Download or read book Experimental Study of Burner Geometry Effect on the Coherent Structures Flashback and Flame Front Dynamics of Unconfined and Confined Partially Premixed Swirling Methane Flames written by Mahmoud M.A. Ahmed and published by . This book was released on 2020 with total page 0 pages. Available in PDF, EPUB and Kindle. Book excerpt: The effect of burner geometry (central fuel nozzle, mixing tube length, and flame confinement) on the ensuing flowfield's coherent structures (precessing vortex core and vortex shedding) and their relationship with some combustion stability parameters, such as flashback and flame front dynamics of a swirling partially premixed methane flame, is experimentally studied. In this investigation, several measurement techniques are employed. These include particle image velocimetry (PIV), Mie scattering, laser Doppler velocimetry (LDV), Bruel & Kjaer microphone, high-speed Schlieren imaging technique, and high-speed luminescence imaging. In addition, proper orthogonal decomposition (POD) is used as a post processing technique to capture the flow-field coherent structures. In the first part of the study, the effect of central nozzle geometry on coherent structures' strength and frequency is examined inside a relatively long mixing tube. Furthermore, the relationship between the strength and frequency of coherent structures and the mean flashback region inside the mixing tube is studied. In the second part, the central nozzle geometry is modified based on the conclusions reached in the first part of the study, and its effect on the suppression of the coherent structures is investigated using a relatively short mixing length. Finally, the effect of nozzle geometries, which exhibit the highest and lowest coherent structures' amplitude, on coherent structures and acoustics modes is studied using different mixing tube lengths in the presence of a flame confinement. The results showed that the central nozzle geometry significantly affects coherent structures' strength and amplitude inside the mixing tube. Moreover, the results revealed a strong relationship between coherent structures' strength and the mean flashback region inside the mixing tube. Furthermore, the central nozzle geometry is found to significantly affect the amplitude of coherent structures for both confined and unconfined swirling partial premixed flames, and acoustics for the confined flames. All in all, it can be concluded that passive techniques can be a viable strategy for mitigating combustion instabilities of partially premixed flames.
Download or read book Turbulent Premixed Flames written by Nedunchezhian Swaminathan and published by Cambridge University Press. This book was released on 2011-04-25 with total page 447 pages. Available in PDF, EPUB and Kindle. Book excerpt: A work on turbulent premixed combustion is important because of increased concern about the environmental impact of combustion and the search for new combustion concepts and technologies. An improved understanding of lean fuel turbulent premixed flames must play a central role in the fundamental science of these new concepts. Lean premixed flames have the potential to offer ultra-low emission levels, but they are notoriously susceptible to combustion oscillations. Thus, sophisticated control measures are inevitably required. The editors' intent is to set out the modeling aspects in the field of turbulent premixed combustion. Good progress has been made on this topic, and this cohesive volume contains contributions from international experts on various subtopics of the lean premixed flame problem.
Download or read book Experimental Investigation of the Dynamics and Structure of Lean premixed Turbulent Combustion written by Frank Tat Cheong Yuen and published by . This book was released on 2009 with total page 306 pages. Available in PDF, EPUB and Kindle. Book excerpt: Turbulent premixed propane/air and methane/air flames were studied using planar Rayleigh scattering and particle image velocimetry on a stabilized Bunsen type burner. The fuel-air equivalence ratio was varied from &phis; = 0:7 to 1.0 for propane flames, and from &phis; = 0:6 to 1.0 for methane flames. The non-dimensional turbulence intensity, u'/ SL (ratio of fluctuation velocity to laminar burning velocity), covered the range from 3 to 24, equivalent to conditions of corrugated flamelets and thin reaction zones regimes. Temperature gradients decreased with the increasing u'/SL and levelled off beyond u'/SL > 10 for both propane and methane flames. Flame front thickness increased slightly as u'/SL increased for both mixtures, although the thickness increase was more noticeable for propane flames, which meant the thermal flame front structure was being thickened. A zone of higher temperature was observed on the average temperature profile in the preheat zone of the flame front as well as some instantaneous temperature profiles at the highest u'/SL. Curvature probability density functions were similar to the Gaussian distribution at all u'/ SL for both mixtures and for all the flame sections. The mean curvature values decreased as a function of u'/ SL and approached zero. Flame front thickness was smaller when evaluated at flame front locations with zero curvature than that with curvature. Temperature gradients and FSD were larger when the flame curvature was zero. The combined thickness and FSD data suggest that the curvature effect is more dominant than that of the stretch by turbulent eddies during flame propagation. Integrated flame surface density for both propane and methane flames exhibited no dependance on u'/S L regardless of the FSD method used for evaluation. This observation implies that flame surface area may not be the dominant factor in increasing the turbulent burning velocity and the flamelet assumption may not be valid under the conditions studied. Dkappa term, the product of diffusivity evaluated at conditions studied and the flame front curvature, was a magnitude smaller than or the same magnitude as the laminar burning velocity.
