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Artykuły w czasopismach na temat "Flamme front instabilities"
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Ayoobi, Mohsen, i Ingmar Schoegl. "Numerical analysis of flame instabilities in narrow channels: Laminar premixed methane/air combustion". International Journal of Spray and Combustion Dynamics 9, nr 3 (5.06.2017): 155–71. http://dx.doi.org/10.1177/1756827717706009.
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Premixed flames propagating within small channels show complex combustion phenomena that differ from flame propagation at conventional scales. Available experimental and numerical studies have documented stationary, non-stationary, or asymmetric modes that depend on properties of the incoming reactant flow as well as channel geometry and wall temperatures. This work seeks to illuminate mechanisms leading to symmetry breaking and limit cycle behavior that are fundamental to these combustion modes. Specifically, four cases of lean premixed methane/air combustion—two equivalence ratios (0.53 and 0.7) and two channel widths (2 mm and 5 mm)—are investigated in a 2D configuration with constant channel length and bulk inlet velocity, where numerical simulations are performed using detailed chemistry. External wall heating is simulated by imposing a linear temperature gradient as a boundary condition on both walls. In the 2 mm channel, both equivalence ratios produce flames that stabilize with symmetric flame fronts after propagating upstream. In the 5 mm channel, flame fronts start symmetrically, although symmetry is broken almost immediately after ignition. Further, 5 mm channels produce non-stationary combustion modes with dramatically different limit cycles: in the leaner case ( φ = 0.53), the asymmetric flame front flops periodically, whereas in the richer case ( φ = 0.7), flames with repetitive extinctions and ignitions (FREI) are observed. To further understand the flame dynamics, reaction fronts and flame fronts are captured and differentiated. Results show that the loss of flame front symmetry originates in a region close to the flame cusp, where flow and chemical characteristics exhibit large gradients and curvatures. Limit cycle behavior is illuminated by investigating flame edges that are formed along the wall, and accompany local or global ignition and extinction processes. In the flopping mode ( φ = 0.53), local ignition and extinction in regions adjacent to the wall result in oblique fronts that advance and recede along the wall and redirect the flow ahead of the flame. In the FREI mode, asymmetric flames propagate much farther upstream, where they experience global extinction due to heat losses, and re-ignite far downstream with opposite flame front orientation. In both cases, an interaction of flow and chemical effects drives the asymmetric limit cycles. The lack of instabilities and asymmetries for the 2mm cases is attributed to insufficient wall separation, which is of the same order of magnitude as the flame thickness.
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Xia, Yongfang, Tingyong Fang, Haitao Wang, Erbao Guo i Jinwei Ma. "Numerical investigation of low-velocity filtration combustion instability based on the initial preheating non-uniformity". E3S Web of Conferences 136 (2019): 02040. http://dx.doi.org/10.1051/e3sconf/201913602040.
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The effects of the initial preheating perturbation on the dynamical behaviors of FGC wave propagation instability for low-velocity FGC in packed bed are studied numerically. The behaviors of the flame front inclination, break, and shrinking instabilities are always observed in experiments. Based on the experimental phenomena, an initial thermal perturbation model is numerically proposed as to predict the deformation behaviors of the flame front instabilities. The typical flame shapes are obtained depending on filtration velocity, equivalence ratio, and initial preheating temperature difference. It is demonstrated that the development of flame front inclination instability is proportional to the magnitude of initial preheating perturbation. At a lower equivalence ratio, the initial thermal perturbation of 300 K leads to the evolution of flame front break. Increasing filtration velocity leads to the appearance of flame front break, due to the intensification of the hydrodynamic instability. In addition, a perculiar instability of flame front shifting is also confirmed with the initial thermal perturbation of 400 K, which results in a fuel leakage of incomplete combustion.
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CLAVIN, P., L. MASSE i F. A. WILLIAMS. "COMPARISON OF FLAME-FRONT INSTABILITIES WITH INSTABILITIES OF ABLATION FRONTS IN INERTIAL-CONFINEMENT FUSION". Combustion Science and Technology 177, nr 5-6 (kwiecień 2005): 979–89. http://dx.doi.org/10.1080/00102200590926950.
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Krikunova, Anastasia. "Numerical simulation of combustion instabilities under the alternating gravity conditions". MATEC Web of Conferences 209 (2018): 00005. http://dx.doi.org/10.1051/matecconf/201820900005.
