Download or read book THE SURFACE STRUCTURE OF TURBULENT PREMIXED HYDROGEN AIR RIM STABILIZED FLAMES AT HIGH REYNOLDS NUMBER PREMIXED TURBULENT FLAMES FLAMES written by MING-SHIN WU and published by . This book was released on 1991 with total page 376 pages. Available in PDF, EPUB and Kindle. Book excerpt: three-dimensional simulation, to remove 2-D deficiencies, appears to be computationally feasible for high Reynolds number conditions of interest for practical applications.
Download or read book Turbulent Premixed Hydrogen Air Flames written by and published by . This book was released on 1991 with total page 108 pages. Available in PDF, EPUB and Kindle. Book excerpt: The properties of turbulent premixed flames were investigated both theoretically and experimentally. Attention was limited to hydrogen/air mixtures burning as either turbulent jet flames or a freely propagating flames in isotropic turbulence. The research has application to a variety to premixed turbulent combustion processes: underwater metal cutting at great depth, primary combustors for high-speed airbreathing propulsion systems, afterburners, fuel/ air explosions, and spark-ignition internal combustion engines. Major findings of this phase of the investigation are as follows: (1) effects of preferential diffusion are relevent for flames at high Reynolds number, retarding and enhancing the distortion of the flame surface by turbulence for stable and unstable conditions, respectively; (2) local turbulent burning velocity, flame brush thickness and the fractal dimension of the flame surface all increase with distance from the flameholder, with larger rates of increases at larger turbulence intensities; (3) estimates of flame properties using contemporary turbulence models were only fair because these methods cannot account for effects of preferential diffusion, distance from the flameholder and finite laminar flame speeds; and (4) the stochastic simulation duplicated measured trends of flame surface properties for neutral preferential diffusion conditions (the only case considered) but underestimated effects of turbulence (particularly near the flame tip) due to the limitations of a two-dimensional simulation.
Download or read book Theories of Turbulent Combustion in High Speed Flows written by Paul A. Libby and published by . This book was released on 1995 with total page 12 pages. Available in PDF, EPUB and Kindle. Book excerpt:
Download or read book Experimental Investigation of the Dynamics and Structure of Lean premixed Turbulent Combustion written by Frank Tat Cheong Yuen and published by . This book was released on 2009 with total page 306 pages. Available in PDF, EPUB and Kindle. Book excerpt: Turbulent premixed propane/air and methane/air flames were studied using planar Rayleigh scattering and particle image velocimetry on a stabilized Bunsen type burner. The fuel-air equivalence ratio was varied from &phis; = 0:7 to 1.0 for propane flames, and from &phis; = 0:6 to 1.0 for methane flames. The non-dimensional turbulence intensity, u'/ SL (ratio of fluctuation velocity to laminar burning velocity), covered the range from 3 to 24, equivalent to conditions of corrugated flamelets and thin reaction zones regimes. Temperature gradients decreased with the increasing u'/SL and levelled off beyond u'/SL > 10 for both propane and methane flames. Flame front thickness increased slightly as u'/SL increased for both mixtures, although the thickness increase was more noticeable for propane flames, which meant the thermal flame front structure was being thickened. A zone of higher temperature was observed on the average temperature profile in the preheat zone of the flame front as well as some instantaneous temperature profiles at the highest u'/SL. Curvature probability density functions were similar to the Gaussian distribution at all u'/ SL for both mixtures and for all the flame sections. The mean curvature values decreased as a function of u'/ SL and approached zero. Flame front thickness was smaller when evaluated at flame front locations with zero curvature than that with curvature. Temperature gradients and FSD were larger when the flame curvature was zero. The combined thickness and FSD data suggest that the curvature effect is more dominant than that of the stretch by turbulent eddies during flame propagation. Integrated flame surface density for both propane and methane flames exhibited no dependance on u'/S L regardless of the FSD method used for evaluation. This observation implies that flame surface area may not be the dominant factor in increasing the turbulent burning velocity and the flamelet assumption may not be valid under the conditions studied. Dkappa term, the product of diffusivity evaluated at conditions studied and the flame front curvature, was a magnitude smaller than or the same magnitude as the laminar burning velocity.
