This thesis deals with turbulent mixing processes occurring in internal combustion engines, when applying exhaust gas recirculation (EGR). EGR is a very efficient way to reduce emissions of nitrogen oxides (NOx) in internal combustion engines. Exhaust gases are recirculated and mixed with the fresh intake air, reducing the oxygen con- centration of the combustion gas and thus the peak combustion temperatures. This temperature decrease results in a reduction of NOx emissions. When applying EGR, one is often faced with non-uniform distribution of exhaust among and inside the cylinders, deteriorating the emission performance. The mixing of exhaust gases and air is governed by the flow in the engine intake manifold, which is characterized by unsteadiness due to turbulence and engine pulsations. Moreover, the density cannot be assumed to be constant due to the presence of large temperature variations.Different flow cases having these characteristics are computed by compressible Large Eddy Simulations (LES). First, the stationary flows in two T-junction type geometries are investigated. The method is validated by comparison with experimental data and the accuracy of the simulations is confirmed by grid sensitivity studies. The flow structures and the unsteady flow modes are described for a range of mass flow ratios between the main and the branch inlet. A comparison to RANS computations showed qualitatively different flow fields.Thereafter, pulsating inflow conditions are prescribed on the branch inlet in or- der to mimic the large pulsations occurring in the EGR loop. The flow modes are investigated using Dynamical Mode Decomposition (DMD).After having established the simulation tool, the flow in a six-cylinder engine is simulated. The flow is studied by Proper Orthogonal Decomposition (POD) and DMD. The mixing quality is studied in terms of cylinder-to-cylinder non-uniformity and temporal and spatial variances. It was found that cycle-averaging of the concentration may give misleading results. A sensitivity study with respect to changes in the boundary conditions showed that the EGR pulsations, have large influence on the results. This could also be shown by POD of the concentration field showing the significance of the pulses for the maldistribution of exhaust gases.Finally, the flow in an intake manifold of a four-cylinder engine is investigated in terms of EGR distribution. For this geometry, pipe bends upstream of the EGR inlet were found to be responsible for the maldistribution. / <p>QC 20130207</p>

Les émissions polluantes à l’échappement des véhicules automobiles sont l'une des principales sources de pollution de l'air dans le monde d'aujourd'hui. Par conséquent, la législation a évolué afin de limiter ces émissions. L'un des aspects clés pour répondre consiste à bien maîtriser les échanges gazeux au sein du moteur à combustion interne. Cette amélioration est possible par l'optimisation de répartiteurs d'admission d'air. Dans ces répartiteurs d'admission d'air, la maitrise de l’écoulement de type tumble est une piste de progrès. Des volets sont installés à la sortie du répartiteur afin d'améliorer le rapport de tumble et donc le mélange air-carburant (VTS-Variable Tumble System). Une autre caractéristique de l'écoulement à l'intérieur des répartiteurs est l'effet des écoulements pulsés qui engendrent des fluctuations de pression assez importante. Par conséquent, le but de cette étude consiste à simuler le flux d'air pulsé à l'intérieur des répartiteurs d'admission et à identifier l'effet des pulsations de pression sur les composants actifs tels que les volets. Le travail de simulation dans la présente thèse a été effectué à partir du code open source CFD OpenFOAM. Dans un premier temps, l'effet des pulsations de pression est simulé à l'intérieur d'un tube d'acier et une méthodologie de simulation est développée. Les résultats de la simulation sont validés à partir de résultats expérimentaux obtenus sur un dispositif spécifique, le banc dynamique. Ensuite, des simulations ont été effectuées sur le répartiteur d'admission principal avec des volets. Tout d’abord, les simulations sont effectuées en régime permanent avec cinq positions d'ouverture différentes du clapet. Les forces et les moments agissant sur le volet en régime permanent sont obtenus et analysés. Puis, des simulations en régime transitoire avec des effets de pulsation de pression sont effectuées. Les résultats de la simulation instationnaire sont comparés aux résultats expérimentaux en termes de fluctuations de pression relative. Les effets des pulsations de pression sur les forces aérodynamiques et les moments agissant sur les volets sont analysés et commentés. / The exhaust emissions from automobiles are one of the major sources of air pollution in today’s world. Thence,research and development is the key feature of the modern automotive industries to meet strict emission legislation. One of the key aspects to meet these requirements is to improve the gas exchange process within internal combustion engines. It is possible by the design optimization of the air intake manifolds for internal combustion engines. One of such advancement in air intake manifolds is variable tumble systems (VTS). In VTS system, tumble flaps are installed at the exit of the manifold runner in order to improve tumble ratio and hence air-fuel mixing. Another feature of the flow inside the intake manifolds is pressure pulsation effect. Therefore, the aim of the Ph.D. work is to simulate the pulsating air flow inside the air intake manifolds and to identify the effect of the pressure pulsations on the active components like tumble flaps. The simulation work in the present thesis has been carried out on open source CFD code OpenFOAM. In a first step, the effect of pressure pulsations is simulated inside a steel tube and a simulation methodology is developed. The results of the simulation are validated on a specific experimental device, the dynamic flow bench. Then,simulations have been carried out on the main intake manifold with tumble flaps. Firstly, the simulations are performed with five different opening positions of the tumble flap in a steady state configuration. The forces and moments acting on the flap in steady state are obtained and analyzed. Then, unsteady simulations with pressure pulsation effects are performed. The results of obtained from unsteady simulation are compared with the experimental results in terms of relative pressure fluctuations. The effect of the pressure pulsation on the aerodynamic forces and moments acting on the tumble flaps are analyzed and explained.