Author: Noah Eugen Klarmann
Publisher:
ISBN: 9783843941280
Category :
Languages : en
Pages :

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Author: Tarek Echekki
Publisher: Springer Science & Business Media
ISBN: 9400704127
Category : Technology & Engineering
Languages : en
Pages : 496

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Book Description
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.

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: S. Ruan
Publisher:
ISBN:
Category :
Languages : en
Pages :

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Book Description
Increasingly stringent regulation of pollutant emission has motivated the search for cleaner and more efficient combustion devices, which remain the primary means of power generation and propulsion for all kinds of transport. Fuel-lean premixed combustion technology has been identified to be a promising approach, despite many difficulties involve, notably issues concerning flame stability and ignitability. A partially premixed system has been introduced to remedy these problems, however, our understanding on this combustion mode needs to be greatly improved to realise its full potential. This thesis aims to further the understanding of various fundamental physical processes in turbulent partially premixed flames. DNS data of a laboratory-scale hydrogen turbulent jet lifted flame is analysed in this study. The partially premixed nature of this flame is established by examining the instantaneous and averaged reaction rates and the "Flame Index", which indicate premixed and diffusion burning modes coexisting. The behaviour of turbulent flame stretch and its relation to other physical processes, in particular the scalar-turbulence interaction, the effects of partial premixing on the displacement speed of iso-scalar surface and its correlation with the surface curvature are explored using DNS data. The scalar gradient alignment characteristics change from aligning with the most compressive strain to aligning with the most extensive one in regions of intensive heat release. This alignment change creates negative normal strain rate which can result in negative surface averaged tangential strain rate. The partial premixing affects the flame surface displacement speed through the mixture fraction dissipation rate and a second derivative in the mixture fraction space. The correlation of curvature and displacement speed is found to be negative in general and the effects of partial premixing act to reduce this negative correlation. The combined effects of the normal strain rate and the displacement speed/curvature correlation contribute to the negative mean flame stretch observed in the flame brush. Scalar dissipation rates (SDR) of the mixture fraction ẼZZ, progress variable Ẽcc and their cross dissipation rates (CDR) ẼcZ are identified as important quantities in the modelling of partially premixed flames. Their behaviours in the lifted flame stabilisation region are examined in a unified framework. It is found that SDR of mixture fraction is well below the quenching value in this region while SDR of progress variable is smaller than that in laminar flames. The CDR changes from weakly positive to negative at the flame leading edge due to the change in scalar gradient alignment characteristics. Axial and radial variation of these quantities are analysed and it is found that Ẽcc is an order of magnitude bigger than ẼZZ. ẼcZ is two orders of magnitude smaller than Ẽcc and it can be either positive or negative depending on local flow and flame conditions. Simple algebraic models show reasonable agreement compared to DNS when a suitable definition of c is used. Further statistics of the scalar gradients are presented and a presumed lognormal distribution is found to give reasonable results for their marginal PDFs and a bivariate lognormal distribution is a good approximation for their joint PDF. Four mean reaction rate closures based on presumed PDF and flamelets are assessed a priori using DNS data. The turbulent flame front structure is first compared with unstrained and strained laminar premixed and dif fusion flamelets. It is found that unstrained premixed flamelets give overall reasonable approximation in most parts of this flame. A joint PDF model which includes the correlation between mixture fraction and progress variable using a "copula" method shows excellent agreement with DNS results while their statistical independence does not hold in the burning regions of this partially premixed flame. The unstrained premixed flamelet with the correlated joint PDF method is identified to be the most appropriate model for the lifted jet flame calculation. This model is then used in the RANS simulation of turbulent jet lifted flames. A new model to include the contribution from diffusion burning and the effects of partial premixing due to SDR of mixture fraction is also identified and included in the calculation. These models are implemented in a commercial CFD code "Fluent" with user defined scalars and functions. It is found that both the correlated joint PDF model and the model accounting for the diffusive burning in partial premixing are important in order to accurately predict flame lift-off height compared to the experiments.

Author: Hongtao Yang
Publisher:
ISBN:
Category : Electronic dissertations
Languages : en
Pages :

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Book Description
Turbulent lean premixed combustion now plays a predominant role in reducing emission of pollutants such as NOx. For turbulent premixed flames located in the thin-reaction-zones regime, small-scale eddies could penetrate into the preheat zone of the flames and enhance the mixing process. In this study, the effects of small-scale turbulence on emission (NOx and CO) formation in premixed flame fronts are investigated through the incorporation of turbulence induced diffusion in the preheat zone of one-dimensional premixed flames. One-dimensional methane/air premixed flames are simulated with the 53-species GRI-Mech 3.0 mechanism at both atmospheric and engine conditions with different turbulence intensities. It is found that the NO generated in flame fronts deceases with increasing intensity of small-scale turbulence and the effect is more profound at high pressures. At high pressures, the turbulence induced diffusion in the preheat zone can reduce the NOx formation in flame fronts by more than 40%. On the other hand, the CO mass fraction in flame fronts increases with increasing intensity of small-scale turbulence. In the cases considered, the CO mass fraction in the flame fronts can increase by more than 55%. In addition, a flamelet-based approach that accounts for the flame thickening effects has been formulated to simulate NOx and CO formation in turbulent lean premixed combustion. In this approach, the species NO and CO are transported and solved in a simulation with chemical source terms being pre-calculated from 1-D premixed flames with detailed chemical kinetics and turbulence induced diffusion. The NO source term can be quantified by its formation in flame fronts and its formation rate in post-flame region. The CO source term can be calculated through its mass fraction at flame fronts, its mass fraction in the post-flame region and an oxidation time scale. The effect of heat loss on NO formation has been studied by investigate the relation between post-flame NO formation rate and flame temperature. Meanwhile, the effect of turbulent-chemistry interaction on NO were studied. The flamelet-based emission model has been implemented into Fluent and 3-Dimensional simulations were conducted in a combustion rig.

Author: Santanu De
Publisher: Springer
ISBN: 9811074100
Category : Science
Languages : en
Pages : 663

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Book Description
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.

Author: Norbert Peters
Publisher: Cambridge University Press
ISBN: 1139428063
Category : Science
Languages : en
Pages : 322

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Book Description
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.

Author: Andrei Lipatnikov
Publisher: CRC Press
ISBN: 1466510250
Category : Science
Languages : en
Pages : 548

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Book Description
Lean burning of premixed gases is considered to be a promising combustion technology for future clean and highly efficient gas turbine combustors. Yet researchers face several challenges in dealing with premixed turbulent combustion, from its nonlinear multiscale nature and the impact of local phenomena to the multitude of competing models. Filling

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:
Publisher:
ISBN:
Category :
Languages : en
Pages : 58

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