A Study of Lean Premixed Swirl-stabilized Combustion of Gaseous Alternative Fuels PDF Download
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Author: Donald McKinley Wicksall
Publisher:
ISBN:
Category : Combustion
Languages : en
Pages : 424
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Author: Donald McKinley Wicksall
Publisher:
ISBN:
Category : Combustion
Languages : en
Pages : 424
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Author: Amin Akbari
Publisher:
ISBN: 9781303167799
Category :
Languages : en
Pages : 195
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Lean blowoff, flashback and pollutant emission associated with lean premixed combustion of alternative fuels stabilized by high swirl are evaluated in this work. Alternative fuel compositions include blends of natural gas and hydrogen. Lean blowoff refers to events where the fuel-to-air ratio is not sufficient to sustain the reaction. Blowoff is often a dynamic process consisting of several stages. Correlations based on constant Damköhler (Da) number are able to estimate the impact of fuel composition on lean blowoff for the conditions studied. The accuracy of estimating initiation of blowoff is superior compared to estimation of subsequent stages of blowoff. Flashback refers to propagation of the reaction upstream into the premixing zone. In high swirl combustion applications, the concept of a quench criterion has been proposed for predicting flashback. For the present work, this concept only holds for some measured cases, which indicates multiple flashback modes even in high swirl combustion applications. The other major combustion challenge is pollutant emission. In this study NOx, CO, and N2O levels are experimentally measured. In addition, a chemical reaction network (CRN) was developed to study the details of emission formation. To develop a CRN, details of the reacting flow were needed. Hence, computational fluid dynamics (CFD) simulations were conducted. To validate CFD simulations, particle image velocimetry (PIV) and OH* chemiluminescence flame front imaging were applied. OH* chemiluminescence was also employed to visualize the flame structure and shape for different fuel compositions. The CRN simulations indicate that the NNH NOx formation pathway dominates the other formation pathways. Thus, conditions that enhance NNH NOx, such as an increase of hydrogen in fuel composition, and decrease of residence time, will result in more total NOx. The CRN also illustrates how the relative contribution of each NOx formation pathway to total NOx changes with adiabatic flame temperature (AFT). The NNH NOx formation pathway is dominant for AFT below 1900K; the Zeldovich mechanism is dominant for AFT above 1900K. In terms of N2O emissions measured and simulated results suggest the levels are negligible even for very low combustion temperatures.
Author: Derek Dunn-Rankin
Publisher: Academic Press
ISBN: 0080550525
Category : Technology & Engineering
Languages : en
Pages : 282
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Book Description
Combustion under sufficiently fuel-lean conditions can have the desirable attributes of high efficiency and low emissions, this being particularly important in light of recent and rapid increases in the cost of fossil fuels and concerns over the links between combustion and global climate change. Lean Combustion is an eminently authoritative, reference work on the latest advances in lean combustion technology and systems. It will offer engineers working on combustion equipment and systems both the fundamentals and the latest developments in more efficient fuel usage and in much-sought-after reductions of undesirable emissions, while still achieving desired power output and performance. This volume brings together research and design of lean combustion systems across the technology spectrum in order to explore the state-of-the-art in lean combustion and its role in meeting current and future demands on combustion systems. Readers will learn about advances in the understanding of ultra lean fuel mixtures and how new types of burners and approaches to managing heat flow can reduce problems often found with lean combustion such as slow, difficult ignition and frequent flame extinction. The book will also offer abundant references and examples of recent real-world applications. Covers all major recent developments in lean combustion science and technology, with new applications in both traditional combustion schemes as well as such novel uses as highly preheated and hydrogen-fueled systems Offers techniques for overcoming difficult ignition problems and flame extinction with lean fuel mixtures Covers new developments in lean combustion using high levels of pre-heat and heat re-circulating burners, as well as the active control of lean combustion instabilities
Author: Megan Karalus
Publisher:
ISBN:
Category : Combustion engineering
Languages : en
Pages : 188
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Book Description