Download or read book Interactions Between Coherent and Turbulent Oscillations and Their Impact on the Dynamics of Flames and Flow Fields written by Ashwini Karmarkar and published by . This book was released on 2022 with total page 0 pages. Available in PDF, EPUB and Kindle. Book excerpt: This primary focus of this dissertation is to investigate the coupling mechanisms by which flow field fluctuations can interact with heat release oscillations and how the coupling mechanisms are impacted by the addition of turbulent fluctuations. This work is particularly motivated by the problem of combustion instability in gas turbine engines. Combustion instability is a type of thermoacoustic instability that occurs due to coupling between the coherent oscillations in heat release rate and the acoustic modes of the combustor. The modulation of heat release rate due to the interaction of the flame front with coherent structures in the flow can be a driver of combustion instability. While there have multiple studies analysing the interaction between flames and coherent structures, many of the experimental studies focus on the low-turbulence regime, which is not representative of realistic engine conditions. More recent studies have analysed flame response and limiting phenomena at high turbulence intensities, although the interaction between competing phenomena of turbulent and coherent oscillations have not been comprehensively studied so far and is therefore a focus contribution of this work. In this dissertation, two configurations are studied -- the canonical rod-stabilized V-flame and a more realistic partially-premixed swirl flame. The canonical configuration allows for more control over individual flow parameters so that the coherent and turbulent fluctuations can be independently controlled and systematically varied. High-speed stereoscopic particle image velocimetry (sPIV) is the primary diagnostic used in this configuration. The coherent oscillations in the flow field are excited by longitudinal acoustic excitation and different configurations of perforated plates in the burner provide varying turbulence intensities. The results from this work conclusively show that the magnitude of turbulence intensity in the flow can significantly impact the flow dynamics, the symmetry of the flow response to external excitation, and the coupling between the flow field and flame fluctuations. The realistic swirling flame configuration is used to characterize the interaction between the precessing vortex core (PVC), which is the consequence of a global hydrodynamic instability, and thermoacoustic instabilities, which are the result of a coupling between combustor acoustics and the unsteady heat release rate of combustion. This study is performed using experimental data obtained from a model gas turbine combustor system to simulate realistic conditions. High-speed stereoscopic particle image velocimetry, OH planar laser-induced fluorescence, and acetone planar laser-induced fluorescence are used to obtain information about the velocity fields, flame, and fuel flow behavior, respectively. The results from this work show that in the cases where the frequency of the PVC overlaps with the frequency of a thermoacoustic mode, the thermoacoustic mode is subsequently suppressed. Further, the thermoacoustic coupling process is driven by both velocity and mixture variations, but the PVC oscillations do not significantly drive variations in the mixture, only the velocity field. Put together, the findings from both configurations provide important insight into the coupling mechanisms that govern the interactions between the various flow field fluctuations and their impact on the unsteady heat release from the flame.