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The work is devoted to the analysis of the methane-air conical flame behaviour under conditions of an alternating gravitational field. Numerical simulation based on the software package FlowVision, has shown the possibility of modeling the flame front instabilities during the transition from the normal gravitational conditions to zero gravity. The appearance of the flame front oscillations is demonstrated under the such conditions. Further studies will provide a complete picture of the behavior of the flame in an alternating gravitational field.
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Altantzis, C., C. E. Frouzakis, A. G. Tomboulides, M. Matalon i K. Boulouchos. "Hydrodynamic and thermodiffusive instability effects on the evolution of laminar planar lean premixed hydrogen flames". Journal of Fluid Mechanics 700 (18.05.2012): 329–61. http://dx.doi.org/10.1017/jfm.2012.136.
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AbstractNumerical simulations with single-step chemistry and detailed transport are used to study premixed hydrogen/air flames in two-dimensional channel-like domains with periodic boundary conditions along the horizontal boundaries as a function of the domain height. Both unity Lewis number, where only hydrodynamic instability appears, and subunity Lewis number, where the flame propagation is strongly affected by the combined effect of hydrodynamic and thermodiffusive instabilities are considered. The simulations aim at studying the initial linear growth of perturbations superimposed on the planar flame front as well as the long-term nonlinear evolution. The dispersion relation between the growth rate and the wavelength of the perturbation characterizing the linear regime is extracted from the simulations and compared with linear stability theory. The dynamics observed during the nonlinear evolution depend strongly on the domain size and on the Lewis number. As predicted by the theory, unity Lewis number flames are found to form a single cusp structure which propagates unchanged with constant speed. The long-term dynamics of the subunity Lewis number flames include steady cell propagation, lateral flame movement, oscillations and regular as well as chaotic cell splitting and merging.
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KUSKE, R., i P. MILEWSKI. "Modulated two-dimensional patterns in reaction–diffusion systems". European Journal of Applied Mathematics 10, nr 2 (kwiecień 1999): 157–84. http://dx.doi.org/10.1017/s095679259800360x.
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New modulation equations for hexagonal patterns in reaction–diffusion systems are derived for parameter régimes corresponding to the onset of patterns. These systems include additional nonlinearities which are not present in Rayleigh–Bénard convection or Swift–Hohenberg type models. The dynamics of hexagonal and roll patterns are studied using a combination of analytical and computational approaches which exploit the hexagonal structure of the modulation equations. The investigation demonstrates instabilities and new phenomena not found in other systems, and is applied to patterns of flame fronts in a certain model of burner stabilized flames.
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Yang, Sheng, Abhishek Saha, Fujia Wu i Chung K. Law. "Morphology and self-acceleration of expanding laminar flames with flame-front cellular instabilities". Combustion and Flame 171 (wrzesień 2016): 112–18. http://dx.doi.org/10.1016/j.combustflame.2016.05.017.
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Steinbacher, Thomas, i Wolfgang Polifke. "Convective Velocity Perturbations and Excess Gain in Flame Response as a Result of Flame-Flow Feedback". Fluids 7, nr 2 (31.01.2022): 61. http://dx.doi.org/10.3390/fluids7020061.
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Convective velocity perturbations (CVPs) are known to play an important role in the response of flames to acoustic perturbations and in thermoacoustic combustion instabilities. In order to elucidate the flow-physical origin of CVPs, the present study models the response of laminar premixed slit flames to low amplitude perturbations of the upstream flow velocity with a reduced order flow decomposition approach: A linearized G-equation represents the shape and heat release rate of the perturbed flame, while the velocity perturbation field is decomposed into irrotational and solenoidal contributions. The former are determined with a conformal mapping from geometry and boundary conditions, whereas the latter are governed by flame front curvature and flow expansion across the flame, which generates baroclinic vorticity. High-resolution CFD analysis provides values of model parameters and confirms the plausibility of model results. This flow decomposition approach makes it possible to explicitly evaluate and analyze the respective contributions of irrotational and solenoidal flows to the flame response, and conversely the effect of flame perturbations on the flow. The use of the popular ad hoc hypothesis of convected velocity perturbation is avoided. It is found that convected velocity perturbations do not result from immediate acoustic-to-hydrodynamic mode conversion, but are generated by flame-flow feedback. In this sense, models for flame dynamics that rely on ad-hoc models for CVPs do not respect causality. Furthermore, analysis of the flame impulse response reveals that for the configuration investigated, flame-flow feedback is also responsible for “excess gain” of the flame response, that is, the magnitude of the flame frequency response above unity.
