Download or read book Dynamics of Interacting Turbulent Flames written by Ankit Tyagi and published by . This book was released on 2019 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: 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.
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 Turbulent Combustion Modeling written by Tarek Echekki and published by Springer Science & Business Media. This book was released on 2010-12-25 with total page 496 pages. Available in PDF, EPUB and Kindle. Book excerpt: Turbulent combustion sits at the interface of two important nonlinear, multiscale phenomena: chemistry and turbulence. Its study is extremely timely in view of the need to develop new combustion technologies in order to address challenges associated with climate change, energy source uncertainty, and air pollution. Despite the fact that modeling of turbulent combustion is a subject that has been researched for a number of years, its complexity implies that key issues are still eluding, and a theoretical description that is accurate enough to make turbulent combustion models rigorous and quantitative for industrial use is still lacking. In this book, prominent experts review most of the available approaches in modeling turbulent combustion, with particular focus on the exploding increase in computational resources that has allowed the simulation of increasingly detailed phenomena. The relevant algorithms are presented, the theoretical methods are explained, and various application examples are given. The book is intended for a relatively broad audience, including seasoned researchers and graduate students in engineering, applied mathematics and computational science, engine designers and computational fluid dynamics (CFD) practitioners, scientists at funding agencies, and anyone wishing to understand the state-of-the-art and the future directions of this scientifically challenging and practically important field.
Download or read book Dynamics of Vortex flame Interactions and Implications for Turbulent Combustion written by T. R. Meyer and published by . This book was released on 2003 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt:
Download or read book The Dynamics and Structure of Turbulent Premixed Flames written by Cal Rising and published by . This book was released on 2021 with total page 108 pages. Available in PDF, EPUB and Kindle. Book excerpt: Modern propulsion and power generation technology operates under highly turbulent conditions to promote increased efficiency. The coupled relationship between the turbulence conditions and imposed pressure gradients on reacting flow dynamics are explored by decomposing the vorticity transport terms to quantify the vorticity budgets under varying conditions. This is performed on a bluff-body reacting flow-field by utilizing the two-dimensional diagnostics of particle image velocimetry (PIV) and CH* chemiluminescence to allow for a resolved velocity field and flame front. The vorticity budget is determined by utilizing a mean conditional fluid element tracking procedure to quantify the evolution of the individual vorticity terms through the flame front.
Download or read book Dynamics of Exothermicity written by Brian Bowen and published by CRC Press. This book was released on 1996-09-15 with total page 410 pages. Available in PDF, EPUB and Kindle. Book excerpt: Covering the dynamics of reactive systems and of explosions, the 15 papers discuss the treatment of turbulent mixing in reactive systems, acoustic interactions with combustion fields, liquid atomization, soot formation, practical applications of combustion in waste incineration and pulse jet ignition in internal combustion engines, detonations phenomena, and mixing effects in explosions. Includes six color plates. No index. Annotation copyrighted by Book News, Inc., Portland, OR
Download or read book Large Eddy Simulations of Premixed Turbulent Flame Dynamics written by Gaurav Kewlani and published by . This book was released on 2014 with total page 300 pages. Available in PDF, EPUB and Kindle. Book excerpt: 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.
Download or read book Modeling and Simulation of Turbulent Combustion written by Santanu De and published by Springer. This book was released on 2017-12-12 with total page 663 pages. Available in PDF, EPUB and Kindle. Book excerpt: This book presents a comprehensive review of state-of-the-art models for turbulent combustion, with special emphasis on the theory, development and applications of combustion models in practical combustion systems. It simplifies the complex multi-scale and nonlinear interaction between chemistry and turbulence to allow a broader audience to understand the modeling and numerical simulations of turbulent combustion, which remains at the forefront of research due to its industrial relevance. Further, the book provides a holistic view by covering a diverse range of basic and advanced topics—from the fundamentals of turbulence–chemistry interactions, role of high-performance computing in combustion simulations, and optimization and reduction techniques for chemical kinetics, to state-of-the-art modeling strategies for turbulent premixed and nonpremixed combustion and their applications in engineering contexts.
