Cette Thèse de doctorat est consacrée à l’étude, par simulation aux grandes échelles ou LES (Larg eeddy simulation), d’un jet pariétal d’hydrogène sous-détendu dans un écoulement transverse supersonique d’air vicié. Cette configuration est représentative des conditions d’écoulement rencontrées dans les moteurs aérobies de type super-statoréacteurs (scramjets). En effet, les futurs systèmes de transport à grande vitesse dépendent fortement du développement de ce type de moteur. Dans de telles conditions, l’écoulement d’air chaud est maintenu supersonique dans la chambre de combustion afin de réduire les effets induits par l’échauffement et la dissociation de l’air. Nous étudions les processus de mélange et de combustion qui se développent en aval du jet de combustible. Ce travail s’appuie sur l’emploi d’un outil de simulation numérique haute fidélité : CREAMS (Compressible REActive Multi-species Solver) développé à l’Institut Pprime. Ce code de calcul met en oeuvre des schémas numériques d’ordre élevé : schéma Runge–Kutta d’ordre 3 pour l’intégration temporelle combiné à un schéma WENO d’ordre 7 et centré d’ordre 8 pour la discrétisation spatiale. Les simulations réalisées dans des conditions inertes permettent de caractériser l’importance des interactions choc/turbulence avec une attention particulière accordée à la description des fluctuations de composition à l’échelle non-résolue (i.e. sous-maille). Compte tenu de leur niveau de résolution,les simulations réactives permettent quant à elles une analyse détaillée du mode de stabilisation et des régimes de combustion turbulente rencontrés fournissant ainsi des informations très précieuses quant à l’adéquation des modélisations existantes pour ces conditions extrêmes. / This dissertation is devoted to the Large-eddy simulation (LES) study of a wall hydrogen underexpanded jet in a supersonic crossflow of vitiated air. This configuration is representative of flow conditions encountered in aerospace engines such as supersonic combustion ramjet (scramjets). Indeed, future of high-speed transport systems heavily depends on the development of this type of engine. Under such conditions, the high temperature flow of vitiated air is maintained supersonic in the combustion chamber to reduce effects of heating and dissociation. The mixing and combustion processes that develop downstream of the fuel jet are studied. This work is based on the use of a high fidelity numerical simulation: CREAMS (Compressible REActive Multi-species Solver) which is developed at the Pprime Institute. This computational solver makes use of high precision numerical schemes: a 3rd order Runge–Kutta scheme for the time integration combines with a 7th order WENO and 8th order centered scheme for the spatial discretisation. Non-reactive simulations allow to characterize the importance of shock/turbulence interactions with special attention paid to the description of the unresolved (i.e. sub-grid scale) scalar fluctuations. The reactive simulations allow to perform a detailed analysis of the stabilization mode and turbulent combustion regimes tha are encountered, thus providing valuable information about the possible adequacy of the available representation for these extreme conditions.

Advanced combustion concepts are required to meet the increasing global energy demand and stringent emission regulations imposed by the governments on automobile manufacturers. Improvement in efficiency and reduction in emissions can be achieved by downsizing the Spark Ignition (SI) engines. The operating range of SI engine is limited by occurrence of knock, pre-ignition and the following super-knock due to boosting of intake pressure, to account for the reduction of power, as a result of downsizing the engine. Super-knock, which represents high momentary pressure accompanied with pressure oscillations, is known to permanently damage the moving component of the engines. Therefore fundamental comprehensive understanding of the mechanism involved in pre-ignition and super-knock are required to design highly efficient spark ignition engines with lower emissions that can meet the increasing government regulations. \nThe thesis focuses on auto-ignition characteristics of endgas and the bulk mixture properties that favor transition of pre-ignition to super-knock. Direct numerical studies indicate that super-knock occurs to due to initiation of premature flame front that transition into detonation. In literature, many sources are reported to trigger pre-ignition. Due to the uncertainty of the information on the sources that trigger pre-ignition, it is extremely difficult to predict and control pre-ignition event in SI engines. Since the information on the source of pre-ignition is not available, the main focus of this work is to understand the physical and chemical mechanisms involved in super-knock, factors that influence super-knock and methods to predict super-knock. \n
Pre-ignition was initiated at known locations and crank angle using a hotspot of known size and strength. Different parametric cases were studied and the location and timing of pre-ignition initiation is found to be extremely important in determining the transition of pre-ignition event to super-knock. Pre-ignition increases the temperature of the endgas and the overall bulk mixture, that transitions the pre-ignition flame front to a detonation. The transition of the flame propagation mode from deflagration to detonation was investigated with different type of analysis methods and all results confirmed the transition of pre-ignition flame front to detonation that results in super- knock.