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NOVICK-COHEN, A., i G. I. SIVASHINSKY. "Hydrodynamic Instabilities in Flame Fronts: Breathing Solutions". Combustion Science and Technology 46, nr 1-2 (kwiecień 1986): 109–11. http://dx.doi.org/10.1080/00102208608959795.
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Zhang, Xinyi, Chenglong Tang, Huibin Yu i Zuohua Huang. "Flame-Front Instabilities of Outwardly Expanding Isooctane/n-Butanol Blend–Air Flames at Elevated Pressures". Energy & Fuels 28, nr 3 (10.03.2014): 2258–66. http://dx.doi.org/10.1021/ef4025382.
Pełny tekst źródłaRozprawy doktorskie na temat "Flamme front instabilities"
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Hok, Jean-Jacques. "Stratégie de modélisation pour la simulation aux grandes échelles d'explosions de mélanges hydrogène-air pauvres". Electronic Thesis or Diss., Université de Toulouse (2023-....), 2024. http://www.theses.fr/2024TLSEP065.
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La crise climatique à laquelle le monde est confronté aujourd'hui exige des actions immédiates pour réduire les émissions de carbone. En particulier, une transition énergétique rapide vers des sources plus propres est nécessaire. Parmi de nombreux candidats, l'hydrogène se distingue en tant que vecteur d'énergie décarboné. Cependant, son stockage et son transport en grandes quantités posent des problèmes de sécurité. Dans le cas d'une fuite accidentelle d'hydrogène, un mélange hautement inflammable peut se former. En cas d'allumage, différents scénarios et régimes de combustion sont possibles, en fonction de différents facteurs tels que la géométrie (dimensions, confinement, présence d'obstacles), la composition du mélange, la température, la pression ou le niveau de turbulence. Ces régimes vont de la déflagration lente à la transition vers la détonation dans le pire des cas. Pour prédire les dommages consécutifs à une explosion, la Mécanique des Fluides Numérique présente l'avantage d'être plus sûre que les expériences et de donner accès à des quantités difficiles ou impossibles à mesurer empiriquement. Cette thèse traite de la prédiction des explosions de mélanges d'hydrogène-air pauvres en utilisant l'approche de Simulation aux Grandes Échelles (SGE ou LES en anglais). Les mélanges pauvres d'H2-air sont caractérisés par leur nombre de Lewis subunitaire, qui traduit un déséquilibre entre les processus de diffusion moléculaire et thermique avec des conséquences majeures : (1) les flammes H2-air pauvres sont très sensibles à l'étirement ; (2) elles sont enclines à développer des cellules sur le front de flamme dues à l'instabilité thermo-diffusive. Les deux constituent des mécanismes d'accélération qui impactent la surpression générée lors de l'explosion. Dans ce travail, nous montrons que l'utilisation du modèle de Flamme Épaissie (TF en anglais) pour simuler les flammes à nombre de Lewis subunitaire : (1) induit une amplification de l'effet d'étirement sur la flamme ; (2) combinée à la faible résolution de maillage en LES, filtre les instabilités de front de flamme. Le couplage de ces mécanismes indésirables peut générer une propagation erronée de la flamme qui remet en question la capacité de prédiction de la LES pour les explosions de mélanges H2-air pauvres. Dans le cadre de cette thèse, une stratégie de modélisation est proposée afin de prédire de manière fiable et précise les explosions d'hydrogène-air pauvre. Un nouveau paradigme est envisagé pour corriger séparément l'amplification des effets d'étirement et modéliser les phénomènes de sous-maille dus à l'instabilité thermo-diffusive. Ces deux corrections sont d'abord développées sur des configurations canoniques, puis étendues et validées sur des configurations d'explosion plus réalistesThe climate crisis the world faces today calls for immediate actions to curb down carbon emissions. In particular, a rapid energy transition towards cleaner sources is necessary. Among many candidates, hydrogen stands out as a carbon-free energy vector. However, its storage and transport in big quantities raise safety concerns. Following a leakage, mixed with the surrounding air, this hydrogen can form a highly flammable mixture. In case of accidental ignition of this mixture, different combustion scenarios and regimes are possible, depending on factors such as geometry (dimensions, confinement, presence of obstacles), mixture composition, temperature, pressure or turbulence level. These regimes range from slow deflagration to the transition to detonation in the worst case. To predict the damage induced by an explosion, Computational Fluid Dynamics has the advantage of being safer than experiments and gives access to quantities hard or impossible to measure empirically. This thesis deals with the prediction of lean hydrogen-air explosions using Large-Eddy Simulation (LES). Lean H2-air mixtures are known for their distinctive