Download or read book Experimental Studies of Transient and Limit Cycle Thermoacoustic Oscillations in a Lean Premixed Swirl Stabilized Combustor written by Efstathios Karlis 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 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 Thermoacoustic Combustion Instability Control written by Dan Zhao and published by Academic Press. This book was released on 2023-02-13 with total page 1145 pages. Available in PDF, EPUB and Kindle. Book excerpt: Thermoacoustic Combustion Instability Control: Engineering Applications and Computer Codes provides a unique opportunity for researchers, students and engineers to access recent developments from technical, theoretical and engineering perspectives. The book is a compendium of the most recent advances in theoretical and computational modeling and the thermoacoustic instability phenomena associated with multi-dimensional computing methods and recent developments in signal-processing techniques. These include, but are not restricted to a real-time observer, proper orthogonal decomposition (POD), dynamic mode decomposition, Galerkin expansion, empirical mode decomposition, the Lattice Boltzmann method, and associated numerical and analytical approaches. The fundamental physics of thermoacoustic instability occurs in both macro- and micro-scale combustors. Practical methods for alleviating common problems are presented in the book with an analytical approach to arm readers with the tools they need to apply in their own industrial or research setting. Readers will benefit from practicing the worked examples and the training provided on computer coding for combustion technology to achieve useful results and simulations that advance their knowledge and research. Focuses on applications of theoretical and numerical modes with computer codes relevant to combustion technology Includes the most recent modeling and analytical developments motivated by empirical experimental observations in a highly visual way Provides self-contained chapters that include a comprehensive, introductory section that ensures any readers new to this topic are equipped with required technical terms

Download or read book Effect of Noise on a Model Thermoacoustic System at its Stability Boundary written by Richard Steinert and published by Springer. This book was released on 2016-04-20 with total page 52 pages. Available in PDF, EPUB and Kindle. Book excerpt: In experiments on a prototypical combustor, Richard Steinert identifies new insights on the impact of noise on the phenomenon known as thermoacoustic instability. The phenomenon is a concerning issue which creates a technical limit on the efficiency and environmental impact of fossil fuels combustion in industrial combustors. It poses a threat to the structural integrity of practical systems such as gas turbine combustors and rocket engines. The experiments demonstrate that thermoacoustic systems feature an interesting noise-induced behaviour known as coherence resonance – a coherent response of dynamical systems close to their stability boundary that is induced by stochastic excitation. The work contained in this publication is an example illustrating the importance of fundamental considerations in solving perplexing engineering issues.

Download or read book Thermoacoustic Instability written by R. I. Sujith and published by Springer Nature. This book was released on 2021-12-14 with total page 484 pages. Available in PDF, EPUB and Kindle. Book excerpt: This book systematically presents the consolidated findings of the phenomenon of self-organization observed during the onset of thermoacoustic instability using approaches from dynamical systems and complex systems theory. Over the last decade, several complex dynamical states beyond limit cycle oscillations such as quasiperiodicity, frequency-locking, period-n, chaos, strange non-chaos, and intermittency have been discovered in thermoacoustic systems operated in laminar and turbulent flow regimes. During the onset of thermoacoustic instability in turbulent systems, an ordered acoustic field and large coherent vortices emerge from the background of turbulent combustion. This emergence of order from disorder in both temporal and spatiotemporal dynamics is explored in the contexts of synchronization, pattern formation, collective interaction, multifractality, and complex networks. For the past six decades, the spontaneous emergence of large amplitude, self-sustained, tonal oscillations in confined combustion systems, characterized as thermoacoustic instability, has remained one of the most challenging areas of research. The presence of such instabilities continues to hinder the development and deployment of high-performance combustion systems used in power generation and propulsion applications. Even with the advent of sophisticated measurement techniques to aid experimental investigations and vast improvements in computational power necessary to capture flow physics in high fidelity simulations, conventional reductionist approaches have not succeeded in explaining the plethora of dynamical behaviors and the associated complexities that arise in practical combustion systems. As a result, models and theories based on such approaches are limited in their application to mitigate or evade thermoacoustic instabilities, which continue to be among the biggest concerns for engine manufacturers today. This book helps to overcome these limitations by providing appropriate methodologies to deal with nonlinear thermoacoustic oscillations, and by developing control strategies that can mitigate and forewarn thermoacoustic instabilities. The book is also beneficial to scientists and engineers studying the occurrence of several other instabilities, such as flow-induced vibrations, compressor surge, aeroacoustics and aeroelastic instabilities in diverse fluid-mechanical environments, to graduate students who intend to apply dynamical systems and complex systems approach to their areas of research, and to physicists who look for experimental applications of their theoretical findings on nonlinear and complex systems.