Download or read book Dynamics of Deflagrations and Reactive Systems written by A. L. Kuhl and published by AIAA (American Institute of Aeronautics & Astronautics). This book was released on 1991 with total page 456 pages. Available in PDF, EPUB and Kindle. Book excerpt: The four companion volumes on Dynamics of Deflagrations and Reactive Systems and Dynamics of Detonations and Explosions present 91 of the149 papers given at the Twelfth International Colloquium on the Dynamics of Explosions and Reactive Systems (ICDERS) held at the University of Michigan in Ann Arbor during July 1989. Four volumes: Dynamics of Deflagrations and Reactive Systems: Flames (Volume 131) and Dynamics of Deflagrations and Reactive Systems: Heterogeneous Combustion (Volume 132) span a broad area, encompassing the processes of coupling the exothermic energy release with the fluid dynamics occurring in any combustion process. Dynamics of Detonations and Explosions: Detonations (Volume 133) and Dynamics of Detonations and Explosions: Explosion Phenomena (Volume 134) principally address the rate processes of energy deposition in a compressible medium and the concurrent nonsteady flow as it typically occurs in explosion phenomena. In this volume, Dynamics of Detonations and Explosions: Detonations, the papers have been arranged into chapters on gaseous detonations, detonation initiation and transmission, nonideal detonations and boundary effects, and multiphase detonations. Although the brevity of this preface does not permit the editors to do justice to all papers, we offer the following highlights of some of the especially noteworthy contributions.
Download or read book Flame Turbulence Interactions written by and published by . This book was released on 1993 with total page 119 pages. Available in PDF, EPUB and Kindle. Book excerpt: The interaction between individual vortices and a premixed laminar flame was investigated in order to characterize the underlying dynamics of flame-turbulence interactions and thereby gain an improved understanding of premixed turbulent flames. In addition, previous two-dimensional flame structure measurements made in turbulent premixed flames were re-analyzed in order to obtain flame curvature and orientation statistics ... Premixed Turbulent Flames, Flame-Vortex Interactions, Turbulence-Flame Interactions, Turbulent Flame Structure.
Download or read book Fluid Mechanics Aspects of Fire and Smoke Dynamics in Enclosures written by Bart Merci and published by CRC Press. This book was released on 2022-10-24 with total page 355 pages. Available in PDF, EPUB and Kindle. Book excerpt: - written by world leading experts in the field - contains many worked-out examples, taken from daily life fire related practical problems - covers the entire range from basics up to state-of-the-art computer simulations of fire and smoke related fluid mechanics aspects, including the effect of water - provides extensive treatment of the interaction of water sprays with a fire-driven flow - contains a chapter on CFD (Computational Fluid Dynamics), the increasingly popular calculation method in the field of fire safety science
Download or read book Dynamics of Heterogeneous Combustion and Reacting Systems written by A. L. Kuhl and published by AIAA. This book was released on 1993 with total page 474 pages. Available in PDF, EPUB and Kindle. Book excerpt:
Download or read book Analysis of Inter scale Turbulence Chemistry Dynamics with Reduced Physics Simulations for Application to Large Eddy Simulation of Premixed Turbulent Combustion written by Paulo Lucena Kreppel Paes and published by . This book was released on 2019 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: Large Eddy Simulation (LES) is a powerful formulation to model turbulent reacting flows with tradeoffs between complexity and resolution. The classical LES framework assumes that the evolution of the more energetic grid-filtered motions are dominated by the dynamical interactions that are explicitly resolved on an "effective grid" that incorporates implicit and/or explicit filtering at the smallest grid-resolvable scales by non-physical friction introduced by the numerical algorithm and modeled terms. The dynamical effects of the unresolved Sub-Filter-Scale (SFS) motions on the evolution of the Resolved-Scale (RS) motions are higher order modulations. However, the application of the classical LES framework to turbulent reacting flows is not clear since dynamically first-order chemical kinetics associated with heat release reside within mostly unresolved SFS thin flame regions. Consequently, key dynamics underlying the function of combustion devices often reside dominantly within unresolved SFS motions in contradiction to the fundamental requirement underlying accurate prediction of resolved-scale dynamics with LES. Furthermore, the topological structure of the flame is necessarily frontal in nature (i.e., sheet-like structure), which poses difficulties for an LES strategy that must model coherent structures that live partially in resolved and partially in subfilter scale fluctuations with a method that treats turbulence eddies as either resolved or subfilter scale. In my research program, we explore the introduction of new modeling elements embedded within current state-of-the-art LES frameworks to capture the impacts of