sub-unity Lewis number, which characterises an unbalance between molecular and heat diffusion processes with major consequences: (1) lean H2-air flames are strongly sensitive to stretch; (2) they are prone to develop flame front cells due to the thermo-diffusive instability. Both constitute accelerating mechanisms which impact the overpressure generated during the explosion. In this work, we show that the Thickened Flame (TF) approach to simulate sub-unity Lewis number flames: (1) induces an amplification of stretch on the flame; (2) combined with the low grid resolution in LES, filters out flame front instabilities. The coupling of these undesired mechanisms can generate an erroneous flame propagation which questions the predictability of LES for lean H2-air explosions. In this thesis, a modelling strategy is proposed to reliably and accurately predict lean hydrogen-air explosions. A new paradigm is considered to separately correct the amplification of stretch effects and model subgrid phenomena due to the thermo-diffusive instability. These two corrections are first developed on canonical configurations and then extended and validated on more realistic explosion configurations
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Radisson, Basile. "Dynamique non linéaire de fronts de flammes : expériences et modélisation". Thesis, Aix-Marseille, 2019. http://www.theses.fr/2019AIXM0124.
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Les flammes de prémélange sont souvent minces devant les échelles de l’écoulement dans lequel elles évoluent. La description de leur dynamique peut alors se réduire à des équations d’évolution pour leur front. Ce manuscrit présente une série d’expériences de laboratoire qui visent à valider la pertinence de telles modélisations. Les expériences sont menées dans une configuration quasi-2D (brûleur de Hele-Shaw) permettant une analyse fine de la dynamique de l’interface. Dans une première partie, l’évolution d’une flamme initialement plane et se propageant dans un écoulement au repos est étudiée. Pour la première fois, une comparaison quantitative de l’évolution non-linéaire avec une équation de type Michelson-Sivashinsky est obtenue. Par ailleurs, on montre que les solutions analytiques de cette équation permettent de prédire certaines propriétés statistiques du front. Ces prédictions restent valables même aux temps longs lorsque le bruit joue un rôle important dans la dynamique. Dans une deuxième partie, l’influence de l’enceinte du brûleur est étudiée. Un nouveau mécanisme de couplage vibroacoustique, propre à cette géométrie confinée,est identifié. Les propriétés de ces modes de structure sont ensuite exploitées pour étudier l’interaction d’une flamme avec un forçage périodique. Enfin, ces flammes quasi-2D, planes en moyenne, sont soumises à un écoulement faiblement turbulent. L’évolution de la vitesse de flamme avec l’intensité du forçage transite d’un régime super-linéaire aux très faibles forçages vers un régime sous-linéaire quand l’intensité turbulente s’approche de la vitesse de flamme laminaireIn many applications where premixed combustion is involved, the flame thickness is weak compared to the scales of the flow. This property allows to describe the flame frontevolution as an interface dynamics. In this manuscript some experiments are performed in order to check the validity of such models. The experiments are carried out in a Hele-Shaw burner. This quasi-bidimensional configuration allows for an accurate analysis ofthe flame front evolution. First, the dynamics of an initially flat flame propagating in aquiescent flow are analyzed. A quantitative comparison of an experimental flame evolution with the one predicted by a Michelson-Sivashinsky type equation is obtained for the firsttime. Moreover, the analytic pole solutions of this model allows us to predict some statisticproperties of the flame front. These predictions are shown to still be valid at large time,where the external noise plays an important role in the observed dynamics. In a second part, flame/burner interactions are investigated. A new vibroacoustic coupling mechanismis identified. Then, harnessing the properties of this vibroacoustic coupling, the flame issubmitted to an oscillating flow. It allows us to explore some characteristics of the flame response to a time dependent external forcing. Finally, the flame is submitted to a weaklyturbulent flow. The influence of the flow fluctuations intensity on the turbulent flamespeed is explored. The flame speed increase is shown to switch from a sublinear regime atsmall forcing to a superlinear one when the forcing intensity is approaching the laminar flame speed value
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Boury, Gaël. "Etudes théoriques et numériques de fronts de flammes plissées : dynamiques non-linéaires libres ou bruitées". Poitiers, 2003. http://www.theses.fr/2003POIT2255.