Download or read book Theoretical and Experimental Studies of Vortex Breakdown in a Lean Premixed Swirl stabilized Combustor written by Chukwueloka O. U. Umeh and published by . This book was released on 2009 with total page 424 pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book Velocity Fields in a High Pressure Liquid Fueled Swirl stabilized Lean Partially premixed Combustor written by Pankaj Saini and published by . This book was released on 2016 with total page 0 pages. Available in PDF, EPUB and Kindle. Book excerpt: This thesis investigates thermoacoustic velocity coupling in a lean, partially-premixed, liquid fueled gas turbine combustor operating at engine relevant conditions. Temporally resolved velocity fields are calculated through stereoscopic particle image velocimetry running at 5 kHz. Low velocity data yield is typical of such high pressure flames, so a new spatio-temporal data reconstruction algorithm is developed and implemented here to aid with the analysis of the measured velocity fields. Velocity measurements from six cases are studied here: gas phase velocities from two non-reacting and two reacting flows, and liquid phase velocities (on fuel droplets) from two other reacting cases. While the acoustic fields and the fuel flux into the combustor are found to have periodic oscillations corresponding to the thermoacoustic frequency, the velocity fields do not exhibit any relevant coherent oscillations. It therefore appears that, for the combustor being studied here, the thermoacoustic oscillations are a result of the fuel flow coupling, as opposed to vortex-acoustic-flame interactions.

Download or read book Applied Mechanics Reviews written by and published by . This book was released on 2000 with total page 776 pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book The Investigation and Semi Empirical Modeling of Thermoacoustic Phase Relationships in a Lean Premixed Prevapourized Combustor at Elevated Pressure written by Joseph Daniel Maxim Cirtwill and published by . This book was released on 2017 with total page 0 pages. Available in PDF, EPUB and Kindle. Book excerpt: This document presents an investigation of the self-excited coupling mechanisms that occur to produce both low-amplitude intermittent, as well as high-amplitude limit-cycle pressure oscillations in an aeronautical gas turbine combustor. Measurements of a lean premixed prevapourized combustor at elevated pressure were conducted using OH* chemiluminescence, pressure transducers, stereoscopic particle image velocimetry and droplet laser scattering. Analysis of the spectra and phase relationships between the measured variables is performed to determine and describe the coupling mechanisms. A semi-empirical model is presented to explain differences in the limit-cycle pressure amplitudes observed under nominally identical operating conditions. Evidence suggests that an oscillating flux of fuel into the combustor is responsible for both intermittent and limit-cycle oscillations, though different coupling relationships are observed in each case. The final amplitude of limit-cycle oscillations is shown to be correlated with changes in the phase difference between the fuel oscillations and the pressure.

Download or read book Combustion Instabilities in Gas Turbine Engines written by Timothy C. Lieuwen and published by AIAA (American Institute of Aeronautics & Astronautics). This book was released on 2005 with total page 688 pages. Available in PDF, EPUB and Kindle. Book excerpt: 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.

