Experimental and Numerical Investigation of the Influence of Multiple-Burner Arrangements on Combustion Instabilities in Turbulent Swirl-Stabilized Flames PDF Download
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Author: Christian Kraus
Publisher:
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Category :
Languages : en
Pages :
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Author: Christian Kraus
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Category :
Languages : en
Pages :
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Author: Paul Palies
Publisher: Academic Press
ISBN: 0128199970
Category : Technology & Engineering
Languages : en
Pages : 402
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Stabilization and Dynamic of Premixed Swirling Flames: Prevaporized, Stratified, Partially, and Fully Premixed Regimes focuses on swirling flames in various premixed modes (stratified, partially, fully, prevaporized) for the combustor, and development and design of current and future swirl-stabilized combustion systems. This includes predicting capabilities, modeling of turbulent combustion, liquid fuel modeling, and a complete overview of stabilization of these flames in aeroengines. The book also discusses the effects of the operating envelope on upstream fresh gases and the subsequent impact of flame speed, combustion, and mixing, the theoretical framework for flame stabilization, and fully lean premixed injector design. Specific attention is paid to ground gas turbine applications, and a comprehensive review of stabilization mechanisms for premixed, partially-premixed, and stratified premixed flames. The last chapter covers the design of a fully premixed injector for future jet engine applications. Features a complete view of the challenges at the intersection of swirling flame combustors, their requirements, and the physics of fluids at work Addresses the challenges of turbulent combustion modeling with numerical simulations Includes the presentation of the very latest numerical results and analyses of flashback, lean blowout, and combustion instabilities Covers the design of a fully premixed injector for future jet engine applications
Author: Gaurav Kewlani
Publisher:
ISBN:
Category :
Languages : en
Pages : 300
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Book Description
High efficiency, low emissions and stable operation over a wide range of conditions are some of the key requirements of modem-day combustors. To achieve these objectives, lean premixed flames are generally preferred as they achieve efficient and clean combustion. A drawback of lean premixed combustion, however, is that the flames are more prone to dynamics. The unsteady release of sensible heat and flow dilatation in combustion processes create pressure fluctuations which, particularly in premixed flames, can couple with the acoustics of the combustion system. This acoustic coupling creates a feedback loop with the heat release that can lead to severe thermoacoustic instabilities that can damage the combustor. Understanding these dynamics, predicting their onset and proposing passive and active control strategies are critical to large-scale implementation. For the numerical study of such systems, large eddy simulation (LES) techniques with appropriate combustion models and reaction mechanisms are highly appropriate. These approaches balance the computational complexity and predictive accuracy. This work, therefore, aims to explore the applicability of these methods to the study of premixed wake stabilized flames. Specifically, finite rate chemistry LES models that can effectively capture the interaction between different turbulent scales and the combustion fronts have been implemented, and applied for the analysis of premixed turbulent flame dynamics in laboratory-scale combustor configurations. Firstly, the artificial flame thickening approach, along with an appropriate reduced chemistry mechanism, is utilized for modeling turbulence-combustion interactions at small scales. A novel dynamic formulation is proposed that explicitly incorporates the influence of strain on flame wrinkling by solving a transport equation for the latter rather than using local-equilibrium-based algebraic models. Additionally, a multiple-step combustion chemistry mechanism is used for the simulations. Secondly, the presumed-PDF approach, coupled with the flamelet generated manifold (FGM) technique, is also implemented for modeling turbulence-combustion interactions. The proposed formulation explicitly incorporates the influence of strain via the