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Artigos de revistas sobre o assunto "Stretched Flames"
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JU, YIGUANG, HONGSHENG GUO, KAORU MARUTA e FENGSHAN LIU. "On the extinction limit and flammability limit of non-adiabatic stretched methane–air premixed flames". Journal of Fluid Mechanics 342 (10 de julho de 1997): 315–34. http://dx.doi.org/10.1017/s0022112097005636.
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Extinction limits and the lean flammability limit of non-adiabatic stretched premixed methane–air flames are investigated numerically with detailed chemistry and two different Planck mean absorption coefficient models. Attention is paid to the combined effect of radiative heat loss and stretch at low stretch rate. It is found that for a mixture at an equivalence ratio lower than the standard lean flammability limit, a moderate stretch can strengthen the combustion and allow burning. The flame is extinguished at a high stretch rate due to stretch and is quenched at a low stretch rate due to radiation loss. A O-shaped curve of flame temperature versus stretch rate with two distinct extinction limits, a radiation extinction limit and a stretch extinction limit respectively on the left- and right-hand sides, is obtained. A C-shaped curve showing the flammability limit of the stretched methane–air flame is obtained by plotting these two extinction limits in the mixture strength coordinate. A good agreement is shown on comparing the predicted results with the experimental data. For equivalence ratio larger than a critical value, it is found that the O-shaped temperature curve opens up in the middle of the stable branch, so that the stable branch divides into two stable flame branches; a weak flame branch and a normal flame branch. The weak flame can survive between the radiation extinction limit and the opening point (jump limit) while the normal flame branch can survive from its stretch extinction limit to zero stretch rate. Finally, a G-shaped curve showing both extinction limits and jump limits of stretched methane–air flames is presented. It is found that the critical equivalence ratio for opening up corresponds to the standard flammability limit measured in microgravity. Furthermore, the results show that the flammability limit (inferior limit) of the stretched methane–air flame is lower than the standard flammability limit because flames are strengthened by a moderate stretch at Lewis number less than unity.
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Clavin, Paul, e José C. Graña-Otero. "Curved and stretched flames: the two Markstein numbers". Journal of Fluid Mechanics 686 (28 de setembro de 2011): 187–217. http://dx.doi.org/10.1017/jfm.2011.318.
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AbstractThe analytical result concerning the Markstein number of adiabatic flames was obtained in 1982 with the one-step Arrhenius model in the limit of a large activation energy. This result is not relevant for real flames. The form of the law expressing the flame velocity in terms of the total stretch rate of the flame front through a single Markstein length is not conserved when the location of the front (surface of zero thickness) changes within the flame thickness. It is shown in this paper that two different Markstein numbers ${\mathscr{M}}_{I} \not = {\mathscr{M}}_{II} $ characterize usual wrinkled flames sustained by a multiple-step chemical network, ${\mathscr{M}}_{I} $ for the modification of the flame velocity due to the curvature of the front and ${\mathscr{M}}_{II} $ for the effect of the flow strain rate. In contrast to ${\mathscr{M}}_{I} $, ${\mathscr{M}}_{II} $ depends on the location of the flame surface within the flame thickness, in such a way that the final result for the flame dynamics is not depending on this choice. The first part of the paper is devoted to present a general method of solution, valid for any multiple-step chemical network. The two Markstein numbers for two-step chain-branching models representing rich hydrogen–air flames and lean hydrocarbon–air flames are then computed analytically in the second part.
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Ju, Yiguang, Kaoru Maruta e Takashi Niioka. "Combustion Limits". Applied Mechanics Reviews 54, n.º 3 (1 de maio de 2001): 257–77. http://dx.doi.org/10.1115/1.3097297.
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Combustion limits and related flame behaviors are reviewed, especially with regard to fundamental problems. As for premixed flames, after a brief historical overview of research on the flammability limit, recent trends of research on planar propagating flames, curved propagating flames, flame balls, and stretched premixed flames are discussed, and then all types of flames are summarized. Finally, instability and dynamics near limits is discussed. With regard to combustion limits of counterflow diffusion flames and droplet flames, their instability is demonstrated, then an explanation of lifted flames and edge flames is presented. Suggestions for future work are also discussed in the concluding remarks. There are 166 references cited in this review article.
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Law, C. K. "Dynamics of stretched flames". Symposium (International) on Combustion 22, n.º 1 (janeiro de 1989): 1381–402. http://dx.doi.org/10.1016/s0082-0784(89)80149-3.
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Mikolaitis, David W. "Stretched spherical cap flames". Combustion and Flame 63, n.º 1-2 (janeiro de 1986): 95–111. http://dx.doi.org/10.1016/0010-2180(86)90114-8.
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Thiesset, F., F. Halter, C. Bariki, C. Lapeyre, C. Chauveau, I. Gökalp, L. Selle e T. Poinsot. "Isolating strain and curvature effects in premixed flame/vortex interactions". Journal of Fluid Mechanics 831 (13 de outubro de 2017): 618–54. http://dx.doi.org/10.1017/jfm.2017.641.
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This study focuses on the response of premixed flames to a transient hydrodynamic perturbation in an intermediate situation between laminar stretched flames and turbulent flames: an axisymmetric vortex interacting with a flame. The reasons motivating this choice are discussed in the framework of turbulent combustion models and flame response to the stretch rate. We experimentally quantify the dependence of the flame kinematic properties (displacement and consumption speeds) to geometrical scalars (stretch rate and curvature) in flames characterized by different effective Lewis numbers. Whilst the displacement speed can be readily measured using particle image velocimetry and tomographic diagnostics, providing a reliable estimate of the consumption speed from experiments remains particularly challenging. In the present work, a method based on a budget of fuel on a well chosen domain is proposed and validated both experimentally and numerically using two-dimensional direct numerical simulations of flame/vortex interactions. It is demonstrated that the Lewis number impact neither the geometrical nor the kinematic features of the flames, these quantities being much more influenced by the vortex intensity. While interacting with the vortex, the flame displacement (at an isotherm close to the leading edge) and consumption speeds are found to increase almost independently of the type of fuel. We show that the total stretch rate is not the only scalar quantity impacting the flame displacement and consumption speeds and that curvature has a significant influence. Experimental data are interpreted in the light of asymptotic theories revealing the existence of two distinct Markstein numbers, one characterizing the dependence of flame speed to curvature, the other to the total stretch rate. This theory appears to be well suited for representing the evolution of the displacement speed with respect to either the total stretch rate, curvature or strain rate. It also explains the limited dependence of the flame displacement speed to Lewis number and the strong correlation with curvature observed in the experiments. An explicit relationship between displacement and consumption speeds is also given, indicating that the fuel consumption rate is likely to be altered by both the total stretch rate and curvature.