This work examines lean premixed flame stability for multi-component fuel mixtures to support fuel flexibility for industrial combustors. A single Jet Stirred Reactor (JSR), a generic recirculation stabilized combustor, along with gaseous fuels of hydrogen, methane, and hydrogen/methane blends are chosen for the study. Experimental data on blowout are collected and a series of models are used to understand the mechanism of extinction in this recirculation-stabilized flame environment. By studying this more generic combustor, the aim is to develop generalizable results and methodologies for understanding and predicting lean blowout of multicomponent fuels. Experimental data approaching blowout are taken for fuels of pure hydrogen, pure methane, and hydrogen/methane blends in 10% by volume increments. The data relate inlet equivalence ratios to experimentally measured temperatures for each fuel approaching blowout and reveal the final blowout condition for each fuel. These blowout data are obtained by holding the air flow rate constant and decreasing the fuel flow rate until the flame is extinguished. Doing so holds the flow field and turbulence parameters approximately constant as blowout is approached. The reactor is stabilized to lower equivalence ratios and temperatures as the percentage of hydrogen in the fuel increases. In order to gain insight on the mechanism controlling blowout, two dimensional, axisymmetric computational fluid dynamic (CFD) simulations are carried out for the lean premixed combustion of both hydrogen and methane as the fuel. Hydrogen requires only 9 species to fully describe its chemistry. Therefore, the detailed mechanism of Li et al. is chosen for the hydrogen simulations. Methane combustion is described by the full GRI-3.0 chemical mechanism with 35 species. To facilitate reasonable computational times a skeletal mechanism of 22 species is developed from GRI-3.0 using the Directed Relation Graph method developed by Lu and Law. The CFD simulations for both hydrogen and methane combustion are run similarly to the experiments. The fuel flow rate is reduced until the CFD model no longer produces a burning solution. Contour plots from the CFD model illustrate the evolution of the flow-field, temperature profiles, and flame structure within the JSR as blowout is approached for both fuels. The modeling suggests that lean blowout in the JSR does not occur in a spatially homogeneous condition, but rather under a zonal structure. Analysis of the models from the perspective of a combusting fluid particle traveling through the jet, into the recirculation zone, and then entraining back into the jet suggests that the blowout condition is dependent on the development of the pool of radicals. The flame remains stable as long as the radical pool develops significantly enough to achieve ignition before the hypothetical combusting fluid particle is re-entrained. As the fuel flow decreases, the induction period increases and the ignition event is pushed further around the recirculation zone. Eventually, the induction period becomes so long that the ignition is incomplete at the point where the recirculating gas is entrained. This threshold leads to overall flame extinction. Two Chemical Reactor Network (CRN) models are developed using the flow field and reaction fields from the detailed CFD models in an attempt to capture the bulk of the physical processes responsible for flame stability. The single Plug Flow Reactor (PFR) model follows the concept of the hypothetical combusting fluid particle and assumes that only convective transport is responsible for stability. This model matches hydrogen blowout well, reproducing the ignition event and the development of the pool of radicals before re-entrainment. While the single PFR model with the UCSD chemical mechanism does predict the blowout temperature across the full range of methane/hydrogen fuel blends well, it fails to adequately predict blowout equivalence ratio for fuels with high methane concentrations. A two PFR model is subsequently developed in which the core jet region (of constant mass flow) exchanges mass with the recirculation region through turbulent diffusive transport. Entrainment of flow by jet action is confined entirely to the recirculation region, represented by the exhaust of the recirculation PFR being convectively re-entrained at its entrance. The two PFR model performs about as well as the single PFR model in predicting blowout for hydrogen in the JSR and shows significant improvement over the single PFR model in both following the experimental data approaching blowout, and predicting the blowout condition for methane. In fact the two PFR model shows good agreement with both equivalence ratio and temperature at blowout across the full range of hydrogen/methane blends. Regardless of the chemical mechanism applied, or whether we consider transport by convection only as in the single PFR model, or transport by both convection and diffusion as in the two PFR model, the story regarding the onset of blowout remains the same and is consistent with that given by CFD as well: the key to the stable operation of the reactor is the ignition event in the recirculation zone, resulting in the development of the radical pool. For pure hydrogen combustion as the fuel flow rate is reduced and the reactor moves towards blowout the destruction of the fuel slows and spreads, and the development of the radical pool moves further around the recirculation zone. The radical pool must develop (i.e. ignition must occur) before re-entrainment or the reactor will extinguish. For methane we similarly see the destruction of methane spread, and the net production of CO, and subsequently the net production of OH move further around the recirculation zone until the re-entrainment of radicals can no longer sustain the combustion. For methane, transport of the CO and radicals through turbulent diffusion appears to be a controlling process in this ignition event. The ignition event for hydrogen, on the other hand, is affected very little by the inclusion of diffusive transport of radicals. This is most likely due to the fact that the breakdown of hydrogen directly produces an H radical that feeds the chain propagating reaction, however the direct breakdown of methane has no such feedback. It is only in the destruction of methane intermediates that the H radical needed to feed the chain propagating reaction is produced.