Download or read book Unsteady Combustor Physics written by Tim C. Lieuwen and published by Cambridge University Press. This book was released on 2012-08-27 with total page 427 pages. Available in PDF, EPUB and Kindle. Book excerpt: Developing clean, sustainable energy systems is a pre-eminent issue of our time. Most projections indicate that combustion-based energy conversion systems will continue to be the predominant approach for the majority of our energy usage. Unsteady combustor issues present the key challenge associated with the development of clean, high-efficiency combustion systems such as those used for power generation, heating or propulsion applications. This comprehensive study is unique, treating the subject in a systematic manner. Although this book focuses on unsteady combusting flows, it places particular emphasis on the system dynamics that occur at the intersection of the combustion, fluid mechanics and acoustic disciplines. Individuals with a background in fluid mechanics and combustion will find this book to be an incomparable study that synthesises these fields into a coherent understanding of the intrinsically unsteady processes in combustors.
Download or read book Premixed Burner Experiments written by and published by . This book was released on 1995 with total page 15 pages. Available in PDF, EPUB and Kindle. Book excerpt: This research program is exploring techniques for improved fuel-air mixing, with the aim of achieving combustor operations up to stoichiometric conditions with minimal NO(subscript x) and maximum efficiency. The experimental studies involve the use of a double-concentric natural gas burner that is operable in either premixed or non-premixed modes, and the system allows systematic variation of equivalence ratio, swirl strength shear length region and flow momentum in each annulus. Flame structures formed with various combinations of swirl strengths, flow throughput and equivalence ratios in premixed mode show the significant impact of swirl flow distribution on flame structure emanating from the mixedness. This impact on flame structure is expected to have a pronounced effect on the heat release rate and the emission of NO(subscript x). Thus, swirler design and configuration remains a key factor in the quest for completely optimized combustion. Parallel numerical studies of the flow and combustion phenomena were carried out, using the RSM and the k-E turbulence models. These results have not only indicated the strengths and limitations of CFD in performance and pollutants emission predictions, but have provided guidelines on the size and strength of the recirculation produced and the spatio-temporal structure of the combustion flowfield. The first stage of parametric studies on geometry and operational parameters at Morgan State University have culminated in the completion of a one-dimensional flow code that is integrated with a solid, virtual model of the existing premixed burner. This coupling will provide the unique opportunity to study the impact of geometry on the flowfield and vice-versa, with particular emphasis on concurrent design optimization.
Download or read book Flashback Mechanisms in Lean Premixed Gas Turbine Combustion written by Ali Cemal Benim and published by Academic Press. This book was released on 2014-12-01 with total page 134 pages. Available in PDF, EPUB and Kindle. Book excerpt: Blending fuels with hydrogen offers the potential to reduce NOx and CO2 emissions in gas turbines, but doing so introduces potential new problems such as flashback. Flashback can lead to thermal overload and destruction of hardware in the turbine engine, with potentially expensive consequences. The little research on flashback that is available is fragmented. Flashback Mechanisms in Lean Premixed Gas Turbine Combustion by Ali Cemal Benim will address not only the overall issue of the flashback phenomenon, but also the issue of fragmented and incomplete research. Presents a coherent review of flame flashback (a classic problem in premixed combustion) and its connection with the growing trend of popularity of more-efficient hydrogen-blend fuels Begins with a brief review of industrial gas turbine combustion technology Covers current environmental and economic motivations for replacing natural gas with hydrogen-blend fuels
Download or read book Boundary Layer Flashback of Swirl Flames written by Dominik Fabian Ebi and published by . This book was released on 2016 with total page 408 pages. Available in PDF, EPUB and Kindle. Book excerpt: Flame flashback in the boundary layer of swirling flows is investigated experimentally in a model swirl combustor. The model combustor features a mixing tube with an axial swirler and an attached center body. The findings provide novel insight into the mechanism facilitating boundary layer flashback of swirl flames. Turbulent, lean-premixed flames of methane and hydrogen are studied at atmospheric pressure and bulk flow velocities up to 5 m/s. Hydrogen contents range from 0% to 95% and equivalence ratios range from 0.4 to 1. The focus in the present work is on the upstream flame propagation inside the mixing tube. Stereoscopic particle image velocimetry (PIV) is applied at kilohertz-rate