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Author: Lu-Yin Wang
Publisher:
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Languages : en
Pages : 0
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Soot formation and evolution in relation with the flow fields were investigated experimentally in turbulent swirl-stabilized non-premixed flames using three different fuels: methane, ethanol and aviation Jet A-1. The studied flames were confined and stabilized in a model gas turbine combustor with a swirl number of ~0.55. Soot volume fraction, fv, and primary soot particle size, dp, were measured using auto-compensating laser-induced incandescence, and planar three-component velocity fields were measured using stereoscopic particle image velocimetry. Measurements of planar laser-induced fluorescence of OH and OH* chemiluminescence were also made for methane and ethanol flames. The OH* field was further Abel-inverted to qualitatively locate the heat release zone. The flow field for all flames featured pronounced inner and outer recirculation zones (IRZ, ORZ), each bounded by their corresponding inner and outer shear layers (ISL, OSL). Abel-inverted OH* intensity maps showed that primary reaction zones occurred in the vicinity of ISL. The central fuel jet penetrating into the IRZ accompanied by a stagnation zone was observed in all methane flames. Soot measurements showed that the overall dp for methane and Jet A-1 flames ranged between 30 nm and 60 nm without discernible trends. In methane flames, peak time-averaged fv occurred between the central jet penetration and the ISL. The decrease and the final disappearance of time-averaged fv were strongly correlated with elevated OH, demonstrating a dominant oxidative attack of OH on soot. With a ~7% increase in air flow rate, the level of soot volume fraction dropped by nearly threefold due to enhanced turbulence intermittency. The appearance of ethanol spray flames, which lacked a bright yellow color, largely differed from others. The absence of soot was confirmed in the laser-induced incandescence measurements. The isothermal flow field of ethanol flames exhibited a large-scale structure of precessing vortex core which was then suppressed under reacting conditions. In Jet A-1 flames, spray pattern changed from V-shaped hollow cone to semi-solid cone when air flow rate increased by 20%, resulting in a 60% reduction in peak time-averaged fv. In contrast to results obtained from the methane flame, soot was found primarily outside the ISL where fuel existed in abundance.
Author: Lu-Yin Wang
Publisher:
ISBN:
Category :
Languages : en
Pages : 0
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Book Description
Soot formation and evolution in relation with the flow fields were investigated experimentally in turbulent swirl-stabilized non-premixed flames using three different fuels: methane, ethanol and aviation Jet A-1. The studied flames were confined and stabilized in a model gas turbine combustor with a swirl number of ~0.55. Soot volume fraction, fv, and primary soot particle size, dp, were measured using auto-compensating laser-induced incandescence, and planar three-component velocity fields were measured using stereoscopic particle image velocimetry. Measurements of planar laser-induced fluorescence of OH and OH* chemiluminescence were also made for methane and ethanol flames. The OH* field was further Abel-inverted to qualitatively locate the heat release zone. The flow field for all flames featured pronounced inner and outer recirculation zones (IRZ, ORZ), each bounded by their corresponding inner and outer shear layers (ISL, OSL). Abel-inverted OH* intensity maps showed that primary reaction zones occurred in the vicinity of ISL. The central fuel jet penetrating into the IRZ accompanied by a stagnation zone was observed in all methane flames. Soot measurements showed that the overall dp for methane and Jet A-1 flames ranged between 30 nm and 60 nm without discernible trends. In methane flames, peak time-averaged fv occurred between the central jet penetration and the ISL. The decrease and the final disappearance of time-averaged fv were strongly correlated with elevated OH, demonstrating a dominant oxidative attack of OH on soot. With a ~7% increase in air flow rate, the level of soot volume fraction dropped by nearly threefold due to enhanced turbulence intermittency. The appearance of ethanol spray flames, which lacked a bright yellow color, largely differed from others. The absence of soot was confirmed in the laser-induced incandescence measurements. The isothermal flow field of ethanol flames exhibited a large-scale structure of precessing vortex core which was then suppressed under reacting conditions. In Jet A-1 flames, spray pattern changed from V-shaped hollow cone to semi-solid cone when air flow rate increased by 20%, resulting in a 60% reduction in peak time-averaged fv. In contrast to results obtained from the methane flame, soot was found primarily outside the ISL where fuel existed in abundance.