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Avula, Murali, e Ishwar K. Puri. "Dioxin formation in stretched flames". Chemosphere 24, n.º 12 (junho de 1992): 1785–98. http://dx.doi.org/10.1016/0045-6535(92)90233-h.
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Mokrin, Sergey, R. V. Fursenko e S. S. Minaev. "Thermal-Diffusive Stability of Counterflow Premixed Flames at Low Lewis Numbers". Advanced Materials Research 1040 (setembro de 2014): 608–13. http://dx.doi.org/10.4028/www.scientific.net/amr.1040.608.
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Dynamics of radiative, near-limit, stretched premixed flames is investigated analytically and numerically. Investigation of counterflow premixed flames stability is important for the development of new combustion technologies such as those associated with low-NOx emission, lean burn and material synthesis. Emphasis is paid on the linear stability of multiple flame regimes. The present analysis, for the first time, gives out a dispersion equation describing growth rate of small spatial perturbations of the flame front. The stability diagram is obtained and the region of instability is distinguished.
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Yousif, Alaeldeen Altag, e Shaharin Anwar Sulaiman. "Experimental Study on Laminar Flame Speeds and Markstein Length of Methane-Air Mixtures at Atmospheric Conditions". Applied Mechanics and Materials 699 (novembro de 2014): 714–19. http://dx.doi.org/10.4028/www.scientific.net/amm.699.714.
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Accurate value of laminar flame speed is an important parameter of combustible mixtures. In this respect, experimental data are very useful for modeling improvement and validating chemical kinetic mechanisms. To achieve this, an experimental characterization on spherically expanding flames propagation of methane-air mixtures were carried out. Tests were conducted in constant volume cylindrical combustion chamber to measure stretched, unstretched laminar flame speed, laminar burning velocity, and flame stretch effect as quantified by the associated Markstein lengths. The mixtures of methane-air were ignited at extensive ranges of lean-to-rich equivalence ratios, under ambient pressure and temperature. This is achieved by high speed schlieren cine-photography for flames observation in the vessel. The results showed that the unstretched laminar burning velocity increased and the peak value of the unstretched laminar burning velocity shifted to the richer mixture side with the increase of equivalence ratio. The flame propagation speed showed different trends at different equivalence ratio for tested mixtures. It was found that the Markstein length was increased with the increase of equivalence ratio.
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Ju, Yiguang, e Yuan Xue. "Extinction and flame bifurcations of stretched dimethyl ether premixed flames". Proceedings of the Combustion Institute 30, n.º 1 (janeiro de 2005): 295–301. http://dx.doi.org/10.1016/j.proci.2004.08.258.
Texto completo da fonteTeses / dissertações sobre o assunto "Stretched Flames"
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YAMAMOTO, Kazuhiro, e Satoru ISHIZUKA. "Temperatures of Positively and Negatively Stretched Flames". Japan Society of Mechanical Engineers, 2003. http://hdl.handle.net/2237/9370.
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Long, Scott R. "Experimental determination of strain rates in stretched laminar diffusion flames". Thesis, This resource online, 1992. http://scholar.lib.vt.edu/theses/available/etd-08222009-040351/.
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Detomaso, Nicola. "Simulation aux grandes échelles de la combustion à volume constant : modélisation numérique des flammes turbulentes en expansion dans les mélanges non homogènes". Electronic Thesis or Diss., Université de Toulouse (2023-....), 2024. http://www.theses.fr/2024TLSEP034.
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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 systèmesClassical 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
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Nanduri, Jagannath Ramchandra. "A COMPUTATIONAL STUDY OF THE STRUCTURE, STABILITY, DYNAMICS, AND RESPONSE OF LOW STRETCH DIFFUSION FLAME". Case Western Reserve University School of Graduate Studies / OhioLINK, 2006. http://rave.ohiolink.edu/etdc/view?acc_num=case1132237973.
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Amato, Alberto. "Leading points concepts in turbulent premixed combustion modeling". Diss., Georgia Institute of Technology, 2014. http://hdl.handle.net/1853/52247.
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The propagation of premixed flames in turbulent flows is a problem of wide physical and technological interest, with a significant literature on their propagation speed and front topology. While certain scalings and parametric dependencies are well understood, a variety of problems remain. One major challenge, and focus of this thesis, is to model the influence of fuel/oxidizer composition on turbulent burning rates.
Classical explanations for augmentation of turbulent burning rates by turbulent velocity fluctuations rely on global arguments - i.e., the turbulent burning velocity increase is directly proportional to the increase in flame surface area and mean local burning rate along the flame. However, the development of such global approaches is complicated by the abundance of phenomena influencing the propagation of turbulent premixed flames. Emphasizing key governing processes and cutting-off interesting but marginal phenomena appears to be necessary to make further progress in understanding the subject.
An alternative approach to understand turbulent augmentation of burning rates is based upon so-called "leading points", which are intrinsically local properties of the turbulent flame. Leading points concepts suggest that the key physical mechanism controlling turbulent burning velocities of premixed flames is the velocity of the points on the flame that propagate farthest out into the reactants. It is postulated that modifications in the overall turbulent combustion speed depend solely on modifications of the burning rate at the leading points since an increase (decrease) in the average propagation speed of these points causes more (less) flame area to be produced behind them. In this framework, modeling of turbulent burning rates can be thought as consisting of two sub-problems: the modeling of (1) burning rates at the leading points and of (2) the dynamics/statistics of the leading points in the turbulent flame. The main objective of this thesis is to critically address both aspects, providing validation and development of the physical description put forward by leading point concepts.