Controlled Auto-Ignition (CAI) also known as Homogeneous Charge Compression Ignition (HCCI) is increasingly seen as a very effective way of lowering both fuel consumption and emissions. Hence, it is regarded as one of the best ways to meet stringent future emissions legislation. It has however, still many problems to overcome, such as limited operating range. This combustion concept was achieved in a production type, 4-cylinder gasoline engine, in two separated tests: naturally aspirated and turbocharged. Very few modifications to the original engine were needed. These consisted basically of a new set of camshafts for the naturally aspirated test and new camshafts plus turbocharger for the boosted test. The first part of investigation shows that naturally aspirated CAI could be readily achieved from 1000 to 3500rpm. The load range, however, decreased noticeably with engine speed due to flow restrictions imposed by the low lift camshafts. Ultra-low levels of NOx emissions and reduced fuel consumption were observed. After baseline experiments with naturally aspirated operation, the capability of turbocharging for extended CAI operation was investigated. The results show that the CAI range could achieve higher load and speed with the addition of the turbocharger. The engine showed increased fuel consumption due to excessive pumping losses. Emissions, however, have been reduced substantially in comparison to the original engine. NOx levels could be reduced by up to 98% when compared to a standard SI production engine.

The Homogeneous Charge Compression Ignition (HCCI) engine is a promising engine concept that emits low concentrations of NOx and particulates and still has a high efficiency. Since the charge is auto-ignited, the auto-ignition quality of the fuel is of major importance. It has been shown in several studies that neither of the classical measures of auto-ignition quality of gasoline-like fuels, RON and MON, can alone describe this in all conditions in HCCI combustion. However, even in such cases it is possible to combine RON and MON into an octane index, OI, that describes the auto-ignition quality well in most conditions. The octane numbers are combined into the OI with the variable K according to the following equation: OI = (1-K)RON + K MON = RON – K S The OI of a sensitive fuel is the equivalent of the octane number of a primary reference fuel with the same resistance to auto-ignition in the tested condition. The K-value is dependent on the temperature and pressure history. A generic parameter Tcomp15, the temperature at 15 bar during the compression, was introduced to describe the temperature and pressure history. It was found that the K-value increases with increasing Tcomp15 and two linear equations have been suggested to describe this relationship. At high or low Tcomp15 it has been found that the sensitivity of the fuel octane quality on combustion phasing is small and the auto-ignition quality defined by the OI scale does no longer play a big role. NO affects the combustion phasing of gasoline-like fuels. This effect is most significant at low concentration where it advances the combustion phasing considerably. At higher conditions its influence is different for different fuels. A sensitive fuel is considered a good HCCI fuel since its OI changes in the same direction as the octane requirement of the engine, which would make the engine management easier. It is also likely that a sensitive fuel will enable a wider operating range. The auto-ignition quality of diesel-like fuels was studied in tests with three different strategies of mixture formation. In these tests it was found that the ignition delay increased with lower cetane number and that the cetane number described the auto-ignition quality well, even for fuels of significantly different physical properties. The experiments were, however, made at a limited range of operating conditions and low load. A good diesel-like HCCI fuel should be easy to vaporize to facilitate homogeneity. It should have a high resistance to auto-ignition, not necessarily the highest, one that allows both high and low loads at a given compression ratio. Finally, it should also function well with the injection system without a significant decrease in injection system life length. / QC 20100917

