Auswahl der wissenschaftlichen Literatur zum Thema „Flammes turbulentes en expansion“

We study the propagation of premixed flames in two-dimensional homogeneous isotropic turbulence using a Navier–Stokes/front-capturing methodology within the context of hydrodynamic theory. The flame is treated as a thin layer separating burnt and unburnt gases, of vanishingly small thickness, smaller than the smallest fluid scales. The method is thus suitable to investigate the flame propagation in the wrinkled flamelet regime of turbulent combustion. A flow-control system regulates the mean position of the flame and the incident turbulence intensity. In this context we study the individual effects of turbulence intensity, turbulence scale, thermal expansion, hydrodynamic strain and hydrodynamic instability on the propagation characteristics of the flame. Results are obtained assuming positive Markstein length, corresponding to lean hydrocarbon–air or rich hydrogen–air mixtures. For stable planar flames we find a quadratic dependence of turbulent speed on turbulence intensity. Upon onset of hydrodynamic instability, corrugated structures replace the planar conformation and we observe a greater resilience to turbulence, the quadratic scaling being replaced by scaling exponents less than one. Such resilience is also confirmed by the observation of a threshold turbulence intensity below which the propagation speed of corrugated flames is indistinguishable from the laminar speed. Turbulent speed is found to increase and later plateau with increasing thermal expansion, this affecting the average flame displacement but not the mean flame curvature. In addition, turbulence integral scale is also observed to affect the propagation of the flame with the existence of an intermediate scale maximizing the turbulent speed. This maximizing scale is smaller for corrugated flames than it is for planar flames, implying that small eddies that will be unable to significantly perturb a planar front could be rather effective in perturbing a corrugated flame. Turbulent planar flames, and more so corrugated flames, were observed to experience a positive mean hydrodynamic strain, which was explained in terms of the overwhelming mean contribution of the normal component of strain. The positive straining causes a decrease in the mean laminar propagation speed which in turn can decrease the turbulent speed. The effect of the flame on the incident turbulent field was examined in terms of loss of isotropy and vorticity destruction by thermal expansion. The latter can be mitigated by a baroclinic vorticity generation which is enhanced for corrugated flames.

AbstractThe thermal expansion induced by the exothermic chemical reactions taking place in a turbulent reactive flow affects the velocity field so strongly that the large-scale velocity fluctuations as well as the small-scale velocity gradients can be governed by chemistry rather than by turbulence. Moreover, thermal expansion is well known to be responsible for counter-gradient turbulent diffusion and flame-generated turbulence phenomena. In the present study, by making use of an original splitting procedure applied to the velocity field, we establish the occurrence of two distinct thermal expansion effects in the flamelet regime of turbulent premixed combustion. The first is referred to as the direct thermal expansion effect. It is associated with a local flamelet crossing contribution as previously considered in early analyses of turbulent transport in premixed flames. The second, denoted herein as the indirect thermal expansion effect, is an outcome of the turbulent wrinkling processes that increases the flame surface area. Based on a splitting procedure applied to the velocity field, the respective influences of the two effects are identified and analysed. Furthermore, the theoretical analysis shows that the thermal expansion induced through the local flames can be treated separately in the usual continuity and momentum equations. This description of the turbulent reactive velocity field, leads also to relate all of the usual turbulent quantities to the reactive scalar field. Finally, algebraic closures for the turbulent transport terms of mass and momentum are proposed and successfully validated through comparison with direct numerical simulation data.

AbstractThe purpose of this paper is to demonstrate the effects of thermal expansion, as a result of heat release arising from exothermic chemical reactions, on the underlying turbulent fluid dynamics and its modelling in the case of turbulent premixed combustion. The thermal expansion due to heat release gives rise to predominantly positive values of dilatation rate within turbulent premixed flames, which has been shown to have significant implications on the flow topology distributions, and turbulent kinetic energy and enstrophy evolutions. It has been demonstrated that the magnitude of predominantly positive dilatation rate provides the measure of the strength of thermal expansion. The influence of thermal expansion on fluid turbulence has been shown to strengthen with decreasing values of Karlovitz number and characteristic Lewis number, and with increasing density ratio between unburned and burned gases. This is reflected in the weakening of the contributions of flow topologies, which are obtained only for positive values of dilatation rate, with increasing Karlovitz number. The thermal expansion within premixed turbulent flames not only induces mostly positive dilatation rate but also induces a flame-induced pressure gradient due to flame normal acceleration. The correlation between the pressure and dilatation fluctuations, and the vector product between density and pressure gradients significantly affect the evolutions of turbulent kinetic energy and enstrophy within turbulent premixed flames through pressure-dilatation and baroclinic torque terms, respectively. The relative contributions of pressure-dilatation and baroclinic torque in comparison to the magnitudes of the other terms in the turbulent kinetic energy and enstrophy transport equations, respectively strengthen with decreasing values of Karlovitz and characteristic Lewis numbers. This leads to significant augmentations of turbulent kinetic energy and enstrophy within the flame brush for small values of Karlovitz and characteristic Lewis numbers, but both turbulent kinetic energy and enstrophy decay from the unburned to the burned gas side of the flame brush for large values of Karlovitz and characteristic Lewis numbers. The heat release within premixed flames also induces significant anisotropy of sub-grid stresses and affects their alignments with resolved strain rates. This anisotropy plays a key role in the modelling of sub-grid stresses and the explicit closure of the isotropic part of the sub-grid stress has been demonstrated to improve the performance of sub-grid stress and turbulent kinetic energy closures. Moreover, the usual dynamic modelling techniques, which are used for non-reacting turbulent flows, have been shown to not be suitable for turbulent premixed flames. Furthermore, the velocity increase across the flame due to flame normal acceleration may induce counter-gradient transport for turbulent kinetic energy, reactive scalars, scalar gradients and scalar variances in premixed turbulent flames under some conditions. The propensity of counter-gradient transport increases with decreasing values of root-mean-square turbulent velocity and characteristic Lewis number. It has been found that vorticity aligns predominantly with the intermediate principal strain rate eigendirection but the relative extents of alignment of vorticity with the most extensive and the most compressive principal strain rate eigendirections change in response to the strength of thermal expansion. It has been found that dilatation rate almost equates to the most extensive strain rate for small sub-unity Lewis numbers and for the combination of large Damköhler and small Karlovitz numbers, and under these conditions vorticity shows no alignment with the most extensive principal strain rate eigendirection but an increased collinear alignment with the most compressive principal strain rate eigendirection is obtained. By contrast, for the combination of high Karlovitz number and low Damköhler number in the flames with Lewis number close to unity, vorticity shows an increased collinear alignment with the most extensive principal direction in the reaction zone where the effects of heat release are strong. The strengthening of flame normal acceleration in comparison to turbulent straining with increasing values of density ratio, Damköhler number and decreasing Lewis number makes the reactive scalar gradient align preferentially with the most extensive principal strain rate eigendirection, which is in contrast to preferential collinear alignment of the passive scalar gradient with the most compressive principal strain rate eigendirection. For high Karlovitz number, the reactive scalar gradient alignment starts to resemble the behaviour observed in the case of passive scalar mixing. The influence of thermal expansion on the alignment characteristics of vorticity and reactive scalar gradient with local principal strain rate eigendirections dictates the statistics of vortex-stretching term in the enstrophy transport equation and normal strain rate contributions in the scalar dissipation rate and flame surface density transport equations, respectively. Based on the aforementioned fundamental physical information regarding the thermal expansion effects on fluid turbulence in premixed combustion, it has been argued that turbulence and combustion modelling are closely interlinked in turbulent premixed combustion. Therefore, it might be necessary to alter and adapt both turbulence and combustion modelling strategies while moving from one combustion regime to the other.