the dynamically dominant inter-scale couplings between RS and SFS motions to improve the predictive accuracy of premixed turbulent combustion evolution at the resolved scales. We aim to systematically refine understanding of the inter-scale interactions between coherent structural features in physical space and in scale space in LES of premixed turbulent combustion. Given the complexity of the interaction between a flame and a complete range of turbulence eddy scales, we analyze reduced physics two-dimensional simulations of the interactions between single-scale vortex arrays and laminar premixed flames, with systematically increasing relative vortex strength creating higher complexity in flame corrugation. To characterize physical-scale space relationships, we apply the Fourier description using a newly developed procedure that removes the broadband Fourier spectral content associated with boundary discontinuities in the non-periodic directions of variables simulated within a finite domain without significant modification of the scales of interest in the original signals. This procedure allows for the analysis of any signal with the Fourier spectral decomposition regardless of the boundary conditions. Using Fourier-space filters, we identify characteristic coherent structural features concurrently in physical and Fourier space in response to flame-eddy interaction and their relative contributions to the SFS and RS variance content of the primary variables of interest. Momentum, energy and species concentrations display different distinct structural features that undergo systematic transition from weak to strong flame-vortex interactions. The primary variables within the dynamical system were classified based on the RS vs. SFS variance content, and distinct structural features in physical and Fourier space were identified for each class. We show that the SFS variance for all variables analyzed is associated with the SFS corrugated flame front, which in 2D Fourier space is associated with a coherent broadband "star-like" pattern that extends from the resolved to the flame subfilter scales. The directional dependences, magnitudes and phase relationships among the Fourier coefficients within the "legs" of the star reflect the power-law spectral representation of fronts and are shown to be closely connected with the direction and magnitude of flame-normal gradients of key variables within the corrugated flame front. We take advantage of the mathematical simplicity of the Fourier spectral description of the nonlinearities in the equations of motion to identify the dominant nonlinear couplings between SFS and RS fluctuations, and from these the SFS content involved in the dominant SFS-RS interactions. In Fourier space the nonlinear terms appear as sums of elemental scale interactions each of which have a well-defined geometrical relationship among wave vectors that form polygons in multidimensional Fourier space. Whereas the shape of the polygon is triangular within advective nonlinearities (triads), it is quadrangular for the chemical nonlinearities (quadrads). This elemental representation of key nonlinearities is used to develop a novel strategy to arrange and down-select the dominant nonlinear inter-scale couplings between SFS and RS motions, from which the corresponding SFS content associated with dynamically dominant RS-SFS dynamics are extracted. The procedure is applied to advective, triadic, and chemical, quadratic, nonlinearities within the LES-filtered governing equations. For primary variables that have most of its energy content at large scales and rapid drops in energy towards small scale, the large-scale features of the dynamically dominant SFS content are shown to be coupled with the smallest resolved scales leading to the corrugations and thickness of the RS flame front. In contrast, the dynamically dominant SFS content of intermediate species involved in heat release rate is shown to follow the smallest corrugations of the flame front reaction zone, which deviate from the RS flame centerline in regions with higher corrugations, such as the flame cusps. The distinct structural features of dynamically dominant SFS content are used for the development of simplified mathematical representations that could be applied within a modeling strategy that directly embeds the interaction between the modeled dominant SFS content and RS evolution within existing LES frameworks to improve the dynamical evolution of resolved-scale motions. From our analysis we develop a number of primary mathematical forms that encapsulate dominant SFS content of momentum, energy and key species variables within advective nonlinearities and show that these produce significant improvements in the time derivatives underlying evolution of the resolved scales. The analysis demonstrates the potential for incorporating directly key energetic and structural features of SFS that significantly impact the evolution of RS motions through key nonlinear dynamic couplings in LES frameworks employing highly simplified mathematical representations. This research lays the groundwork for a Galerkin-like modeling strategy that incorporates highly reduced numbers of basis functions that encapsulate previously determined dominant nonlinear couplings between subfilter-scale structure and resolved-scale evolution.