Les responsabilités environnementales font que les constructeurs automobiles visent à acquérir des connaissances approfondies sur les phénomènes physico-chimiques des chambres de combustion des moteurs pour la compréhension et le contrôle des émissions polluantes. En dépit des performances remarquables dans le domaine du calcul numérique intensif de très haute performance, les simulations numériques des chambres de combustion des moteurs à combustion interne ne permettent pas encore une description détaillée des processus chimiques. L’objectif de ces travaux est d’améliorer les modèles de combustion turbulente basés sur la Simulation aux Grandes Echelles en développant une méthode de tabulation de la cinétique chimique représentative de la combustion dans un cycle moteur. Une attention toute particulière est portée sur la capacité de la méthode de tabulation à reproduire la formation des espèces polluantes lors des phases de compression, d’auto-allumage et de détente d’un moteur à combustion interne. Cette méthode suppose que la chimie dans une chambre de combustion interne peut être approchée par une tabulation des résultats issus de calculs de réacteurs isochores 0-D, les coordonnées de la table étant: la variable de progrès, l’énergie et la masse volumique. La capacité prédictive de la technique de tabulation a été testée par des simulations d’auto-allumage en volume variable. La méthode de tabulation "Tabulated Thermo Chemistry" (TTC) a été initialement développée pour coupler la cinétique chimique à des solveurs Navier-Stokes compressibles dans des situations d’écoulement à faible nombre de Mach. Dans ce travail, la méthode TTC a été adaptée pour la combustion à pression variable. Une version TTC dédiée à la combustion dans les moteurs est donc implantée dans un code instationnaire LES compressible. La méthode de couplage a été testée avec succès tant sur des configurations simplifiées que sur une Machine à Compression Rapide. / Due to environmental concerns, automotive manufacturers aim at acquiring knowledge of physical and chemical phenomena inside the combustion chamber to understand and control pollutant emissions. Despite the outstanding performances in the domain of high performance intensive numerical calculation, the numerical simulations of the combustion chambers of the internal combustion engines do not allow a detailed description of the chemical processes. The present study aims to improve the turbulent combustion models based on Large Eddy Simulation approach by developing an efficient cost cutting tabulation method to fit chemistry in engine combustion modeling. A particular attention is paid to the capacity of the tabulation method to reproduce the pollutant species formation during the compression stroke, the reaction phase, and the power stroke of an ICE. This method assumes that IC engine chemistry can be mapped by a collection of 0-D reactor computation data, using for coordinates: the progress variable, the energy and the density. In a first step, the chemical prediction capability of this technique is validated on an auto-igniting variable volume simulation. The Tabulated Thermo Chemistry (TTC) method was initially developed to couple the chemical kinetics with compressible Navier-Stokes solvers for low Mach number flows. In this work, the TTC method was adapted in the framework of variable pressure combustion. A TTC version dedicated to engine combustion is thus coupled in an unsteady compressible LES code. The method of coupling was successfully tested both on simplified configurations and on a Rapid Compression Machine.

Extensive analysis of the physical and chemical effects controlling the operation of combustion modes driven by auto-ignition is presented in this thesis. Specifically, the study integrates knowledge attained by analyzing the effects of fuel molecular structure on auto-ignition, quantity or quality of charge dilution, and in-cylinder temperature and pressure on burning characteristics in single and multiple injection strategies employed in compression ignition (CI), partially premixed combustion (PPC) and homogenous charge compression ignition (HCCI) engines. In the first section of the thesis, a multiple injection strategy aimed to produce heat at a constant pressure, commonly known as isobaric combustion, has been studied. Then, to eliminate the complexity of spray-to-spray interactions observed with isobaric combustion, the second section of the thesis is focused on compression ignition (CI) through single injection. In the final section, the presentation will move towards moderate conditions with high dilution, in which combustion becomes dominated by chemical kinetics. At these conditions, there is emerging evidence that certain fuels exhibit unusual heat release characteristics where fuel releases heat in three distinctive stages. Overall, the thesis discusses factors controlling the auto-ignition for CI, PPC and HCCI engines that can provide valuable insights to improve their operation. Isobaric combustion in CI engine involves large interactions between physical and chemical effects. Injection of spray jets into oxygen-deprived regions catalyzes the mechanism for soot production – urging to employ either multiple injectors, low reactivity fuel or an additional expansion stage. Fuels – regardless of their auto-ignition tendency – share the same combustion characteristics in the high load CI, where auto-ignition is controlled by only the injector’s physical specifications. Such observation is a showcase of the fuel flexible engines that has the potential of using sustainable fuels – without being restrained by the auto-ignition properties of the fuel. The thesis provides evidence from experiment and simulation that three-stage auto-ignition is indeed a phenomenon driven by chemical kinetics. Three-stage auto-ignition opens the perspective to overcome the limitation of the high-pressure rise rates associated with HCCI engine.