scalar dissipation rate and can result in more accurate predictions especially for highly unsteady flame configurations. Specifically, the dissipation rate is incorporated as an additional coordinate to presume the PDF and strained flamelets are utilized to generate the chemistry databases. These LES solvers have been developed and applied for the analysis of reacting flows in several combustor configurations, i.e. triangular bluff body in a rectangular channel, backward facing step configuration, axi-symmetric bluff body in cylindrical chamber, and cylindrical sudden expansion with swirl, and their performance has been be validated against experimental observations. Subsequently, the impact of the equivalence ratio variation on flame-flow dynamics is studied for the swirl configuration using the experimental PIV data as well as the numerical LES code, following which dynamic mode decomposition of the flow field is performed. It is observed that increasing the equivalence ratio can appreciably influence the dominant flow features in the wake region, including the size and shape of the recirculation zone(s), as well as the flame dynamics. Specifically, varying the heat loading results in altering the dominant flame stabilization mechanism, thereby causing transitions across distinct- flame configurations, while also modifying the inner recirculation zone topology significantly. Additionally, the LES framework has also been applied to gain an insight into the combustion dynamics phenomena for the backward-facing step configuration. Apart from evaluating the influence of equivalence ratio on the combustion process for stable flames, the flame-flow interactions in acoustically forced scenarios are also analyzed using LES and dynamic mode decomposition (DMD). Specifically, numerical simulations are performed corresponding to a selfexcited combustion instability configuration as observed in the experiments, and it is observed that LES is able to suitably capture the flame dynamics. These insights highlight the effect of heat release variation on flame-flow interactions in wall-confined combustor configurations, which can significantly impact combustion stability in acoustically-coupled systems. The fidelity of the solvers in predicting the system response to variation in heat loading and to acoustic forcing suggests that the LES framework can be suitably applied for the analysis of flame dynamics as well as to understand the fundamental mechanisms responsible for combustion instability. KEY WORDS - large eddy simulation, LES, wake stabilized flame, turbulent premixed combustion, combustion modeling, artificially thickened flame model, triangular bluff body, backward facing step combustor, presumed-PDF model, flamelet generated manifold, axi-symmetric bluff body, cylindrical swirl combustor, particle image velocimetry, dynamic mode decomposition, combustion instability, forced response.
Author: Bart Merci
Publisher: Springer Science & Business Media
ISBN: 9400714092
Category : Technology & Engineering
Languages : en
Pages : 180
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Book Description
This book reflects the outcome of the 1st International Workshop on Turbulent Spray Combustion held in 2009 in Corsica (France). The focus is on reporting the progress of experimental and numerical techniques in two-phase flows, with emphasis on spray combustion. The motivation for studies in this area is that knowledge of the dominant phenomena and their interactions in such flow systems is essential for the development of predictive models and their use in combustor and gas turbine design. This necessitates the development of accurate experimental methods and numerical modelling techniques. The workshop aimed at providing an opportunity for experts and young researchers to present the state-of-the-art, discuss new developments or techniques and exchange ideas in the areas of experimentations, modelling and simulation of reactive multiphase flows. The first two papers reflect the contents of the invited lectures, given by experts in the field of turbulent spray combustion. The first concerns computational issues, while the second deals with experiments. These lectures initiated very interesting and interactive discussions among the researchers, further pursued in contributed poster presentations. Contributions 3 and 4 focus on some aspects of the impact of the interaction between fuel evaporation and combustion on spray combustion in the context of gas turbines, while the final article deals with the interaction between evaporation and turbulence.
