Dissertations / Theses on the topic 'Bluff body stabilized flame'

Thesis (Ph.D.)--Aerospace Engineering, Georgia Institute of Technology, 2009.
Committee Chair: Lieuwen, Tim; Committee Member: Gaeta, Rick; Committee Member: Menon, Suresh; Committee Member: Seitzman, Jerry; Committee Member: Zinn, Ben.

Thesis (M. S.)--Aerospace Engineering, Georgia Institute of Technology, 2010.
Committee Chair: Tim Lieuwen; Committee Member: Jeff Jagoda; Committee Member: Suresh Menon. Part of the SMARTech Electronic Thesis and Dissertation Collection.

Combustion instability plagues the combustion community in a wide range of applications. This un-solved problem is especially prevalent and expensive in aerospace propulsion and ground power generation. The challenges associated with understanding and predicting combustion instability lie in the flame response to the acoustic field. One of the more complicated flame response mechanisms is the velocity coupled flame response, where the flame responds dynamically to the acoustic velocity as well as the vortically induced velocity field excited by the acoustics. This vortically induced, or hydrodynamic, velocity field holds critical importance to the flame response but is computationally expensive to predict, often requiring high fidelity CFD computations. Furthermore, its behavior can be a strong function of the numerous flow parameters that change over the operability map of a combustor. This research focuses on a nominally two dimensional bluff body combustor, which has rich hydrodynamic stability behavior with a manageable number of stability parameters. The work focuses first on experimentally characterizing the dynamical flow and flame behavior. Next, the research shifts focus toward hydrodynamic stability theory, using it to explain the physical phenomena observed in the experimental work. Additionally, the hydrodynamic stability work shows how the use of simple, model analysis can identify the important stability parameters and elucidate their governing physical roles. Finally, the research explores the forced response of the flow and flame while systematically varying the underlying hydrodynamic stability characteristics. In the case of longitudinal combustion instability of highly preheated bluff body combustors, it shows that conditions where an acoustic mode frequency equals the hydrodynamic global mode frequency are not especially dangerous from a combustion instability standpoint, and may actually have a reduced heat release response. This demonstrates the very non-intuitive role that the natural hydrodynamic flow stability plays in the forced heat release response of the flame. For the fluid mechanics community, this work contributes to the detailed understanding of both unforced and forced bluff body combustor dynamics, and shows how each is influenced by the underlying hydrodynamics. In particular, it emphasizes the role of the density-shear layer offset, and shows how its extreme sensitivity leads to complicated flow dynamics. For the flow-combustor community as a whole, the work reviews a pre-existing method to obtain the important flow stability parameters, and demonstrates a novel way to link those parameters to the governing flow physics. For the combustion instability community, this thesis emphasizes the importance of the hydrodynamic stability characteristics of the flow, and concludes by offering a paradigm for consideration of the hydrodynamics in a combustion instability problem.

Combustor blowout is a very serious concern in modern land-based and aircraft engine combustors. The ability to sense blowout precursors can provide significant payoffs in engine reliability and life. The objective of this work is to characterize the blowout phenomenon and develop a sensing methodology which can detect and assess the proximity of a combustor to blowout by monitoring its acoustic signature, thus providing early warning before the actual blowout of the combustor. The first part of the work examines the blowout phenomenon in a piloted jet burner. As blowout was approached, the flame detached from one side of the burner and showed increased flame tip fluctuations, resulting in an increase in low frequency acoustics. Work was then focused on swirling combustion systems. Close to blowout, localized extinction/re-ignition events were observed, which manifested as bursts in the acoustic signal. These events increased in number and duration as the combustor approached blowout, resulting an increase in low frequency acoustics. A variety of spectral, wavelet and thresholding based approaches were developed to detect precursors to blowout. The third part of the study focused on a bluff body burner. It characterized the underlying flame dynamics near blowout in greater detail and related it to the observed acoustic emissions. Vorticity was found to play a significant role in the flame dynamics. The flame passed through two distinct stages prior to blowout. The first was associated with momentary strain levels that exceed the flames extinction strain rate, leading to flame holes. The second was due to large scale alteration of the fluid dynamics in the bluff body wake, leading to violent flapping of the flame front and even larger straining of the flame. This led to low frequency acoustic oscillations, of the order of von Karman vortex shedding. This manifested as an abrupt increase in combustion noise spectra at 40-100 Hz very close to blowout. Finally, work was also done to improve the robustness of lean blowout detection by developing integration techniques that combined data from acoustic and optical sensors.

