Lumping and Reduction of Detailed Kinetic Schemes: an Effective Coupling

When it comes to handling large hydrocarbon molecules and describing the pyrolysis and combustion behavior of complex mixtures, the potential and limitations of detailed chemistry require a careful investigation. Indeed, as they involve a large number of species and reactions, detailed kinetic mechanisms often make the model predictions computationally expensive, thus strongly restricting their potential. Therefore, the automatic generation of detailed mechanisms with several thousands of molecular species and elementary reactions, very useful in many circumstances and a prerequisite to derive reduced models, may become useless from a more general application viewpoint. In order to overcome these limitations, in this paper a proper strategy to obtain suitable mechanisms for multidimensional computational fluid dynamics (CFD) applications is presented and discussed. It couples the advantages of two reduction techniques: chemical lumping of species and reactions and a flexible and reliable reduction technique aimed at eliminating unimportant species and reactions. It is shown that a central advantage of semidetailed kinetic models, already reduced with a chemical lumping, is their easier and more effective applicability to successive automatic reductions. Kinetic schemes of n-heptane and n-dodecane oxidation, reduced to 100 and 120 species, respectively, are obtained and compared with experimental data and with the complete original model. These dimensions easily allow the successive use of these reduced kinetic models particularly when the aim is to embody them within computational fluid dynamic models. In particular, simulations of steady-state, axisymmetric, laminar diffusion flames were performed comparing the original and the reduced kinetic mechanisms. The results demonstrated not only the reliability of the reduced mechanisms, but, more importantly, highlighted the great computational advantages, especially in terms of CPU times, of reduced kinetics for the simulation of multidimensional systems. Similar or even more apparent benefits are expected when reducing lumped kinetic schemes of combustion of real transportation fuels, such as gasoline, jet, and diesel fuels.