The recirculation zone length behind a bluff body is influenced by the turbulence intensity at the base of the body in isothermal flows and also the heat release and its interaction with turbulence in reacting flows. This relationship is observed to be nonlinear and is controlled by the balance of forces acting on the recirculation zone, which arise from the pressure and turbulence fields. The pressure force is directly influenced by the volumetric expansion resulting from the heat release, whereas the change in the turbulent shear force depends on the nonlinear interaction between turbulence and combustion. This behaviour is elucidated through a control volume analysis. A scaling relation for the recirculation zone length is deduced to relate the turbulence intensity and the amount of heat release. This relation is verified using the large eddy simulation data from 20 computations of isothermal flows and premixed flames that are stabilised behind the bluff body. The application of this scaling to flames in an open environment and behind a backward facing step is also explored. The observations and results are explained on a physical basis.

The second-order structure functions (SFs) of the velocity field, which characterize the velocity difference at two points, are widely used in research into non-reacting turbulent flows. In the present paper, the approach is extended in order to study the influence of combustion-induced thermal expansion on turbulent flow within a premixed flame brush. For this purpose, SFs conditioned to various combinations of mixture states at two different points (reactant–reactant, reactant–product, product–product, etc.) are introduced in the paper and a relevant exact transport equation is derived in the appendix. Subsequently, in order to demonstrate the capabilities of the newly developed approach for advancing the understanding of turbulent reacting flows, the conditioned SFs are extracted from three-dimensional (3-D) direct numerical simulation data obtained from two statistically 1-D planar, fully developed, weakly turbulent, premixed, single-step-chemistry flames characterized by significantly different (7.53 and 2.50) density ratios, with all other things being approximately equal. Obtained results show that the conditioned SFs differ significantly from standard mean SFs and convey a large amount of important information on various local phenomena that stem from the influence of combustion-induced thermal expansion on turbulent flow. In particular, the conditioned SFs not only (i) indicate a number of already known local phenomena discussed in the paper, but also (ii) reveal a less recognized phenomenon such as substantial influence of combustion-induced thermal expansion on turbulence in constant-density unburned reactants and even (iii) allow us to detect a new phenomenon such as the appearance of strong local velocity perturbations (shear layers) within flamelets. Moreover, SFs conditioned to heat-release zones indicate a highly anisotropic influence of combustion-induced thermal expansion on the evolution of small-scale two-point velocity differences within flamelets, with the effects being opposite (an increase or a decrease) for different components of the local velocity vector.

The interrelation between Reynolds stresses and their dissipation rate tensors for different Karlovitz number values was analysed using a direct numerical simulation (DNS) database of turbulent statistically planar premixed H2-air flames with an equivalence ratio of 0.7. It was found that a significant enhancement of Reynolds stresses and dissipation rates takes place as a result of turbulence generation due to thermal expansion for small and moderate Karlovitz number values. However, both Reynolds stresses and dissipation rates decrease monotonically within the flame brush for large Karlovitz number values, as the flame-generated turbulence becomes overridden by the strong isotropic turbulence. Although there are similarities between the anisotropies of Reynolds stress and its dissipation rate tensors within the flame brush, the anisotropy tensors of these quantities are found to be non-linearly related. The predictions of three different models for the dissipation rate tensor were compared to the results computed from DNS data. It was found that the model relying upon isotropy and a linear dependence between the Reynolds stress and its dissipation rates does not correctly capture the turbulence characteristics within the flame brush for small and moderate Karlovitz number values. In contrast, the models that incorporate the dependence of the invariants of the anisotropy tensor of Reynolds stresses were found to capture the components of dissipation rate tensor for all Karlovitz number conditions.

A laboratory-scale chamber is convenient for combustion scenarios in the practical analysis of industrial explosions and devices such as internal combustion engines. The safety risks in hazardous areas can be assessed and managed during accidents. Increased hydrogen usage in renewable energy production requires increased attention to the safety issues since hydrogen produces higher explosion overpressures and flame speed and can cause more damage than methane or propane. This paper reports numerical simulation of turbulent hydrogen combustion and flame propagation in the University of Sydney's small-scale combustion chamber. It is used for the investigation of turbulent premixed propagating flame interaction with several solid obstacles. Obstructions in the direction of flow cause a complex flame front interaction with the turbulence generated ahead of it. For numerical analysis, OpenFOAM CFD software was chosen, and a custom-built turbulent combustion solver based on the progress variable model—flameFoam—was used. Numerical results for validation purposes show that the pressure behaviour and flame propagation obtained using RANS and TFC models were well reproduced. The interaction between larger-scale flow features and flame dynamics was obtained corresponding to the experimental or mode detailed LES modelling results from the literature. The analysis revealed that as the propagating flame reached and interacted with obstacles and the recirculation wake was created behind solid obstacles, leaving traces of an unburned mixture. The expansion of flames due to narrow vents generates turbulent eddies, which cause wrinkling of the flame front.

Le moteur downsizé à allumage commandé constitue l'une des voies principales explorées par les constructeurs automobiles pour améliorer le rendement et réduire les émissions de dioxyde de carbone des motorisations essence. Il s'agit de combiner une réduction de la cylindrée avec une forte suralimentation afin d'améliorer le rendement du moteur, en particulier à faibles et moyennes charges. Leur mise au point est limitée par l'augmentation des combustions anormales, dont le contrôle par forte dilution peut également entraîner l'apparition de variabilités cycliques importantes. Actuellement, la compréhension des nombreux paramètres intervenant dans l'apparition de ces phénomènes et de leurs interactions, reste encore imparfaite. Dans ce contexte, l'objectif de ce travail est de contribuer à la compréhension des mécanismes impliqués dans les processus de propagation des flammes turbulentes. Cette étude est réalisée dans une enceinte de combustion sphérique haute pression haute température, équipée de ventilateurs générant une turbulence homogène et isotrope. La première partie de ce travail est consacrée à l'étude de la combustion prémélangée laminaire isooctane/air. Dans un deuxième temps, l'aérodynamique de l'écoulement dans l'enceinte est finement caractérisée par Vélocimétrie Laser Doppler et Vélocimétrie par Images de Particules. Enfin, la propagation des flammes turbulentes est étudiée en termes de vitesse à partir de visualisations par ombroscopie. Une loi unifiée, permettant de décrire la propagation des flammes turbulentes indépendamment des conditions thermodynamiques initiales, de l'intensité de la turbulence et de la nature du mélange réactif est notamment proposée.

