Literatura científica selecionada sobre o tema "Light scattering. particle image velocimetry. OH* chemiluminescence"

Since lean premixed combustion allows for fuel-efficiency and low emissions, it is nowadays state of the art in stationary gas turbines. In the long term, it is also a promising approach for aero engines, when safety issues like lean blowout (LBO) and flame flashback in the premixer can be overcome. While for the use of hydrogen the LBO limits are extended, the flashback propensity is increased. Thus, axial air injection is applied in order to eliminate flashback in a swirl-stabilized combustor burning premixed hydrogen. Axial injection constitutes a nonswirling jet on the central axis of the radial swirl generator which influences the vortex breakdown (VB) position. In the present work, changes in the flow field and their impact on flashback limits of a model combustor are evaluated. First, a parametric study is conducted under isothermal test conditions in a water tunnel employing particle image velocimetry (PIV). The varied parameters are the amount of axially injected air and swirl number. Subsequently, flashback safety is evaluated in the presence of axial air injection in an atmospheric combustor test rig and a stability map is recorded. The flame structure is measured using high-speed OH* chemiluminescence imaging. Simultaneous high-speed PIV measurements of the reacting flow provide insight in the time-resolved reacting flow field and indicate the flame location by evaluating the Mie scattering of the raw PIV images by means of the qualitative light sheet (QLS) technique. The isothermal tests identify the potential of axial air injection to overcome the axial velocity deficits at the nozzle outlet, which is considered crucial in order to provide flashback safety. This effect of axial air injection is shown to prevail in the presence of a flame. Generally, flashback safety is shown to benefit from an elevated amount of axial air injection and a lower swirl number. Note that the latter also leads to increased NOx emissions, while axial air injection does not. Additionally, fuel momentum is indicated to positively influence flashback resistance, although based on a different mechanism, an explanation of which is suggested. In summary, flashback-proof operation of the burner with a high amount of axial air injection is achieved on the whole operating range of the test rig at inlet temperatures of 620 K and up to stoichiometric conditions while maintaining single digit NOx emissions below a flame temperature of 2000 K.

Sooting ethylene/air flames were investigated experimentally in a dual swirl gas turbine model combustor with good optical access at atmospheric pressure. The goals of the investigations were a detailed characterization of the soot formation and oxidation processes under gas turbine relevant conditions and the establishment of a data base for the validation of numerical combustion simulations. The flow field was measured by stereoscopic particle image velocimetry, the soot volume fractions by laser-induced incandescence, the heat release by OH chemiluminescence imaging and the temperatures by coherent anti-Stokes Raman scattering. Two flames are compared: a fuel-rich partially premixed flame with moderate soot concentrations and a second one with the same parameters but additional injection of secondary air. Instantaneous as well as average distributions of the measured quantities are presented and discussed. The measured soot distributions exhibit a high temporal and spatial dynamic. This behavior correlates with broad temperature probability density functions. With injection of secondary air downstream of the flame zone the distributions change drastically. The data set, including PDFs of soot concentration, temperature and flow velocity, is unique in combining different laser diagnostics with a combustor exhibiting a more challenging geometry than existing validation experiments.

AbstractA gas turbine model combustor (Turbomeca Burner) for premixed methane/air flames has been operated at atmospheric pressure in two different modes of premixing. In the partially premixed mode, fuel was injected into the air flow within the swirl generator shortly upstream of the combustion chamber while in the perfectly premixed mode fuel and air were mixed far upstream. The main objective of this work is the study of the influence of the mode of premixing on the combustion behavior. Stereoscopic particle image velocimetry has been applied for the measurement of the flow field, OH chemiluminescence imaging for the visualization of the flame shapes and single-shot laser Raman scattering for the determination of the joint probability density functions of major species concentrations, mixture fraction and temperature. The mixing and reaction progress and effects of turbulence-chemistry interactions are characterized by scatterplots showing the correlations between different quantities. To isolate effects of mixing from combustion instabilities that were frequently observed in this combustor, operating conditions without thermo-acoustic oscillations or coherent flow structures were chosen. While the mode of premixing had no major influence on the general flame behavior characteristic differences were observed with respect to flame anchoring, the flow field in the inner recirculation zone and the CO concentration level. The results further extend the data base of previous experimental and numerical investigations with this burner.