To address the first sub-problem, a comparison between numerical simulations of one-dimensional laminar flames in different geometrical configurations and statistics from a database of direct numerical simulations (DNS) is detailed. In this thesis, it is shown that the leading portions of the turbulent flame front display a structure that on average can be reproduced reasonably well by results obtained from model geometries with the same curvature. However, the comparison between model laminar flame computations and highly curved flamelets is complicated by the presence of negative (i.e., compressive) strain rates, due to gas expansion. For the highest turbulent intensity investigated, local consumption speeds, curvatures, strain rates and flame thicknesses approach the maximum values obtained by the laminar model geometries, while other cases display substantially lower values.
To address the second sub-problem, the dynamics of flame propagation in simplified flow geometries is studied theoretically. Utilizing results for Hamilton-Jacobi equations from the Aubry-Mather theory, it is shown how the overall flame front progation under certain conditions is controlled only by discrete points on the flame. Based on these results, definitions of leading points are proposed and their dynamics is studied. These results validate some basic ideas from leading points arguments, but also modify them appreciably. For the simple case of a front propagating in a one-dimensional shear flow, these results clearly show that the front displacement speed is controlled by velocity field characteristics at discrete points on the flame only when the amplitude of the shear flow is sufficiently large and does not vary too rapidly in time. However, these points do not generally lie on the farthest forward point of the front. On the contrary, for sufficiently weak or unsteady flow perturbations, the front displacement speed is not controlled by discrete points, but rather by the entire spatial distribution of the velocity field. For these conditions, the leading points do not have any dynamical significance in controlling the front displacement speed. Finally, these results clearly show that the effects of flame curvature sensitivity in modifying the front displacement speed can be successfully interpreted in term of leading point concepts.
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Hinton, Nathan Ian David. "Measuring laminar burning velocities using constant volume combustion vessel techniques". Thesis, University of Oxford, 2014. http://ora.ox.ac.uk/objects/uuid:5b641b04-8040-4d49-a7e8-aae0b0ffc8b5.
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The laminar burning velocity is an important fundamental property of a fuel-air mixture at given conditions of temperature and pressure. Knowledge of burning velocities is required as an input for combustion models, including engine simulations, and the validation of chemical kinetic mechanisms. It is also important to understand the effect of stretch upon laminar flames, to correct for stretch and determine true (unstretched) laminar burning velocities, but also for modelling combustion where stretch rates are high, such as turbulent combustion models. A constant volume combustion vessel has been used in this work to determine burning velocities using two methods: a) flame speed measurements during the constant pressure period, and b) analysis of the pressure rise data. Consistency between these two techniques has been demonstrated for the first time. Flame front imaging and linear extrapolation of flame speed has been used to determine unstretched flame speeds at constant pressure and burned gas Markstein lengths. Measurement of the pressure rise during constant volume combustion has been used along with a numerical multi-zone combustion model to determine burning velocities for elevated temperatures and pressures as the unburned gas ahead of the spherically expanding flame front is compressed isentropically. This burning velocity data is correlated using a 14 term correlation to account for the effects of equivalence ratio, temperature, pressure and fraction of diluents. This correlation has been modified from an existing 12 term correlation to more accurately represent the dependence of burning velocity upon temperature and pressure. A number of fuels have been tested in the combustion vessel. Biogas (mixtures of CH4 and CO2) has been tested for a range of equivalence ratios (0.7–1.4), with initial temperatures of 298, 380 and 450 K, initial pressures of 1, 2 and 4 bar and CO2 fractions of up to 40% by mole. Hydrous ethanol has been tested at the same conditions (apart from 298 K due to the need to vaporise the ethanol), and for fractions of water up to 40% by volume. Binary, ternary and quaternary blends of toluene, n-heptane, ethanol and iso-octane (THEO) have been tested for stoichiometric mixtures only, at 380 and 450 K, and 1, 2 and 4 bar, to represent surrogate gasoline blended with ethanol. For all fuels, correlation coefficients have been obtained to represent the burning velocities over wide ranging conditions. Common trends are seen, such as the reduction in burning velocity with pressure and increase with temperature. In the case of biogas, increasing CO2 results in a decrease in burning velocity, a shift in peak burning velocity towards stoichiometric, a decrease in burned gas Markstein length and a delayed onset of cellularity. For hydrous ethanol the reduction in burning velocity as H2O content is increased is more noticeably non-linear, and whilst the onset of cellularity is delayed, the effect on Markstein length is minor. Chemical kinetic simulations are performed to replicate the conditions for biogas mixtures using the GRI 3.0 mechanism and the FlameMaster package. For hydrous ethanol, simulations were performed by Carsten Olm at Eötvös Loránd University, using the OpenSMOKE 1D premixed flame solver. In both cases, good agreement with experimental results is seen. Tests have also been performed using a single cylinder optical engine to compare the results of the hydrous ethanol tests with early burn combustion, and a good comparison is seen. Results from tests on THEO fuels are compared with mixing rules developed in the literature to enable burning velocities of blends to be determined from knowledge of that of the pure components alone. A variety of rules are compared, and it is found that in most cases, the best approximation is found by using the rule in which the burning velocity of the blend is represented by weighting by the energy fraction of the individual components.
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Marshall, Andrew. "Turbulent flame propagation characteristics of high hydrogen content fuels". Diss., Georgia Institute of Technology, 2015. http://hdl.handle.net/1853/53859.
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Increasingly stringent pollution and emission controls have caused a rise in the use of combustors operating under lean, premixed conditions. Operating lean (excess air) lowers the level of nitrous oxides (NOx) emitted to the environment. In addition, concerns over climate change due to increased carbon dioxide (CO2) emissions and the need for energy independence in the United States have spurred interest in developing combustors capable of operating with a wide range of fuel compositions. One method to decrease the carbon footprint of modern combustors is the use of high hydrogen content (HHC) fuels. The objective of this research is to develop tools to better understand the physics of turbulent flame propagation in highly stretch sensitive premixed flames in order to predict their behavior at conditions realistic to the environment of gas turbine combustors.