La dynamique des flammes de prémélange est étudiée par deux approches numériques différentes. La première résout les équations compressibles de Navier-Stokes avec une chimie simplifiée (DNS). Afin de réduire les coûts de calcul, nous analysons et développons un schéma numérique à grille décalée. Le traitement des ondes acoustiques aux sorties est connu pour rendre les flammes cylindriques légèrement carrées. Ces déformations non-physiques sont expliquées en mettant en évidence la modélisation insuffisamment précise de l'accélération du fluide lorsque l'écoulement est oblique à la sortie. Une étude paramétrique et statistique de flammes turbulentes est menée en 2D et une simulation parallèle 3D est réalisée dans un domaine de (3cm)3. En considérant la flamme infiniment mince, l'approche EEM diminue considérablement les coûts de calcul. Les mêmes simulations sont réalisées et comparées aux résultats de DNS pour tester la capacité du modèle EEM à fournir des résultats quantitatifs.

Le cycle thermodynamique classique des turbines à gaz n'a subi aucune modification majeure au cours des dernières décennies, et les améliorations d'efficacité les plus importantes ont été obtenues en réduisant les pertes thermiques, en augmentant le taux de compression et la température maximale. Malgré les efforts visant à améliorer les performances des chambres de combustion, les technologies actuelles pourraient ne pas être à la hauteur des contraintes environnementales de plus en plus strictes. Par conséquent, une percée technologique est essentielle pour façonner l'avenir des moteurs thermiques. La combustion à gain de pression (PGC) émerge comme l'une des solutions les plus prometteuses, introduisant de nouveaux cycles thermodynamiques où la pression augmente tout au long du processus de combustion. Cela peut conduire à une augmentation d'entropie plus faible, bénéficiant à l'efficacité globale du cycle.Plusieurs concepts de PGC sont actuellement étudiés par la communauté scientifique, allant de la déflagration, telle que la combustion à volume constant (CVC), à la détonation, notamment la combustion à détonation rotative (RDC). La simulation numérique est utilisée pour évaluer les performances de ces systèmes et pour mieux comprendre leur comportement afin de les améliorer avant de procéder à des essais expérimentaux. La simulation aux grandes échelles (LES) a un rôle important dans ce domaine grâce à sa capacité à prédire fidèlement les écoulements réactifs. Cependant, avec la complexité croissante des systèmes de combustion, des modèles physiques avancés sont cruciaux pour assurer des simulations prédictives.Dans ce travail, la combustion à volume constant est évaluée et les principaux défis numériques posés par ces systèmes de combustion sont examinés. L'allumage, la combustion à haute pression, la dilution, l'interaction flamme-turbulence, les effets d'étirement, les flux de chaleur font partie intégrante de la physique que les systèmes CVC englobent, et leur interaction conduit à des phénomènes physiques complexes qui doivent être modélisés. Les modèles numériques développés dans ce travail sont principalement examinés dans des cas test, puis appliqués dans le calcul de la chambre à volume constant CV2, opérée au laboratoire Pprime (Poitiers, France).D'abord, des nouvelles conditions limites sont dérivées de la théorie des tuyères pour mimer les effets des soupapes d'admission et d'échappement. Les propriétés d'écoulement sont imposées dynamiquement à la fois à l'entrée et à la sortie de ces systèmes contrôlés par des vannes.Une chimie globale pour les mélanges propane/air est dérivée pour différentes pressions, températures et compositions de gaz frais. La cinétique chimique est optimisée pour différentes concentrations de diluants, composés des gaz brûlés tels que le dioxyde de carbone et la vapeur d'eau. Comme les moteurs à piston, les chambres CVC fonctionnent cycliquement, et chaque cycle de combustion est influencé par les gaz résiduels provenant des cycles précédents. Pour cette raison, un modèle numérique détaillant la composition locale des mélanges inflammables dilués est proposé pour fournir toutes les informations sur les gaz frais nécessaires à la cinétique et au modèle de combustion. Basé sur une généralisation du Thickened Flame (TF), un nouveau modèle de combustion, le Stretched-Thickened Flame (S-TF) model, est développé pour surmonter les limitations du modèle TF dans la prédiction des effets d'étirement sur la vitesse de combustion des flammes laminaire. Cela est crucial pour capturer efficacement les événements transitoires des flammes propagative, fondamentaux dans les chambres CVC. Enfin, dans le cadre de la modélisation de l'allumage, le modèle de dépôt d'énergie est couplé avec le modèle S-TF.Les modèles développés dans cette thèse sont ensuite appliqués à la chambre CV2, mettant en évidence leur impact positif dans la prédiction de la physique transitoire impliquée dans ces systèmes
Classical gas turbine thermodynamic cycle has undergone no major changes over the last decades and the most important efficiency improvements have been obtained reducing thermal losses and raising the overall pressure ratio and peak temperature. Despite the efforts in research and development aiming at enhancing especially combustion chambers performances, current technologies may fall short of complying the increasingly stringent environmental constraints. Consequently, a technological breakthrough is essential to shape the future of thermal engines. Pressure Gain Combustion (PGC) emerges as one of the most promising solutions, introducing new thermodynamic cycles where, unlike the Brayton cycle, pressure increases across the combustion process. This can lead to a lower entropy raise, benefiting the overall cycle efficiency.Several PGC concepts are currently studied by the combustion community, ranging from deflagration, such as constant volume combustion (CVC), to detonation, including Rotating Detonation Combustion (RDC) and Pulse Detonation Engine (PDE). Numerical simulation is used to assess the performance of these systems as well as better understand their behavior for improvements before performing experimental tests. Large Eddy Simulation (LES) has assumed an increasingly significant role in combustion science thanks to its high capability in capturing reacting flows. However, with the increasing complexity of combustion systems, advanced physical models are crucial to ensure predictive simulations.In this work, constant volume combustion technology is assessed and the main numerical challenges posed by these combustion systems are scrutinized. Ignition, high pressure combustion, dilution, flame-turbulence interaction, flame-stretch effects, heat fluxes are just part of the physics that CVC systems encompass and their interplay leads to complex physical phenomena that have to be modeled. The numerical models developed in this work are primarily scrutinized in simple test cases and then applied in complete 3D LES framework to compute the constant volume combustion chamber CV2, operated at Pprime laboratory (Poitiers, France).First, novel boundary conditions, based on NSCBC formalism, are derived from nozzle theory to mimic intake and exhaust valve effects. With this strategy no moving part is introduced in the LES and the flow properties are imposed both at the inlet and the outlet of these valves-controlled systems.Second, a two-step chemistry for propane/air mixtures is derived for multiple pressure, temperature and composition of fresh gases. The chemical kinetics is optimized for different concentration of dilutants, composed by burnt products such as carbon dioxide and water vapor. Like piston engines, constant volume chambers operate cyclically and each combustion event is affected by the residual burnt gases coming from previous cycles. For this reason, a numerical model to detail the local composition of diluted flammable mixtures is proposed to provide all the fresh gas information required by the kinetics and the combustion model. Based on a generalization of the classical Thickened Flame (TF) model, a new combustion model, the Stretched-Thickened Flame (S-TF) model, is developed to overcome the TF model limitations in predicting stretch effects on the laminar flame burning velocity. This is crucial to well capture transient events of propagating flames, which are fundamental in CVCs.Eventually, the ignition modeling is assessed and the Energy Deposition model is coupled with the S-TF model by tracking the kernel size in time.The models developed in this thesis are then applied to the CV2 chamber, highlighting their positive impact in capturing the unsteady physics involved in such systems