Download or read book Dynamics of Deflagrations and Reactive Systems written by A. L. Kuhl and published by AIAA (American Institute of Aeronautics & Astronautics). This book was released on 1991 with total page 456 pages. Available in PDF, EPUB and Kindle. Book excerpt: The four companion volumes on Dynamics of Deflagrations and Reactive Systems and Dynamics of Detonations and Explosions present 91 of the149 papers given at the Twelfth International Colloquium on the Dynamics of Explosions and Reactive Systems (ICDERS) held at the University of Michigan in Ann Arbor during July 1989. Four volumes: Dynamics of Deflagrations and Reactive Systems: Flames (Volume 131) and Dynamics of Deflagrations and Reactive Systems: Heterogeneous Combustion (Volume 132) span a broad area, encompassing the processes of coupling the exothermic energy release with the fluid dynamics occurring in any combustion process. Dynamics of Detonations and Explosions: Detonations (Volume 133) and Dynamics of Detonations and Explosions: Explosion Phenomena (Volume 134) principally address the rate processes of energy deposition in a compressible medium and the concurrent nonsteady flow as it typically occurs in explosion phenomena. In this volume, Dynamics of Detonations and Explosions: Detonations, the papers have been arranged into chapters on gaseous detonations, detonation initiation and transmission, nonideal detonations and boundary effects, and multiphase detonations. Although the brevity of this preface does not permit the editors to do justice to all papers, we offer the following highlights of some of the especially noteworthy contributions.
Download or read book Dominant Interscale Dynamics in Premixed Turbulent Combustion for Application to Large Eddy Simulation written by Yash Girish Shah and published by . This book was released on 2021 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: The large-eddy simulation (LES) approach has become an important tool for engineering design and optimization of combustion devices. In a properly designed LES, the larger-scale flow variations are resolved by the effective numerical grid, which incorporates the implicit/explicit filtering introduced by the spatial filtering from the grid, artificial diffusion from numerical schemes, and modeled terms. The dynamical consequences of the remaining subfilter-scale (SFS) fluctuations below the effective grid to the evolution of grid-resolved scales (RS) motions in dynamically relevant physical variables are modeled. The nonlinearities in the dynamical evolution of resolved-scale variables that result from the inherent nonlinear coupling between the resolved and subfilter scales are often not sufficiently well captured in turbulent combustion, as the intermediate species that participate in the heat release process are dominantly only at chemical length and time scales well within the subfilter scales. In this research work, we explore new LES modeling strategies that represent the RS-SFS interactions in the evolution of RS primary variables (momentum, energy and species concentrations) more directly by approximating the SFS content that contributes dominantly to the dynamical evolution of RS quantities using simple mathematical forms. To achieve this, we first obtain a highly resolved three-dimensional Direct Numerical Simulation (DNS) dataset of flame-turbulence interactions that capture the essential RS-SFS interactions in primary variables over a wide range of scales. To systematically analyze the RS-SFS interscale couplings for LES, we then obtain the scale content as the Fourier space representation of the inherently inhomogeneous DNS dataset by applying a discontinuity removal procedure that removes the unphysical gradients naturally introduced at the boundaries of the computational domain by the periodic extension that occurs with the Fourier transform over a bounded domain. The Fourier space representation of primary variables in turbulence-flame dynamical interactions are systematically analyzed. Variables that display high gradients across the flame front over length scales comparable to or larger than flame scales are found to have higher Fourier variance contributions at wavenumbers below flame scale wavenumber, while Fourier variance contributions from variables that are localized only within the flame are found to be distributed to higher wavenumbers in Fourier space. Using this Fourier space representation, we systematically determine the scales of the energy-dominant flow variations in momentum and enthalpy that are resolved by LES and objectively identify the RS and SFS fluctuations for other primary variables. Variables that display frontal variations surrounding the flame are found to be dominantly resolved while those variables that are localized only within the flame are found to be dominantly subfilter scale. These differences in Fourier variance distributions are shown to have implications to the extent of RS-SFS interactions between these variables. We take advantage of the mathematical property of the Fourier spectral description that allows the nonlinearities from the advective transport and chemical reaction rates in the dynamical system to be expressed as elemental sums over triadic interactions involving three wavevectors and quadrad interactions involving four wavevectors between the RS and SFS fluctuations. Using this elemental representation, the SFS content that contributes dominantly to the dynamical evolution of