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Les flammes de prémélange, souvent minces, sont assimilées à des interfaces actives. Des équations d'évolution pour leur front sont obtenues par développement asymptotique en contraste de densité. Selon ces dernières, la dynamique de flamme devrait être correctement gouvernée par les seules interactions entre hydrodynamique non visqueuse, non linéarité géométrique liée à la propagation (Huygens), changement de densité et géométrie d'ensemble, si certaines symétries globales soient respectées (invariances Galiléenne, par translation ou rotation) ou leur brisure prise en compte. Cette thèse est confortée par l'examen de 3 configurations : flammes accrochées soumises à un écoulement tangentiel fort et des stimuli externes, influence de gravités faibles, expansion 3-D. Les méthodes mises en œuvre sont analytiques et pseudo-spectrales. Dans chaque cas des lois d'échelle simples pour leur plissement sont identifiées. Celles-ci sont en accord au moins qualitatif avec les expériences disponibles. Pour chaque cas, des problèmes ouverts sont mentionnésUsually, premixed flames are thin. We view them as active interfaces. Evolution Equations for their front are obtained from asymptotic expansions in the density-contrast. Flame dynamics seems accurately controlled only by the interplay amongst elliptic hydrodynamics, a geometric non-linearity coming from the flame normal propagation (Huygens), the change in density, and the overall geometry, provided minimal symmetries (Galilean, translation, rotation) are fulfilled, or explicitly broken. Examining three configurations confirms the thesis, namely: flames anchored in the presence of a strong tangential blowing and external forcing, influence of a weak gravity field, 3-Dimensional expansions. Our methods are analytical and pseudo-spectral. In each case, scaling laws for wrinkling are identified. These are in good agreement with available experiments. Open problems are also evoked
Streszczenia konferencji na temat "Flamme front instabilities"
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BYCHKOV, VITALIY. "FLAME FRONT INSTABILITIES AND DEVELOPMENT OF FRACTAL FLAMES". W Conference on Fractals 2002. WORLD SCIENTIFIC, 2002. http://dx.doi.org/10.1142/9789812777720_0021.
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Baghirzade, Mammadbaghir, Md Nayer Nasim, Behlol Nawaz, Jonathan Aguilar, Martia Shahsavan, Mohammadrasool Morovatiyan i John Hunter Mack. "Analysis of Premixed Laminar Combustion of Methane With Noble Gases as a Working Fluid". W ASME 2021 Internal Combustion Engine Division Fall Technical Conference. American Society of Mechanical Engineers, 2021. http://dx.doi.org/10.1115/icef2021-67516.
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Abstract Hydrodynamic and diffusional-thermal instabilities affect the flame dynamics, which result in non-planar flame fronts with self-accelerating cellularities and wrinkles. In premixed flames, the driving mechanism for perturbations is hydrodynamic instabilities, which are associated with thermal expansion. Under high-pressure conditions, such as in spark-ignition engines, the flame curvature and morphology might be influenced by the hydrodynamic instabilities. This study focuses on the replacement of nitrogen with a noble gas (argon and krypton) as the working fluid in the premixed combustion of methane to investigate its effect on flame stability and dynamics. The utilization of noble gases can also enhance the ideal thermal efficiency of internal combustion engines due to the higher specific heat ratio they possess and may also reduce the NOx emissions markedly because of the lack of nitrogen in the working fluid. The experiments are conducted for various equivalence ratios (φ = 0.8, 1.0, 1.2) in a constant volume combustion chamber (CVCC) at atmospheric and elevated initial pressures and atmospheric temperature. As an outcome of this study, to understand the influence of krypton on methane combustion, spherically propagating flames are analyzed in terms of the laminar flame burning velocity, cellular instability, unburned gas Markstein length, and flame morphology utilizing a Z-type Schlieren optical diagnostic technique and fractal analysis, which is a promising approach to analyze flame surfaces. The fractal dimension of the flame fronts is calculated by a box-counting algorithm. The results are compared against the previously examined case studies in which argon was used as the primary working fluid.
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Gopalakrishnan, Harish Subramanian, Andrea Gruber i Jonas Moeck. "Computation of Intrinsic Instability and Sound Generation From Autoignition Fronts". W ASME Turbo Expo 2022: Turbomachinery Technical Conference and Exposition. American Society of Mechanical Engineers, 2022. http://dx.doi.org/10.1115/gt2022-82480.
