This study explores the reactions and related species of NOx pollutants in methane flames in order to understand their production and consumption during the combustion process. To do this, several analytical simulations were run to explore the behavior of nitrogen species in the pre-flame, post- flame, and reaction layer regions. The results were then analyzed in order to identify all "steady-state" species in the flame as well as the determine all the unnecessary reactions and species that are not required to meet a defined accuracy. The reductions were then applied and proven to be viable.

Performing high-fidelity plasma simulations remains computationally expensive because of their large dimension and complex chemistry. Atmospheric re-entry plasmas for instance, involve hundreds of species in thousands of reactions used in detailed physical models. These models are very complex as they describe the non-equilibrium phenomena due to finite-rate processes in the flow. Chemical non-equilibrium arises because of the many dissociation, ionization and excitation reaction at various time-scales. Vibrational, rotational, electronic and translational temperatures characterize the flow and exchange energy between species, which leads to thermal non-equilibrium.With the current computational resources, detailed three-dimensional simulations are still out of reach. Detailed calculations using the full dynamics are often restricted to a zero- or one-dimensional description. A trade-off has to be made between the level of accuracy of the model and its computational cost. This thesis presents various methods to develop accurate reduced kinetic models for plasma flows. Starting from detailed chemistry, high-fidelity reductions are achieved through the application of either physics-based techniques, such as presented by the binning methods and time-scale based reductions, either empirical techniques given by principal component analysis. As an original contribution to the existing methods, the physics-based techniques are combined with principal component analysis uniting both communities. The different techniques are trained on a 34 species collisional-radiative model for argon plasma by comparing shock relaxation simulations.The best performing method is applied on the large N-N2 mechanism containing 9391 species and 23 million reactions calculated by the NASA Ames Research Center. As a preliminary step, the system dynamics is analyzed to improve our understanding of the various processes occurring in plasma flows. The reactions are analyzed and classified according to their importance. A deep investigation of the kinetics enables finding the main variables and parameters characterizing the plasma, which can thereafter be used to develop or improve existing reductions.As a result, a novel coarse grain model has been developed for argon by binning the electronic excited levels and the ionized species into 2 Boltzmann averaged energy bins. The ground state is solved individually together with the free electrons, reducing the species mass conservation equations from 34 to 4. Principal component analysis has been transferred from the combustion community to plasma flows by investigating the Manifold-Generated and Score-PCA techniques. PCA identifies low dimensional manifolds empirically, projecting the full kinetics to its base of principal components. A novel approach combines the binning techniques with PCA, finding an optimized model for reducing the N3 rovibrational collisional model. / Doctorat en Sciences de l'ingénieur et technologie / info:eu-repo/semantics/nonPublished

With the increase of global temperature and decrease of fossil fuel sources, biofuels become an excellent alternative in present days. Because of its oxygenated nature, biofuels are found to be more environmentally friendly over fossil fuels. Therefore, in this study, initially two different biofuels: ethanol and 2,5 dimethyl furan (DMF) are considered as an additive to gasoline which shows a significant improvement in its combustion characteristics. Due to this promising result for further studies of these biofuel, details chemical kinetic study of biofuels is considered in this work through generating its mechanism for engine relevant conditions. Detail chemical mechanism PCRL-Mech1 is generated for ethanol which is applicable for wide range of operating conditions. The mechanism is successfully validated with available experimental data of laminar burning speed (LBS) and ignition delay time (IDT). Species concentration at different reactor conditions are also considered for the comparison which shows an excellent agreement. Detail mechanism generation for another newer biofuel anisole is also considered because of its favorable features in combustion properties and potential source of biomass. Anisole is a higher hydrocarbon aromatic component and comparative newer fuel which has limited experimental data. However, with that available experimental data, the developed anisole mechanism shows a good agreement predicting LBS and IDT results. The chemical kinetics of this fuel is also analyzed through reaction path flux and sensitivity analyses. Although, detail mechanisms have higher accuracy, they would be very expensive when using in multiscale computational fluid dynamics (CFD) modeling. Therefore, different mechanism reduction schemes are considered to reduce the mechanism size. Initially direct relation graph (DRG), direct relation graph with error propagation (DRGEP) and sensitivity analysis is implemented to generate a skeleton mechanism for PCRL-Mech1, which successfully reduced its size. In addition, the rate-controlled constraint equilibrium (RCCE) analysis is considered as a reduction scheme. The constraints for RCCE calculation are selected through approximate singular value decomposition of actual degree of disequilibrium (ASVDADD) analysis. A good comparison of temperature profile of RCCE simulation proves the success of ASVDADD method.

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La stabilisation et la propagation d’une flamme laminaire pré-mélangée méthane/air dans un canal étroit, sont revisitées à partir de simulations numériques. La combustion est modélisée à l’aide d’une chimie et de propriétés de transport complexes, ainsi que du couplage des transferts thermiques à l’interface et dans les parois. Premièrement, une procédure de réduction des mécanismes chimiques adaptée à cette application est appliquée. Deuxièmement, la réponse de la forme de flamme lorsque soumise à diverses conditions thermiques est analysée en termes de vitesse de propagation et de topologie de l’écoulement au voisinage du front de réaction. Troisièmement, le mécanisme de transfert thermique déclenchant la propagation de flamme lorsque celle-ci est soumise à un préchauffage est montré être principalement convectif. Pour finir, le rôle prépondérant de la gravité, via l’action du moment barocline, sur la symétrie des flammes se propageant dans des canaux étroits, est démontré. / The flow physics controlling the stabilization and propagation of a methane/air laminar premixed flame in a narrow channel is revisited from numerical simulations. Combustion is described with complex chemistry and transport properties, along with a coupled simulation of heat transfer at and within the wall. First, a chemistry mechanism reduction procedure fitted to this application is applied. Second, the response of the premixed flame shape to various heat transfer conditions is analyzed in terms of flame propagation velocity and flow topology in the vicinity of the reactive front. Third, the heat transfer mechanism triggering the flame movement when this last is submitted to an upstream wall preheating is revealed to be mainly convective. To finish, the preponderant role of gravity, via an impact on the baroclinic torque, in the symmetry breaking of small-scale channel flames is demonstrated.