Author: Zachary Alexander LaBry
Publisher:
ISBN:
Category :
Languages : en
Pages : 220
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One of the most difficult challenges facing the development of modern gas turbines-for power generation, and propulsion-is the mitigation of dynamic instabilities in the presence of efficiency and emissions constraints. Dynamic instabilities-self-excited, self-sustaining oscillations which link the combustor acoustics to the combustion process-can result in significant levels of thermal and mechanical stress on combustion systems, leading to reduced operational lifetime, potentially dangerous failure modes, and significant deviations from the desired operating conditions. Due to the complexity of the problem, with the relevant time and length scales of the system--from the chemistry to the acoustics-spanning several orders of magnitude, even sophisticated numerical techniques have been severely limited in their ability to make reliable predictions, leaving the task of finding and eliminating modes of instability to a lengthy and expensive trial-and-error process. Lean-premixed combustion, one of the leading technologies for low emission combustors, is particularly susceptible to these types of instabilities. The sealed systems that are necessary to maintain a reaction in a lean mixture do not attenuate acoustics well, which often results in high-amplitude pressure fluctuations. In this thesis, we focus on developing a better predictive framework for the onset of combustion instabilities in a swirl-stabilized, lean-premixed combustor. We correlate the self-excited acoustic behavior with quantifiable system properties that can be generalized across different fuel blends. This work is predicated on the idea that self-excited combustion instability arises from the selective amplification of the noise inherent in a turbulent combustion system, and that the frequency-based response of the flame is a function of the flame geometry. In the first part of the thesis, we focus on the flame geometry, identifying several discrete transitions that take place in the swirl-stabilized flame as we adjust the equivalence ratio. By comparing the transitions across several CH4/H2 fuel blends, and using statistical techniques to interrogate the global effect of the small-scale flow-flame interactions, we find that the extinction strain rate-the flow-driven rate of change in flame surface area at which the chemistry is no longer -sufficiently fast to maintain the reaction-is directly linked to the flame transitions. The swirl-stabilized flow features several critical regions with large and unsteady velocity derivatives, particularly, a pair of shear layers that divide the incoming flow of reactants from an inner and an outer recirculation zone. As the extinction strain rate increases with increasing equivalence ratio, the flame transitions through these critical regions, manifesting as discrete changes in the flame geometry. In the second part, we address the correlation between self-excited instability and the forced acoustic response. By modifying the pressure boundary conditions, we decouple the flame from the acoustics over a domain of interest (defined by a range of equivalence ratios that correspond to the onset of dynamic instability in the coupled system). We then apply external acoustic forcing at a single frequency to ascertain the response of the flame to each particular forcing frequency by means of a flame transfer function. This enables us to consider the frequency-by-frequency response of the flame to its own internally generated noise. We show that the onset of instability is well-predicted by the overlap of the natural acoustic frequencies of the combustor (predicted using a non-linear flame response model) with those frequencies for which the phase of the flame transfer function satisfies the well-known Rayleigh criterion, which is a necessary condition for the presence of self-excited combustion instability. By examining both the forced response and the self-excited instability across several different fuel blends, we go on to show that both behaviors correlate well with the flame geometry, which we have already shown to be dictated by the extinction strain rate of the particular fuel blend. We go on to collapse both sets of data on the strained flame consumption speed taken at the limit of the extinction strain rate, and in doing so, present a framework for predicting the operating conditions under which the combustor in the coupled configuration will go unstable based on measurements and correlations from the uncoupled configuration. Furthermore by taking the consumption speed at the extinction limit, we are correlating the geometry and dynamics with a parameter that is solely a function of mixture properties. This provides the basis for a framework for predicting instability from properties that are more readily measured or simulated, and provides and explicit means of converting these results to different fuel mixtures.
Author: Nedunchezhian Swaminathan
Publisher: Cambridge University Press
ISBN: 1139498584
Category : Technology & Engineering
Languages : en
Pages : 447
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Book Description
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.
Author:
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ISBN:
Category : Aeronautics
Languages : en
Pages : 1460
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Book Description
Lists citations with abstracts for aerospace related reports obtained from world wide sources and announces documents that have recently been entered into the NASA Scientific and Technical Information Database.
Author: Tim C. Lieuwen
Publisher: Cambridge University Press
ISBN: 1139576836
Category : Technology & Engineering
Languages : en
Pages : 427
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Book Description
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.