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Abstract Burning carbon-free fuels such as hydrogen in gas turbines promises power generation with minimal emissions of greenhouse gases. A two-stage sequential combustor architecture with a propagation-stabilized flame in the first stage and an autoignition-stabilized flame in the second stage allows for efficient combustion of hydrogen fuels. However, interactions between the autoignition-stabilized flame and the acoustic modes of the combustor may result in self-sustained thermoacoustic oscillations, which severely affect the stable operation of the combustor. In this paper, we study an ‘intrinsic’ thermoacoustic feedback mechanism in which acoustic waves generated by unsteady heat release rate oscillations of the autoignition front propagate upstream and induce flow perturbations in the incoming reactant mixture, which, in turn, act as a disturbance source for the ignition front. We first perform detailed reactive Navier-Stokes (DNS) and Euler computations of an autoignition front in a one-dimensional setting to demonstrate the occurrence of intrinsic instability. Self-excited ignition front oscillations are observed at a characteristic frequency and tend to become more unstable as the acoustic reflection from the boundaries is increased. The Euler computations yield identical unsteady ignition front behaviour as the DNS computations, suggesting that inviscid mechanisms control the instability. In the second part of this work we present a simplified framework based on the linearized Euler equations (LEE) to compute the sound field generated by an unsteady autoignition front. Unsteady autoignition fronts create sources of sound due to local fluctuations in gas properties, in addition to heat release oscillations, which must be accounted for. The LEE predictions of the fluctuating pressure field in the combustor agree well with the DNS data. The findings of the present work are essential for understanding and modeling thermoacoustic instabilities in reheat combustors with autoignition-stabilized flames.
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Oravecz, Lisa M., Indrek S. Wichman i Sandra L. Olson. "Instability of Flame Spread in Microgravity". W ASME 1999 International Mechanical Engineering Congress and Exposition. American Society of Mechanical Engineers, 1999. http://dx.doi.org/10.1115/imece1999-1118.
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Abstract Results from the first part of an experimental study of flame spread instability are presented. The instabilities were investigated in the NASA drop facilities because the particular instabilities being examined were most pronounced in microgravity, when the influences of buoyancy were minimized. The flame front over thin cellulosic samples broke apart into separate flamelets which interacted with one another and oscillated (frequency ∼ 1 Hz). Different heat-sink backings, which were used to promote flame instability and flamelet productions are examined and described. Preliminary experiments in the NASA 5 second drop tower (Zero-G) drop facility are discussed.
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Malanoski, Michael, Michael Aguilar, Jacqueline O’Connor, Dong-hyuk Shin, Bobby Noble i Tim Lieuwen. "Flame Leading Edge and Flow Dynamics in a Swirling, Lifted Flame". W ASME Turbo Expo 2012: Turbine Technical Conference and Exposition. American Society of Mechanical Engineers, 2012. http://dx.doi.org/10.1115/gt2012-68256.
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Flames in high swirl flow fields with vortex breakdown often stabilize aerodynamically in front of interior flow stagnation points. In contrast to shear layer stabilized flames with a nearly fixed, well defined flame attachment point, the leading edge of aerodynamically stabilized flames can move around substantially, due to both the inherent dynamics of the vortex breakdown region, as well as externally imposed oscillations. Motion of this flame stabilization point relative to the flow field has an important dynamical role during combustion instabilities, as it creates flame front wrinkles and heat release fluctuations. For example, a prior study has shown that nonlinear dynamics of the flame response at high forcing amplitudes were related to these leading edge dynamics. This heat release mechanism exists alongside other flame wrinkling processes, arising from such processes as shear layer rollup and swirl fluctuations. This paper describes an experimental investigation of acoustic forcing effects on the dynamics of leading edge of a swirl stabilized flame. Vortex breakdown bubble dynamics were characterized using both high-speed particle image velocimetry (PIV) and line-of-sight high-speed CH* chemiluminescence. A wide array of forcing conditions was achieved by varying forcing frequency, amplitude, and acoustic field symmetry. These results show significant differences in instantaneous and time averaged location of the flow stagnation points. They also show motion of the flame leading edge that are of the same order of magnitude as corresponding particle displacement associated with the fluctuating velocity field. This observation suggests that heat release fluctuations associated with leading edge motion may be just as significant in controlling the unsteady flame response as the flame wrinkles excited by velocity fluctuations.
