numerical simulation of turbulent propane-air combustion with nonhomogeneous reactants

High-resolution two-dimensional numerical simulations have been performed f or premixed turbulent propane-air flames propagating into regions of nonhom ogeneous reactant stoichiometry. Simulations include complex chemical kinet ics, realistic molecular transport, and fully resolved hydrodynamics (no tu rbulence model). Aerothermochemical conditions (pressure, temperature, stoi chiometry, and turbulence velocity scale) approach those in an automotive g asoline direct-injection (GDI) engine at a low-speed, part-load operating c ondition. Salient findings are as follows: (1) There is no leakage of the p rimary fuel (propane) behind an initial thin premixed heat-release zone; Th is "primary premixed flame" can be described using a monotonic progress var iable and laminar premixed flamelet concepts. (2) For the conditions simula ted, differences in global heat release and flame area (length) between hom ogeneous and nonhomogeneous reactants having the same overall stoichiometry are small. (3) Beyond three-to-four flame thicknesses behind the primary f lame, practically all hydrocarbon fuel has broken down into CO and H-2. (4) The rate of heat release in the "secondary reaction zone" behind the prima ry premixed flame is governed by turbulent mixing and the kinetics of CO2 p roduction. Mixture-fraction-conditioned secondary heat release, CO, and CO2 production rates are qualitatively similar to results from a first-order c onditional moment closure (CMC) model; CMC gives poor results for H-2, H2O, and radical species. Description of the secondary heat release using stead y laminar diffusion flamelet concepts is problematic. (5) Of the chemical s pecies considered, HCO mass fraction or the product of CH2O and OH mass fra ctions correlates best with local heat-release rate [1], (6) Computational considerations demand modifications to chemical mechanisms involving C3H7 a nd CH3CO. Specific changes are proposed to strike a satisfactory balance be tween accuracy and computational efficiency over a broad range of reactant stoichiometry. (C) 2000 by The Combustion Institute.