to provide the time-resolved, three-component velocity field. The flame front is detected simultaneously based on the acquired Mie scattering images. Simultaneous high-speed chemiluminescence imaging provides the overall flame shape and global propagation direction. In addition to the planar measurements, a technique capable of detecting the instantaneous, time-resolved, 3D flame front topography is developed and applied successfully. Oil droplets, which vaporize in the preheat zone of the flame, serve as the marker for the flame front. The droplets are illuminated with a laser and imaged from four different views followed by a tomographic reconstruction to obtain the volumetric particle field. The velocity field in the unburnt gas is measured using tomographic PIV. The resulting data include the simultaneous 3D flame front and volumetric velocity field at 5 kHz. Flashback is found to occur in the form of large-scale, convex-shaped flame tongues, which swirl in the bulk flow direction as they propagate in the negative axial direction along the center body wall. Gas dilatation associated with the heat release imposes a blockage effect on the approach flow, which causes a 3D deflection of streamlines. As a result, a region of negative axial velocity forms along the leading side of the flame tongues, which facilitates flashback. These regions of negative axial velocity, already observed in previous studies, are shown to be the result of a predominantly swirling fluid motion as opposed to boundary layer separation or flow recirculation. The effect of hydrogen addition on flashback is investigated. Flashback occurs at significantly leaner conditions for hydrogen-rich flames, but the mechanism driving flashback is found to be independent of the hydrogen content for the conditions investigated in the present work. Quantitative differences in the flame-flow interaction between methane and hydrogen-rich flashbacks are discussed in detail.
Download or read book Flame Flashback in Wall Boundary Layers of Premixed Combustion Systems written by Christian Thomas Eichler and published by . This book was released on 2011 with total page 212 pages. Available in PDF, EPUB and Kindle. Book excerpt:
Download or read book Incipient Behaviour of Flashback in a Lean Premixed Swirl Burner written by Christopher Eric Schneider and published by . This book was released on 2019 with total page 0 pages. Available in PDF, EPUB and Kindle. Book excerpt: The dynamics of flame flashback were studied in a lean premixed swirl burner with central bluff-body. A range of conditions with varied flow velocities, inlet temperatures, and hydrogen/methane flow rates were investigated. Intermittent movement of the flame into the feed tube, was found over a range of conditions, with consistent trends as the system moved from stable operation to complete flashback. Statistical analysis of chemiluminescence data showed a strong link between characteristic behaviours of the system, such as the statistical frequency of upstream propagating flame protrusions, and the magnitude of the flashback, independent of the inlet conditions. Effects of conditions on the flashback magnitude and abruptness of transition are described. Existing metrics for predicting flashback were found to be inadequate for describing the observed dynamics. Laser diagnostics revealed only a slight statistical drop in axial flow velocity upstream of the flame, which appeared to strengthen as the flame moved upstream.
Download or read book Experimental investigation of the effects of burner geometry on resonantly pulsed non premixed jet flames written by Joe Whiteaker and published by . This book was released on 2003 with total page 210 pages. Available in PDF, EPUB and Kindle. Book excerpt:
Download or read book An Experimental Study of Burner Stabilized Turbulent Flames in Premixed Reactants written by M. D. Fox and published by . This book was released on 1962 with total page 33 pages. Available in PDF, EPUB and Kindle. Book excerpt: Current concepts of flame propagation in premixed, turbulent gas streams are examined. This leads to the conclusion that the link between theory and experiment is entirely inadequate and incapable of improvement by existing methods. A series of new methods is implemented in an attempt to short-circuit the unprofitable chain of hypothesis and experiment which has hampered the identification of dubious steps. Methods of introducing uniform turbulence at relatively slow flows and improvements in light sources allow analysis of the approach flow by photographing particles illuminated by an interrupted Tyndall beam. Three new optical deflection methods are used to give a measure of the randomness of flame-front orientation, of the time-mean structure of the flame and of the instantaneous shape of the corrugated front. It is found that this corrugated surface propagates at a velocity considerably in excess of the normal laminar burning velocity. Quantitative analysis of the frequency of "peaks" and "valleys" on the surface, together with comparative data from the apex of laminar flames, suggests an explanation in terms of the effects of curvature and, secondarily, of the influence of small scale turbulence.