Combustion systems utilizing bluff bodies to stabilize the combustion processes can experience oscillatory heat release due to the alternate shedding of coherent, von Kármán vortices under certain operating conditions. This phenomenon needs to be understood in greater detail, since unsteady burning due to vortex shedding can lead to combustion instabilities and flame extinction in practical combustion systems. The primary objective of this study was to elucidate the influence of combustion process heat release upon the Bénard-von Kármán (BVK) instability in reacting bluff body wakes. For this purpose, spatial and temporal heat release distributions in bluff body-stabilized combustion of liquid Jet-A fuel with high-temperature, vitiated air were characterized over a wide range of operating conditions. Upon comparing the spatial and temporal heat release distributions, the fuel entrainment and subsequent heat release in the near-wake were found to strongly influence the onset and amplitude of the BVK instability. As the amount of heat release in the near-wake decreased, the BVK instability increased in amplitude. This was attributed to the corresponding decrease in the local density gradient across the reacting shear layers, which resulted in less damping of vorticity due to gas expansion. The experimental results were compared to the results of a parallel, linear stability analysis in order to further understand the influence of the combustion processes in the near-wake upon the wake instability characteristics. The results of this analysis support the postulate that oscillatory heat release due to BVK vortex shedding is the result of local absolute instability in the near-wake, which is eliminated only if the temperature rise across the reacting shear layers is sufficiently high. Furthermore, the results of this thesis demonstrate that non-uniform fuelling of the near-wake reaction zone increases the likelihood of absolutely unstable, BVK flame dynamics due to the possibility of near-unity products-to-reactants density ratios locally, especially when the reactants temperature is high.

A sub-grid scale closure for Large Eddy Simulation (LES) of turbulent combustion, based on physical space filtering of laminar flames is presented. The proposed formalism relies on a presumed probability density function (PDF) derived from the filtered laminar flames and flamelet tabulated chemistry. The combustion LES filter size is not fixed in this novel approach when sub-grid scale wrinkling occurs, but calibrated depending on the local level of unresolved scalar fluctuations. The model was validated by simulating 1D filtered laminar flames and 2D Bunsen flames. Subsequently, the model was tested on a 3D turbulent scenario by performing LES of the premixed and stratified configurations of the Cambridge swirl burner, experimentally studied by Sweeney and co-workers. Comparison of simulation and experiments for both the premixed and stratified configurations showed good agreement emphasizing the model characteristiscs. Instantaneous and time averaged LES data were analyzed to extract

Des données expérimentales sur les grosses structures ainsi que les corrélations entre certaines grandeurs physiques, telles que la vitesse et la température, sont nécessaires à la compréhension de la flamme turbulente. Pour obtenir de tels renseignements, on utilise l'association de deux techniques qui sont l'anémométrie Doppler laser pour les mesures de vitesse et le thermocouple à fil fin compensé numériquement pour la température. De par leur façon d'acquérir les données, aléatoire pour l'une, à fréquence fixe pour l'autre, ces deux méthodes obligent un traitement spécifique des mesures lors de leur étude couplée. L'étude de la compensation de l'inertie thermique du thermocouple, à partir de la constance de temps du capteur et du signal temporel de température, a permis de dégager les points clés de la méthode et d'en donner des limites. L'utilisation de ces deux techniques dans une flamme turbulente non prémélangée méthane-air, stabilisée par écoulement derrière un obstacle a permis d'expliquer les mécanismes de réallumage de la flamme principale par transferts de gaz chauds issus de la zone de recirculation. Le caractère périodique de ces transferts a également été montré par l'étude temporelle des signaux de température.