An experimental analysis of confined premixed turbulent methane/air and hydrogen/air jet flames is presented. A generic lab scale burner for high-velocity preheated jets equipped with an optical combustion chamber was designed and set up. The size and operating conditions were configured to enable flame stabilization by recirculation of hot combustion products. The geometry of the rectangular confinement and an off-center positioning of the jet nozzle were chosen to resemble one burner nozzle of a FLOX®-based combustor. The off-center jet arrangement caused the formation of a pronounced lateral recirculation zone similar to the one in previously investigated FLOX®-combustors (Lückerath et al., 2007. “FLOX® Combustion at High Pressure with Different Fuel Compositions,” ASME J. Eng. Gas Turbines Power, 130(1), pp. 011505; Lammel et al., 2010. “FLOX® Combustion at High Power Density and High Flame Temperatures,” ASME J. Eng. Gas Turbines Power, 132(12), p. 121503ff). The analysis was accomplished by different laser measurement techniques. Flame structures were visualized by OH* chemiluminescence imaging and planar laser-induced fluorescence of the OH radical. Laser Raman scattering was used to determine concentrations of the major species and the temperature. Velocity fields were measured with particle image velocimetry. Results of measurements in two confined jet flames are shown. The mixing of fresh gas with recirculating combustion products and the stabilization of the methane flame are discussed in detail. The presented findings deliver important information for the understanding of confined jet flames operated with different fuels. The obtained data sets can be used for the validation of numerical simulations as well.

L'augmentation du trafic aérien pousse la recherche sur la combustion à obtenir une compréhension détaillée des processus physiques et chimiques qui se produisent dans le moteur de l'avion ; les principaux objectifs sont 1) d'améliorer le processus de combustion et 2) de réduire les émissions gazeuses et particulaires. Une solution au premier problème réside dans la combustion à une pression et une température plus élevées, mais cela peut avoir un impact sur le second problème, notamment en ce qui concerne la production de suie et les émissions de NOx. Pour trouver une solution, il est nécessaire de développer des outils expérimentaux appliqués dans des conditions de combustion représentatives de celles rencontrées dans les moteurs d'avion afin de capturer les phénomènes complexes se produisant à l'intérieur de ces moteurs. Pour ce faire, un banc d'essai aéronautique semi-industriel équipé de diagnostics optiques adaptés mis en œuvre/disponible à l'ONERA Palaiseau permet d'entrevoir des informations sur ces processus de combustion. En conséquence, cette recherche vise à développer et à mettre en œuvre des techniques optiques pour caractériser les particules de suie dans les chambres de combustion des moteurs aéronautiques. L'incandescence induite par laser (LII) est la principale technique sur laquelle portent les efforts. Cette technique est basée sur le modèle de rayonnement de la loi de Planck. La technique LII est utilisée en raison de sa grande sensibilité pour la détection de la fraction volumique de la suie et de sa flexibilité pour sa mise en œuvre dans diverses configurations optiques. En outre, le couplage de la technique LII avec d'autres techniques optiques présente un grand intérêt pour la compréhension des mécanismes et des paramètres conduisant à la formation de la suie. Premièrement, la fluorescence induite par laser (LIF) est ajoutée comme technique complémentaire pour détecter les précurseurs de suie connus sous le nom d'hydrocarbures aromatiques polycycliques (HAP). Deuxièmement, la diffusion de la lumière fournit des informations sur la distribution des gouttelettes de combustible liquide imbrûlées et des particules de suie à l'intérieur de la chambre de combustion. Troisièmement, la chimiluminescence OH* montre la zone de réaction et le dégagement de chaleur. Enfin, le transport des HAPs et de la suie ou les interactions avec le champ d'écoulement sont abordés via la vélocimétrie par image de particules (PIV)
The growth of air traffic urges combustion research to get a detailed understanding of the physical and chemical processes occurring in the aircraft engine; the main objectives are 1) to improve the combustion process and 2) to lower gaseous and particulate emissions. A solution to the first issue lies in the combustion at higher pressure and temperature, but this can impact the second issue, particularly concerning the production of soot and NOx emissions. To find a solution, it is necessary to develop experimental tools applied in representative combustion conditions relative to those encountered in aircraft engines in order to capture complex phenomena occurring inside these engines. To do this, a semi-industrial aeronautical test rig equipped with suitable optical diagnostics implemented/available at ONERA Palaiseau offers a glimpse of information into these combustion processes. As a result, this research aims to develop and implement optical techniques for characterizing soot particles in aeronautical engine-relevant combustors. Laser-Induced Incandescence (LII) is the primary technique on which efforts are directed. This technique is based on Planck's law radiation model. LII technique is used due to its high sensitivity for detecting the soot volume fraction and flexibility for its implementation in various optical configurations. In addition, the coupling of LII with other optical techniques presents a high interest in understanding the mechanisms and parameters leading to soot formation. First, Laser-Induced Fluorescence (LIF) is added as a complementary technique to detect soot precursors known as Polycyclic-Aromatic Hydrocarbons (PAHs). Secondly, light scattering provides information on the distribution of unburnt liquid fuel droplets and possible soot particle localization inside the combustor. Thirdly, OH* chemiluminescence illustrates the reaction zone and heat release. Finally, the PAHs and soot transportation or interactions with the flow field are tackled via particle image velocimetry (PIV)

