Modeling of Alkane Oxidation using Constituents and Species

Abstract

A chemical kinetics reduction model is proposed for alkane oxidation in air that is baaed on a parallel methodology to that used in turbulence modeling in the context of Large Eddy Simulation. The objective of kinetic modeling is to predict the heat release and temperature evolution. In an a priori step, a categorization of time scales is first conducted to identify scales that must be modeled and scales that must be computed using progress variables based on the model for the other scales. First, a decomposition of heavy (carbon number greater or equal to 3) hydrocarbons into constituents is proposed. Examination of results obtained using the LLNL heptane-oxidation database in conjunction with Chemkin II shows that (i) with appropriate scaling, the total constituent mole fraction behaves in a self-similar manner and the total constituent molar density rate follows a quasi-steady behavior, and (ii) the light species can be partitioned into two subsets according to whether they are quasi-steady (nine species) or unsteady (11 species). The twelve progress variables represented by the total constituent molar density and the molar densities of the unsteady light species are defined to be a base from which the system's behavior can be reproduced. This is a dramatic reduction from the 160 species (progress variables) and 1540 reactions in the LLNL set 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). A summary of the model is presented explaining the curve fits that constitute the model, namely (1) for the constituent molar density rate a long with the corresponding enthalpy production rate, (2) for the quasi-steady species mole fraction, and (3) for the contribution from the heavy species to the unsteady light species reaction rates. The proposed kinetic mechanism is valid over a pressure range from atmospheric to 60 bar, temperatures from 600 K to 2500 K and equivalence ratio1 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. Highlights of the a priori model results are illustrated for a variety of initial conditions. Results from a posteriori tests are shown in which the model predictions for the unsteady light species and the temperature are compared to the equivalent quantities baaed on the LLNL dataset.

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