This thesis presents the results of an experimental study into the flame propagation characteristics of highly stretch-sensitive, turbulent premixed flames generated in a low swirl burner (LSB). This study uses a scaling law, developed in an earlier thesis from leading point concepts for turbulent premixed flames, to collapse turbulent flame speed data over a wide range of conditions. The flow and flame structure are characterized using high speed particle image velocimetry (PIV) over a wide range of fuel compositions, mean flow velocities, and turbulence levels. The first part of this study looks at turbulent flame speeds for these mixtures and applies the previously developed leading points scaling model in order to test its validity in an alternate geometry. The model was found to collapse the turbulent flame speed data over a wide range of fuel compositions and turbulence levels, giving merit to the leading points model as a method that can produce meaningful results with different geometries and turbulent flame speed definitions. The second part of this thesis examines flame front topologies and stretch statistics of these highly stretch sensitive, turbulent premixed flames. Instantaneous flame front locations and local flow velocities are used to calculate flame curvatures and tangential strain rates. Statistics of these two quantities are calculated both over the entire flame surface and also conditioned at the leading points of the flames. Results presented do not support the arguments made in the development of the leading points model. Only minor effects of fuel composition are noted on curvature statistics, which are mostly dominated by the turbulence. There is a stronger sensitivity for tangential strain rate statistics, however, time-averaged values are still well below the values hypothesized from the leading points model. The results of this study emphasize the importance of local flame topology measurements towards the development of predictive models of the turbulent flame speed.
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Taylor, Simon Crispin. "Burning velocity and the influence of flame stretch". Thesis, University of Leeds, 1991. http://etheses.whiterose.ac.uk/2099/.
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A new technique is presented for determining burning velocities and stretch effects in laminar flames, and applied to a range of fuel/air mixtures. The speeds of expanding spherical flames, measured by high-speed schlieren cine-photography, are shown to vary with flame radius. A simple phenomenological model has been developed to analyse the data and obtain the one-dimensional flame speed by extrapolation to infinite radius. The validity of the simple model has been tested by using it to analyse the results of detailed simulations of expanding spherical flames. The true one-dimensional flame speeds in this case are known from planar flame modelling using the same kinetic scheme. The simple model predicted flame speeds within 2% of the true values for hydrogen/air mixtures over most of the stoichiometric range. This demonstrates that the extrapolation procedure is sound and will produce reliable results when applied to experimental data. Since the flame speeds derived from experiments are one-dimensional values, multiplying them by the density ratio gives one-dimensional burning velocities (s,'). Maximum burning velocities of hydrogen, methane, ethane, propane and ethylene mixtures with air were 2.85 ms-', 0.37 ms-', 0.41 ms-', 0.39 ms-' and 0.66 ms-' respectively. These are considerably smaller than most burner-derived values. The discrepancies can be explained by flow divergence and stretch effects perturbing burner measurements. The rate at which the measured flame speed approaches its limiting value depends on flame thickness and flame stretch. By subtracting the flame thickness term, the influence of flame stretch, expressed as the Markstein length, can be derived. Again values are given across the whole stoichiometric range of all fuels listed above, and form the most complete set of Markstein lengths reported to date. The Markstein lengths are negative in lean hydrogen and methane and in rich ethane and propane mixtures: this means that stretch increases the burning rate. They are positive in all other mixtures, showing that stretch decreases the burning rate. The results are in line with predictions based on Lewis number considerations. An alternative method of deriving one-dimensional burning velocities and Markstein lengths has been investigated. Burning velocities were measured at different stretch rates in flames in stagnation-point flow. Particle tracking was used to derive burning velocities referred to the hot side of the flame from the upstream values. The two burning velocities extrapolated to different one-dimensional values, both of which differed slightly from the expanding flame results. The suggested reason is that the upstream velocity gradient is not an accurate measure of the stretch experienced by the flame. Markstein lengths were consistent with those from the expanding flame method but the uncertainties were much larger. The method in its present form is therefore useful qualitatively but not quantitatively.
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li, zhiliang. "EXPERIMENTAL AND CFD INVESTIGATIONS OF LIFTED TRIBRACHIAL FLAMES". Doctoral diss., University of Central Florida, 2010. http://digital.library.ucf.edu/cdm/ref/collection/ETD/id/3048.
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Experimental measurements of the lift-off velocity and lift-off height, and numerical simulations were conducted on the liftoff and stabilization phenomena of laminar jet diffusion flames of inert-diluted C3H8 and CH4 fuels. Both non-reacting and reacting jets were investigated, including effects of multi-component diffusivities and heat release (buoyancy and gas expansion). The role of Schmidt number for non-reacting jets was investigated, with no conclusive Schmidt number criterion for liftoff previously known in similarity solutions. The cold-flow simulation for He-diluted CH4 fuel does not predict flame liftoff; however, adding heat release reaction leads to the prediction of liftoff, which is consistent with experimental observations. Including reaction was also found to improve liftoff height prediction for C3H8 flames, with the flame base location differing from that in the similarity solution - the intersection of the stoichiometric and iso-velocity contours is not necessary for flame stabilization (and thus lift-off). Possible mechanisms other than that proposed for similarity solution may better help to explain the stabilization and liftoff phenomena. The stretch rate at a wide range of isotherms near the base of the lifted tribrachial flame were also quantitatively plotted and analyzed.Ph.D.
Department of Mechanical, Materials and Aerospace Engineering
Engineering and Computer Science
Mechanical Engineering PhD
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Roldo, Ismael. "Estudo experimental e teórico de chamas em escoamento de estagnação imersas em meios porosos inertes". reponame:Biblioteca Digital de Teses e Dissertações da UFRGS, 2015. http://hdl.handle.net/10183/127905.