A successful transition to clean energy hinges on meeting the world's growing energy demand while reducing greenhouse gas emissions. Achieving this will require significant growth in electricity generation from clean and carbon-free energy sources. Several energy providers have already begun the transition from traditional carbon-based fuels to cleaner alternatives, such as hydrogen and hydrogen enriched natural gas. However, there are still many technical challenges that must be addressed when applying these fuels in gas turbines. The application of hydrogen or hydrogen/natural gas blends to advanced class gas turbines, which have higher operating pressures and temperatures has raised concerns about the potential for leakages or fuel sequencing operations where flammable mixtures of fuel and air could auto-ignite. Public information on the auto-ignition of hydrogen in air at atmospheric pressure is well documented. Such data shows the auto-ignition temperature of hydrogen is roughly 100 °C lower than that of methane. Studies also show that as pressure increases, methane's auto-ignition temperature decreases. However, there was insufficient information in the published literature to characterize the influence of pressure on auto-ignition for hydrogen fuel applications. This study describes the test methodology used to evaluate conditions where auto-ignition occurs for various fuel-air mixtures operating at pressures between 1-30 atmospheres and equivalence ratios between 0.2-1.6. Testing was completed with hydrogen, natural gas and blends at various equivalence ratios using a heated volume with multiple reactant delivery methods. Testing was performed for natural gas to validate the test and data collection methods cited in prior published literature. Results indicate that at atmospheric pressures, an increase in hydrogen concentration results in a reduced auto-ignition temperature. However, at 30 atmospheres, the auto-ignition temperature increased with higher hydrogen concentrations. iv Variations of auto-ignition delay times were also observed during the testing and are compared to modeling predictions, providing insight into auto-ignition characteristics.