Download or read book Fundamentals of Turbulent and Multiphase Combustion written by Kenneth Kuan-yun Kuo and published by John Wiley & Sons. This book was released on 2012-04-24 with total page 914 pages. Available in PDF, EPUB and Kindle. Book excerpt: Detailed coverage of advanced combustion topics from the author of Principles of combustion, Second Edition Turbulence, turbulent combustion, and multiphase reacting flows have become major research topics in recent decades due to their application across diverse fields, including energy, environment, propulsion, transportation, industrial safety, and nanotechnology. Most of the knowledge accumulated from this research has never been published in book form—until now. Fundamentals of Turbulent and Multiphase Combustion presents up-to-date, integrated coverage of the fundamentals of turbulence, combustion, and multiphase phenomena along with useful experimental techniques, including non-intrusive, laser-based measurement techniques, providing a firm background in both contemporary and classical approaches. Beginning with two full chapters on laminar premixed and non-premixed flames, this book takes a multiphase approach, beginning with more common topics and moving on to higher-level applications. In addition, Fundamentals of Turbulent and Multiphase Combustion: Addresses seven basic topical areas in combustion and multiphase flows, including laminar premixed and non-premixed flames, theory of turbulence, turbulent premixed and non-premixed flames, and multiphase flows Covers spray atomization and combustion, solid-propellant combustion, homogeneous propellants, nitramines, reacting boundary-layer flows, single energetic particle combustion, and granular bed combustion Provides experimental setups and results whenever appropriate Supported with a large number of examples and problems as well as a solutions manual, Fundamentals of Turbulent and Multiphase Combustion is an important resource for professional engineers and researchers as well as graduate students in mechanical, chemical, and aerospace engineering.
Download or read book Lean Premixed Flame Structure in Intense Turbulence written by Sastri Purushottama Nandula and published by . This book was released on 2003 with total page 364 pages. Available in PDF, EPUB and Kindle. Book excerpt:
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 Experimental Investigation on the Effects of Free Stream Turbulence and Fuel Type on Structure and Blowoff Characteristics of Turbulent Premixed Bluff body Stabilized Flames written by Bikram Roy Chowdhury and published by . This book was released on 2017 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: An experimental investigation on the effect of different levels of turbulence intensity and properties of the fuel/air mixture on the structure and characteristics of lean flames stabilized on an axisymmetric bluff body is described in this thesis. Simultaneous imaging of hydroxyl (OH) and formaldehyde (CH2O) by planar laser induced fluorescence and particle image velocimetry (PIV) were used to study the interaction between the flame and the flow field. CH2O fluorescence and the pixel-by-pixel multiplication of OH and CH2O fluorescence signals were utilized to mark preheat and heat release regions respectively. In addition, high-speed chemiluminescence imaging was performed to understand the time resolved characteristics of the flame. The first part of the thesis focuses on the characteristics of stably burning lean methane/-, propane/- and ethylene/air flames when subjected to low (4 %), moderate (14 %) and intense (24 and 30%) levels of free stream turbulence. The flame front structure was observed to be strongly dependent on the free stream turbulence level of the incoming fuel/air mixture as well on the properties of the fuel/air mixture. Formation of cusps and unburnt mixture fingers were observed as the turbulence intensity was increased from 4 to 14 % but, the heat release region remained continuous. Under intense turbulence conditions, methane/- and ethylene/air (f = 0.85) flames exhibited localized extinctions along the flame sheet and flamelet merging events which created isolated pockets of reactants in the flame envelope. In addition to these features, propane/- and ethylene/air (f=0.655) flames exhibited the occurrence of flame fragmentation events and the general shape of these flames were observed to intermittently switch from a symmetric (varicose) to asymmetric (sinuous) mode. Several properties were measured to characterize the effects of turbulence – flame interaction which includes the average preheat and reaction zone thicknesses, strain rates and curvature along the flame front, burning fraction, flame brush thickness, flame surface density, area ratio and turbulent flame speed. The next part of the thesis focuses on blowoff dynamics of lean methane/-, propane/- and ethylene/air flames for mean velocities of 5, 10 and 15 m/s and subjected to free stream turbulence levels from 4 to 30%. Apart from the propane/air flames at an apporach velcoity of 5 m/s and turbulence intensity of 30 %, increasing turbulence intensity was found to reduce the flame stability. The blowoff equivalence ratios of propane/air flames was observed to be higher than methane/- and ethylene/air