Numerical modeling of reacting systems with detailed kinetic mechanisms

Realistic numerical simulations of pyrolysis and combustion require not only accurate modeling of fluid dynamics, but also a detailed characterization of chemical reactions. In recent years, increasing efforts have been devoted to the development of more complex reaction mechanisms, with high levels of detail and comprehensiveness. Clearly, thanks to new advancements in chemical kinetics, the size of mechanisms tends to grow with time. Recent trends suggest that in the near future mechanisms with more than ~ 20,000 species will be available. While such large mechanisms may provide very detailed information about the chemistry of combustion and pyrolysis, it is very expensive to accommodate them in numerical simulations. The computational cost can be prohibitive even for ideal systems (i.e., reacting systems in which no transport phenomena are included) or one-dimensional laminar flames. Even if the size (i.e., number of species and reactions) of detailed kinetic mechanisms is the most important challenge for the numerical simulations, also the stiffness (due to the wide range of chemicals times) of the nonlinear chemical equations governing the evolution of species plays a fundamental role in controlling the performance and the robustness of numerical algorithms. The points mentioned above suggest the need of using specifically conceived numerical techniques and tools for carrying out numerical simulations of reacting systems involving detailed kinetics (thousands of species and reactions). The purpose of this contribution is to present the equations of reacting systems typically adopted for developing and validating large kinetic mechanisms and discuss the basic numerical techniques to solve them efficiently. Moreover, because of their importance in the interpretation of results of simulations involving complex chemistry, numerical tools such as sensitivity analysis, rate of production, and reaction path analysis are introduced and described.