Download or read book Modeling and Control of Thermoacoustics in a One dimensional Combustor written by Xiaoling Chen and published by . This book was released on 2021 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: This dissertation examines the model-based optimization of sensor placement, estimation, and control for the active suppression of thermoacoustic instabilities in a one-dimensional combustor. This research is motivated by the increasing use of lean premixed combustion for emission reduction in gas turbine combustors. Thermoacoustic instability is a potentially damaging side effect of lean premixed combustion, caused by the unstable coupling between acoustics and unsteady heat release. There is extensive existing literature on the suppression of this instability, using both passive means such as Helmholtz resonators and active stability control. Much of the work on active combustion stability control relies on the injection of an additional acoustic excitation or fuel supply to break the above undesirable unstable coupling, thereby suppressing instability. Researchers have shown the promise of active combustion instability control both in simulations and laboratory experiments, for both single and multiple modes of instability. Active combustion stability control remains relatively scarce in industrial practice, despite the rich existing literature indicating its potential for success. Several important research questions need to be answered in order to help bridge this gap. First, while much of the recent research on active combustion stability control assumes one-dimensional combustion dynamics, an open question remains regarding the importance of other dynamic effects in the combustor, such as the dynamic interactions between multiple flames. Second, the degree to which the placement of sensors and actuators in a combustor affects the accuracy with which combustion instability dynamics can be estimated remains relatively unexplored in the literature. Third, the suitability of traditional linear model-based estimation and control techniques for stabilizing combustion instabilities with nonlinear heat release dynamics also remains relatively unexplored in the literature. The overarching goal of this dissertation is to address the above gaps using a combination of optimal sensor placement, optimal estimation, and optimal control. Towards this goal, the dissertation makes six specific contributions to the literature: 1. First, the author performs an experimental comparison between thermoacoustic instabilities in single- versus multi-nozzle combustion systems (Chapter 2). This study shows that the dynamic interactions between multiple flames in a multi-nozzle combustor have a non-trivial impact on thermoacoustic instability, especially the time scales of the transient instabilities. This helps characterize and understand the constraints on the practical applicability of one-dimensional combustion instability models for multi-nozzle systems, including the models used in the remainder of this dissertation. 2. Second, the author optimizes the design of a laboratory characterization experiment for a one-dimensional combustor (Chapter 3). This optimization utilizes Fisher information analysis for optimal combustion instability characterization, for the first time. 3. Third, the dissertation shows, using a mix of simulation-based and experimental studies, that the above use of optimal experimental design improves combustion instability parameterization accuracy (Chapter 4). Moreover, by furnishing more accurate combustion instability models, one is able to achieve higher levels of confidence in the robustness of the resulting combustion stability controllers. 4. Fourth, the dissertation presents a novel algorithm that makes it possible to estimate combustion heat release rates from multi-microphone measurements of the resulting acoustic signatures, in a manner that does not require the modeling of heat release dynamics (Chapter 5). This is important because it simplifies the online estimation of heat release dynamics, compared to model-based estimation methods requiring a heat release model. 5. Fifth, the dissertation studies the impact of sensor placement on the observability and LQG control of combustion instabilities governed by a linear $n-\tau$ heat release model (Chapter 6). This work highlights the importance of placing acoustic sensors at specific locations like the pressure mode anti-node points, including the acoustically closed combustor boundary. 6. Finally, the author develops a computational framework for the co-simulation of linear combustor acoustics, model-based combustion stability control, and nonlinear heat release dynamics governed by a level-set solver (Chapter 7). To the best of the author's knowledge, this is the first framework in the literature enabling the simulation-based study of the efficacy of linear control for combustion instabilities characterized by nonlinear heat release dynamics. In making the above contributions to the literature, this dissertation builds on the well-established idea that linear model-based estimation and control can be effective in suppressing combustion instability. The novelty of the dissertation lies in: 1. Pushing the above idea further by examining the degree to which its efficacy can be enhanced further through the use of information theory to optimize sensor placement and experimental design for estimation/control applications. 2. Building a framework that makes it possible to study the efficacy of model-based linear estimation/control in the context of thermo-acoustic instabilities driven by nonlinear heat release dynamics.