La propagation de flammes en expansion dans un prémélange propane-air est étudiée pour des mélanges présentant des répartitions hétérogènes de combustible. Dans une première partie, une flamme laminaire est allumée dans une stratification symétrique de combustible. Pendant la propagation de la flamme, des enregistrements tomographiques sont effectués pour différents temps de propagation et différentes répartitions initiales de combustible. Dès le début de la propagation, la flamme dans son ensemble est influencée par la stratification. La courbure moyenne peut être déduite du comportement de la flamme homogène dans des conditions stoechiométriques à travers une loi linéaire ; cette loi dépend de l'amplitude initiale de la fraction de mélange 𝑍 et du temps de propagation. Des mesures simultanées de vitesse absolue de déplacement et d'émission de radicaux OH* et CH*, combinées avec des mesures de 𝑍 par P. L. I. F. Montrent la faible contribution du terme -2𝘋/𝑍 𝑛. Grad(𝑍) présent dans la définition de la vitesse de déplacement du front de flamme. On montre que "l'effet mémoire" de la stratification est principalement dû au réservoir de radicaux et de température induit, à l'inertie de l'allumage et à l'expansion thermique liée aux zones brûlant à la stoechiométrie. Dans une seconde partie, pour une turbulence de grille donnée, une comparaison entre des flammes de prémélanges homogènes et hétérogènes est réalisée, et ce, pour différentes fractions de mélanges moyennes et niveaux de fluctuations. L'étude géométrique des flammes montre l'importance du niveau d'hétérogénéités sur la propagation pour les différentes conditions. Dans les premiers temps de propagation, les fluctuations de fraction de mélange n'affectent pas la taille de la flamme, pour des conditions moyennes stoechiométriques. Dans les instants suivants, un niveau optimum d'hétérogénéités peut être déterminé, sous forme d'un couple (𝑍, 𝑍'), pour lequel la propagation de la flamme est équivalente aux flammes homogènes ayant une fraction de mélange moyenne supérieure. Ce soutien est dû au réservoir chimique et thermique induit, permettant d'obtenir une combustion dans un milieu en dessous des limites classiques d'inflammabilité.

En vue d'atteindre la neutralité carbone d'ici 2050, l’Union européenne envisage l'hydrogène comme un vecteur énergétique prometteur afin de réduire la consommation des ressources fossiles. Alors que les piles à combustible et les véhicules électriques occupent déjà une place importante dans la décarbonation du secteur des transports, l'hydrogène est également considéré comme une alternative aux carburants conventionnels pour les véhicules lourds. Toutefois, de nombreux obstacles liés aux propriétés physico-chimiques ainsi qu’à la combustion pauvre en hydrogène sont encore à l’étude : apparition de phénomènes de combustion anormale, production d'oxydes d'azote, instabilités dues aux effets thermodiffusifs. Cette thèse contribue à la compréhension du processus d’auto-inflammation des mélanges pauvres hydrogène/air ainsi qu’au phénomène de propagation des flammes prémélangées laminaires et turbulentes. Dans une première partie, des mesures de délais d’auto-inflammation hydrogène/air et hydrogène/air/oxydes d’azote sont réalisées à l’aide d’une machine à compression rapide afin de revisiter et valider un mécanisme cinétique dans des conditions similaires à celles rencontrées dans un moteur à allumage commandé. Dans une deuxième partie, des flammes laminaires prémélangées en expansion sont étudiées au sein d’une chambre de combustion sphérique à volume constant, en faisant varier la température initiale ainsi que la dilution à la vapeur d’eau et en considérant les instabilités intrinsèques liées aux propriétés physico-chimiques de l’hydrogène : instabilités thermodiffusives, hydrodynamiques et liées à la pesanteur. Dans une dernière partie,des flammes turbulentes prémélangées en expansion sont caractérisées par génération d’une turbulence homogène et isotrope au sein d’une chambre sphérique. Une étude paramétrique est réalisée par rapport à un cas de référence en faisant varier l’intensité turbulente, la pression initiale et la richesse. Finalement, une corrélation turbulente est proposée afin de décrire la propagation de ces flammes et en vue d’être utilisée dans des modèles numériques
In order to reach carbon neutrality by 2050, the European Union is considering hydrogen as a promising energy carrier to reduce reliance on fossil fuels. While fuel cells and electric vehicles already play an important role in decarbonizing the transport sector, hydrogen is also seen as an alternative to conventional fuels for heavy-duty vehicles. Yet, a number of challenges linked to the physico-chemical properties of lean hydrogen combustion are still under investigation: abnormal combustion phenomena, production of nitrogen oxides,instabilities due to thermodiffusive effects, to state a few. This thesis contributes to the understanding of the auto-ignition process in lean hydrogen/air mixtures, as well as the propagation of laminar and turbulent premixed flames. First, measurements of hydrogen/air and hydrogen/air/nitrogen oxides ignition delay times are carried out using a rapid compression machine, to update and validate a kinetic mechanism under spark ignition engine-like conditions. Second, outwardly propagating spherical premixed laminar flames were studiedin a constant-volume combustion chamber, varying the initial temperature and steam dilution, and considering the intrinsic instabilities linked to the physico-chemical properties of hydrogen namely thermodiffusive,hydrodynamic and gravity-related instabilities. Then, expanding premixed turbulent flames are characterized by the generation of a homogeneous and isotropic turbulence zone within a spherical chamber. A parametric study is conducted by varying turbulent intensity, initial pressure and equivalence ratio. Finally, a turbulent correlation is proposed to describe the turbulent propagation of such flames, for use in numerical models