Les préoccupations environnementales actuelles ont conduit les constructeurs automobiles à proposer de nouvelles technologies dans le but de réduire les émissions de CO2. Parmi ces technologies, le downsizing appliqué aux moteurs turbocompressés à allumage commandé est une des solutions privilégiées, car permettant d'atteindre des points de fonctionnement fortement chargés, avec un meilleur rendement thermique. Cependant, les fortes charges favorisent l'apparition de cliquetis, un phénomène potentiellement dommageable pour le moteur et qui l'empêche d'exploiter tout son potentiel. Du fait des variabilités cycliques de combustion dans le moteur, le cliquetis, qui dépend des conditions locales dans la chambre de combustion, peut apparaître uniquement sur quelques cycles, à différents endroits et instants. Dans cette thèse, une approche par Simulation aux Grandes Échelles (SGE) a été choisi, dans le but d'étudier et d'améliorer notre compréhension du cliquetis. L'étude se base sur la SGE d'un moteur industriel, le RENAULT 1.2 TCe 115. Un premier ensemble de 30 cycles a été simulé sur un point de fonctionnement de référence, correspondant à un point cliquetant dans la base de données banc d'essais fournie par RENAULT. Les résultats de simulation ont été comparés aux résultats expérimentaux, aussi bien en termes de variabilités cycliques de combustion que de cliquetis. A la suite, un balayage d'avance à l'allumage a été simulé pour étendre la base de données LES à des points plus faiblement et plus fortement cliquetants. La base de données résultante se compose de 150 cycles de combustion, utilisés pour développer des méthodologies et outils, dans le but de mieux caractériser le cliquetis et d'approfondir sa compréhension. L'accès numérique à toute grandeur dans la chambre de combustion, combiné à la description séparée dans cette simulation entre la flamme de pré-mélange initiée par la bougie et l'auto-inflammation dans les gaz frais, ont permis de caractériser le cliquetis en se focalisant sur son origine : l'auto-inflammation. A la suite, les méthodologies et outils développés ont soutenu une analyse détaillée des mécanismes qui contrôlent l'apparition du cliquetis. En particulier, le lien entre le cliquetis et les variabilités cycliques de combustion a été exploré. Les résultats mettent notamment en évidence l'impact des variabilités cycliques, aussi bien de la vitesse de propagation que de la forme de la flamme de pré-mélange, sur le cliquetis. / The rising concerns about the environment have led car manufacturers to come up with new engine technologies, in order to reduce the impact of internal combustion engines on CO2 emissions. In this context, downsizing of turbocharged spark-ignition engines has become a commonly used technology, the advantage of which is to operate the engine under thermally more efficient high loads. However, these high loads favour the appearance of potentially damaging knock phenomena, which prevent the engine to fully exploit its potential. Because of cyclic combustion variability (CCV) in the engine, knock, which depends on the local conditions inside the combustion chamber, can appear at different locations and timings and not in all engine cycles. In this thesis, a Large-Eddy Simulation (LES) approach was selected to investigate and further improve our understanding of the appearance of knock. The study is based on the LES of a production engine, the RENAULT 1.2 TCe 115. For this engine, a set of 30 cycles was initially simulated at a single operating point, corresponding to a knocking point in the test bench database from RENAULT. The results were compared to experimental findings, both in terms of CCV and knock. Subsequently, a spark-timing sweep was simulated in order to enlarge the LES database to also include weaker and stronger knock levels. The resulting LES, which consists of 150 combustion cycles, was used to develop methodologies and tools with the objective to better characterize and understand knock. The computational access to any quantity inside the combustion chamber, together with the separate description with the present LES approach between the spark-triggered premixed flame propagation and auto-ignition, were exploited to characterize knock focusing on its source: autoignition in the fresh gases. Then, the developed methodologies and tools supported a detailed analysis of the mechanisms that control the knock onset. In particular, its link with CCV was explored. The results point out the impact of the cyclic variability in the premixed flame propagation speed and shape on knock.

This thesis reports a model to predict the auto-ignition time of single droplets of n-paraffin fuel mixtures using the method of continuous thermodynamics. The model uses experimental data for pure fuels to fit rate parameters for a single-step global chemical reaction equation; from this, correlations for rate parameters as a function of species molecular mass are derived, which are integrated to produce a continuous thermodynamics expression for mixture reaction rate. Experiments were carried out using the suspended droplet-moving furnace technique. The model was then tested and compared to experimental data for three continuous mixtures with known compositions: one ranging from ¬n-octane to n-hexadecane, the second ranging from n-dodecane to n-eicosane, and the third being a combination of the first two mixtures to produce a “dumbbell” mixture. Discrete and continuous mixture models of the ASTM standard distillation test were compared to design the experimental mixtures and provide the distribution parameters of the continuous mixtures intended to simulate them. The results of calculations were found to agree very well with measured ignition times for the mixtures.

This thesis reports a model to predict the auto-ignition time of single droplets of n-paraffin fuel mixtures using the method of continuous thermodynamics. The model uses experimental data for pure fuels to fit rate parameters for a single-step global chemical reaction equation; from this, correlations for rate parameters as a function of species molecular mass are derived, which are integrated to produce a continuous thermodynamics expression for mixture reaction rate. Experiments were carried out using the suspended droplet-moving furnace technique. The model was then tested and compared to experimental data for three continuous mixtures with known compositions: one ranging from ¬n-octane to n-hexadecane, the second ranging from n-dodecane to n-eicosane, and the third being a combination of the first two mixtures to produce a “dumbbell” mixture. Discrete and continuous mixture models of the ASTM standard distillation test were compared to design the experimental mixtures and provide the distribution parameters of the continuous mixtures intended to simulate them. The results of calculations were found to agree very well with measured ignition times for the mixtures.