resolved-scales advective nonlinearity is identified by applying the triads downselection procedure [75]. We find that RS-SFS interactions involving SFS content from significantly larger scales compared to the smallest resolvable scales in the DNS are required to adequately estimate the resolved-scale advective nonlinearity in LES. These dynamically dominant SFS for RS advective nonlinearity span over a broader range of wavenumbers for dominantly SFS variables compared to variables that are dominantly resolved-scale. To study the dominant RS-SFS interactions in the chemical nonlinearity, a new two-stage downselection procedure is developed in this work, which expresses the quadrad interactions between the reaction rate constant and the species concentrations to the resolved chemical reaction rates first into triadic interactions between the chemical reaction rate constant and the product of species concentrations. The product of species concentrations is then expressed as a triadic sum over interactions between individual species concentrations and the corresponding dynamically dominant SFS is extracted from both stages using triad downselect procedure for second-order nonlinearities. The dynamically dominant SFS resulting from this procedure is found to be considerably reduced from the full SFS and is shown to be effective in adequately approximating the chemical reaction rates at resolved scales through RS-SFS interactions. The structure underlying the distribution of these dynamically dominant SFS fluctuations in species concentrations are identified for key species in representative reactions in regions where the incorporation of the SFS content is impactful to the estimation of chemical reaction rates in LES. The dynamically dominant SFS species structure is found in two groupings: ``single-banded'' structure characterized by one distinct peak, and ``double-banded'' structure characterized by two peaks of opposite signs. Species that are produced and consumed within the flame are observed to have single-banded structure and species displaying a frontal behavior are observed to have double-banded structure in their dynamically dominant SFS concentrations on average. The local structure of the dynamically dominant SFS species concentrations surrounding the flame is impacted by neighboring flame-flame interactions as well as by variations in flame curvature. The impacts of the flame-flame interactions are strong when the dynamically dominant SFS species structure has ``large'' length scales with concentration peaks significantly displaced from the flame front. Finally, mathematical forms to approximate the mean single and double banded structure in the dynamically dominant SFS concentrations are proposed for application within a structure-based SFS modeling strategy which directly embeds the interaction between the modeled dominant SFS content and the RS evolution within existing LES frameworks. This research lays the groundwork for future LES model developments that utilize this strategy for improving LES predictions of resolved-scale dynamics.
Download or read book Physics of Turbulent Jet Ignition written by Sayan Biswas and published by Springer. This book was released on 2018-05-03 with total page 230 pages. Available in PDF, EPUB and Kindle. Book excerpt: This book focuses on developing strategies for ultra-lean combustion of natural gas and hydrogen, and contributes to the research on extending the lean flammability limit of hydrogen and air using a hot supersonic jet. The author addresses experimental methods, data analysis techniques, and results throughout each chapter and: Explains the fundamental mechanisms behind turbulent hot jet ignition using non-dimensional analysis Explores ignition characteristics by impinging hot jet and multiple jets in relation to better controllability and lean combustion Explores how different instability modes interact with the acoustic modes of the combustion chamber. This book provides a potential answer to some of the issues that arise from lean engine operation, such as poor ignition, engine misfire, cycle-to-cycle variability, combustion instability, reduction in efficiency, and an increase in unburned hydrocarbon emissions. This thesis was submitted to and approved by Purdue University.
Download or read book Turbulent Combustion written by Norbert Peters and published by Cambridge University Press. This book was released on 2000-08-15 with total page 322 pages. Available in PDF, EPUB and Kindle. Book excerpt: The combustion of fossil fuels remains a key technology for the foreseeable future. It is therefore important that we understand the mechanisms of combustion and, in particular, the role of turbulence within this process. Combustion always takes place within a turbulent flow field for two reasons: turbulence increases the mixing process and enhances combustion, but at the same time combustion releases heat which generates flow instability through buoyancy, thus enhancing the transition to turbulence. The four chapters of this book present a thorough introduction to the field of turbulent combustion. After an overview of modeling approaches, the three remaining chapters consider the three distinct cases of premixed, non-premixed, and partially premixed combustion, respectively. This book will be of value to researchers and students of engineering and applied mathematics by demonstrating the current theories of turbulent combustion within a unified presentation of the field.