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:
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Category :
Languages : en
Pages : 17
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A novel means has been developed for using weak swirl to stabilize freely propagating open premixed turbulent flames (swirl numbers between 0.05 to 0.3). By injecting a small amount of air tangentially into the co-flow of a concentric burner, stationary flames can be maintained above the burner exit for a large range of mixture, turbulence and flow conditions. The absence of physical surfaces in the vicinity of the flame provides free access to laser diagnostics. Laser Doppler anemometry and laser Mie scattering measurements of four flames with and without incident turbulence show that their features are typical of wrinkled laminar flames. The most distinct characteristics is that flame stabilization does not rely on flow recirculation. Centrifugal force induced by swirl causes flow divergence, and the flame is maintained at where the local mass flux balances the burning rate. The flame speeds can be estimated based on the centerline velocity vector, which is locally normal to the flame brush. This flame geometry is the closest approximation to the 1-D planar flame for determining fundamental properties to advance turbulent combustion theories. 18 refs.
Author: Paul Palies
Publisher: Academic Press
ISBN: 0128199970
Category : Technology & Engineering
Languages : en
Pages : 402
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Book Description
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
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.
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Languages : en
Pages : 31
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This study investigated the effects of turbulence on the spatial distribution of the soot particle and OH fields in turbulent C2H4 air/jet diffusion flames. Measurements obtained using planar laser-induced incandescence (LII) for soot volume fraction and laser-induced fluorescence (LIF) for both OH, and polycyclic aromatic hydrocarbons (PAH) formed the basis for investigating soot formation and destruction processes in these flames. These laser-based techniques were applied to the flame independently as well as simultaneously. Extensive information on the structure of the soot and OH fields was obtained from two-dimensional imaging experiments. Imaging results for soot, OH and PAH indicated three distinct soot formation/oxidation regions; a rapid soot growth region, in which OH and soot particles lie in distinctly different radial locations; a mixing dominated region controlled by large-scale fluid motion; and, finally, a soot oxidation region in which the OH and soot fields overlap spatially, resulting in the rapid oxidation of soot particles. Detailed quantitative analyses including soot volume fraction, OH and soot zone thickness variations, and probabilities distributions for soot and OH were performed. The measurements of soot and OH zone thickness showed that the soot zone thickness varied nearly linearly in the formation region, while approximately a doubling of thickness of the OH zone was evident over the studied Reynolds number range (4000-23000). The probability density function results tor soot, OH, and PAH indicated that OH and PAH are spatially interrelated with respect to soot formation and oxidation processes
Author: Zachary Alexander LaBry
Publisher:
ISBN:
Category :
Languages : en
Pages : 220
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One of the most difficult challenges facing the development of modern gas turbines-for power generation, and propulsion-is the mitigation of dynamic instabilities in the presence of efficiency and emissions constraints. Dynamic instabilities-self-excited, self-sustaining oscillations which link the combustor acoustics to the combustion process-can result in significant levels of thermal and mechanical stress on combustion systems, leading to reduced operational lifetime, potentially dangerous failure modes, and significant deviations from the desired operating conditions. Due to the complexity of the problem, with the relevant time and length scales of the system--from the chemistry to the acoustics-spanning several orders of magnitude, even sophisticated numerical techniques have been severely limited in their ability to make reliable predictions, leaving the task of finding and eliminating modes of instability to a lengthy and expensive trial-and-error process. Lean-premixed combustion, one of the leading technologies for low emission combustors, is particularly susceptible to these types of instabilities. The sealed systems that are necessary to maintain a reaction in a lean mixture do not attenuate acoustics well, which often results in high-amplitude pressure fluctuations. In this thesis, we focus on developing a better predictive framework for