Author:
Publisher:
ISBN:
Category :
Languages : en
Pages :
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Author: Paul Palies
Publisher: Academic Press
ISBN: 0128199970
Category : Technology & Engineering
Languages : en
Pages : 402
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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: Omid Askari
Publisher:
ISBN:
Category : Combustion chambers
Languages : en
Pages : 271
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Book Description
This dissertation investigates the combustion and injection fundamental characteristics of different alternative fuels both experimentally and theoretically. The subjects such as lean partially premixed combustion of methane/hydrogen/air/diluent, methane high pressure direct-injection, thermal plasma formation, thermodynamic properties of hydrocarbon/air mixtures at high temperatures, laminar flames and flame morphology of synthetic gas (syngas) and Gas-to-Liquid (GTL) fuels were extensively studied in this work. These subjects will be summarized in three following paragraphs. The fundamentals of spray and partially premixed combustion characteristics of directly injected methane in a constant volume combustion chamber have been experimentally studied. The injected fuel jet generates turbulence in the vessel and forms a turbulent heterogeneous fuel-air mixture in the vessel, similar to that in a Compressed Natural Gas (CNG) Direct-Injection (DI) engines. The effect of different characteristics parameters such as spark delay time, stratification ratio, turbulence intensity, fuel injection pressure, chamber pressure, chamber temperature, Exhaust Gas recirculation (EGR) addition, hydrogen addition and equivalence ratio on flame propagation and emission concentrations were analyzed. As a part of this work and for the purpose of control and calibration of high pressure injector, spray development and characteristics including spray tip penetration, spray cone angle and overall equivalence ratio were evaluated under a wide range of fuel injection pressures of 30 to 90 atm and different chamber pressures of 1 to 5 atm. Thermodynamic properties of hydrocarbon/air plasma mixtures at ultra-high temperatures must be precisely calculated due to important influence on the flame kernel formation and propagation in combusting flows and spark discharge applications. A new algorithm based on the statistical thermodynamics was developed to calculate the ultra-high temperature plasma composition and thermodynamic properties. The method was applied to compute the thermodynamic properties of hydrogen/air and methane/air plasma mixtures for a wide range of temperatures (1,000-100,000 K), pressures (10−6-100 atm) and different equivalence ratios within flammability limit. In calculating the individual thermodynamic properties of the atomic species, the Debye-Huckel cutoff criterion has been used for terminating the series expression of the electronic partition function. A new differential-based multi-shell model was developed in conjunction with Schlieren photography to measure laminar burning speed and to study the flame instabilities for different alternative fuels such as syngas and GTL. Flame instabilities such as cracking and wrinkling were observed during flame propagation and discussed in terms of the hydrodynamic and thermo-diffusive effects. Laminar burning speeds were measured using pressure rise data during flame propagation and power law correlations were developed over a wide range of temperatures, pressures and equivalence ratios. As a part of this work, the effect of EGR addition and substitution of nitrogen with helium in air on flame morphology and laminar burning speed were extensively investigated. The effect of cell formation on flame surface area of syngas fuel in terms of a newly defined parameter called cellularity factor was also evaluated. In addition to that the experimental onset of auto-ignition and theoretical ignition delay times of premixed GTL/air mixture were determined at high pressures and low temperatures over a wide range of equivalence ratios.