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O interesse no desenvolvimento de sistemas eficientes de combustão para reduzir a poluição ambiental e aumentar a eficiência de queima tem chamado a atenção para a combustão em meios porosos inertes. A recirculação de calor, induzida pela matriz sólida a partir dos produtos quentes para os reagentes frios, aumenta a temperatura da chama melhorando a sua estabilidade e permitindo a utilização de combustíveis com baixo poder calorífico. Um estudo teórico recente mostra que uma chama estabilizada por um plano de estagnação imersa em um meio poroso pode, sob certas condições, estender os limites de inflamabilidade de uma mistura de ar e combustível. Por outro lado, o plano de estagnação é um problema que simula o efeito da taxa de deformação do escoamento sobre a estabilidade da chama, o que é relevante para várias configurações de queimador poroso. Portanto, o foco deste trabalho é o estudo da combustão em um queimador poroso com um plano de estagnação. Um experimento é conduzido com empacotamento de esferas, onde uma chama pode ser estabilizada por plano de estagnação devido a um anteparo. A razão de equivalência e a taxa de deformação são controladas pelos fluxos de ar e de combustível e da distância entre injetor e anteparo. A posição da chama é aproximadamente determinada pelo campo de temperaturas medidas por termopares. Complementarmente é realizada uma análise numérica simplificada do problema na qual se pode verificar o efeito da taxa de deformação sobre a estabilidade de chamas em queimadores porosos. Os resultados mostram que é possível estabilizar chamas no interior do meio poroso com plano de estagnação, porém, não foi possível atribuir um aumento de temperatura devido ao aumento da taxa de deformação.The interest in developing efficient combustion systems to reduce environmental pollution and increase the burning efficiency has called attention to the combustion in inert porous media. The heat recirculation, induced by the solid matrix, from the hot products to the incoming cold reactants, increases the flame temperature and improves its stability, allowing for the use of fuels with low heat content. A recent study shows theoretically that a flame stabilized by a stagnation plane immersed in a porous medium may, under certain conditions, to extend the flammability limits of a mixture of fuel and air. On the other hand, the stagnation plane imposes a certain strain rate on the flow field, which is relevant to various porous burner configurations. Therefore, the focus of this work is the study of combustion in a porous burner with a stagnation plane. An experiment is conducted with packing bed of spheres where a flame can be stabilized against a stagnation plane. The equivalence ratio and the strain rate are controlled by the flows of air and fuel and the distance between the injector and the stagnation plane. The flame position is approximately determined by the temperature field measured by thermocouples. In addition, it is performed a simplified numerical analysis of the problem in which one can see the effect of the strain rate on the stability of flames in porous burners. The results show that it is possible to stabilize flames within the porous medium with stagnation plane, however, it has not been possible to assign a temperature increase due to the increased strain rate.
Livros sobre o assunto "Stretched Flames"
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Pitz, R. W. Comparison of reaction zones in turbulent lifted diffusion flames to stretched laminar flamelets. Washington, D.C: American Institute of Aeronautics and Astronautics, 1992.
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National Aeronautics and Space Administration (NASA) Staff. Stretch-Induced Quenching in Flame-Vortex Interactions. Independently Published, 2019.
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Clasen, Mathias. Sizing Up the Beast. Oxford University Press, 2017. http://dx.doi.org/10.1093/oso/9780190666507.003.0002.
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Horror fiction has been a legitimate object of academic study for several decades now. There are many competing theoretical approaches to horror and the Gothic, but the most prevalent approaches are seriously flawed. Constructivist approaches, which see horror as a product of historical circumstance, ignore the genre’s psychological and biological underpinnings and its deep history. Horror stretches back in time beyond the Gothic novel through folk tales to earlier oral narratives. Psychoanalytical approaches, which build on Freud’s theories of psychology, are scientifically obsolete and have a distorting effect on the subject matter, reducing horror to representations of psychosexual complexes. The chapter critically discusses existing approaches to horror, as well as horror as an affectively defined genre, and it argues for a consilient, biocultural approach which integrates other viable approaches within a framework based on biology and which builds on current social science.
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Freud, Sophie. Living in the Shadow of the Freud Family. Praeger, 2007. http://dx.doi.org/10.5040/9798400680328.
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I had to do something to escape Hitler's clutches, writes Esti Freud. Yet she waits with her then-16-year-old daughter, Sophie in Paris until German canons can be heard in the distance before deciding to escape by bicycle across France, as Sophie keeps looking back to see whether German tanks will overtake them. Both women survive and, in their own ways, come to feel a need to keep a personal record of those tumultuous times. Thus, in a memoir written at age 79, Esti Fraud, daughter-in-law of Sigmund Freud and wife of his oldest son, Martin, looks back on her life starting before the 20th century, lived on three continents, and stretched through two world wars and the Holocaust. Twenty years after her mothers' death, daughter Sophie turned to Esti's memoir as the scaffold for this book, expanding it through family letters, archival material, and her own diary penned as a teenager. Out of these documents, Sophie Freud has created a many-voiced mosaic, including letters and insights from a wide cast of characters who tell the story of a famous family—and of a century. This work gives an insider's, in-law view of the family Freud, its foundations, and flaws. The relationship between Esti, daughter of a wealthy Vienna attorney and her husband Martin Freud is foreshadowed by the young lovers' fathers. At first meeting Esti, Sigmund told his son the glamorous woman was too beautiful for the clan, meaning her splendor belied a lifestyle not conducive to the frugal Freud ways. And Esti's father, on hearing of her love for Martin, expressed regret she was involved with a man who was not a financially favorable linkage, and that his family was not respectable since patriarch Sigmund was just another psychiatrist, and one who writes pornography books at that. Thus begins the ill-fated relationship that would rock two families and a generation of children to come. Sophie weaves into the text letters she inherited, including letters from Martin while he was a prisoner of war, and excerpts from her own diary, kept as an adolescent. The resulting mosaic will fascinate—and perhaps disturb—readers interested in Freud and psychoanalysis, as well as those intrigued by relationships and family.
Capítulos de livros sobre o assunto "Stretched Flames"
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Isaac, K. M. "Characteristics of Stretched Hydrogen-Air Diffusion Flames at High Pressures". In Transition, Turbulence and Combustion, 203–16. Dordrecht: Springer Netherlands, 1994. http://dx.doi.org/10.1007/978-94-011-1034-1_19.