This thesis aims at improving our knowledge on swirl combustors. The work presented here is based on Large Eddy Simulations (LES) coupled to an advanced combustion model: the Conditional Moment Closure (CMC). Numerical predictions have been systematically compared and validated with detailed experimental datasets. In order to analyze further the physics underlying the large numerical datasets, Proper Orthogonal Decomposition (POD) has also been used throughout the thesis. Various aspects of the aerodynamics of swirling flames are investigated, such as precession or vortex formation caused by flow oscillations, as well as various combustion aspects such as localized extinctions and flame lift-off. All the above affect flame stabilization in different ways and are explored through focused simulations. The first study investigates isothermal air flows behind an enclosed bluff body, with the incoming flow being pulsated. These flows have strong similarities to flows found in combustors experiencing self-excited oscillations and can therefore be considered as canonical problems. At high enough forcing frequencies, double ring vortices are shed from the air pipe exit. Various harmonics of the pulsating frequency are observed in the spectra and their relation with the vortex shedding is investigated through POD. The second study explores the structure of the Delft III piloted turbulent non-premixed flame. The simple configuration allows to analyze further key combustion aspects of combustors, with further insights provided on the dynamics of localized extinctions and re-ignition, as well as the pollutants emissions. The third study presents a comprehensive analysis of the aerodynamics of swirl flows based on the TECFLAM confined non-premixed S09c configuration. A periodic component inside the air inlet pipe and around the central bluff body is observed, for both the inert and reactive flows. POD shows that these flow oscillations are due to single and double helical vortices, similar to Precessing Vortex Cores (PVC), that develop inside the air inlet pipe and whose axes rotate around the burner. The combustion process is found to affect the swirl flow aerodynamics. Finally, the fourth study investigates the TECFLAM configuration again, but here attention is given to the flame lift-off evident in experiments and reproduced by the LES-CMC formulation. The stabilization process and the pollutants emission of the flame are investigated in detail.

L'objectif de ce travail est de faire ressortir l'influence de l'aérodynamique du sillage d'un obstacle sur les mécanismes de stabilisation de la flamme non prémélangée. Cette aérodynamique est régie non seulement par la dynamique des écoulements mais aussi et surtout par la géométrie de l'accroche-flamme. Elle a été étudiée sur une configuration expérimentale non confinée de deux écoulements (combustible et air) séparés, concentriques, et dans le cas de deux stabilisateurs simplifiés (disque et tulipe). Des visualisations directes ont été réalisées et complétées par des mesures quantitatives des champs de vitesses en écoulements isotherme et réactif grâce à un système d'Anémométrie Doppler Laser à deux composantes. Les champs thermiques ont été obtenus au moyen d'un couple thermoélectrique à fil fin. Une caractérisation fine de la zone de recirculation a permis de mettre en évidence plusieurs phénomènes importants: (i) Le disque, provoquant une divergence importante dans l'écoulement annulaire, engendre une zone de recirculation plus longue, plus large et plus intense que celle observée dans le cas de la tulipe et pour laquelle l'effet du développement de couches limites est prépondérant, (ii) La compétition en terme de débit de quantité de mouvement "jet central/écoulement de retour" permet de distinguer le régime "jet dominant" de celui "écoulement dominant", (iii) En écoulement réactif, c'est la diminution de la masse volumique dans l'environnement proche du jet central qui facilite sa pénétration, (iv) L'effet de la forme du stabilisateur s'évalue en écoulement réactif de façon comparable au cas isotherme: l'aérodynamique de l'écoulement de retour engendré par le disque est plus perturbatrice que celle créée par la tulipe. L'analyse du développement et de la structure de la flamme a mis en évidence trois régimes de stabilisation principaux (développement, anneau, renfermement), deux régimes de transition et un domaine d'extinction. En complément de cette analyse, l'étude plus approfondie du régime "flamme avec anneau" a fait ressortir que l'anneau est une flamme triple obtenue à l'interface de deux écoulements: un prémélange de combustible, d'air et de produits de combustion et un écoulement d'air
The objective of this work was to point out the influence of the aerodynamic behind a bluff-body on the mechanisms of stabilisation of a non premixed flame. This aerodynamic behaviour is controlled not only by the dynamics of the flows but also, and particularly, by the geometry of the stabiliser. This analysis has been developed on a non-confined experimental set-up consisting of two separated and concentric flows (fuel and air), and for two stabilisers, disk and tulip shape burners. Direct visualisations were performed and were completed with velocity field measurements for cold and reacting flows by means of a two-component laser Doppler Anemometer. Temperature fields were also obtained by using a thermocouple. The characterisation of the recirculation zone allowed to bring to light several interesting phenomena: (i) The disk burner induces an important deviation of the annular flow and creates a larger, wider and more intense recirculation zone than the one observed with the tulip burner, for whiçh the effect of the boundary-layer development is predominant, (ii) The competition between the central jet and the recirculating flow, in terms of momentum flux, allows to differentiate the "dominant jet regime" from the "dominant flow regime", (iii) For the reacting flow, the decrease of the density in the zone surrounding the central jet facilitates its penetration, (iv) The effect of the burner shape for the reacting flow can be evaluated in a similar way as for the cold flow: the aerodynamic of the recirculating flow due to the disk enhances stronger perturbations than those generated by the tulip. The analysis of the development of the flame and of its structure has emphasised three main stabilisation regimes (development, ring flame, recirculating flame), two transition regimes and a domain of extinction. To complete this analysis, a study of the "ring flame" regime has been developed. It pointed out that the ring is a triple flame formed at the interface of two flows: a premixed fuel-air-combustion products flow and an air flow