Author: Ankit Tyagi
Publisher:
ISBN:
Category :
Languages : en
Pages :
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This dissertation investigates the physics of interactions between turbulent premixed flames. It is known that multiple flame configurations provide better stability characteristics compared to a large single-flame. However, the advantages of multiple flames are limited by flame proximity as flame-flame interactions tend to reduce the burning efficiency of the reactant gases. Previous studies have shown that interactions between multiple flames directly impact the flame structure and its propagation, resulting in reduced burning efficiency. Previous experimental studies of interacting flames addressed flame-flame interactions investigating their effects on combustor stability and efficiency from a global perspective. However, the local flame-flame interaction physics was not addressed comprehensively, in part because these studies were limited to specific flow and flame configurations. In particular, these studies focused on swirling flames in bluff-body configurations typical of modern combustor geometries. Furthermore, these studies lacked flowfield measurements and were limited to flame structure and heat release rate measurements due to the complex nature of the experimental configurations. Much of the work to date on understanding the local physics of interactions comes from direct numerical simulations (DNS), but these studies treated idealized configurations of limited practical utility.To bridge these two gaps, an experimental investigation of flame-flame interactions was performed using a dual-burner rig, composed of two flames, built to facilitate precise variations in flame spacing. This rig was designed to operate in different configurations. These facilitated the focus on local interaction physics. In particular, the rig was built to study interacting V-flames and Bunsen flames. Moreover, the design of the dual-burners permitted conducting studies of nonreacting flow interactions with flames to better understand local physics of the flame. Direct flame and flow measurements were performed to characterize the mutual interaction of flame and the local flowfield. In particular, flame structure and flow were characterized using synchronized OH-planar laser-induced fluorescence (OH-PLIF) and stereoscopic-particle image velocimetry (s-PIV). These measurements were performed at a sampling rate of 10 kHz to obtain converged statistics on flame-flame interactions. A novel image processing technique was implemented for robust detection and characterization of flame-flame interaction events from OH-PLIF images.Using this experimental approach, the following studies were conducted: i) effects of flame spacing on flame structure of interacting V-flames, ii) effects of multiple flames on frequency, topology, and orientation of local flame-flame interactions, iii) effects of high mean-shear flow on flame-flame interactions, and iv) effects of pocket formation on flame dynamics. In the first study, flame spacing variations in V-flames were found to directly impact flame attachment. For smaller flame spacings, recirculation of hot combustion products near the bluff-bodies facilitated a secondary flame branch attachment in the shear layers in the interaction regions. For larger flame spacing, the secondary attachment became intermittent, indicating that closer flame spacing resulted in better attachment and stability characteristics for these flames. In the second study, the presence of adjacent flames was found to directly impact the frequencies of flame-flame interaction events. Dual-flames showed lower reactant-side interactions rates and higher product-side interactions rates when compared with single-flames. For dual-flames, comparisons between interaction orientation and local strain rate orientation showed that compressive forces led to flame front merging or pinch-off. The third study, which focused on how mean shear affects the local flame dynamics, found that high-mean shear flows entrained the flame away from the center of the burner. This entrainment directly reduced interaction event frequencies along the flame branch closest to the high mean-shear flow, while interaction event frequency in the other branch increased. Finally, flame pocket formation was investigated and results showed that a majority of the reactant pockets burned-out, while a majority of the product pockets merged with the flame surface. These results suggested that pocket behavior in turbulent flames can change local flame dynamics and it is important to capture these effects to accurately predict flame behavior. Additionally, limitations of planar high-speed imaging techniques were explored and a statistical framework, using probabilistic models, was presented in the context of reactant pocket propagation. The outcome of this work provided improved uncertainty estimation for planar measurements in three-dimensional flows.This experimental investigation provided deeper insights into the local physics of flame-flame interactions, in practical configurations, using detailed flame and flow measurements. The presence of adjacent flames influenced the attachment characteristics and local flame structure that directly impacted the stability of these multiple flame configurations. Local compressive forces facilitated the occurrence of these events, highlighting the importance of changes to the flowfield due to adjacent flames. Pocket formation, which directly affected reactant gas burning efficiency, was found to occur frequently. Taken together, these results provided comprehensive insights into the effects of flame-flame interactions that enhance our understanding of the nature of interacting flames.
Author: Fenando F. Grinstein
Publisher: Cambridge University Press
ISBN: 1107137047
Category : Science
Languages : en
Pages : 481
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Book Description
Reviews our current understanding of the subject. For graduate students and researchers in computational fluid dynamics and turbulence.