the onset of combustion instabilities in a swirl-stabilized, lean-premixed combustor. We correlate the self-excited acoustic behavior with quantifiable system properties that can be generalized across different fuel blends. This work is predicated on the idea that self-excited combustion instability arises from the selective amplification of the noise inherent in a turbulent combustion system, and that the frequency-based response of the flame is a function of the flame geometry. In the first part of the thesis, we focus on the flame geometry, identifying several discrete transitions that take place in the swirl-stabilized flame as we adjust the equivalence ratio. By comparing the transitions across several CH4/H2 fuel blends, and using statistical techniques to interrogate the global effect of the small-scale flow-flame interactions, we find that the extinction strain rate-the flow-driven rate of change in flame surface area at which the chemistry is no longer -sufficiently fast to maintain the reaction-is directly linked to the flame transitions. The swirl-stabilized flow features several critical regions with large and unsteady velocity derivatives, particularly, a pair of shear layers that divide the incoming flow of reactants from an inner and an outer recirculation zone. As the extinction strain rate increases with increasing equivalence ratio, the flame transitions through these critical regions, manifesting as discrete changes in the flame geometry. In the second part, we address the correlation between self-excited instability and the forced acoustic response. By modifying the pressure boundary conditions, we decouple the flame from the acoustics over a domain of interest (defined by a range of equivalence ratios that correspond to the onset of dynamic instability in the coupled system). We then apply external acoustic forcing at a single frequency to ascertain the response of the flame to each particular forcing frequency by means of a flame transfer function. This enables us to consider the frequency-by-frequency response of the flame to its own internally generated noise. We show that the onset of instability is well-predicted by the overlap of the natural acoustic frequencies of the combustor (predicted using a non-linear flame response model) with those frequencies for which the phase of the flame transfer function satisfies the well-known Rayleigh criterion, which is a necessary condition for the presence of self-excited combustion instability. By examining both the forced response and the self-excited instability across several different fuel blends, we go on to show that both behaviors correlate well with the flame geometry, which we have already shown to be dictated by the extinction strain rate of the particular fuel blend. We go on to collapse both sets of data on the strained flame consumption speed taken at the limit of the extinction strain rate, and in doing so, present a framework for predicting the operating conditions under which the combustor in the coupled configuration will go unstable based on measurements and correlations from the uncoupled configuration. Furthermore by taking the consumption speed at the extinction limit, we are correlating the geometry and dynamics with a parameter that is solely a function of mixture properties. This provides the basis for a framework for predicting instability from properties that are more readily measured or simulated, and provides and explicit means of converting these results to different fuel mixtures.
Author: Taylor Rault
Publisher:
ISBN:
Category :
Languages : en
Pages : 0
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The effects of ethanol, n-butanol, and m-xylene addition to swirl-stabilized Jet A-1 spray flames were investigated in a model combustor with 94 mm x 94 mm cross-section and 188 mm length. Dopants were added in concentrations up to 20% by energy content for a 10 kW total thermal output; two air flow rates were used for each blend. Droplet size distributions, time-averaged flow fields, as well as time-averaged soot volume fractions and primary particle sizes were measured using the Fraunhofer diffraction technique, stereoscopic particle image velocimetry, and auto-compensating laser-induced incandescence, respectively. Measurable differences in spray distributions and flow fields were observed for all additives compared to neat Jet A-1. In all blend flames, an additional time-averaged soot volume fraction peak absent for neat Jet A-1 was observed below 30 mm above the combustor base. While total soot loading generally increased with ethanol and m-xylene addition, it decreased with n-butanol addition.
Author: Hemant P. Mungekar
Publisher:
ISBN:
Category :
Languages : en
Pages : 512
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Author:
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ISBN:
Category :
Languages : en
Pages : 192
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