Etude de l'initiation et de la propagation de l'auto-inflammation dans un milieu présentant des inhomogénéités de température

Des simulations de l'auto-allumage par compression sont réalisées dans une configuration de Machine à Compression Rapide (MCR). Dans un premier temps, des résultats de simulations aux grandes échelles sont comparés avec des mesures expérimentales. Ensuite, la DNS de cette MCR est utilisée pour analyser les mécanismes contrôlant les noyaux précurseurs de l'allumage. Il est montré que les fluctuations locales de compression, quantifiées par l'historique de la divergence du champ de vitesse, ont un effet dominant sur l'allumage

Numerical studies on supersonic spray combustion in high-temperature shear flows in a scramjet combustor

Numerical simulation is applied to detail the combustion characteristics of n-decane sprays in highly compressible vortices formed in a supersonic mixing layer. The multi-phase reacting flow is modeled, in which the shear flow is solved Eulerianly by means of direct numerical simulation, and the motions of individual sub-grid point-mass fuel droplets are tracked Lagrangianly. Spray combustion behaviors are studied under different ambient pressures. Results indicate that ignition kernels are formed at high-strain vortex braids, where the scalar dissipation rates are high. The flame kernels are then strongly strained, associated with the rotation of the shearing vortex, and propagate to envelop the local vortex. It is observed that the flammable mixtures entrained in the vortex are burned from the edge to the core of the vortex until the reactants are completely consumed. As the ambient pressure increases, the high-temperature region expands so that the behaviors of spray flames are strongly changed. An overall analysis of the combustion field indicates that the time-averaged temperature increases, and the fluctuating pressure decreases, resulting in a more stable spray combustion under higher pressures, primarily due to the acceleration of the chemical reaction

Analysis of the initiation and development of autoignition after a rapid compression of a turbulent reactive mixture : Application to the context of CAI/HCCI

La stratégie de combustion par auto-inflammation d’une charge homogène en composition s’intègre dans une démarche de réduction des émissions de particules et de NOx, tout en conservant les rendements thermiques élevés des moteurs Diesel classiques. Pour contrôler ce nouveau mode de combustion, une compréhension fine des mécanismes de couplage entre l'aérodynamique et la thermochimie est nécessaire. Des simulations numériques directes d'un écoulement homogène, turbulent, réactif et subissant une compression, ont été effectués. Deux régimes d'auto-inflammation ont ainsi pu être définis. Le premier, dit quasi-homogène, est caractérisé par une auto-inflammation en masse d'un volume important du mélange réactif et s'accompagne de fortes ondes de pression. Dans le second régime, dit localisé, les noyaux s'initient de manière plus sporadique dans l'espace et dans le temps et aucune onde de pression significative n'est générée lors de l'auto-allumage.Combustion by autoignition of a homogeneous charge aims at reducing particulate matter as well as NOx emissions, while maintaining higher thermal efficiency of conventional diesel engines. To control this new mode of combustion, a fine understanding of the mechanisms of coupling between aerodynamics and thermochemistry is required. Direct Numerical Simulations of a turbulent reactive flow, undergoing a compression, have been performed. This study led to identification of two regimes. The first, known as quasi-homogeneous, is characterized by volumetric autoignition of large zones of the reactive mixture and results in the generation of strong pressure waves, which are potentially dangerous for the structure of engines. In the second regime, called localized, hot spots are initiated more sporadically in space and time, and their topology is such that no significant pressure wave is generated

Analyse de l'initiation et du développement de l'auto-inflammation après compression rapide d’un mélange turbulent réactif : Application au contexte CAI/HCCI

Combustion by autoignition of a homogeneous charge aims at reducing particulate matter as well as NOx emissions, while maintaining higher thermal efficiency of conventional diesel engines. To control this new mode of combustion, a fine understanding of the mechanisms of coupling between aerodynamics and thermochemistry is required. Direct Numerical Simulations of a turbulent reactive flow, undergoing a compression, have been performed. This study led to identification of two regimes. The first, known as quasi-homogeneous, is characterized by volumetric autoignition of large zones of the reactive mixture and results in the generation of strong pressure waves, which are potentially dangerous for the structure of engines. In the second regime, called localized, hot spots are initiated more sporadically in space and time, and their topology is such that no significant pressure wave is generated.La stratégie de combustion par auto-inflammation d’une charge homogène en composition s’intègre dans une démarche de réduction des émissions de particules et de NOx, tout en conservant les rendements thermiques élevés des moteurs Diesel classiques. Pour contrôler ce nouveau mode de combustion, une compréhension fine des mécanismes de couplage entre l'aérodynamique et la thermochimie est nécessaire. Des simulations numériques directes d'un écoulement homogène, turbulent, réactif et subissant une compression, ont été effectués. Deux régimes d'auto-inflammation ont ainsi pu être définis. Le premier, dit quasi-homogène, est caractérisé par une auto-inflammation en masse d'un volume important du mélange réactif et s'accompagne de fortes ondes de pression. Dans le second régime, dit localisé, les noyaux s'initient de manière plus sporadique dans l'espace et dans le temps et aucune onde de pression significative n'est générée lors de l'auto-allumage