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Meyer, Michael Peter, e Rune Peter Lindstedt. "Evaluation of Hazard Correlations for Hydrogen-Rich Fuels Using Stretched Transient Flames". In Green Energy and Technology, 197–222. Singapore: Springer Singapore, 2021. http://dx.doi.org/10.1007/978-981-16-2648-7_9.
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Reames, Donald V. "A Turbulent History". In Solar Energetic Particles, 19–48. Cham: Springer International Publishing, 2021. http://dx.doi.org/10.1007/978-3-030-66402-2_2.
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AbstractLarge solar energetic-particle (SEP) events are clearly associated in time with eruptive phenomena on the Sun, but how? When large SEP events were first observed, flares were the only visible candidate, and diffusion theory was stretched to explain how the particles could spread through space, as widely as observed. The observation of coronal mass ejections (CMEs), and the wide, fast shock waves they can drive, provided better candidates later. Then small events were found with 1000-fold enhancements in 3He/4He that required a different kind of source—should we reconsider flares, or their open-field cousins, solar jets? The 3He-rich events were soon associated with the electron beams that produce type III radio bursts. It seems the radio astronomers knew of both SEP sources all along. Sometimes the distinction between the sources is blurred when shocks reaccelerate residual 3He-rich impulsive suprathermal ions. Eventually, however, we would even begin to measure the source-plasma temperature that helps to better distinguish the SEP sources.
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Scioli, Anthony. "The Psychology of Hope: A Diagnostic and Prescriptive Account". In Historical and Multidisciplinary Perspectives on Hope, 137–63. Cham: Springer International Publishing, 2020. http://dx.doi.org/10.1007/978-3-030-46489-9_8.
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Abstract In this chapter, I review psychology’s contributions to the study of hope. To close potential gaps in this interdisciplinary volume, I include work in psychiatry and nursing. The nearly 400-year history of psychological reflections on hope reveals extended stretches of neglect, alternating with brief flashes of interest. Shifting scientific paradigms are partly to blame. However, I suggest that the greatest challenge for investigators seeking scientific consensus on the topic may be cultural and sociopolitical. I begin with a review of the most significant writings and research on hope, dating back to the seventeenth century. I examine goal-related approaches in greater depth, due to their strong influence on the field of psychology. The latter half of this chapter is more critical and prescriptive. For a deeper commentary, I rely on Markus’s (Meas Interdisciplinary Res Perspect 6:54–77, 2008) distinction between constructs and concepts as well as Danziger’s (Naming the mind: How psychology found its language. Sage Publications, 1997) observation on how psychology found its lexicon. This middle, diagnostic section includes a review of philosophy of science criteria for evaluating theories. I transition to general prescriptions for achieving a better understanding of hope, organized around Bacon’s “four idols” of the mind, and add specific suggestions for future research. I conclude with a summary of recent work within our hope lab.
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"Predicted Structure of Stretched and Unstretched Methane-Air Diffusion Flames". In Dynamics of Flames and Reactive Systems, 305–19. New York: American Institute of Aeronautics and Astronautics, 1985. http://dx.doi.org/10.2514/5.9781600865701.0305.0319.
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"Charikleia, Condemned as a Poisoner, Rescued by Divine Intervention". In Women’s Religions in the Greco-Roman World, editado por Ross Shepard Kraemer, 374. Oxford University PressNew York, NY, 2004. http://dx.doi.org/10.1093/oso/9780195170658.003.0119.
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Abstract author, translation, text, and bibliography: See entry 22. The executioners built a gigantic bonfire and then lit it. As the flames took hold, Charikleia begged a moment’s grace from the guards who held her, promising that she would mount the pyre without the use of force. She stretched her arms towards that quarter of the sky whence the sun was beaming, and prayed in a loud voice: “O Sun and Earth and you spirits above and beneath the earth who watch and punish the sins of men, bear me witness that I am innocent of the charges laid against me and that I gladly suffer death because of the unendurable agonies that fate inflicts on me. Receive me mercifully, but with all possible speed exact retribution from that she-devil, that evil adulteress—Arsake—who has contrived all this to rob me of my beloved.” At these words there was general uproar. While some of the crowd were still making up their minds to halt the execution for a second trial, others were already moving to do so, but, before they could act, Charikleia climbed onto the pyre and positioned herself at the very heart of the fire. There she stood for some time without taking any hurt. The flames flowed around her rather than licking against her; they caused her no harm but drew back wherever she moved towards them, serving merely to encircle her in splendor and present a vision of her standing in radiant beauty in a frame of light, like a bride in a chamber of flame.
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"Simulation of Stretched Premixed CH4-Air and C3H8-Air Flames with Detailed Chemistry". In Dynamics of Reactive Systems Part I: Flames; Part II: Heterogeneous Combustion and Applications, 195–214. Washington DC: American Institute of Aeronautics and Astronautics, 1988. http://dx.doi.org/10.2514/5.9781600865879.0195.0214.
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Michalski, Krzysztof. "Time Flows, the Child Plays". In The Flame of Eternity, traduzido por Benjamin Paloff. Princeton University Press, 2011. http://dx.doi.org/10.23943/princeton/9780691143460.003.0002.
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This chapter introduces the images of the grazing cows, the child at play, and the person observing them with envy and emotion. It argues that these images are supposed to confront us with human life, concentrated in the lived moment and simultaneously tearing the past from the future. It is life stretched out from “yesterday” to “tomorrow” and thereby burdened with memory and guilt—and at the same time innocent and oblivious, growing out of time in its every instant: the connection between time and its sickness, eternity. Such a concept of the human condition, the chapter shows, carries far-reaching consequences.
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"Stretch Effects in Planar Premixed Hydrogen-Air Flames". In Dynamics of Flames and Reactive Systems, 61–74. New York: American Institute of Aeronautics and Astronautics, 1985. http://dx.doi.org/10.2514/5.9781600865701.0061.0074.
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Hecht, Jeff. "Fibers of Glass". In City of Light, 28–33. Oxford University PressNew York, NY, 1999. http://dx.doi.org/10.1093/oso/9780195108187.003.0003.
