Elementary-reaction chemical kinetics of hydrocarbon oxidation consists of hundreds to thousands of species and thousands of reactions. As such, it is impossible to use it in models and codes involving turbulence because computations are unfeasible due to lack of memory and computer speed. The solution is to reduce the elementary chemical kinetics to a much smaller set of representative reactions. A kinetic reduction has been shown to work very well for isooctane and its mixtures with n-pentane, iso-hexane, and n-heptane.
A model previously developed for reducing n-heptane oxidation kinetics has been slightly modified to address the oxidation of iso-octane, primary reference fuels (PRFs), and iso-octane mixtures with n-pentane and iso-hexane. The conceptual model was based on the identification of constituents as the building blocks of the heavy species, and on reducing the number of species progress variables to follow the total constituent molar density and the molar densities of the unsteady light species. A normalized surrogate temperature variable had been defined for which a normalized total constituent molar density exhibited a self-similar behavior for initial temperatures in the cold ignition regime, and for a wide range of initial pressures and equivalence ratios. Also, the molar densities of O2 and H2O exhibited a quasi-linear behavior versus the normalized variable.
On going from n-heptane to iso-octane and its mixtures, examination of the LLNL full mechanisms using CHEMKIN II showed that the constituent list was augmented by one additional entity, but the molar density of this total constituent remained self-similar versus the normalized surrogate temperature variable for initial temperature values in the cold ignition regime, and the wide range of initial pressures and equivalence ratios previously examined. When the higher-than-cold ignition temperature regime was explored, a family of self-similar curves versus the normalized surrogate temperature variable was obtained according to the initial temperature. The molar densities of O2 and H2O remained quasi-linear versus the normalized surrogate temperature variable.
In a previous n-heptane model, mathematical fits versus the normalized surrogate temperature variable were required in a four-dimensional space to solve the equations and obtain the species and temperature evolution; these fits were now replaced with tabulation in the context of constituents and light species. The favorable predictive aspect of the model should be contrasted with its relative simplicity, as it has a specified form that is not dynamically changed during the computation and only requires solutions of 11 species progress variables, all of which are light species; the model has much increased range with respect to other reduced models, even with respect to those having a larger number of species. The advantage of such a simple model becomes increasingly significant with increasing carbon atoms of the fuel because of the proliferation of relatively heavy species with increasing carbon number; it would be very advantageous for performing computations for heavy fuels.