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Fu, X., H. Wen, Q. Xie i B. Wang. "Research on Characteristics of Thermoacoustic Instabilities in Air-Methane-Ammonia Premixed Swirl-Stabilized Combustors". W Proceedings of the 10th INTERNATIONAL SEMINAR ON FLAME STRUCTURE Novosibirsk, Russia October 9-13, 2023. Crossref, 2023. http://dx.doi.org/10.53954/9785605098669_197.
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As an excellent hydrogen carrier, ammonia is a very good clean energy source. However, due to the poor combustion performance of ammonia, other fuels can be blended to improve the combustion performance. In this paper, the combustion characteristics of ammonia/methane composite fuel and analyzed, and the ability of ammonia/methane ratio to alleviate the thermoacoustic instabilities of burners is discussed, which is of great significance for the development of low-emission and high-reliability combustors. In this study, the pressure and heat release rate oscillation characteristics in the combustion chamber, and the evolution of the flame surface driven by velocity perturbation are investigated. The flow shape and instantaneous OH profile are measured with high-speed camera and OH-PLIF technique, respectively. Analysis shows that its vibration pattern and frequency are similar to the axial first-order acoustic mode; both the heat release rate and the pressure fluctuate at the same frequency. The inlet velocity perturbation leads to the reciprocating retraction and expansion of the flame front as well as the further change of the flame wrinkle. The spatiotemporal varying in the flame surface area are the key mechanism for driving the fluctuation of the heat release rate. When the swirling flame interacts with the combustor wall, the flame surface will be broken out, and the flame surface area will be extended to the maximum. In addition, the stoichiometric ratio and ammonia/methane mixing ratio have a great influence on the thermoacoustic instability of premixed swirl combustion. As the equivalence ratio changes from the lean to the rich, the burner undergoes a transition process: from the stable combustion, quasiperiodic oscillation, limit cycle oscillation, quasi-periodical oscillation, until the stable combustion. Blending ratio mainly change the flame heat release power, flame shape, flame propagation velocity and the interaction between the flame and the combustor wall. As the ratio of ammonia gas increases, the burner changes from limit cycle oscillation to the stable combustion, and the flame heat release power also decreases. Therefore, blending ammonia in methane is a good attempt to alleviate thermoacoustic coupling, and optimizing the overall airflow velocity, equivalence ratio and blending ratio is the key to get the best performance.
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Lee, Doh-Hyoung, i Tim C. Lieuwen. "Acoustic Nearfield Characteristics of a Premixed Flame in a Longitudinal Acoustic Field". W ASME Turbo Expo 2001: Power for Land, Sea, and Air. American Society of Mechanical Engineers, 2001. http://dx.doi.org/10.1115/2001-gt-0040.
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The occurrence of self excited, combustion driven oscillations pose significant problems in lean, premixed gas turbine combustors. The interactions between flames and longitudinal acoustic oscillations play a key role in many of these instabilities. This paper analyzes the acoustic field in the vicinity of a premixed flame front in order to clarify these interactions. Specifically, it describes a numerical analysis of the acoustic characteristics of a premixed flame that is excited by longitudinal disturbances. Solutions are determined from an integral formulation of the acoustic wave equation that is solved via boundary element techniques. Analyses of these results are performed to characterize the deviations of the acoustic field from one-dimensionality. First, as can be anticipated from quasi one-dimensional considerations, the flame’s reflection coefficient is lower than that predicted by purely one-dimensional calculations. The results also show that the acoustic pressure is nearly one-dimensional in the vicinity of the flame. Finally, they show that the velocity oscillations are strongly multidimensional, even in acoustically compact flames. At very low frequencies, these local velocity oscillations are in phase with each other, while at higher frequencies, their phase may change significantly along the flame. The results of this study show that the multidimensional nature of the acoustic velocity oscillations in the near field of the flame must be taken into consideration in analyses of the interactions between acoustic waves and flames.
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Shrivastava, Sourabh, Ishan Verma, Rakesh Yadav i Pravin Nakod. "Solution-Based Mesh Adaption Criteria Development for Accelerating Flame Tracking Simulations". W ASME Turbo Expo 2022: Turbomachinery Technical Conference and Exposition. American Society of Mechanical Engineers, 2022. http://dx.doi.org/10.1115/gt2022-82620.
