Mecanismos virtuais de cinética química aplicados à modelagem de misturas complexas de combustíveis gasosos

Abstract

Incorporating detailed chemical kinetics into the numerical modeling of turbulent combustion is an arduous task, especially when one wants to study industrial-scale problems more realistically. In fact, detailed kinetic mechanisms are constructed to be comprehensive, consisting of numerous species and reactions to accurately model a wide range of thermo-chemical conditions. Therefore, it is necessary to evaluate hundreds of transport equations for the mass fractions of the species present, besides the correct appreciation of each reaction existing in the kinetic mechanism. In most cases of interest, the computational resources required to perform such a task are prohibitive. An alternative frequently used in the literature is the reduction of detailed mechanisms using specific conditions of interest as parameters. While this is an efficient strategy in cases involving the burning of simple fuels, it tends to produce mechanisms that are still considered extensive in cases involving complex fuel mixtures. The present thesis uses the strategy of constructing optimized virtual kinetic mechanisms in the modeling of a complex mixture of gaseous fuels characteristic of an industrial CO boiler. This methodology aims transfering to a simplified mechanism the kinetic complexity of a detailed reference mechanism. To this end, firstly kinetic mechanisms available in the literature are evaluated under conditions of interest. A detailed mechanism is proposed and used as a reference. Classical techniques of kinetic complexity reduction are applied to this mechanism to identify the minimum number of species/reactions necessary to represent the conditions of interest. Subsequently, the same detailed mechanism is used as a reference in the optimization processes of the virtual kinetic mechanism, composed of virtual reactions and virtual species. The accuracy of the optimized mechanism is evaluated in one-dimensional premixed laminar flame calculations. The results obtained indicate that the methodology can be successfully applied to complex mixtures of gaseous fuels, just as it was used in studies of hydrocarbon-air mixtures. However, it was shown that the process of optimizing simultaneous operating conditions, necessary for generalizing the model to the entire range of interest, needs to be revisited in problems requiring explicit optimization of the thermodynamic properties of the virtual intermediate species.