A Simplified Model of Alkane Oxidation

Abstract

A simplified model is proposed for the kinetics of alkane oxidation in air, based on a decomposition of heavy (carbon number greater or equal to 3) hydrocarbons into a 13 constituent radical base. The behavior of this base is examined in test computations for n-heptane utilizing Chemkin II with LLNL data inputs, placing emphasis on modeling to predict the heat release and temperature evolution. A normalized temperature was constructed which when used to plot the total constructed molar density divided by the product of the equivalence ratio and a nondimensional pressure, reveals a self-similar behavior of the plotted variable over a wide range of initial pressures and equivalence ratios. Examination of the LLNL kinetics shows that the total constituent molar density rate follows a quasi-steady behavior. This reaction rate was curve fitted along with the corresponding enthalpy production. The fits are shown against the normalized temperature for various equivalence ratios and initial nondimensional pressures and comparisons with the LLNL kinetics are very favorable. The model reduces the LLNL n-heptane mechanism from 160 species (progress variables) and 1540 reactions to 12 progress variables, 16 quasi-steady rates (associated with heavy species), 162 conventional reaction rates (light species) and 11 other functional forms. (i.e. fits for the mean heavy-species heat capacity at constant pressure, the enthalpy release rate of the heavy species, and the molar fraction of quasi-steady light species). The proposed kinetic mechanism is valid over a pressure range from atmospheric to 60 bar, temperatures from 600 K to 2500 K and equivalence ratios from 0.125 to 8. This range encompasses diesel, HCCI and gas turbine engines, including cold ignition; and NO_x, CO and soot pollutant formation in the lean and rich regimes, respectively.

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