Depuis plusieurs années, les constructeurs automobiles suivent la voie du « downsizing » pour le développement des moteurs à allumage commandé. Ce procédé basé sur la réduction des cylindrées moteur combinée à la suralimentation a déjà fait ses preuves quant à son intérêt dans l’augmentation du rendement et la réduction des émissions polluantes des moteurs à essence. Les nouvelles conditions thermodynamiques, de turbulence et de dilution de ces moteurs engendrant de nouvelles possibilités de dilution dans les mélanges air/carburant, elles amènent également de nouvelles problématiques quant aux combustions anormales observées et l’apparition d’importantes variabilités cycliques. Ces travaux de thèse s’insèrent dans l’objectif de compréhension du comportement des flammes de prémélange d’isooctane/air en expansion dans des conditions représentatives d’un moteur « downsizé ». Leur étude a dans un premier temps été réalisée dans des conditions laminaires afin d’extraire les vitesses de flammes et longueurs de Markstein associées aux différents mélanges réactifs, et notamment sous forte dilution. Des corrélations ont alors été développées pour répondre aux besoins des modèles de simulation. Un nouveau dispositif de diagnostic optique a ensuite été employé pour améliorer la visualisation des flammes turbulentes en expansion. Une corrélation de coefficient correctif est ici développée pour remédier à la surestimation de vitesse engendrée par une visualisation Schlieren de ces flammes turbulentes. Une étude approfondie de l’influence des conditions thermodynamiques initiales, de la turbulence, ainsi que des caractéristiques diffusives du mélange air/carburant a par ailleurs été conduite afin d’isoler l’effet de chacun de ces paramètres sur le développement et la propagation de la flamme turbulente. Enfin l’effet d’une évolution simultanée des conditions thermodynamiques initiales similaire à celle d’une compression moteur a été étudié pour mieux appréhender les changements de comportement des flammes turbulentes dans des conditions plus représentatives du moteur à allumage commandé
For several years, “downsizing” is used by car manufacturers to develop new spark ignition engines. This method based on the reduction of engine size combined with an increase of intake pressure (turbocharger) is well known to reduce pollutant emissions and increase efficiency. New thermodynamic, turbulent and dilution conditions could be used with these new engines but they can bring new issues like unusual combustion or cyclic variability. This thesis took place to improve the understanding of premixed expanding isooctane/air flames behavior under downsized engine-like conditions. As a first step, this work is conducted under laminar conditions to extract laminar burning velocities and Markstein lengths of the different mixtures, especially under high dilution. New correlations are then developed to answer the needs of numerical models. A new optical dispositive is then used to improve the visualization of turbulent expanding flames. A corrective coefficient correlation is proposed to avoid the overestimated values of turbulent burning speed generated by Schlieren visualization with such turbulent flames. A deep survey of starting conditions (temperature, pressure, turbulence, dissipative characteristics of air/fuel mixtures) influence is done to investigate the effect of each parameters on the development and the propagation of the turbulent flame. Finally, the effect of a coupled rise of initial temperature and pressure, similar to an engine compression, is studied to better understand the changes of flame behavior under more realistic spark-ignition engine conditions

Jusqu’à aujourd’hui, les moteurs automobiles et aéronautiques étaient conçus pour fonctionner avec des combustibles fossiles. Pour relever les défis économiques et environnementaux du monde actuel et de la transition énergétique, des carburants de substitution sont développés et testés. Ils sont utilisés pour remplacer directement les carburants classiques ou bien sous forme de mélanges pour obtenir les propriétés thermochimiques souhaitées. Cependant, l’impact de ces nouveaux carburants sur les performances des chambres de combustion reste partiellement connu. Dans cette perspective, des simulations haute-fidélité de la combustion turbulente de carburants de substitution ne peuvent être réalisées qu’à la condition de prendre en compte une description détaillée multi-composants des mélanges de liquides et de gaz. L’objectif de cette thèse est de contribuer à la modélisation instationnaire des flammes de spray en présence de phénomènes complexes multi-composants : évaporation différentielle, mélange multi-espèces, nombreuses réactions chimiques en phase gazeuse. À cet effet, le carburant est traité comme un ensemble de mélanges à plusieurs composants, qui peuvent être différents dans les phases liquide et gazeuse en fonction de la précision requise. Différents modèles pour les phénomènes susmentionnés sont disponibles dans la littérature. Le principal défi est le couplage de ces différentes approches et leur validation dans des conditions réalistes et complexes. Premièrement, l’approche multi-composants choisie pour la phase gazeuse, basée sur le transport d’un grand nombre d’espèces et sur la chimie de type Arrhénius, est validée pour les flammes prémélangées. La configuration de flamme sphérique en expansion a été choisie pour étudier la vitesse de consommation de la flamme, paramètre important de la combustion. En collaboration avec l’équipe expérimentale du laboratoire CORIA, un formalisme pour l’évaluation précise de la vitesse de consommation de flamme est établi pour les flammes en expansion sphérique confinées et non confinées. Ce formalisme permet d’avoir une comparaison précise des résultats expérimentaux et numériques pour les flammes méthane / air et iso-octane / air et de valider ainsi le modèle de transport en phase gazeuse. Deuxièmement, nous nous sommes concentrés sur le processus physique d’évaporation. Le modèle d’évaporation multi-composants d’Abramzon-Sirignano est implémenté dans le solveur fluide YALES2 en se basant sur une approche de point-force pour les gouttelettes de carburant. Ce modèle a été adapté pour permettre de décrire l’évaporation d’un ou plusieurs composants avec ou sans évaporation différentielle. En tant que tel, le modèle est capable de traiter divers surrogates de carburant. Ce modèle d’évaporation est comparé au modèle de Spalding et validé sur les résultats expérimentaux de Chauveau et al. [33], Nomura et al. [158], Ghassemi et al. [82] et Daïf et al. [47] pour une gouttelette à un seul composant, puis une gouttelette isolée à deux composants avec et sans convection. Enfin, la simulation aux grandes échelles (LES) d’une flamme de spray complexe n-heptane/air est réalisée avec une chimie analytiquement réduite (ARC, [169, 205]). Cette flamme a été étudiée expérimentalement au laboratoire CORIA avec des diagnostics haute-fidélité pour caractériser la structure de la flamme et fournir des données quantitatives telles que la vitesse et la température en phase gazeuse ainsi que les distributions locales de taille et de vitesse des gouttelettes de carburant. La comparaison avec les données expérimentales [225] et avec les simulations réalisées dans le cadre du 6ème atelier sur la combustion turbulente de spray (TCS6), montre que les LES actuelles reproduisent fidèlement l’écoulement gazeux et les propriétés de la phase dispersée. Cette configuration ouvre la voie à la simulation de flammes de spray encore plus complexes avec des combustibles à plusieurs composants
Until recently, automotive and aeronautical engines were designed to operate with fossil fuels. To better meet the economic and environmental challenges of the modern world and of the energy transition, alternative fuels are developed and tested. They are used to replaceconventional fuels or as a blend to achieve the desired thermo-chemical properties. However, the impact of these new fuels on the performance of combustion chambers remains partially known. From this perspective, high-fidelity simulations of turbulent combustion of alternative fuels can be reached only if a detailed multi-component description of the liquid and gas mixtures is considered. The objective of this thesis is to contribute to the unsteady modeling of spray flames where complex multi-component phenomena occur : differential evaporation, multi-species mixing, gas phase chemical reactions. To this aim, the fuel is treated as a set of multi-component mixtures, which may be different in the liquid and gas phases depending on the required accuracy. Different models for the aforementioned phenomena are available in the literature, and the main challenge is the coupling of these different approaches and their validation in realistic and complex conditions. First, the chosen multi-component approach for the gas phase, based on the transport of a large number of species and on finite-rate chemistry, is validated for premixed flames. The expanding spherical flame configuration was chosen to study the flame consumption speed, which is an important parameter in combustion. In collaboration with the experimental team at the CORIA laboratory, a flame consumption speed formalism is established for non-confined and confined spherical expanding flames. This formalism enables to have a precise comparison of experimental and numerical results for methane/air and iso-octane/air flames and to validate the gas phase models. Second, we focused on the physical process of evaporation. The multi-component evaporation model of Abramzon-Sirignano is implemented in the YALES2 flow solver based on a point-particle approach for the fuel droplets. This model is adapted to enable the description of single- or multi-component evaporation with or without differential evaporation. As such, the model is capable of dealing with various fuel surrogates. The evaporation model is compared to the Spalding model and validated on experimental results of Chauveau et al. [33], Nomura et al. [158], Ghassemi et al. [82] and Daïf et al. [47] for a single component droplet and then two-component isolated droplet with and without convection. Finally, the 3D Large-Eddy Simulation (LES) of a complex n-heptane/air spray flame is conducted with analytical reduced chemistry (ARC, [169, 205]). This flame was experimentally studied at the CORIA laboratory with high fidelity diagnostics to characterize the flame structure and provide quantitative data such as gas-phase velocity and temperature as well as local droplet size and velocity distributions. Comparison with the experimental data [225] and with the simulations carried out within the framework of the 6th Workshop on Turbulent Combustion of Spray, shows that the current LES accurately reproduce the gas flow and properties of the dispersed phase. This configuration paves the way for the simulation of even more complex spray flames with multi-component fuels