Download or read book Experimental Study of Turbulent Premixed Combustion in V shaped Flames written by Sina Kheirkhah and published by . This book was released on 2016 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: Characteristics of turbulent premixed flames were investigated experimentally. The investigations were performed using Mie scattering, Particle Image Velocimetry, Rayleigh scattering, and broad-band luminosity imaging techniques. Methane-air flames associated with a relatively wide range of turbulence intensities, fuel-air equivalence ratios, and mean bulk flow velocities were investigated. For a relatively moderate value of turbulence intensity, a new concept is introduced and utilized to provide a detailed description associated with interaction of turbulent flow and flame front. The concept pertains to reactants velocity estimated at the vicinity of the flame front and is referred to as the edge velocity. Specifically, it is shown that fluctuations of the flame front position are induced by fluctuations of the edge velocity. For a relatively wide range of turbulence intensity, several characteristics of turbulent premixed flames, namely, front topology, brush thickness, surface density, and consumption speeds are investigated. For the first time, several flame front structures are identified and studied. It is shown that, due to formation of these front structures, the regime of turbulent premixed combustion transitions from the regime of counter-gradient diffusion to that of the gradient diffusion. In addition to these, a comprehensive study is performed to investigate influence of flame configuration on several flame front characteristics. It is obtained that, although changing the flame configuration influences several flame characteristics, the trends associated with the effects of governing parameters on the characteristics are nearly independent of the flame configuration.
Download or read book Flame flow Interaction During Premixed and Stratified Swirl Flame Flashback in an Annular Swirl Combustor written by Rakesh Ranjan (Ph. D.) and published by . This book was released on 2018 with total page 292 pages. Available in PDF, EPUB and Kindle. Book excerpt: The interaction between a propagating flame and the approach flow is critical to the understanding of boundary layer flashback of swirling flames. In this work, I investigated this interaction during flashback using high-speed luminosity imaging and simultaneous three-dimensional particle image velocimetry. The mean axial velocity through the mixing tube is kept at 2.5 m/s while the hydrogen enrichment of the fuel is varied up to 87%. These flashback experiments are conducted at pressures ranging from 1 to 5 atm. To understand the flame-flow interaction physics, I developed a novel analysis methodology for low-turbulence fully-premixed methane-air swirl flame flashback, by stacking the planar flame profiles and three-dimensional velocity data. In the quasi-reconstructed velocity field, the motion of an approaching fluid parcel is analyzed in the frame-of-reference of the propagating flame. For the first time, the role of inertial forces in swirling flame-flow interaction is revealed. Subsequently, I investigated the effect of fuel-air partial premixing on the flashback behavior at atmospheric and elevated pressures. A swirler-based fuel-injection system was used to create fuel-air stratification in the radial direction. For elevated pressure measurements, an optically accessible elevated pressure chamber was designed and constructed to conduct flashback experiments up to 5 atm. The spatial distribution of the equivalence ratio under non-reacting conditions was investigated using planar laser-induced fluorescence with acetone as the fuel tracer. It was observed that fuel-air pockets were distributed across the mixing tube width, although in an average sense, the fuel-air mixture was radially stratified. The global behavior of upstream flame propagation is reported for different levels of hydrogen-enrichment. For stratified hydrogen-rich flashback, the propagation path of the flame changes from the inner wall to outer wall induced by the faster chemistry of stoichiometric mixtures that are frequently present near the outer wall. This behavior of hydrogen-rich flashback persists even at elevated pressures up to 5 atm, although the propagation of the flame occurs as a wide flame tongue as opposed to the acute-tipped flame structures present in the atmospheric cases
Download or read book Industrial and Process Furnaces written by Barrie Jenkins and published by Butterworth-Heinemann. This book was released on 2013-10-23 with total page 677 pages. Available in PDF, EPUB and Kindle. Book excerpt: Furnaces sit at the core of all branches of manufacture and industry, so it is vital that these are designed and operated safely and effi-ciently. This reference provides all of the furnace theory needed to ensure that this can be executed successfully on an industrial scale. Industrial and Process Furnaces: Principles, 2nd Edition provides comprehensive coverage of all aspects of furnace operation and design, including topics essential for process engineers and operators to better understand furnaces. This includes: the combustion process and its control, furnace fuels, efficiency, burner design and selection, aerodynamics, heat release profiles, furnace atmosphere, safety and emissions. These elements and more are brought together to illustrate how to achieve optimum design and operation, with real-world case studies to showcase their application. Up-to-date and comprehensive reference encompassing not only best practice of operation but the essential elements of furnace theory and design, essential to anyone working with furnaces, ovens and combustion-based systems. More case studies, more worked examples. New material in this second edition includes further application of Computational Fluid Dynamics (CFD), with additional content on flames and burners, costs, efficiencies and future trends.
