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Author: Publisher: ISBN: Category : Languages : en Pages : 0
Book Description
To allow implementation of chemical kinetic schemes of arbitrary complexity in computational design codes for gas-turbine combustion, a new microstructural turbulent combustion model was developed. The fine structure of turbulent combustion was represented by PSR (Perfectly Stirred Reactor) theory. The theory is the intense-combustion analog of flamelet theory. Residence times in the PSR were related to the scalar dissipation, and turbulence-chemistry interactions were closed by using the probability distribution function for scalar dissipation in a turbulent flow. Calculations compared very favorably with Raman data on temperature and species from three turbulent bluff-body stabilized laboratory flames: (i) a non-premixed CO/H2/N2-air flame, (ii) a non-premixed CH4/H2-air flame, and (iii) a premixed CH4-air flame. With this success, the model was applied to two practical combustors: (iv) an axially-staged combustion system which produces about half the NOx of a conventional combustor while offering greater operability, and operates in an unusual regime of turbulence-chemistry interactions, and (v) a conventional aircraft engine combustor. In the latter case, a kinetic scheme with over 121 species and 996 elementary reactions was demonstrated. In both cases, the calculated results agreed well with temperature and species data. The physical model developed here was used directly in the industry-standard pressure-corrected mean Navier-Stokes/assumed-shape pdf/k-epsilon type of CFD code, which affords significant geometric flexibility and rapid convergence for gas-turbine combustor flowfields.
Author: Publisher: ISBN: Category : Languages : en Pages : 0
Book Description
To allow implementation of chemical kinetic schemes of arbitrary complexity in computational design codes for gas-turbine combustion, a new microstructural turbulent combustion model was developed. The fine structure of turbulent combustion was represented by PSR (Perfectly Stirred Reactor) theory. The theory is the intense-combustion analog of flamelet theory. Residence times in the PSR were related to the scalar dissipation, and turbulence-chemistry interactions were closed by using the probability distribution function for scalar dissipation in a turbulent flow. Calculations compared very favorably with Raman data on temperature and species from three turbulent bluff-body stabilized laboratory flames: (i) a non-premixed CO/H2/N2-air flame, (ii) a non-premixed CH4/H2-air flame, and (iii) a premixed CH4-air flame. With this success, the model was applied to two practical combustors: (iv) an axially-staged combustion system which produces about half the NOx of a conventional combustor while offering greater operability, and operates in an unusual regime of turbulence-chemistry interactions, and (v) a conventional aircraft engine combustor. In the latter case, a kinetic scheme with over 121 species and 996 elementary reactions was demonstrated. In both cases, the calculated results agreed well with temperature and species data. The physical model developed here was used directly in the industry-standard pressure-corrected mean Navier-Stokes/assumed-shape pdf/k-epsilon type of CFD code, which affords significant geometric flexibility and rapid convergence for gas-turbine combustor flowfields.
Author: Publisher: ISBN: Category : Languages : en Pages : 56
Book Description
The goal of this research is a quantitative understanding of turbulence-chemistry interactions as they pertain to combustion in aeropropulsion engines. The two principal classes of accomplishment are: (1) The first-ever stochastic joint velocity-composition pdf simulations of bluff-body stabilized flames, and comparison with Raman data on major species, temperature and mixture fraction (mean and rms quantities of each) in the same burner. Fuels have been CO/H2 mixtures (whose reduced chemistry is modeled with two compositional variables) and methane (five variables). This method of merging pdf transport and CFD codes can be used to combine the pdf model with any of the CFD codes used in design. There is thus a clear path for transitioning the results of the research. Remaining issues included (i) a chemistry scheme for jet fuels (not just CO/H2 and CH4), tested in turbulence and not only in the the laminar context. and (ii) more species and temperature data in the regime of high turbulence intensity, so that the model can be tested in the regime of real engines. (2) The "Partially Stirred Reactor" or PaSR model was developed toward the first two of these goals. The unsteady evolution of a full chemistry scheme is computed in the presence of turbulence of prescribed frequency. jg p.2.
Author: Nedunchezhian Swaminathan Publisher: Cambridge University Press ISBN: 1139498584 Category : Technology & Engineering Languages : en Pages : 447
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: Norbert Peters Publisher: Cambridge University Press ISBN: 1139428063 Category : Science Languages : en Pages : 322
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: Tarek Echekki Publisher: Springer Science & Business Media ISBN: 9400704127 Category : Technology & Engineering Languages : en Pages : 496
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: National Aeronautics and Space Administration (NASA) Publisher: Createspace Independent Publishing Platform ISBN: 9781723483301 Category : Languages : en Pages : 132
Book Description
Studies in the mathematical modeling of the high-speed turbulent combustion has received renewal attention in the recent years. The review of fundamentals, approaches and extensive bibliography was presented by Bray, Libbi and Williams. In order to obtain accurate predictions for turbulent combustible flows, the effects of turbulent fluctuations on the chemical source terms should be taken into account. The averaging of chemical source terms requires to utilize probability density function (PDF) model. There are two main approaches which are dominant in high-speed combustion modeling now. In the first approach, PDF form is assumed based on intuitia of modelliers (see, for example, Spiegler et.al.; Girimaji; Baurle et.al.). The second way is much more elaborate and it is based on the solution of evolution equation for PDF. This approach was proposed by S.Pope for incompressible flames. Recently, it was modified for modeling of compressible flames in studies of Farschi; Hsu; Hsu, Raji, Norris; Eifer, Kollman. But its realization in CFD is extremely expensive in computations due to large multidimensionality of PDF evolution equation (Baurle, Hsu, Hassan). Zaitsev, S. and Buriko, Yu. and Guskov, O. and Kopchenov, V. and Lubimov, D. and Tshepin, S. and Volkov, D. Glenn Research Center TURBULENT COMBUSTION; MATHEMATICAL MODELS; HIGH SPEED; TURBULENT FLOW; COMBUSTION CHEMISTRY; COMPUTATIONAL FLUID DYNAMICS; REACTION KINETICS; COMBUSTION; BIBLIOGRAPHIES; PREMIXED FLAMES...
Author: David M. Mann Publisher: ISBN: Category : Chemical reactions Languages : en Pages : 302
Book Description
Partial contents: Supercritical droplet behavior; Fundamentals of acoustic instabilities in liquid-propellant rockets; Modeling liquid jet atomization proceses; Liquid-propellant droplets dynamics and combustions in supercritical forced convective environments; Contributions of shear coaxial injectors to liquid rocket motor combustion instabilities; High pressure combustion studies under combustion driven oscillatory flow conditions; Droplet collision on liquid propellant combustion; Combustion and plumes; Development of a collisional radiative emission model for strongly nonequilibrium flows; Energy transfer processes in the production of excited states in reacting rocket flows; modeling nonequilibrium radiation in high altitude plumes; kinetics of plume radiation, and of HEDMs and metallic fuels combustion; Nonsteady combustion mechanisms of advanced solid propellants; Chemical mechanisms at the burning surface. p15
Author: Paulo Lucena Kreppel Paes Publisher: ISBN: Category : Languages : en Pages :
Book Description
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.
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Author: Nedunchezhian Swaminathan
Author: Norbert Peters
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Author: Tarek Echekki
Author: National Aeronautics and Space Administration (NASA)
Author: David M. Mann
Author: Paul A. Libby
Author: Paulo Lucena Kreppel Paes