Download or read book Flow Diagnostic of Swirl Stabilized Combustion Without and with Porous Inert Media for Mitigation of Combustion Noise and Thermo acoustic Instabilities written by Joseph Warren Meadows and published by . This book was released on 2014 with total page 191 pages. Available in PDF, EPUB and Kindle. Book excerpt: Study of combustion dynamics has gained significant research attention since low-emission systems are increasingly employed in the industry. In particular, combustion noise and thermo-acoustic instabilities are of great importance in highly critical applications such as power generation, jet propulsion engines, and rocket propulsion systems. Recently, porous inert media (PIM), also referred to as foam insert, has shown promise in mitigating combustion noise and thermo-acoustic instabilities in lean premixed (LPM), swirl-stabilized combustion at atmospheric pressure and elevated pressures. In this study, the flow field without and with PIM is investigated to understand the underlying mechanisms responsible for mitigating thermo-acoustic instabilities. Experiments are conducted for LPM combustion and lean direct injection (LDI) combustion. First, time-resolved PIV technique is utilized to measure the non-reacting flow field without and with PIM. Although the flow field inside the annulus of the foam insert was optically inaccessible, measurements immediately downstream provide insight into the instantaneous flow field and turbulence characteristics. The study highlights the role of the foam insert on vorticity, velocity, shear layer spreading angle, recirculation zone dynamics, and turbulent kinetic energy; which ultimately affects the acoustics behavior of the combustor in a favorable manner. The effect of PIM on the dominant turbulent structures in the flow field is quantified using proper orthogonal decomposition (POD) technique. Next, flow field measurements are acquired for LPM swirl-stabilized combustion without and with PIM. The turbulent structures similar to the non-reacting flow field are also present in the reacting flow field, with notable difference in size and shape. The instantaneous and average flow fields provide insight into the effects of PIM on the velocity and turbulence fields. POD analysis is used to quantify the effect of PIM on the dominant turbulent structures, and PIM is shown to distribute the turbulent energy from the large scale structures to smaller scale structures. By harmonically reconstructing the flow field at the frequency of thermo-acoustic instability, the feedback mechanism is found to be the vortical structures in the corner recirculation zones, and PIM is shown to eliminate the feedback mechanism. The efficacy of PIM in mitigating combustion noise and thermo-acoustic instabilities is demonstrated for liquid fuel combustion utilizing the LDI concept. In this system, the flame stabilizes downstream of the dump plane due to a balance of flow velocity and flame speed of the fuel-air mixture created upstream. The ring shaped PIM is placed at the dump plane of the combustor to alter the flow field in an advantageous manner. Sound pressure levels (SPL) and CO and NOx emissions are measured for combustion without and with PIM inserts. Effect of atomizing air to liquid mass ratio on SPL suggests equivalence ratio oscillations are the driving force for thermo-acoustic instabilities. Results show that the PIM insert reduces broad band combustion noise, mitigates peak instabilities occurring at the first longitudinal mode of the natural frequency of the combustor, and facilitates thermal feedback from the flame to the fuel atomization process. Different insert geometries were examined and they all reduced SPLs, but the converging foam geometry provided the best performance. Finally, flow fields of LDI combustion are experimentally measured using time-resolved PIV technique without and with PIM. The instantaneous flow field highlights the role of PIM on the fluctuating velocity field. The driving mechanism for thermo-acoustic instability is identified by analyzing the fluctuating flow field, and PIM is found to decrease the driving force for thermo-acoustic instability. The average flow field is used to show the effect of PIM on the turbulence and POD analysis is used to quantify the effect of PIM on the turbulent structures. The study identifies spatial and temporal non-homogeneities in equivalence ratio as the feedback mechanisms for exciting thermo-acoustic instabilities in LDI swirl-stabilized combustion. In general, PIM decreases the driving force while increasing the dampening force in both LPM and LDI combustion systems.