Download or read book Flame Structure written by M. D. Fox and published by . This book was released on 1961 with total page 74 pages. Available in PDF, EPUB and Kindle. Book excerpt: Current concepts of flame propagation in premixed, turbulent gas streams are examined. This leads to the conclusion that the link between theory and experiment is entirely inadequate and incapable of improvement by existing methods. A series of new method is implemented in an attempt to short-circuit the unprofitable chain of hypothesis and experiment which has hampered the identification of dubious steps. Methods of introducing uniform turbulence at relatively slow flows and improvements in light sources allow analysis of the approach flow by photographing particles illuminated by an interrupted Tyndall beam. Three new optical deflection methods are used to give a measure of the randomness of flame-front orientation, of the time-mean structure of the flame and of the instantaneous shape of the corrugated front. It is found that this corrugated surface propagates at a velocity considerably in excess of the normal laminar burning velocity. Quantitative analysis of the frequency of 'peaks' and 'valleys' on the surface, together with comparative data from the apex of laminar flames, suggests an explanation in terms of the effects of curvature and, secondarily, of the influence of small scale turbulence. (Author).
Download or read book Effects of Leading Edge Flame Behavior on Flame Stabilization and Blowout written by and published by . This book was released on 2004 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: The goal of this work was to identify the mechanisms that effect stabilization of hydrocarbon jet flames. Methane, nitrogen, and co-flowing air were regulated and directed through a burner that created fully-developed fuel flow with concurrent air. The behavior of the reaction zone at the leading-edge was analyzed from digital images obtained from a camera optimally positioned to capture the movements of the entire flame front. Low Reynolds number flows allowed for the investigation of hysteretic behavior. The hysteresis regime refers to the situation where the jet flame has dual positions favorable to flame stabilization: attached and lifted. Results indicate that flame height in hysteresis is significantly impacted by high velocities of co-flow and that past a critical value a local minimum will be created. Fully turbulent lifted flames were also studied to determine the fluctuations in the height of lifted methane flames in the presence of air co-flow. The partially-premixed flame front of the lifted flame fluctuates in the axial direction, with the fluctuations becoming greater in flames stabilized further downstream. These fluctuations are also observed in flames where blowout is imminent. The height and rate of these fluctuations are studied with respect to average height, flow velocities, and Reynolds number. Additionally, the mechanisms that cause jet-flame blowout, particularly in the presence of air co-flow, are not completely understood. Two types of experiments are described, and the data report that a predictor of blowout is the prior disappearance of the axially-oriented flame branch which is consistently witnessed despite a turbulent flameÃØâ'Ơâ"Øs inherent variable behavior. The conclusions are supported by experiments with nitrogen-diluted flames. A blowout parameter is also calculated for methane flames in co-flow and diluted methane flames that can be used to predict at what flow velocities blowout will occur. This work analyzes flames near the bu.