flames. As blowoff was approached, the flame front and shear layer vortices entangled inducing high local strain rates on the flame front that exceed the extinction strain rate resulting in significant breaks along the reaction zone. At conditions near blowoff, significant increase in the frequency of breaks along the reaction zone was observed for low and moderate turbulence conditions. For the higher turbulence conditions, fragmentation of the flame along with the presence of sinuous wakes was observed which aided in the penetration of reactants into the recirculation zone. Velocity vectors near the flame holes indicate the penetration of the reactants into the recirculation zone. Mostly similar sequence of events was observed for methane/-, propane/- and ethylene/air flames near blowoff. Several properties weremeasured to characterize the near blowoff flames which include the strain rate and curvature statistics along the flame front, burning fraction, asymmetric index and the average duration of the blowoff event. Based on the observation from the experiments, turbulent flame speed was attributed to be the primary factor in governing the blowoff equivalence ratio. This point of view was examined by comparing the mean strain rate of methane/- and ethylene/air flames at the equivalence ratio corresponding to near blowoff for propane/air flames.
Download or read book Experimental Investigation of Premixed Turbulent Hydrocarbon Air Bunsen Flames written by Parsa Tamadonfar and published by . This book was released on 2016 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: Through the influence of turbulence, the front of a premixed turbulent flame is subjected to the motions of eddies that leads to an increase in the flame surface area, and the term flame wrinkling is commonly used to describe it. If it is assumed that the flame front would continue to burn locally unaffected by the stretch, then the total turbulent burning velocity is expected to increase proportionally to the increase in the flame surface area caused by wrinkling. When the turbulence intensity is high enough such that the stretch due to hydrodynamics and flame curvature would influence the local premixed laminar burning velocity, then the actual laminar burning velocity (that is, flamelet consumption velocity) should reflect the influence of stretch. To address this issue, obtaining the knowledge of instantaneous flame front structures, flame brush characteristics, and burning velocities of premixed turbulent flames is necessary. Two axisymmetric Bunsen-type burners were used to produce premixed turbulent flames, and three optical measurement techniques were utilized: Particle image velocimetry to measure the turbulence statistics; Rayleigh scattering method to measure the temperature fields of premixed turbulent flames, and Mie scattering method to visualize the flame front contours of premixed turbulent flames. Three hydrocarbons (methane, ethane, and propane) were used as the fuel in the experiments. The turbulence was generated using different perforated plates mounted upstream of the burner exit. A series of comprehensive parameters including the thermal flame front thickness, characteristic flame height, mean flame brush thickness, mean volume of the turbulent flame region, two-dimensional flame front curvature, local flame front angle, two-dimensional flame surface density, wrinkled flame surface area, turbulent burning velocity, mean flamelet consumption velocity, mean turbulent flame stretch factor, mean turbulent Markstein length and number, and mean fuel consumption rate were systematically evaluated from the experimental data. The normalized preheat zone and reaction zone thicknesses decreased with increasing non-dimensional turbulence intensity in ultra-lean premixed turbulent flames under a constant equivalence ratio of 0.6, whereas they increased with increasing equivalence ratios from 0.6 to 1.0 under a constant bulk flow velocity. The normalized preheat zone and reaction zone thicknesses showed no overall trend with increasing non-dimensional longitudinal integral length scale. The normalized preheat zone and reaction zone thicknesses decreased by increasing the Karlovitz number, suggesting that increasing the total stretch rate is the controlling mechanism in the reduction of flame front thickness for the experimental conditions studied in this thesis. In general, the leading edge and half-burning surface turbulent burning velocities were enhanced with increasing equivalence ratio from lean to stoichiometric mixtures, whereas they decreased with increasing equivalence ratio for rich mixtures. These velocities were enhanced with increasing total turbulence intensity. The leading edge and half-burning surface turbulent burning velocities for lean/stoichiometric mixtures were observed to be smaller than that for rich mixtures. The mean turbulent flame stretch factor displayed a dependence on the equivalence ratio and turbulence intensity. Results show that the mean turbulent flame stretch factors for lean/stoichiometric and rich mixtures were not equal when the unstrained premixed laminar burning velocity, non-dimensional bulk flow velocity, non-dimensional turbulence intensity, and non-dimensional longitudinal integral length scale were kept constant.