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Abstract The difference between glass rods and glass fibers is merely a matter of diameter. Anyone who can be trusted with a chemistry set can draw a fiber easily from a glass rod. Hold the two ends of a rod several inches long and put the middle in a hot flame. The heat softens the glass, melting the rigid solid into a thick, viscous liquid. After the rod becomes flexible, pull the two ends apart while removing the rod from the flame. The molten glass stretches into a long, tapered thread, which solidifies almost instantly as air cools it. Although the material remains glass, the thin filament is flexible and seems much less brittle than the rod.
Trabalhos de conferências sobre o assunto "Stretched Flames"
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Buckmaster, J., e J. Buckmaster. "The effects of radiation on stretched flames". In 35th Aerospace Sciences Meeting and Exhibit. Reston, Virigina: American Institute of Aeronautics and Astronautics, 1997. http://dx.doi.org/10.2514/6.1997-238.
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Guo, Hongsheng, Stuart W. Neill e Gregory J. Smallwood. "A Numerical Investigation of NOx Formation in Counterflow CH4/H2/Air Diffusion Flames". In ASME 2006 International Mechanical Engineering Congress and Exposition. ASMEDC, 2006. http://dx.doi.org/10.1115/imece2006-14458.
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A detailed numerical study was carried out for the effect of hydrogen enrichment on flame structure and NOx formation in counterflow CH4/air diffusion flames. Detailed chemistry and complex thermal and transport properties were employed. The enrichment fraction was changed from 0 (pure CH4) to 1.0 (pure H2). The result indicates that for flames with low to moderate stretch rates, with the increase of the enrichment fraction from 0 to 0.5~0.6, NO emission index keeps almost constant or only slightly increases. When the enrichment fraction is increased from 0.5~0.6 to about 0.9, NO emission index quickly increases, and finally NO formation decreases again when pure hydrogen flame condition is approached. However, for flames with higher stretch rates, with the increase of hydrogen enrichment fraction from 0 to 1.0, the formation of NO first quickly increases, then slightly decreases and finally increases again. Detailed analysis suggests that the variation of the characteristics in NO formation in stretched CH4/air diffusion flames is caused by the change of flame structure and NO formation mechanism, when the enrichment fraction and stretch rate are changed.
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Abou-Ellail, Mohsen, Ryo S. Amano, Samer Elhaw, Karam Beshay e Hatem Kayed. "A Skewed Two-Dimensional Probability Density Function for Methane-Air Turbulent Diffusion Flames". In 2010 14th International Heat Transfer Conference. ASMEDC, 2010. http://dx.doi.org/10.1115/ihtc14-23068.
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The present paper describes a mathematical model for turbulent methane-air jet diffusion flames. The mathematical model solves density-weighted governing equations for momentum, mass continuity, turbulent kinetic energy and its dissipation rate. The combustion model solves density-weighted transport equations for the mixture fraction “f”, its variance “g” and its skewness “s”. These variables are used to compute one part of the probability density function (PDF) in mixture fraction domain. The second part of the PDF is computed from the numerical solutions of the mixture fraction dissipation rate “χ” and its variance χ˜″2. The resulting two-dimensional PDF is defined in the mixture-fraction-scalar-dissipation-rate 2D space. The flamelet combustion sub-model is used to compute the mean flame temperature, density and species mass fractions. The flamelet model provides instantaneous state relationships for the stretched flamelets up to the extinction limit. The mean flame properties are computed through the integration of the stretched flamelet state relationships over the two-dimensional PDF. The present 2D probability density function model can predict rim-attached flames as well as unstable lifted flames. This is because the flamelet model provides information on the flame instability arising from the stretching effects of highspeed flowing gases. The new two-dimensional probability density function is used to predict the flame properties of a free jet methane-air flame for which experimental data exists.
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Rajagopalan, Hari Priya, Renee Cole, David Wu, Benjamin Emerson e Timothy Lieuwen. "Turbulent Burning Velocity of High Hydrogen Flames". In ASME Turbo Expo 2024: Turbomachinery Technical Conference and Exposition. American Society of Mechanical Engineers, 2024. http://dx.doi.org/10.1115/gt2024-121349.
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Abstract The turbulent burning velocity (ST) is one of the most important combustion properties controlling combustor operability limits, directly influencing blowoff, flashback, and combustion instabilities. Hydrogen has particularly significant influences on the turbulent flame speed. This paper presents new H2/CH4 data of high pressure, high hydrogen turbulent burning velocities. The data sets were designed to address fundamental questions as well as provide engineering/design relevant insights. This paper presents new scaling analysis of fuel composition, pressure, and preheat temperature effects on turbulent burning velocity. We also discuss the importance of considering what is being held constant (temperature, flame speed, Reynolds number, etc.) when one is analyzing these sensitivities. Data shows that hydrogen fraction and pressure causes an increase in turbulent flame speed; whether quantified as raw ST,GC, or normalized as ST,GC/SL,0 or ST,GC/SL,max, (where SL,0 and SL,max are the unstretched and stretched laminar flame speed respectively). We also propose that observed increases in ST,GC with pressure are due to increases in Reynolds number, and not a kinetics/stretch sensitivity effect. With increasing preheat temperature, ST,GC increases while its normalized value (ST,GC/SL,0 and ST,GC/SL,max) can either increase or decrease, depending upon fuel composition. We also show how these sensitivities vary, depending on whether these comparisons are made at constant raw turbulence intensity urms, or at constant normalized urms/SL,0 or urms/SL,max.
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Venkateswaran, Prabhakar, Andrew D. Marshall, David R. Noble, Jerry M. Seitzman e Tim C. Lieuwen. "Turbulent Consumption Speed Scaling of H2/CO Blends". In ASME 2011 Turbo Expo: Turbine Technical Conference and Exposition. ASMEDC, 2011. http://dx.doi.org/10.1115/gt2011-45401.