Quantification of the Pre-ignition Front Propagation in DNS of Rapidly Compressed Mixture

WOS:000347407100011International audienceA method to estimate the propagation speed of a three-dimensional ignition front in Direct Numerical Simulation (DNS) is discussed. The objective is to contribute to the design of advanced numerical tools for the study of sporadic pre-ignition kernels leading to violent pressure waves, which may for instance damage moving parts in engines. Estimating the speed of a propagating ignition front in three-dimensional DNS, before it is occurring, is not an easy task, because this speed scales as the inverse of the spatial gradient of the time left till ignition, which is a priori an unknown quantity. The proposed approach introduces, for every point of the DNS, an estimation of the time left till ignition, which is obtained from reactors dynamically parameterized from the time evolving DNS results. This provides a three-dimensional distribution of ignition delays at every instant in time of the DNS fields. The time evolution of the ignition speed is then computed from the space derivative of the ignition delay field. Only the pre-ignition phase is examined and the demonstration of the method is made with oversimplified chemistry, in order to apply it to an existing rapid compression machine, in which the mixture composition is homogeneous, whereas the temperature distribution is non-uniform due to heat-transfer at wall. The two expected distinct ignition regimes are reported. In the first, ignition propagates at the speed of sound, or even above, and occurs over a large portion of the combustion chamber, with a strong and sudden pressure increase. The second ignition regime is much more localized in space and with a propagation mechanism pertaining to a deflagration mode. A method is also discussed to delineate in the DNS between ignition influenced by either the constant pressure or the constant volume canonical behaviors

Analyse de l'initiation et du développement de l'auto-inflammation après compression rapide d'un mélange turbulent réactif (Application au contexte CAI/HCCI)

La stratégie de combustion par auto-inflammation d une charge homogène en composition s intègre dans une démarche deréduction des émissions de particules et de NOx, tout en conservant les rendements thermiques élevés des moteurs Diesel classiques. Pour contrôler ce nouveau mode de combustion, une compréhension fine des mécanismes de couplage entre l'aérodynamique et la thermochimie est nécessaire.Des simulations numériques directes d'un écoulement homogène, turbulent, réactif et subissant une compression, ont été effectués. Duex régimes d'auto-inflammation ont ainsi pu être définis. Le premier, dit quasi-homogène, est caractérisé par une auto-inflammation en masse d'un volume important du mélange réactif et s'accompagne de fortes ondes de pression. Dans le second régime, dit localisé, les noyaux s'initient de manière plus sporadique dans l'espace et dans le temps et aucune onde de pression significative n'est générée lors de l'auto-allumage.Combustion by autoignition of a homogeneous charge aims at reducing particulate matter as well as NOx emissions, whilemaintaining higher thermal efficiency of conventional diesel engines. To control this new mode of combustion, a fine understanding of the mechanisms of coupling between aerodynamics and thermochemistry is required.Direct Numerical Simulations of a turbulent reactive flow, undergoing a compression, have been performed. This study led to identification of two regimes. The first, known as quasi-homogeneous, is characterized by volumetric autoignition of large zones of the reactive mixture and results in the generation of strong pressure waves, which are potentially dangerous for the structure of engines. In the second regime, called localized, hot spots are initiated more sporadically in space and time, and their topology is such that no significant pressure wave is generated.ROUEN-INSA Madrillet (765752301) / SudocSudocFranceF

Composition-space premixed flamelet solution with differential diffusion for in situ flamelet-generated manifolds

International audienceCanonical flame problems have been widely used in the combustion literature to tabulate detailed chemistry. Prior to three-dimensional flame simulations, reference laminar premixed flames are usually solved in physical-space, and, diffusion flames in either physical, or, mixture fraction space. Composition-space solutions would be convenient for premixed flame also, because all the points are relevant for chemistry, as a result of the zoom inside the flame zone; however, differential diffusion is not always easy to introduce with accuracy in these moving-frame coordinate systems, parameterized by their physical-space gradients (or scalar dissipation rates). A projection is discussed in this paper that ensures that differential diffusion is properly accounted for, in any composition-space coordinates, thus allowing for perfect matching between physical- and composition-space solutions, even for premixed flames. Both a diffusion velocity correction, which is necessary to properly conserve mass with Fick’s law, and a differential diffusion effect between the composition-space moving with the flow and fast diffusing species, are introduced. A procedure for rapidly building converged composition-space solutions for premixed flamelets is then proposed and tested. It provides the framework for an efficient in situ calculation of complex chemistry with differential diffusion, to be applied to three-dimensional unsteady flame simulations. The objective is to avoid building a priori look-up tables, whose range of validity is strongly limited by their boundary conditions, which are fixed once for all, therefore lacking of generic character, specifically when pressure, composition and enthalpy of fresh gases are varying in space and time

Self-ignition scenarios after rapid compression of a turbulent mixture weakly-stratified in temperature

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Turbulent Combustion Modeling: New Approaches for Highly Refined Simulations

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