Download or read book Structure of Turbulent Premixed Flames written by Evan Timothy Hurlburt and published by . This book was released on 1989 with total page 190 pages. Available in PDF, EPUB and Kindle. Book excerpt:
Download or read book Premixed Turbulent Flame Propagation written by D. A. Santavicca and published by . This book was released on 1990 with total page 15 pages. Available in PDF, EPUB and Kindle. Book excerpt: The results of an experimental study of turbulence-flame interactions and their effect on turbulent flame propagation are reported. Experiments are conducted in a new turbulent flow system which is capable of producing relative turbulence intensities as high as 100 percent. Using a freely propagating, one-dimensional flame configuration, measurements are made using LDV of the mean velocity, turbulence intensity, integral time scale, energy spectrum, Reynolds stress, and integral length scale, at a fixed location both before and after flame arrival. A complete set os such measurements has been made at one operating condition which characterize both the magnitude of flame generated turbulence and its anisotropic nature. The freely propagating, one dimensional flame configuration has also been used to study the effect of turbulence on turbulent flame structure. Two dimensional Mie scattering is used to obtain a two-dimensional slice of the turbulent flame surface. Such measurements have been made at 15 different conditions over very broad range of turbulence Reynolds and Damkohler numbers and have been analyzed to determine the fractal nature of the turbulent flame surface. The results of these measurements show that premixed turbulent flame surfaces are fractal throughout the reaction sheet regime. Keywords: Premixed turbulent combustion; Turbulent flame interactions; Turbulent flame propagation; Turbulent flame structure. (JHD).
Download or read book Applied Mechanics Reviews written by and published by . This book was released on 1987 with total page 934 pages. Available in PDF, EPUB and Kindle. Book excerpt:
Download or read book Experimental Study of Premixed Flames in Intense Isotropic Turbulence written by and published by . This book was released on 1994 with total page 22 pages. Available in PDF, EPUB and Kindle. Book excerpt: A methodology for investigating premixed turbulent flames propagating in intense isotropic turbulence has been developed. The burner uses a turbulence generator developed by Videto and Santavicca and the flame is stabilized by weak-swirl generated by air injectors. This set-up produces stable premixed turbulent flames under a wide range of mixture conditions and turbulence intensities. The experiments are designed to investigate systematically the changes in flame structures for conditions which can be classified as wrinkled laminar flames, corrugated flames and flames with distributed reaction zones. Laser Doppler anemometry and Rayleigh scattering techniques are used to determine the turbulence and scalar statistics. In the intense turbulence, the flames are found to produce very little changes in the mean and rams velocities. Their flame speed increase linearly with turbulence intensity as for wrinkled laminar flames. The Rayleigh scattering pdfs for flames within the distributed reaction zone regime are distinctly bimodal. The probabilities of the reacting states (i.e. contributions from within the reaction zone) is not higher than those of wrinkled laminar flame. These results show that there is no drastic changes in flame structures at Karlovitz number close to unity. This suggest that the Klimov-Williams criterion under-predicts the resilience of wrinkled flamelets to intense turbulence.
