Un des principaux enjeux de l'industrie aéronautique est la recherche du moteur au meilleur rendement possible, pour satisfaire des contraintes économiques, techniques et environnementales. Les turbomachines bénéficient d'un constant perfectionnement depuis plus de 60 ans, et cette technologie semble avoir atteint un plateau. Une rupture technologique est aujourd'hui nécessaire, comme la combustion à volume constant (CVC). Le gain attendu est suffisant pour tenter de remplacer les systèmes actuels où la combustion se fait à pression constante. La combustion à isovolume fait appel à des mécanismes encore rarement maitrisés dans le contexte aéronautique. Sa compréhension passe par des expérimentations et des modèles théoriques et numériques. L’objectif de cette thèse est de développer une théorie et un outil de simulation LES (Large Eddy Simulation) appliqué au cas du concept ‘thermoréacteur’. Ainsi, la première étape a consisté à mettre en place un outil de simulation 0D traduisant l’évolution d’un cycle moteur de type CVC (Combustion à Volume Constant). Certains modèles utilisés dans cet outil 0D sont basés sur des corrélations expérimentales. D'autres présentent des paramètres à déterminer à partir de simulations numériques. La simulation 3D d’un système de type CVC est envisageable aujourd’hui grâce aux progrès récents des méthodes LES. Ainsi, des simulations du thermoréacteur ont pu être réalisées, et confrontées aux résultats expérimentaux obtenus au laboratoire Pprime sur trois points de fonctionnement. Les variabilités cycle à cycle observées expérimentalement ont été analysées dans les calculs LES. Les vitesses importantes au niveau de l'allumage et le taux de résidus du cycle précédent semblent être les principaux facteurs à l'origine de ces variations cycle à cycle. / A major challenge for the aircraft industry is to improve engine efficiency and to reduce pollutant emissions for economic, technical and environmental reasons. Aeronautical gas turbines have enjoyed a constant improvement for more than 60 years. This technology seems to have reached such efficiency levels that a technological breakthrough is necessary. Constant Volume Combustion (CVC) offers significant gain in consumption and could replace classical constant pressure combustion technologies, currently used in aeronautical engines. Mechanisms involved in isovolume combustion are not accurately controlled in the context of aeronautical chambers. Experimental, theoretical and numerical studies should provide a better understanding of CVC devices. The objective of this thesis is to develop simulation tools to study the thermoreacteur concept. First, a zero-dimensional (0D) simulation tool is developed to describe the evolution of a CVC cycle. Models based on experimental correlations are used to build the 0D tool. Parameters have to be determined from numerical simulations. Today, the 3D simulation of a CVC system is possible thanks to the recent progress of the LES (Large Eddy Simulation) methods developed at CERFACS. Simulations of the thermoreacteur concept have been carried out, and compared to experimental results obtained at the Pprime laboratory. Three operating points have been calculated. The main conclusion is the existence of significant cyclic variations which are observed in the experiment and analyzed in the LES: the local flow velocity at spark timing and the level of residuals gases are the major factors leading to cyclic variations.

Deep knowledge of combustion phenomena is of great scientific and technological interest. In fact, better design of combustion equipments (furnaces, boilers, engines, etc) can contribute both in the energy efficiency and in the reduction of pollutant formation. One of the limitations to design combustion equipments, or even predict simple flames, is the resolution of the mathematical formulation. Analytical solutions are not feasible, and recently numerical techniques have received enormous interest. Even though the ever-increasing computational capacity, the numerical resolution requires large computational resources due to the inherent complexity of the phenomenon (viz. multidimensional flames, finite rate kinetics, radiation in participating media, turbulence, etc). Thus, development of capable mathematical models reducing the complexity and the stiffness as well as efficient numerical techniques are of great interest.The main contribution of the thesis is the analysis and application of the laminar flamelet concept to the numerical simulation of both laminar and turbulent non-premixed flames. Assuming a one-dimensional behavior of combustion phenomena in the normal direction to the flame front, and considering an appropriate coordinates transformation, flamelet approaches reduce the complexity of the problem.The numerical methodology employed is based on the finite volume technique and a parallel multiblock algorithm is used obtaining an excellent parallel efficiency. A post-processing verification tool is applied to assess the quality of the numerical solutions.Before dealing with flamelet approaches, a co-flow partially premixed methane/air laminar flame is studied for different levels of partial premixing. A comprehensive study is performed considering different mathematical formulations based on the full resolution of the governing equations and their validation against experimental data from the literature. Special attention is paid to the prediction of pollutant formation.After the full resolution of the governing equations, the mathematical formulation of the flamelet equations and a deep study of the hypothesis assumed are presented. The non-premixed methane/air laminar flame is considered to apply the flamelet modelling approach, comparing the results with the simulations obtained with the full resolution of the governing equations. Steady flamelets show a proper performance to predict the main flame features when differential diffusion and radiation are neglected, while unsteady flamelets are more suitable to account for these effects as well as pollutant formation. Assumptions of the flamelet equations, the scalar dissipation rate modelling, and the evaluation of the Lagrangian flamelet time for unsteady flamelets are specially analysed. For the numerical simulation of turbulent flames, the mathematical formulation based on mass-weighted time-averaging techniques, using RANS EVM two-equation models is considered. The laminar flamelet concept with a presumed PDF is taken into account. An extended Eddy Dissipation Concept model is also applied for comparison purposes. A piloted non-premixed methane/air turbulent flame is studied comparing the numerical results with experimental data from the literature. A clear improvement in the prediction of slow processes is shown when the transient term in the flamelet equations is retained. Radiation is a key aspect to properly define the thermal field and, consequently, species such as nitrogen oxides. Finally, the consideration of the round-jet anomaly is of significant importance to estimate the flame front position.In conclusion, flamelet modelling simulations are revealed to be an accurate approach for the numerical simulation of laminar and turbulent non-premixed flames. Detailed chemistry can be taken into account and the stiffness of the chemistry term is solved in a pre-processing task. Pollutant formation can be predicted considering unsteady flamelets.