Thermoacoustic instability is of fundamental and applied interest in both scientific research and practical applications. This study aims to explore several very important sub-aspects in this field and contribute to a better understanding of thermoacoustic instability as encountered in typical gas turbines and rocket engines. Atomization has been recognized as a key mechanism in driving applied thermoacoustic instability. In this regard, this study mainly focuses on the atomization process relevant for delineation of thermoacoustic instability, contributing to a comprehensive understanding of the effect of acoustics on primary and secondary atomization. Firstly, a tree-based adaptive solver and VOF method are employed to simulate the jet primary atomization. The code is validated by theoretical, numerical and experimental results to demonstrate its capability and accuracy in terms of atomization in both low-speed and high-speed regime. Perturbation frequency and amplitude have shown to affect the atomization significantly. Besides, the effect of acoustic forcing on liquid ligament has also been numerically investigated. A volume source term is introduced to extend the solver to model the compressible effects in the presence of acoustic forcing. The influence of acoustic wave number, amplitude and frequency has been examined in detail. In terms of modelling the thermoacoustic instability, bifurcation analysis is carried out for a time-delayed thermoacoustic system using the Method of Line approach. Good predictions have been obtained to capture the nonlinear behaviors inherent in the system. Moreover, model-based simulation and control of thermoacoustic instability have been conducted. A low-order wave-based network model for acoustics is coupled with nonlinear flame describing function to predict the nonlinear instability characteristics in both frequency and time domain. Furthermore, active feedback control is implemented. Two different controllers have been designed to eliminate the thermoacoustic instability to an acceptably low level and may be employed in a practical manner.

La réduction des émissions polluantes est actuellement un enjeu majeur au sein du secteur aéronautique. Parmi les solutions développées par les motoristes, la combustion en régime pauvre apparaît comme une technologie efficace pour réduire l’impact de la combustion sur l’environnement.Or, ce type de technologie favorise l’apparition d’instabilités de combustion issues d’un couplage thermo-acoustique. Des études expérimentales précédemment menées à l’ONERA ont mis en évidence l’importance de l’atomisation au sein d’un injecteur multipoint sur le phénomène d’instabilités de combustion. L’objectif de cette thèse est de mettre en place la méthodologie multi-échelle pour reproduire les phénomènes de couplage entre l’atomisation du jet liquide en présence d’un écoulement gazeux transverse (configuration simplifiée d’un point d’injection d’un injecteur multipoint) et d’une perturbation acoustique imposée, représentative de l’effet d’une instabilité de combustion. Ce type d’approche pourra, à terme, être utilisé pour la simulation instationnaire LES d’un système de combustion, et permettra de déterminer les temps caractéristiques de convection du carburant liquide pouvant affecter les phénomènes d’évaporation et de combustion, et donc l’apparition des instabilités de combustions. Afin de valider cette approche,les résultats issus des simulations sont systématiquement comparés aux observations expérimentales obtenues dans le cadre du projet SIGMA. Dans un premier temps, une simulation du jet liquide en présence d’un écoulement gazeux transverse est réalisée. Cette simulation a permis de valider l’approche multi-échelle : pour cela, les grandes échelles du jet, ainsi que les mécanismes d’atomisation reproduits par les simulations, sont analysés. Ensuite, l’influence d’une perturbation acoustique sur l’atomisation du jet liquide est étudiée. Les comportements instationnaires du jet et du spray issu de l’atomisation sont comparés aux résultats expérimentaux à l’aide des moyennes temporelles et des moyennes de phase. / The reduction of polluting emissions is currently a major issue in the aeronautics industry.Among the solutions developed by the engine manufacturers, lean combustion appears as an effectivetechnology to reduce the impact of combustion on the environment. However, this type oftechnology enhances the onset of combustion instabilities, resulting from a thermo-acoustic coupling.Experimental studies previously conducted at ONERA have highlighted the importanceof atomization in a multipoint injector to the combustion instabilities. The aim of this thesis isto implement the multi-scale methodology to reproduce the coupling phenomena between theatomization of the liquid jet in the presence of a crossflow (which is a simplified configuration ofan injection point of a multipoint injector) and an imposed acoustic perturbation, representativeof the effect of combustion instabilities. This type of approach can ultimately be used for the unsteadysimulation of a combustion system, and will determine the characteristic convection timesof the liquid fuel that can affect the phenomena of evaporation and combustion, and therefore theappearance of combustion instabilities. In order to validate this approach, the results obtainedfrom the simulations are systematically compared with the experimental observations obtainedwithin the framework of the SIGMA project. First, a simulation of the liquid jet in gaseous crossflowis performed. This simulation enabled us to validate the multi-scale approach : to this end,the large scales of the jet, as well as the atomization mechanisms reproduced by the simulations,are analyzed. Then, the influence of an acoustic perturbation on the atomization of the liquidjet is studied. The unsteady behavior of the jet and the spray resulting from the atomization arecompared with the experimental results using time averages and phase averages.