碩士
國立臺灣大學
機械工程研究所
84
This study is an analysis of the characteristics of flow field and combustion in a bluff-body burner. The nature of the isothermal field is used to predict the stability of flame. FLOW3D is developed by AEA Technology company and which can be used to predict the characteristics of flow field and the structure of flame. The k-ε turbulence model and EBU-Arrhenius Combustion Model with Finite-Rate Reaction were used in the program. The flow fields are investigated by changing the fuel-jet velocity and air-jet velocity. the result include the combusting and isothermal fields, the interaction of combustion and flow field and the correlation of velocity, temperature and mixture fields. Finally, the isothermal velocity and mixture composition fields were used to predict the flame stability and have a satisfactory result.

Thesis (M.S.)--State University of New York at Buffalo, 2007.
Title from PDF title page (viewed on Nov. 13, 2007) Available through UMI ProQuest Digital Dissertations. Thesis adviser: Madnia, Cyrus K., Taulbee, Dale B. Includes bibliographical references.

A bluff-body has been employed as the flame stabilization scheme for many combustion devices such as gas turbines and aviation engines. Although the bluff-body flame holder has a key advantage of generating a hot gas recirculation zone behind it and assist in stable combustion, it also induces flow field and combustion instabilities such as unstable vortex shedding, which can adversely affect the flame stability and lead to blow-off. The understanding of the physical mechanism of flame stabilization and blow-off processes has been one of the critical subjects in premixed combustion systems under highly turbulent conditions. As considering this, the present dissertation presents insight of flame stabilization and blow-off mechanisms using several series of computational studies and detailed analysis using diagnostic approaches. Two-dimensional direct numerical simulations are conducted to examine flame/flow and blow-off dynamics in lean premixed hydrogen-air and syngas-air flames stabilized on a meso-scale bluff-body in a square channel. Several distinct effects on flame stabilization and blow-off dynamics are investigated, such as reduced confinement, hydrodynamic instability, flame time scale, and differential diffusion effects. For the analysis, a proper time scale analysis is attempted to characterize the flame blow-off mechanism, which turns out to be consistent with the classic blow-off theory of Zukoski and Marble. The combined approach of computational singular perturbation and tangential stretch rate is applied to examine chemical characteristics in blow-off dynamics. As an extension from Eulerian to Lagrangian viewpoint, Lagrangian particle tracking analysis of post-processing the pre-computed results is performed to examine the local characteristics during the critical transient event of local extinction and recovery.