Download or read book Flame Structure written by M. D. Fox and published by . This book was released on 1961 with total page 74 pages. Available in PDF, EPUB and Kindle. Book excerpt: Current concepts of flame propagation in premixed, turbulent gas streams are examined. This leads to the conclusion that the link between theory and experiment is entirely inadequate and incapable of improvement by existing methods. A series of new method is implemented in an attempt to short-circuit the unprofitable chain of hypothesis and experiment which has hampered the identification of dubious steps. Methods of introducing uniform turbulence at relatively slow flows and improvements in light sources allow analysis of the approach flow by photographing particles illuminated by an interrupted Tyndall beam. Three new optical deflection methods are used to give a measure of the randomness of flame-front orientation, of the time-mean structure of the flame and of the instantaneous shape of the corrugated front. It is found that this corrugated surface propagates at a velocity considerably in excess of the normal laminar burning velocity. Quantitative analysis of the frequency of 'peaks' and 'valleys' on the surface, together with comparative data from the apex of laminar flames, suggests an explanation in terms of the effects of curvature and, secondarily, of the influence of small scale turbulence. (Author).
Download or read book Stabilization and Dynamic of Premixed Swirling Flames written by Paul Palies and published by Academic Press. This book was released on 2020-07-03 with total page 402 pages. Available in PDF, EPUB and Kindle. Book excerpt: Stabilization and Dynamic of Premixed Swirling Flames: Prevaporized, Stratified, Partially, and Fully Premixed Regimes focuses on swirling flames in various premixed modes (stratified, partially, fully, prevaporized) for the combustor, and development and design of current and future swirl-stabilized combustion systems. This includes predicting capabilities, modeling of turbulent combustion, liquid fuel modeling, and a complete overview of stabilization of these flames in aeroengines. The book also discusses the effects of the operating envelope on upstream fresh gases and the subsequent impact of flame speed, combustion, and mixing, the theoretical framework for flame stabilization, and fully lean premixed injector design. Specific attention is paid to ground gas turbine applications, and a comprehensive review of stabilization mechanisms for premixed, partially-premixed, and stratified premixed flames. The last chapter covers the design of a fully premixed injector for future jet engine applications. Features a complete view of the challenges at the intersection of swirling flame combustors, their requirements, and the physics of fluids at work Addresses the challenges of turbulent combustion modeling with numerical simulations Includes the presentation of the very latest numerical results and analyses of flashback, lean blowout, and combustion instabilities Covers the design of a fully premixed injector for future jet engine applications
Download or read book Direct Numerical Simulation of Lean Premixed Turbulent Flames at High Karlovitz Numbers Under Elevated Pressures written by Xujiang Wang and published by . This book was released on 2019 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt:
Download or read book Lewis Number Effects on Turbulent Premixed Flame Structure written by and published by . This book was released on 1992 with total page 23 pages. Available in PDF, EPUB and Kindle. Book excerpt: The influence of the Lewis number on turbulent flame front geometry is investigated in a premixed turbulent stagnation point flame. A laser tomography technique is used to obtain the flame shape, a fractal analysis of the multiscale flame edges is performed and the distribution of local flame front curvature is determined. Lean H[sub 2]/Air and C[sub 3]H[sub 8]/Air mixtures with similar burning rates were investigated with Lewis numbers of 0.33 and 1.85 respectively. At the conditions studied the laminar H[sub 2]/Air mixture is unstable and a cellular structure is observed. Turbulence in the reactant is generated by a perforated plate and the turbulent length scale (3mm) and intensity (7%) at the nozzle exit are fixed. The equivalence ratio is set so that the burning velocity is the same for all the cases. Results show clearly that the turbulent flame surface area is dependent on the Lewis number. For a Lewis number less than unity surface area production is observed. The shape of the flame front curvature distribution is not found to be very sensitive to the Lewis number. For the H[sub 2]/Air mixture the distribution is skewed toward the positive values indicating the presence of cusps while for the C[sub 3]H[sub 8]/Air mixture the distribution is more symmetrical. In both cases the average curvature is found to be zero, and if the local burning speed varies linearly with curvature, the local positive and negative burning velocity variations due to curvature will balance.