Author: Andres Colorado
Publisher:
ISBN: 9781339564050
Category :
Languages : en
Pages : 178
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This study provides an experimental and numerical examination of pollutant emissions and stability of gaseous fueled reactions stabilized with two premixed-fuel-flexible and ultra-low NOx burner technologies. Both burners feature lean combustion technology to control the formation of nitrogen oxides (NOx). The first fuel--flexible burner is the low-swirl burner (LSB), which features aerodynamic stabilization of the reactions with a divergent flow-field; the second burner is the surface stabilized combustion burner (SSCB), which features the stabilization of the reactions on surface patterns.For combustion applications the most commonly studied species are: NOx, carbon monoxide (CO), and unburned hydrocarbons (UHC). However these are not the only pollutants emitted when burning fossil fuels; other species such as nitrous oxide (N2O), ammonia (NH3) and formaldehyde (CH2O) can be directly emitted from the oxidation reactions. Yet the conditions that favor the emission of these pollutants are not completely understood and require further insight.The results of this dissertation close the gap existing regarding the relations between emission of pollutants species and stability when burning variable gaseous fuels. The results of this study are applicable to current issues such as: 1. Current combustion systems operating at low temperatures to control formation of NOx. 2. Increased use of alternative fuels such as hydrogen, synthetic gas and biogas. 3. Increasing recognition of the need/desire to operate combustion systems in a transient manner to follow load and to offset the intermittency of renewable power. 4. The recent advances in measurement methods allow us to quantify other pollutants, such as N 2O, NH3 and CH2O.Hence in this study, these pollutant species are assessed when burning natural gas (NG) and its binary mixtures with other gaseous fuels such as hydrogen (H2), carbon dioxide (CO2), ethane (C 2H6) and propane (C3H8) at variable operation modes including: ignition; lean blowoff; and variable air to fuel ratio. Some remarkable results of this dissertation include: • At a fixed fire rate (117kW) the addition of hydrogen to NG raises the emission of NO x for the reactions stabilized with the LSB. Under the same conditions, the addition of H2 to NG will reduce the emission levels of the reactions stabilized with the SSCB. • It was found experimentally that nitrous oxide (N2O) is emitted during ignition and blowoff events. • Ammonia (NH3) is also emitted during ignition and blowoff events. • It was found experimentally that at high concentrations of hydrogen in NG (H2>70%), reactions aerodynamically stabilized with the LSB will emit significant amounts of N2O.
Author: Timothy C. Lieuwen
Publisher: AIAA (American Institute of Aeronautics & Astronautics)
ISBN:
Category : Science
Languages : en
Pages : 688
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Book Description
This book offers gas turbine users and manufacturers a valuable resource to help them sort through issues associated with combustion instabilities. In the last ten years, substantial efforts have been made in the industrial, governmental, and academic communities to understand the unique issues associated with combustion instabilities in low-emission gas turbines. The objective of this book is to compile these results into a series of chapters that address the various facets of the problem. The Case Studies section speaks to specific manufacturer and user experiences with combustion instabilities in the development stage and in fielded turbine engines. The book then goes on to examine The Fundamental Mechanisms, The Combustor Modeling, and Control Approaches.
Author: Arthur H. Lefebvre
Publisher: CRC Press
ISBN: 1420086057
Category : Science
Languages : en
Pages : 560
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Book Description
Reflecting the developments in gas turbine combustion technology that have occurred in the last decade, Gas Turbine Combustion: Alternative Fuels and Emissions, Third Edition provides an up-to-date design manual and research reference on the design, manufacture, and operation of gas turbine combustors in applications ranging from aeronautical to po