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This paper describes measurements and analysis of global turbulent consumption speeds, ST,GC, of hydrogen/carbon monoxide (H2/CO) mixtures. The turbulent flame properties of such mixtures are of fundamental interest because of their strong stretch sensitivity and of practical interest since they are the primary constituents of syngas fuels. Data are analyzed at mean flow velocities and turbulence intensities of 4 < U0 < 50 m/s and 1 < u′rms/SL,0 < 100, respectively, for H2/CO blends ranging from 30–90% H2 by volume. Data from two sets of experiments are reported. In the first, fuel blends ranging from 30–90% H2 and mixture equivalence ratio, Φ, were adjusted at each fuel composition to have nominally the same un-stretched laminar flame speed, SL,0. In the second set, equivalence ratios were varied at constant H2 levels. The data clearly corroborate results from other studies that show significant sensitivity of ST,GC to fuel composition. For example, at a fixed u′rms, ST,GC of a 90% H2 case (at Φ = 0.48) is a factor of three times larger than the baseline Φ = 0.9, CH4/air mixture that has the same SL,0 value. We also describe physics-based correlations of these data, using leading points concepts and detailed kinetic calculations of their stretch sensitivities. These results are used to develop an inequality for negative Markstein length flames that bounds the turbulent flame speed data and show that the data can be collapsed using the maximum stretched laminar flame speed, SL,max, rather than SL,0.
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Foley, C. W., I. Chterev, J. Seitzman e T. Lieuwen. "High Resolution PIV and CH-PLIF Measurements and Analysis of a Shear Layer Stabilized Flame". In ASME Turbo Expo 2015: Turbine Technical Conference and Exposition. American Society of Mechanical Engineers, 2015. http://dx.doi.org/10.1115/gt2015-43387.
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Understanding the mechanisms and physics of flame stabilization and blowoff of premixed flames is critical towards the design of high velocity combustion devices. In the high bulk flow velocity situation typical of practical combustors, the flame anchors in shear layers where the local flow velocities are much lower. Within the shear layer, fluid strain deformation rates are very high and the flame can be subjected to significant stretch levels. The main goal of this work was to characterize the flow and stretch conditions that a premixed flame experiences in a practical combustor geometry and to compare these values to calculated extinction values. High resolution, simultaneous PIV and CH-PLIF measurements are used to capture the flame edge and near-field stabilization region. When approaching lean limit extinction conditions, we note characteristic changes in the stretch and flow conditions experienced by the flame. Most notably, the flame becomes less critically stretched when fuel/air ratio is decreased. However, at these lean conditions, the flame is subject to higher mean flow velocities at the edge, suggesting less favorable flow conditions are present at the attachment point of the flame as blowoff is approached. These measurements suggest that blowoff of the flame from the shear layer is not directly stretch extinction induced, but rather the result of an imbalance between the speed of the flame edge and local tangential flow velocity.
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Medina Martinez, Urbano A., Harshavardhana A. Uranakara, Takuya Tomidokoro, Lorenzo Angelilli e Hong G. Im. "Understanding extinction of stretched premixed hydrogen-air flames using the tangential stretching rate". In AIAA SCITECH 2024 Forum. Reston, Virginia: American Institute of Aeronautics and Astronautics, 2024. http://dx.doi.org/10.2514/6.2024-0400.
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Salusbury, Sean D., Ehsan Abbasi-Atibeh e Jeffrey M. Bergthorson. "The Effect of Lewis Number on Instantaneous Flamelet Speed and Position Statistics in Counter-Flow Flames With Increasing Turbulence". In ASME Turbo Expo 2017: Turbomachinery Technical Conference and Exposition. American Society of Mechanical Engineers, 2017. http://dx.doi.org/10.1115/gt2017-64821.
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Differential diffusion effects in premixed combustion are studied in a counter-flow flame experiment for fuel-lean flames of three fuels with different Lewis numbers: methane, propane, and hydrogen. Previous studies of stretched laminar flames show that a maximum reference flame speed is observed for mixtures with Le ≳ 1 at lower flame-stretch values than at extinction, while the reference flame speed for Le ≪ 1 increases until extinction occurs when the flame is constrained by the stagnation point. In this work, counter-flow flame experiments are performed for these same mixtures, building upon the laminar results by using variable high-blockage turbulence-generating plates to generate turbulence intensities from the near-laminar u′/SLo=1 to the maximum u′/SLo achievable for each mixture, on the order of u′/SLo=10. Local, instantaneous reference flamelet speeds within the turbulent flame are extracted from high-speed PIV measurements. Instantaneous flame front positions are measured by Rayleigh scattering. The probability-density functions (PDFs) of instantaneous reference flamelet speeds for the Le ≳ 1 mixtures illustrate that the flamelet speeds are increasing with increasing turbulence intensity. However, at the highest turbulence intensities measured in these experiments, the probability seems to drop off at a velocity that matches experimentally-measured maximum reference flame speeds in previous work. In contrast, in the Le ≪ 1 turbulent flames, the most-probable instantaneous reference flamelet speed increases with increasing turbulence intensity and can, significantly, exceed the maximum reference flame speed measured in counter-flow laminar flames at extinction, with the PDF remaining near symmetric for the highest turbulence intensities. These results are reinforced by instantaneous flame position measurements. Flame-front location PDFs show the most probable flame location is linked both to the bulk flow velocity and to the instantaneous velocity PDFs. Furthermore, hydrogen flame-location PDFs are recognizably skewed upstream as u′/SLo increases, indicating a tendency for the Le ≪ 1 flame brush to propagate farther into the unburned reactants against a steepening average velocity gradient.
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DANCEY, C., e S. LONG. "Experimental investigation of the strain rate field in stretched laminar H2/air diffusion flames". In 23rd Fluid Dynamics, Plasmadynamics, and Lasers Conference. Reston, Virigina: American Institute of Aeronautics and Astronautics, 1993. http://dx.doi.org/10.2514/6.1993-3068.
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Yoshida, A., Y. Momomoto, H. Naito e Y. Saso. "Effect of water mist on temperature and burning velocity of stretched propane-air premixed flames". In MULTIPHASE FLOW 2013. Southampton, UK: WIT Press, 2013. http://dx.doi.org/10.2495/mpf130191.
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