Cette these porte sur l'emission sonore des flammes turbulentes de premelange de propane/air et plus particulierement sur le bruit lie aux fluctuations temporelles de surface de flamme. La premiere partie de ce travail est consacree a l'etude de bruleurs permettant de controler la turbulence et par suite les plissements des fronts de flamme. Un bruleur de type jet turbulent a ete retenu. L'ecoulement avec et sans combustion est etudie a l'aide des mesures d'anemometrie doppler laser. Les caracteristiques statistiques de la flamme sont obtenues par tomographie laser et les mesures de pression acoustique sont realisees grace a un microphone calibre. Les parametres de controle sont: l'echelle integrale et l'intensite de turbulence, la richesse du melange, le debit de gaz et la vitesse de combustion laminaire. Dans la deuxieme partie, les resultats des mesures acoustiques sont compares aux modeles theoriques de clavin-siggia (1991), strahle (1985) lorqu'il y a suffisamment d'echelles de plissement du front, il est montre que les puissances mesurees peuvent etre du meme ordre de grandeur que les puissances calculees et que les variations en fonction des differents parametres prevues par les modeles sont observees. Au-dela d'une certaine frequence, le spectre d'emission sonore suit une loi de puissance dont l'exposant varie avec la richesse entre -5/2 et -7/2, alors que le modele clavin-siggia conduit a une valeur constante egale a -5/2. Par ailleurs, l'existence d'un bruit haute frequence obeissant a la meme loi de puissance, n'est pas decrite par le modele. Il semble donc que d'autres mecanismes de generation du son sont a prendre en compte comme la disparition brutale des poches et des langues de gaz frais, les phenomenes d'extinction et de rallumage ou encore le developpement de l'instabilite de landau due a l'expansion des gaz

Afin d'améliorer la connaissance sur la combustion et les transferts thermiques dans le moteur diesel, une étude globale des flammes pariétales turbulentes de diffusion a été effectuée. Le système d'équations fondamentales qui décrit cette flamme est tout d'abord établi. Certains modèles de turbulence font apparaître des solutions pour les champs de vitesse, d'enthalpie et de concentrations dans la couche limite. La décomposition du coefficient de transfert de chaleur en partie inerte et partie réactive est possible en résolvant l'équation d'énergie avec un terme de chaleur de réaction. Une méthode de corrélation pour mesurer la vitesse moyenne et une méthode de mesure du flux de chaleur surfacique par analyse de la seule température de paroi sont développées. Les champs physiques de la flamme pariétale sont décrits par des échelles d'épaisseur, vitesse et température maximales évoluant en lois de puissance en fonction de la distance radiale. Les lois logarithmiques et la loi en puissance 1/7 décrivent mal les profils de vitesse et d'enthalpie dans la couche limite de flamme pariétale. La solution de Deissler, basée sur l'hypothèse de Von Karman, reproduit bien les profils mesures. Quelques caractéristiques de la turbulence sont déduites de l'analyse des fluctuations de température. Les influences de différents paramètres sur les champs physiques de la flamme sont étudiées, en particulier la température de paroi. Une formulation d'un nombre de Nusselt est obtenue.

La simulation numérique directe est utilisée dans cette thèse pour étudier les flammes de diffusion turbulentes. Les simulations ont été réalisées dans deux configurations distinctes, une couche de mélange et un écoulement turbulent homogène et isotrope. L'objectif général de ce travail est l'étude détaillée des notions introduites par les modèles de flammelettes et en particulier par le modèle de flamme cohérente. Les résultats obtenus sur les taux de réaction le long du front de flamme et sur la densité de surface de flamme montrent qu'une modélisation fondée sur le concept de flamme cohérente est satisfaisante

In the present work, two different turbulent diffusion flames are investigated for soot predictions using the presumed shape multi-environment Eulerian PDF (EPDF) as turbulence-chemistry closure. In this approach, the chemical equation is represented by multiple reactive scalars and finite number of Delta functions are used to describe the shape of joint composition PDF, while the truncated series expansion in spherical harmonics (P1 approximation) is used to solve the radiative heat-transfer equation. The absorption coefficient is modeled using the weighted sum of gray gases model (WSGG) considering four fictitious gases. The soot volume fraction is predicted using acetylene based soot inception model (Moss-Brookes model). The model accounts for inception, surface growth and oxidation processes of soot. An equilibrium based approach is used to determine the OH radical concentration, required for soot oxidation. A single variable PDF in terms of temperature is used to include the turbulence-chemistry effects on soot. An effective absorption coefficient is calculated to include the influence of radiative heat transfer on soot. The combined tool is used to determine the soot formation in two hydrocarbon flames (Delft flame III, pilot stabilized natural gas flame and an unconfined C2H4/air jet flame). The soot formation rate decreases with the inclusion of radiation for both the flames and indicate the need for delineation of radiative heat transfer. The effects of soot-turbulence interaction are consistent with available literature. The effect of collision efficiency on oxidation rate can be clearly explicated from the predictions of C2H4/air flame.