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 Suppression of Thermoacoustic Instabilities in a Swirl Combustor Through Microjet Air Injection written by Zachary Alexander LaBry and published by . This book was released on 2010 with total page 87 pages. Available in PDF, EPUB and Kindle. Book excerpt: Thermoacoustic or combustion instability, a positive feedback loop coupling heat release rate and acoustic oscillations in a combustor, is one of the greatest challenges currently facing the development of new gas turbine systems for propulsion and power generation. Traditional gas turbine designs have bypassed the problem of combustion instability by designing non-premixed combustors around a fixed operating point. Increasing trends toward lower emissions and greater fuel flexibility have placed more emphasis on developing lean-premixed combustors that are stable over a range of operating conditions. This thesis explores two aspects of combustion instability in the context of a swirl-stabilized, lean-premixed combustor: the role of the major coherent flow structures, and the potential for using secondary air injection to passively suppress combustion instability. Microjets inject air into the combustion chamber in the flame anchoring zone. These microjet injectors attempt to modify the flow field so as to break the feedback mechanism between the chamber acoustics and the heat release rate. Eight microjet injector configurations are studied. Flow is injected axially into the outer recirculation zone or radially into the inner recirculation zone. The injectors inject air with either no swirl, the same swirl direction as the main air flow, or the opposite swirl direction as the main air flow. Chamber acoustics are measured using sensitive microphones. The flame and flow field are interrogated using high-speed imaging and stereoscopic particle image velocimetry. The bulk of this work was conducted for lean propane/air flames, slightly above the lean blowoff limit. Two modes of instability were examined: the 1/4 wave mode at 40 Hz, and the 3/4 wave mode at 105 Hz. Without microjet injection, the combustor transitions directly from the 1/4 wave mode instability to the 3/4 wave mode instability as the equivalence ratio is increased above 0.58. Counter-swirling radial microjets injecting air into the inner recirculation zone increased the lower limit of the 3/4 wave mode to an equivalence ratio of 0.62 and reduced the amplitude of the 1/4 wave mode, effectively creating a stable operating regime for equivalence ratios between the two modes. Microjet injector tests indicate that the inner recirculation zone has a dominant role in the dynamic stabilization of the flame. This observation is confirmed by stereoscopic PIV measurements that reveal periodic formation and collapse of the vortex breakdown bubble in the 3/4 wave mode and vortex shedding in the inner recirculation zone in the 1/4 wave mode.

Download or read book Unsteady Combustion written by F. Culick and published by Springer Science & Business Media. This book was released on 2012-12-06 with total page 560 pages. Available in PDF, EPUB and Kindle. Book excerpt: This book contains selected papers prepared for the NATO Advanced Study Institute on "Unsteady Combustion", which was held in Praia da Granja, Portugal, 6-17 September 1993. Approximately 100 delegates from 14 countries attended. The Institute was the most recent in a series beginning with "Instrumentation for Combustion and Flow in Engines", held in Vimeiro, Portugal 1987 and followed by "Combusting Flow Diagnostics" conducted in Montechoro, Portugal in 1990. Together, these three Institutes have covered a wide range of experimental and theoretical topics arising in the research and development of combustion systems with particular emphasis on gas-turbine combustors and internal combustion engines. The emphasis has evolved roughly from instrumentation and experimental techniques to the mixture of experiment, theory and computational work covered in the present volume. As the title of this book implies, the chief aim of this Institute was to provide a broad sampling of problems arising with time-dependent behaviour in combustors. In fact, of course, that intention encompasses practically all possibilities, for "steady" combustion hardly exists if one looks sufficiently closely at the processes in a combustion chamber. The point really is that, apart from the excellent paper by Bahr (Chapter 10) discussing the technology of combustors for aircraft gas turbines, little attention is directed to matters of steady performance. The volume is divided into three parts devoted to the subjects of combustion-induced oscillations; combustion in internal combustion engines; and experimental techniques and modelling.