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Abstract Accurate flame tracking plays a vital role in predicting the combustion characteristics of a system. This is even more critical for systems that evolve over time. Predicting relight performance of an aero combustor, predicting flame propagation due to gas leakage from a storage tank or during the thermal runaway of batteries, are some examples of such dynamic systems. Predicting accurate flame position also plays an important role in deriving the correct pollutant formation rate from a combustion system. The challenge with flame tracking through a 3D computational fluid dynamics (CFD) simulation comes from the requirement to have a good resolution of gradients along the flame front. This requirement can push the overall mesh count of any industrial cases to a very large value (several million-mesh count). Further, the global drive towards using hydrogen or hydrogen blended fuels for different combustion applications pushes the limits on having even finer cells since hydrogen is a fast-burning fuel and has a much thinner flame front compared to hydrocarbons. Solution-based mesh adaption approaches have been widely studied and tested by different research groups to generate the required finer meshes in the critical regions on the fly while keeping the overall mesh count to a manageable level. However, these approaches are typically applicable for a set of problems, and therefore, there is a need for a generic approach suitable for a broader range of problems. This work explores various parameters and specific weightage factors to predict correct flame-tracking outcomes for different types of flames. The selections of flow quantities (flow-variables, their gradients, curvatures) are performed using simple flames and flow configurations. The functions based on selected flow-quantities derived from these studies are then tested to predict the results for the more complex set of published flames like the Engine Combustion Network (ECN) spray flame and Knowledge for Ignition, Acoustics and Instabilities (KIAI) five-burner configuration (liquid and gas fuel). Derived adaption criteria are found to predict the correct flame tracking behavior in terms of transient evolution of flame front, flame propagation, and ignition timing of burners. The parameters used for the study are identified keeping genericity as the key point, and thus making sure that the derived adaption functions can be applied across different types of fuel blends, combustion systems (gaseous or liquid fuel-based systems) and combustion models, for example species transport or mixture fraction-based models.
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Iga, Yuka, Makoto Hiranuma, Takashi Shimura i Toshiaki Ikohagi. "Numerical Study of Cavitation Instabilities Arising in Cascade With Slit". W ASME 2005 Fluids Engineering Division Summer Meeting. ASMEDC, 2005. http://dx.doi.org/10.1115/fedsm2005-77299.
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In this study, unsteady cavitation and cavitation instabilities are numericaly analyzed around flate-plate cascade with slit in water/ air working fluid. In order to investigate the influence of location of slit between front and rear blades on cavitation characteristics, three kinds of two-stage cascade are arranged to have different length of front blade. Through comparisons with the results of previous study about cascade without slit, it is confirmed that cavitation instabilities are suppressed by slit effect in some cases without decline of cavitation performance of the cascade.
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Eriksson, Pontus. "The Zimont TFC Model Applied to Premixed Bluff Body Stabilized Combustion Using Four Different RANS Turbulence Models". W ASME Turbo Expo 2007: Power for Land, Sea, and Air. ASMEDC, 2007. http://dx.doi.org/10.1115/gt2007-27480.
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The Volvo Aero Corp. (VAC) Triangular Bluff Body Stabilized Combustion rig VR-1 has been extensively researched both in terms of experiments and theoretical treatment. Previous CFD work has concentrated on Reynolds Averaged Navier-Stokes (RANS) models combined with the Level Set Flamelet Library approach. Large Eddy Simulation (LES) has also been applied to the case. In this paper the Zimont Turbulent Flame Closure (TFC) model has been investigated in conjunction with the k-ε, k-ω, SST k-ω and RSM RANS model implementations in ANSYS CFX 10.0. It is shown that the various RANS models generate significantly different results in terms of turbulent velocity and integral length scale fields. These parameters influence the computed turbulent flame speed. The turbulent viscosity fields also differ substantially between the various RANS models. This will affect the computed degree of flame front diffusion. For the investigated case; the TFC model combined with the k-ω model fairly accurately captures the recirculation zone length and overall turbulent flame speed. The measured case however displays Kelvin-Helmholtz induced oscillations of the shear layers behind the bluff body. This will combine with the free-stream turbulence and turbulence generated along the upstream surfaces of the bluff body to distort the flame sheets. The two flame fronts will also be subjected to other (unquantified) combustion related instabilities. The combined effect is not captured well in steady state RANS. The analysis is therefore seen to grossly under-predict flame front diffusion, regardless of turbulence model.