Fuel flexibility will be a key issue for the operation of future stationary gas turbines because of the increasing amount of off-spec natural gas qualities from new resources and upcoming new fuels derived from biomass which will be more important in the near future. The performance of gas turbines in terms of flame stability and low emission combustion must be at least maintained also with these new fuels. Therefore, the impact of fuel variation on combustion characteristics must be known for the combustor design. This paper addresses the effect of hydrogen and propane addition on flame characteristics like lean blowout (LBO), emissions (NOx, CO), flame positions and turbulent flame speeds for flames at gas turbine relevant conditions. Hydrogen enriched fuels are typical constituents of gasification fuels such as those obtained from biomass, while propane is considered a typical higher hydrocarbon present in off-spec natural gas. Turbulent, lean premixed flames of different fuels (methane, methane/hydrogen and methane/propane) have been studied in a generic, axis-symmetric, high-pressure gas turbine combustor. Flame stabilization is achieved aerodynamically via a recirculation zone induced by the combustor geometry with sudden expansion. Turbulence at the combustor inlet is generated using a turbulence grid (perforated plate). LBO limits are detected using the global OH chemiluminescence flame signal collected with a photo-multiplier and a data acquisition system together with the exhaust gas temperature measured with a thermocouple. The species concentrations (CO2, O2, CO, NOx) are measured by exhaust gas analyzers. Flame front positions and turbulent flame speeds are determined with Laser Induced Fluorescence measurements of the OH radical (OH-PLIF). Flame characteristics will be presented for the following fuel/air mixtures at a mixture preheating temperature of 673 K: pure methane, H2-enriched flames containing up to 50% hydrogen by volume, methane/propane mixtures containing up to 50% propane by volume. LBO limits, NOx emissions will be presented for different pressures. Most probable flame front positions and turbulent flame speeds are presented at a pressure of 5 bars for fuel mixtures between pure methane and 50% of each additive (propane and hydrogen). Experiments have revealed that a premixed mixture of 50% hydrogen and 50% methane, by volume, can significantly extend the lean blowout limit by up to 22% compared to pure methane. Because of a 120 K lower flame temperature a drastic reduction of the NOx emission (about 57%) is observed. Addition of hydrogen also significantly decreases the flame position (50%), changes the shape of the flame front and because of a higher global reaction rate increases the turbulent flame speed. Experiments with different methane/propane mixtures showed an increase (approximately 25–30%) of the NOx concentration at a propane content of 50%. Additionally, the flame stabilizes closer to the combustor inlet for higher propane contents.

Abstract In order to clarify the mechanism of modulation of turbulence structures such as quasi-streamwise vortices affected by a flame propagating toward a wall, we perform a direct numerical simulation of wall turbulence with premixed hydrogen-air combustion using a detailed chemical reaction mechanism. As a result, existing quasi-streamwise vortices in turbulence near the wall are found to be suppressed, disappearing as the flame approaches. Hence, the turbulent flow tends to become laminar. Moreover, according to the analysis of the vorticity transport equation, it is found that the suppression is due to thermal expansion of the flame rather than an increase in viscosity. From the viewpoint of chemical reactions, it is revealed that thermal expansion inside turbulence vortices is mainly caused by reactions involving H2 and H2O2.

Simulations of turbulent non-premixed swirling flames based on the Sydney swirl burner experiments under different flame characteristics are conducted using large eddy simulations (LES). The simulations attempt to capture the unsteady flame oscillations and explore the underlying instability modes responsible for a centre jet precession and the large scale recirculation zone oscillation. The selected flame series known as SMH flames have a fuel mixture of methane-hydrogen (50:50 by volume). The LES program solved the governing equations on a structured Cartesian grid using finite volume method and the subgrid turbulence and combustion models used the localized dynamic form of Smagorinsky eddy viscosity model and the steady laminar flamelet model respectively. The results show that the LES predicts two types of instability modes near fuel jet region and the bluff body stabilized recirculation zone region. The Mode I instability defined as cyclic precession of a centre jet is identified using time periodicity of the centre jet in flames SMH1 and SMH2. The Mode II instability defined as cyclic expansion and collapse of the recirculation zone is identified using time periodicity of the recirculation zone in flame SMH3. The calculated frequency spectrums found reasonably good agreement with the experimental precession frequencies. Overall, the LES yield a good qualitative and quantitative agreement with the experimental observations, although some discrepancies are apparent.

A radial swirler with vane passage fuel injection using a radial fuel spoke with one fuel hole per passage was investigated using CFD at 0.5 equivalence ratio and 600K inlet temperature at 1 bar. Experimental measurements of the internal flame composition from water cooled gas sample probes were the experimental results used for comparison. Three combustion models were compared: flamelet with two difference kinetic schemes; PDF transport with two step chemistry and finite rate eddy dissipation model. Both models consistently underpredicted the turbulent flame thickness to 90% heat release by a factor of about 2. The PDF model with postprocessing NOx predictions over estimated the NOx emissions considerably and the best model was the flamelet model with full chemistry. The under prediction of the turbulent reaction zone thickness was concluded to be due to inadequate modelling of strained flame quenching for very lean flames with large laminar flame thickness and very low burning velocities. This flamelet model was applied to predict the influence of the radial swirler outlet geometry on the flame development, fuel and air mixing and NOx emissions. A dump expansion from the radial swirler outlet was compared with the addition of a shroud at the outlet and with the addition of a 60mm long outlet throat. The shroud was shown to increase the peak turbulence and confine it very close to the shroud lip. This improved the fuel and air mixing and lowered the predicted NOx from 2.7ppm to 1.2ppm with the shrouded swirler and 0.3ppm with the 60mm outlet throat and mixing length.

Radial swirlers with vane passage natural gas injection, similar to those used in some industrial low NOx gas turbines, were investigated for their flame structure both experimentally and using CFD. The radial swirler NOx emissions at 600K and 1 atmosphere pressure were shown to be 3–4 ppm at 15% oxygen at 1800K and 1–2 ppm at 1700K. These levels were similar to the best published low NOx emissions using any flame stabilizer design. A flame at O̸ = 0.5 and 600K air temperature was investigated for its structure using a 10mm OD water cooled gas sample probe with a 1mm gas sample inlet on the upstream side of the probe. This showed that the mixing in the vane passage and outlet duct was very good. The maximum unmixedness at the first traverse location, 10mm downstream of the dump expansion zone, was 20% of the mean and the unmixedness was less than 5% within 30mm from the dump expansion. The flame structure was shown to involve a thick turbulence reaction zone of about 100mm thickness to the 90% heat release point. The CFD predictions were made using the RSM and k-ε turbulence models and the flamelet combustion model with a strain rate library. The isothermal aerodynamics predictions were in good agreement with others for similar geometries. There was an inner and outer recirculation zone with a swirling shear layer between. The peak turbulent kinetic energy was predicted to be on the inside of the shear layer. The experimental results showed that the flame developed in this region of high turbulence and low axial velocities. The flamelet model was less successful at predicting the flame development. The NOx results were predicted to be 2ppm less than the experimental results, due to the shorter predicted heat release region with associated lower prompt NOx.