Download or read book Impact of Fuel and Oxidizer Composition on Premixed Flame Stabilization in Turbulent Swirling Flows written by Soufien Taamallah and published by . This book was released on 2016 with total page 214 pages. Available in PDF, EPUB and Kindle. Book excerpt: The world relies on fossil fuels as its main energy source (86.7% in 1973, 81.7% in 2012). Several factors including the abundance of resources and the existing infrastructure suggest that this is likely to continue in the near future (potentially 75% in 2040). Meanwhile climate change continues to be a pressing concern that calls for the development of low CO2 energy systems. Among the most promising approaches are pre-combustion capture technologies, e.g., coal gasification and natural gas reforming that produce hydrogen-rich fuels. Another approach is oxy-combustion in which air is replaced by a mixture of O2/CO2/H2O as the oxidizer stream. However, modern gas turbines have been optimized to operate on methane-air combustion and several challenges, notably thermo-acoustic instability, arise when using other fuels or oxidizers because of their different thermochemical and transport properties. While these phenomena constitute a major challenge under conventional operations, using hydrogen-rich fuels or CO2-rich oxidizer exacerbates the problem by modifying the combustor stability map in ways that are not well understood. In this thesis, we identify combustion modes most prone to dynamics, predict the onset of thermo-acoustic instability over a wide range of fuel and oxidizer compositions, and define parameters that can scale the data. To this end, a combination of experimental and numerical tools were deployed. We carried out a series of experiments in an optically accessible laboratory-scale swirl-stabilized combustor typical of those found in modern gas turbines, using high-speed chemiluminescence to examine the flame macrostructure; high-speed Particle Image Velocimetry and OH Planar Laser Induced Fluorescence to probe the flow and flame microstructure. Numerical simulations were used to complement experiments and examine the complex three-dimensional two-way interaction between the flame and the turbulent swirling flow. Experimental data were used to construct the stability maps for different CH4-H2 mixtures and analyze the dynamic flame macrostructures and their transitions. A comparison with acoustically uncoupled combustion shows that the onset of thermo-acoustic instability is concomitant with a specific transition associated with the intermittent appearance of the flame in the outer recirculation zone (ORZ) and stabilization along the outer shear layer (forming between the swirling jet and the ORZ, as revealed by the PIV-PLIF data). The sudden onset of large amplitude limit cycle oscillations and the observed hysteresis suggest the existence of a sub-critical Hopf bifurcation typically characterized by a bistable or "triggering" zone; the flame intermittency in the ORZ can potentially provide the disturbance required to trigger these oscillations. Using a dual-camera method to track chemiluminescence in space and time, this flame transition was found to originate from a reacting kernel that detaches from the inner shear layer flame (forming between the jet and the vortex breakdown zone), reaching the ORZ and spinning at a specific frequency; its characteristic Strouhal number is independent of the Reynolds number and the fuel/oxidizer, only a function of the swirl strength. We propose a new Karlovitz number based criterion that defines the transition on a flow time - flame time space, the former being the inverse of the spinning frequency and the latter being the flame extinction strain rate. According to this scaling, the flame survives in the ORZ if and when it can overcome the region's bulk strain rate. This criterion is valid over a wide range of operating, fuel and oxidizer composition, covering a wide range of fast to slow chemistry scenarios. Given the role of this flame transition in triggering the instability, the same criterion is applicable to predicting the onset of thermo-acoustics. The interaction of the turbulent swirling flow with the flame is further examined using large eddy simulations. Numerical simulations show that the experimentally observed large scale flame structures along the inner shear layer are due to a helical vortex core that originates at the swirler's centerbody. This vortical structure stays aligned with the centerline in the combustor upstream section, but bends and reaches the inner shear layer-stabilized flame around the sudden expansion where it causes the flame wrinkling. We propose that the flame kernel igniting the ORZ/ OSL observed in the experiment may be related to the interaction between the helical vortical structure and the outer shear layer.

Download or read book International Aerospace Abstracts written by and published by . This book was released on 1999 with total page 934 pages. Available in PDF, EPUB and Kindle. Book excerpt: