Download or read book Coherent Structures of Flames written by Maya Mincheva and published by . This book was released on 1996 with total page 124 pages. Available in PDF, EPUB and Kindle. Book excerpt:
Download or read book Coherent Structures in Turbulent Flames II written by Andrew J. Yule and published by . This book was released on 1980 with total page 11 pages. Available in PDF, EPUB and Kindle. Book excerpt: The aim of this investigation is to obtain increased understanding of turbulent combustion in flows related to propulsion systems. Simultaneous measurements of fluctuating velocity, temperature and local ionization level, have been made with arrays of probes, with variation in probe separations and flame initial conditions. The stored data are processed in a variety of ways, including the use of novel conditional sampling techniques. Information is thus derived on turbulent structure, eddy structure, eddy convection, eddy-reaction interaction, reaction zone structure and mixing. It is demonstrated that individually eddies can exist in flames for substantially longer distances than is apparent from classical space-time cross correlations. Similar techniques are also used in turbulent sprays and in a flame impinging ona flat plate. (Author).
Download or read book Coherent Structures in Turbulent Flames written by N. A. Chigier and published by . This book was released on 1983 with total page 32 pages. Available in PDF, EPUB and Kindle. Book excerpt: The objective of this research was to obtain increased understanding of turbulent combustion in flows related to propulsion systems. Measurement and data analysis techniques, and the boundary conditions of various flames were designed to provide improved concepts and fundamental experimental data, to aid the development of modelling techniques for practical combustion systems. The specific approach of the study was to quantify the roles of large eddies (coherent structures) in reacting flows. The experiments and measurement techniques which have been developed to attain this aim were described in previous Interim Scientific Reports. Measurements during the past year have been made in the initial region of gaseous jet flames with separate variation in Reynolds number and equivalence ratio. These measurements have produced detailed data on the structure of the initial regions of jet diffusion flames, as a function of systematic variation of initial burner nozzle conditions. This data is already recognized to be of considerable use for combustion modelling. Two associated experiments, investigating the structures of turbulent liquid fuel sprays and impinging flames have been carried out during the period of the contract.
Download or read book Coherent Structures in Turbulent Flames written by A. J. Yule and published by . This book was released on 1979 with total page 37 pages. Available in PDF, EPUB and Kindle. Book excerpt: Diagnostic and data analysis techniques have been used to investigate the structure of turbulent jet reacting flows with well defined initial and boundary conditions. In the transition region of a turbulent jet flame a double-vortex structure has been found: with outer vortices, which may originate from Kelvin-Helmholtz or Rayleigh type instabilities, and inner vortices which are influenced strongly by one or more combustion instability mechanisms. Measurements of mean velocity, turbulence intensity, ionization current, mean temperature and peak frequencies are presented together with data derived from high speed color schlieren photography and laser schlieren spectra.
Download or read book Experimental Study of Burner Geometry Effect on the Coherent Structures Flashback and Flame Front Dynamics of Unconfined and Confined Partially Premixed Swirling Methane Flames written by Mahmoud M.A. Ahmed and published by . This book was released on 2020 with total page 0 pages. Available in PDF, EPUB and Kindle. Book excerpt: The effect of burner geometry (central fuel nozzle, mixing tube length, and flame confinement) on the ensuing flowfield's coherent structures (precessing vortex core and vortex shedding) and their relationship with some combustion stability parameters, such as flashback and flame front dynamics of a swirling partially premixed methane flame, is experimentally studied. In this investigation, several measurement techniques are employed. These include particle image velocimetry (PIV), Mie scattering, laser Doppler velocimetry (LDV), Bruel & Kjaer microphone, high-speed Schlieren imaging technique, and high-speed luminescence imaging. In addition, proper orthogonal decomposition (POD) is used as a post processing technique to capture the flow-field coherent structures. In the first part of the study, the effect of central nozzle geometry on coherent structures' strength and frequency is examined inside a relatively long mixing tube. Furthermore, the relationship between the strength and frequency of coherent structures and the mean flashback region inside the mixing tube is studied. In the second part, the central nozzle geometry is modified based on the conclusions reached in the first part of the study, and its effect on the suppression of the coherent structures is investigated using a relatively short mixing length. Finally, the effect of nozzle geometries, which exhibit the highest and lowest coherent structures' amplitude, on coherent structures and acoustics modes is studied using different mixing tube lengths in the presence of a flame confinement. The results showed that the central nozzle geometry significantly affects coherent structures' strength and amplitude inside the mixing tube. Moreover, the results revealed a strong relationship between coherent structures' strength and the mean flashback region inside the mixing tube. Furthermore, the central nozzle geometry is found to significantly affect the amplitude of coherent structures for both confined and unconfined swirling partial premixed flames, and acoustics for the confined flames. All in all, it can be concluded that passive techniques can be a viable strategy for mitigating combustion instabilities of partially premixed flames.
Download or read book Coherent Structures in Complex Systems written by D. Reguera and published by Springer. This book was released on 2008-01-11 with total page 466 pages. Available in PDF, EPUB and Kindle. Book excerpt: A rich variety of real-life physical problems which are still poorly understood are of a nonlinear nature. Examples include turbulence, granular flows, detonations and flame propagation, fracture dynamics, and a wealth of new biological and chemical phenomena which are being discovered. Particularly interesting among the manifestations of nonlinearity are coherent structures. This book contains reviews and contributions reporting on the state of the art regarding the role of coherent structures and patterns in nonlinear science.
Download or read book Blowoff Velocity and Turbulent Flame Spread Correlations Based on Coherent Structures in Turbulent Flows written by Krishnan Radhakrishnan and published by . This book was released on 1979* with total page 33 pages. Available in PDF, EPUB and Kindle. Book excerpt:
Download or read book Coherent Structures in the Near Field of Swirling Turbulent Jets and Flames Investigated by PIV and PLIF written by Vladimir Dulin and published by . This book was released on 2022 with total page 0 pages. Available in PDF, EPUB and Kindle. Book excerpt: The results of the experimental study of coherent flow structures in turbulent jets with different swirl rates are reported. The focus is placed on analysis of their impact on mixing of a passive scalar in the jets and on regular flame front deformations during combustion of fuel-lean and fuel-rich methane/air mixtures in case of a strongly swirling jet with vortex breakdown. The measurements are performed by applying simultaneously the particle image velocimetry (PIV) and planar laser-induced fluorescence (PLIF) techniques (acetone and HCHO for the nonreacting and reacting flows, respectively). The PIV data are processed by a proper orthogonal decomposition (POD), and the PLIF data are conditionally sampled according to the correlation coefficients of the velocity POD modes. The coherent velocity fluctuations are associated with regular patterns in the concentration fluctuation fields and flame front deformations. These patterns correspond to unsteady mixing by large-scale vortex structures in the outer mixing layer and also to variations of the entrainment rate for the flows with swirl.
Download or read book Interactions Between Coherent and Turbulent Oscillations and Their Impact on the Dynamics of Flames and Flow Fields written by Ashwini Karmarkar and published by . This book was released on 2022 with total page 0 pages. Available in PDF, EPUB and Kindle. Book excerpt: This primary focus of this dissertation is to investigate the coupling mechanisms by which flow field fluctuations can interact with heat release oscillations and how the coupling mechanisms are impacted by the addition of turbulent fluctuations. This work is particularly motivated by the problem of combustion instability in gas turbine engines. Combustion instability is a type of thermoacoustic instability that occurs due to coupling between the coherent oscillations in heat release rate and the acoustic modes of the combustor. The modulation of heat release rate due to the interaction of the flame front with coherent structures in the flow can be a driver of combustion instability. While there have multiple studies analysing the interaction between flames and coherent structures, many of the experimental studies focus on the low-turbulence regime, which is not representative of realistic engine conditions. More recent studies have analysed flame response and limiting phenomena at high turbulence intensities, although the interaction between competing phenomena of turbulent and coherent oscillations have not been comprehensively studied so far and is therefore a focus contribution of this work. In this dissertation, two configurations are studied -- the canonical rod-stabilized V-flame and a more realistic partially-premixed swirl flame. The canonical configuration allows for more control over individual flow parameters so that the coherent and turbulent fluctuations can be independently controlled and systematically varied. High-speed stereoscopic particle image velocimetry (sPIV) is the primary diagnostic used in this configuration. The coherent oscillations in the flow field are excited by longitudinal acoustic excitation and different configurations of perforated plates in the burner provide varying turbulence intensities. The results from this work conclusively show that the magnitude of turbulence intensity in the flow can significantly impact the flow dynamics, the symmetry of the flow response to external excitation, and the coupling between the flow field and flame fluctuations. The realistic swirling flame configuration is used to characterize the interaction between the precessing vortex core (PVC), which is the consequence of a global hydrodynamic instability, and thermoacoustic instabilities, which are the result of a coupling between combustor acoustics and the unsteady heat release rate of combustion. This study is performed using experimental data obtained from a model gas turbine combustor system to simulate realistic conditions. High-speed stereoscopic particle image velocimetry, OH planar laser-induced fluorescence, and acetone planar laser-induced fluorescence are used to obtain information about the velocity fields, flame, and fuel flow behavior, respectively. The results from this work show that in the cases where the frequency of the PVC overlaps with the frequency of a thermoacoustic mode, the thermoacoustic mode is subsequently suppressed. Further, the thermoacoustic coupling process is driven by both velocity and mixture variations, but the PVC oscillations do not significantly drive variations in the mixture, only the velocity field. Put together, the findings from both configurations provide important insight into the coupling mechanisms that govern the interactions between the various flow field fluctuations and their impact on the unsteady heat release from the flame.
Download or read book Unsteady Combustor Physics written by Tim C. Lieuwen and published by Cambridge University Press. This book was released on 2012-08-27 with total page 427 pages. Available in PDF, EPUB and Kindle. Book excerpt: Developing clean, sustainable energy systems is a pre-eminent issue of our time. Most projections indicate that combustion-based energy conversion systems will continue to be the predominant approach for the majority of our energy usage. Unsteady combustor issues present the key challenge associated with the development of clean, high-efficiency combustion systems such as those used for power generation, heating or propulsion applications. This comprehensive study is unique, treating the subject in a systematic manner. Although this book focuses on unsteady combusting flows, it places particular emphasis on the system dynamics that occur at the intersection of the combustion, fluid mechanics and acoustic disciplines. Individuals with a background in fluid mechanics and combustion will find this book to be an incomparable study that synthesises these fields into a coherent understanding of the intrinsically unsteady processes in combustors.
Download or read book Turbulence Coherent Structures Dynamical Systems and Symmetry written by Philip Holmes and published by Cambridge University Press. This book was released on 2012-02-23 with total page 403 pages. Available in PDF, EPUB and Kindle. Book excerpt: Describes methods revealing the structures and dynamics of turbulence for engineering, physical science and mathematics researchers working in fluid dynamics.
Download or read book Swirling Flows and Flames written by Toufik Boushaki and published by BoD – Books on Demand. This book was released on 2019-06-26 with total page 120 pages. Available in PDF, EPUB and Kindle. Book excerpt: Swirl flows are used in a wide range of industrial applications. In non-reacting cases, examples of applications include vortex amplifiers and reactors, heat exchangers, jet pumps, and cyclone separators. In reacting cases, swirlers are widely used in combustion systems, such as gas turbines, industrial furnaces, boilers, gasoline and diesel engines, and many other practical heating devices. Effects of using swirl on flow and combustion are significant and varied, and concern, for example, aerodynamics, mixing, flame stability, intensity of combustion, and pollutant emissions. The purpose of this book is to present recent research efforts to understand and characterize swirling flows of different types and in different applications. These include gaseous, liquid, and solid fuels in order to enhance combustion systems and their energy efficiency. Swirl flows are very complex and the studies proposed in this project are based on different means, including theoretical calculations, numerical modeling, and experimental measurements.
Download or read book Numerical Simulations of Coherent Structures in a Jet Flame with a Noncircular Cross Section written by and published by . This book was released on 1989 with total page 15 pages. Available in PDF, EPUB and Kindle. Book excerpt: The presence of coherent structures in turbulent shear flows and their importance in enhancing mixing and entrainment of chemical species has been established in recent years through extensive experimental and numerical studies. When non-premixed combustion is considered, the combustion processes could be enhanced by enhancing the fuel-air mixing and entrainment processes associated with these large coherent structures. In recent experimental studies, the entrainment characteristics of jets with different cross sections were studied, and it was determined that an elliptical jet with a certain cross-sectional shape was more efficient in entraining fluid than a circular jet with the same jet area. This increase was attributed to the complex self-induction process and the subsequent three-dimensional deformation of the elliptical vortex ring during its propagation. To understand the complex flow features associated with elliptical jets with and without heat release, a numerical study was undertaken. A new three-dimensional hybrid Navier-Stokes code was developed using spectral techniques and compact schemes to study the spatial evolution of elliptical jets. The simulation code is now ready for carrying out a detailed study of the effect of jet geometry on the mixing and combustion processes. Keywords: Direct numerical simulations; Elliptical jet flow; Compact schemes; Spectral methods; Turbulent flows; Reacting flows; Heat release; Combustion.
Download or read book Self sustaining Mechanisms of Wall Turbulence written by Ronald Lee Panton and published by Computational Mechanics. This book was released on 1997 with total page 448 pages. Available in PDF, EPUB and Kindle. Book excerpt: Why is wall turbulence self-sustaining? In this book well-regarded researchers not only discuss what they know and believe, but also speculate on ideas that still require numerical or experimental testing and verification. An initial brief history of boundary layer structure research is followed by chapters on experimental information and specific topics within the subject. There are then sections on computational aspects.
Download or read book A Gallery of Combustion and Fire written by Charles E. Baukal, Jr. and published by Cambridge University Press. This book was released on 2020-09-03 with total page 193 pages. Available in PDF, EPUB and Kindle. Book excerpt: A Gallery of Combustion and Fire is the first book to provide a graphical perspective of the extremely visual phenomenon of combustion in full color. It is designed primarily to be used in parallel with, and supplement existing combustion textbooks that are usually in black and white, making it a challenge to visualize such a graphic phenomenon. Each image includes a description of how it was generated, which is detailed enough for the expert but simple enough for the novice. Processes range from small scale academic flames up to full scale industrial flames under a wide range of conditions such as low and normal gravity, atmospheric to high pressures, actual and simulated flames, and controlled and uncontrolled flames. Containing over 500 color images, with over 230 contributors from over 75 organizations, this volume is a valuable asset for experts and novices alike.
Download or read book Fluid Dynamic Structures in Diffusion Flames written by A. Cabelli and published by . This book was released on 1987 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: Experiments were carried out in a combustion facility which allowed optical and other measurements to be made on a diffusion flame formed by a jet of fuel gas and a co-flowing jet of air. Acoustic perturbations could be imposed to the flow of fuel with a loudspeaker mounted at the bottom of a settling chamber in the fuel line. A two mirror schlieren system was used to image the gas flames and a colour filter was placed at the focus of the second mirror to produce coloured schlieren images when a continuous white light source was used. Two types of coherent structures were observed, located concentrically along the axis of the flame. The outer structures, found outside the luminous part of the flame, outlined the boundary between products of combustion and the cold airstream, while the inner structures, located well inside the luminous flame, marked the interface between the cold fuel and the reaction zone. The structures were periodic and their frequencies identified by spectral analysis of optical and pressure signals. The signal from a pressure probe placed in the flame and from a phototransistor focused at the corresponding position in the schlieren image, correlated well. Over the range of Reynolds numbers tested, the outer structures exhibited a dominant frequency of approximately 13 Hz whereas the faster inner structures occurred at approximately 256 Hz. Acoustic excitation at these frequencies made the corresponding structures more coherent and influenced the development of the instability close to the nozzle exit. The response of the experimental equipment to band limited white noise, with the jet not ignited, revealed spectral peaks at the frequency of the inner structures which confirm the nexus between the flow and the acoustic field (even without external excitation). The wavelength of these structures (and their convection velocity) did not vary over the range of Reynolds numbers considered, at a given distance from the nozzle exit, but increased slightly with axial position. When the flame was acoustically forced at a frequency different from the preferred frequency, their wavelength remained unchanged but the velocity changed to accommodate the change in frequency.
Download or read book Combustion written by J. Warnatz and published by Springer Science & Business Media. This book was released on 2006-09-23 with total page 389 pages. Available in PDF, EPUB and Kindle. Book excerpt: This book provides a rigorous treatment of the coupling of chemical reactions and fluid flow. Combustion-specific topics of chemistry and fluid mechanics are considered and tools described for the simulation of combustion processes. This edition is completely restructured. Mathematical Formulae and derivations as well as the space-consuming reaction mechanisms have been replaced from the text to appendix. A new chapter discusses the impact of combustion processes on the atmosphere, the chapter on auto-ignition is extended to combustion in Otto- and Diesel-engines, and the chapters on heterogeneous combustion and on soot formation are heavily revised.