A swirl burner was designed to experimentally study the impact of spark location on ignition efficiency and detailed ignition scenarios until flame stabilization or blow-off were established, following experimental observations. Premixed and non-premixed configurations were investigated for the same turbulent flow, in order to evaluate the fuel heterogeneities on ignition efficiency. Attention was paid to providing accurate data on cold flow velocity field statistics (obtained by stereoscopic PIV) and fuel mole fraction field statistics (obtained by PLIF on acetone). Ignition probability maps were established for all conditions by using laser-induced spark for a constant level of deposited energy. No systematic correlations were observed between local flow properties and ignition probability, which leads to the conclusion that history of the flame kernel inside the combustion chamber, must be taken into account to fully explain the ignition mechanism. From this conclusion, ignition scenarios were built using fast flame visualization and dynamic pressure record. Different steps of the ignition process were identified according to the location of the spark. In order to evaluate ignition probability according to spark location and flow conditions (velocity, turbulence and mixing), we extended the predictive model of Neophytou et al. [1], with some modifications, to examine whether it can be applied to ignition of swirling premixed flames. Flame particles are emitted by the spark and tracked in the flow with a Langevin equation by using non-reactive velocity fields obtained by PIV. Physical criteria are proposed to represent flame particles generation, expansion and extinction. Results indicate a relatively good agreement with the experimental database and the ignition scenarios are also well reproduced.

The influence of the structure of perfectly premixed flames on NOx-formation is investigated theoretically. Since a network of reaction kinetics modules and model flames is used for this purpose, the results obtained are independent of specific burner geometries. Calculations are presented for a mixture temperature of 630K, an adiabatic flame temperature of 1840K and 1 and 15 bars combustor pressure. In particular, the following effects are studied separately from each other: - molecular diffusion of temperature and species - flame strain - local quench in highly strained flames and subsequent reignition - turbulent diffusion (no preferential diffusion) - small scale mixing (stirring) in the flame front Either no relevant influence or an increase in NOx-production over that of the one-dimensional laminar flame is found. As a consequence, besides the improvement of mixing quality, a future target for the development of low-NOx burners is to avoid excessive turbulent stirring in the flame front. Turbulent flames that exhibit locally and instantaneously near laminar structures (“flamelets”) appear to be optimal. Using the same methodology, the scope of the investigation is extended to lean-lean staging, since a higher NOx-abatement potential can be expected in principle. As long as the chemical reactions of the second stage take place in the boundary between the fresh mixture of the second stage and the combustion products from upstream, no advantage can be expected from lean-lean staging. Only if the primary burner exhibits much poorer mixing than the second stage can lean-lean staging be beneficial. In contrast, if full mixing between the two stages prior to afterburning can be achieved (lean-mix-lean technique), the combustor outlet temperature can in principle be increased somewhat without NO-penalty. However, the complexity of such a system with a larger flame tube area to be cooled will increase the reaction zone temperatures, so that the full advantage cannot be realised in an engine. Of greater technical relevance is the potential of a lean-mix-lean combustion system within an improved thermodynamic cycle. A reheat process with sequential combustion is perfectly suited for this purpose, since, firstly, the required low inlet temperature of the second stage is automatically generated after partial expansion in the high pressure turbine, secondly, the efficiency of the thermodynamic cycle has its maximum and, thirdly, high exhaust temperatures are generated, which can drive a powerful Rankine cycle. The higher thermodynamic efficiency of this technique leads to an additional drop in NOx-emissions per power produced.

This paper reports proper orthogonal decomposition (POD) analyses for the velocity fields measured in a test burner. The Cambridge/Sandia Stratified Swirl Burner has been used in various studies as a benchmark for high resolution scalar and velocity measurements, for comparison with numerical model prediction. Flow field data was collected for a series of bluff-body stabilized premixed and stratified methane/air flames at turbulent, globally lean conditions (ϕ = 0.75) using high speed stereoscopic particle image velocimetry (HS-SPIV). In this paper, a modal analysis was performed to identify the large scale flow structures and their impact on the flame dynamics. The high speed PIV system was operated at 3 kHz to acquire a series of 4096 sequential flow field images both for reactive and non-reactive cases, sufficient to follow the large-scale spatial and temporal evolution of flame and flow dynamics. The POD analysis allows identification of vortical structures, created by the bluff body, and in the shear layers surrounding the stabilization point. In addition, the analysis reveals that dominant structures are a strong function of the mixture stratification in the flow field. The dominant energetic modes of reactive and non-reactive flows are very different, as the expansion of gases and the high temperatures alter the unstable modes and their survival in the flow.

The effect of centrifugal force on flame propagation velocity of stoichiometric propane-, kerosene-, and n-octane-air turbulent premixed flames was numerically examined. The quasi-turbulent numerical model was set in an unsteady two-dimensional geometry with finite length in the transverse and streamwise directions but with infinite length in the spanwise direction. There was relatively good comparison between literature-reported measurements and predictions of propane-air flame propagation velocity as a function of centrifugal force. It was found that for all mixtures the flame propagation velocity increases with centrifugal force. It reaches a maximum then falls off rapidly with further increases in centrifugal force. The results of this numerical study suggest there are no distinct differences among the three mixtures in terms of the effect of centrifugal force on the flame propagation velocity. There are, however, quantitative differences. The numerical models are set in a non-inertial, rotating reference frame. This rotation imposes a radially outward (centrifugal) force. The ignited mixture at one end of the tube raises the temperature and its heat release tends to laminarize the flow. The attained density difference combined with the direction of the centrifugal force promotes Rayleigh-Taylor instability. This instability with thermal expansion and turbulent flame speed constitute the flame propagation mechanism towards the other tube end. A wave is also originated but propagates faster than the flame. During propagation the flame interacts with eddies that wrinkle and/or corrugate the flame. The flame front wrinkles interact with streamtubes that enhance Landau-Darrieus (hydrodynamic) instability, giving rise to a corrugated flame. Under strong stretch conditions the stabilizing equidiffusive-curvature mechanism fails and the flame front breaks up, allowing inflow of unburned mixture into the flame. This phenomenon slows down the flame temporarily and then the flame speeds up faster than before. However, if corrugation is large and the inflow of unburned mixture into the flame is excessive, the latter locally quenches and slows down the flame. This occurs when the centrifugal force is large, tending to blowout the flame. The wave in the tube interacts continuously with the flame through baroclinic torques at the flame front that further enhances the above mentioned flame-eddies interactions. Only at low centrifugal forces the wave intermingles several times with the flame before the averaged flame propagation velocity is determined. The centrifugal force does not substantially increase the turbulent flame speed as commented by previous experimental investigations. The results also suggest that an ultra-compact combustor (UCC) with high-g cavity (HGC) will be limited to centrifugal force levels in the 2000–3000g range.