Studies On The Efficient Reduction Methods For The Combustion Chemical Kinetic Mechanism Of Fuel

With the development of the computer technology, numerical simulation plays an increasingly important role in the study of combustion as a complementary or even replacement tool of the traditional, high-cost experiments for both scientific discoveries and engineering designs. In recently years, due to the growing serious energy and environmental problems, and the necessity of understanding the combustion process in detail, the simulation method based on chemical kinetic mechanisms has received considerable attentions and has a rapid development. It is shown that the numerical simulation using detailed mechanisms has great advantages and potential in calculating the flame propagation speed, ignition delay time, pollutant formation, and other combustion characteristics, which provide reliable theoretical and data support in exploring clean, efficient energy and new alternative fuels. Nevertheless, the detailed mechanisms typically contain hundreds of species and thousands of elementary reactions, and the direct use of large-scale mechanisms in reactive flow simulations requires expensive computer resources and large amounts of CPU time. Since that, the reduction of detailed chemical kinetic mechanisms becomes one of the most important processes in the reactive flow modelling. Therefore, in order to make the numerical simulation of reactive flow computationally affordable and comprehensively accurate, the development of computational approaches for rigorous reduction of detailed mechanisms is essential.In this thesis, a number of accurate and efficient mechanism reduction methods are proposed on the basis of analyzing the existing research achievements. In order to validate the proposed methods, these detailed chemistries of several typical fuels are reduced, and the corresponding skeletal mechanisms are used in the simulation of reactive flow.The main works and results are summarized as follows:(1) On the basis of the path flux analysis(PFA) approach and full consideration of indirect relation between the species, the path flux analysis with multi generations(MPFA) reduction method is developed. The detailed mechanisms of methane, n-heptane and methyl decanoate are reduced by the PFA and MPFA methods, and the simulation results of the corresponding skeletal mechanisms are compared. It is demonstrated that the MPFA method which takes the indirect relations of multi generations into account is superior to the PFA method. Meanwhile, the relations among the number of generations, computational time and reduction precision in the PFA series methods have been discussed. It is proved that the PFA with three generations is much better than that with two generation. Nevertheless, the application of the PFA method with four or more generations will lead to increasing computational time but litter improvement of accuracy.(2) For the PFA and MPFA methods, the physical meanings are confused in defining the interaction coefficients between species, which may lead to the unreasonable calculations in the rates of the production and consumption flux. Aiming at the above shortcomings, an improved path flux analysis with multi generations(IMPFA) method which improves the definitions of the interaction coefficients among species is proposed. Compared with the reduction results of PFA and MPFA, it is revealed that the IMPFA method can reasonably identify the strength of the interaction relationship between species. As for the IMPFA and MPFA methods with the same number of generations, the former method can obtain much more reasonable skeletal mechanisms.(3) Because the MPFA method is based on the correlations of the reaction rates between species, it cannot analyze the impact of each species or reactions on a certain characteristic of the mechanism directly, and some unimportant species and reactions may be still retained in the skeletal mechanisms. Therefore, an integrated reduction method MPFASA which combines path flux analysis with multi generations(MPFA) and sensitivity analysis(SA) is proposed. By taking the advantages of the MPFA and SA, the MPFASA method can reduce the mechanisms more efficiently. According to the reduction results of methane and n-heptane, it is confirmed that the further reduced mechanisms with high precision can be obtained by the MPFASA method.