Since lean premixed combustion allows for fuel-efficiency and low emissions, it is nowadays state of the art in stationary gas turbines. In the long term, it is also a promising approach for aero engines, when safety issues like lean blowout (LBO) and flame flashback in the premixer can be overcome. While for the use of hydrogen the LBO limits are extended, the flashback propensity is increased. Thus, axial air injection is applied in order to eliminate flashback in a swirl-stabilized combustor burning premixed hydrogen. Axial injection constitutes a non-swirling jet on the central axis of the radial swirl generator which influences the vortex breakdown position. In the present work changes in the flow field and their impact on flashback limits of a model combustor are evaluated. First, a parametric study is conducted under isothermal test conditions in a water tunnel employing particle image velocimetry (PIV). The varied parameters are the amount of axially injected air and swirl number. Subsequently, flashback safety is evaluated in the presence of axial air injection in an atmospheric combustor test rig and a stability map is recorded. The flame structure is measured using high-speed OH* chemiluminescence imaging. Simultaneous high-speed PIV measurements of the reacting flow provide insight in the time-resolved reacting flow field and indicate the flame location by evaluating the Mie scattering of the raw PIV images by the means of the Qualitative Light Sheet (QLS) technique. The isothermal tests identify the potential of axial air injection to overcome the axial velocity deficits at the nozzle outlet, which is considered crucial in order to provide flashback safety. This effect of axial air injection is shown to prevail in the presence of a flame. Generally, flashback safety is shown to benefit from an elevated amount of axial air injection and a lower swirl number. Note, that the latter also leads to increased NOx emissions, while axial air injection does not. Additionally, fuel momentum is indicated to positively influence flashback resistance, although based on a different mechanism, an explanation of which is suggested. In summary, flashback-proof operation of the burner with a high amount of axial air injection is achieved on the whole operating range of the test rig at inlet temperatures of 620 K and up to stoichiometric conditions while maintaining single digit NOx emissions below a flame temperature of 2000 K.

An experimental analysis of confined premixed turbulent methane/air and hydrogen/air jet flames is presented. A generic lab scale burner for high-velocity preheated jets equipped with an optical combustion chamber was designed and set up. The size and operating conditions were configured to enable flame stabilization by recirculation of hot combustion products. The geometry of the rectangular confinement and an off-center positioning of the jet nozzle were chosen to resemble one burner nozzle of a FLOX®-based combustor. The off-center jet arrangement caused the formation of a pronounced lateral recirculation zone similar to the one in previously investigated FLOX®-combustors [1, 2]. The analysis was accomplished by different laser measurement techniques. Flame structures were visualized by OH* chemiluminescence imaging and planar laser-induced fluorescence of the OH radical. Laser Raman scattering was used to determine concentrations of the major species and the temperature. Velocity fields were measured with particle image velocimetry. Results of measurements in two confined jet flames are shown. The mixing of fresh gas with recirculating combustion products and the stabilization of the methane flame are discussed in detail. The presented findings deliver important information for the understanding of confined jet flames operated with different fuels. The obtained data sets can be used for the validation of numerical simulations as well.

Sooting ethylene/air flames were investigated experimentally in a dual swirl gas turbine model combustor with good optical access at atmospheric pressure. The goals of the investigations were a detailed characterization of the soot formation and oxidation processes under gas turbine relevant conditions and the establishment of a data base for the validation of numerical combustion simulations. The flow field was measured by stereoscopic particle image velocimetry, the soot volume fractions by laser-induced incandescence, the heat release by OH chemiluminescence imaging and the temperatures by coherent anti-Stokes Raman scattering. Two flames are compared: a fuel-rich partially premixed flame with moderate soot concentrations and a second one with the same parameters but additional injection of secondary air. Instantaneous as well as average distributions of the measured quantities are presented and discussed. The measured soot distributions exhibit a high temporal and spatial dynamic. This behaviour correlates with broad temperature probability density functions. With injection of secondary air downstream of the flame zone the distributions change drastically. The data set, including PDFs of soot concentration, temperature and flow velocity, is unique in combining different laser diagnostics with a combustor exhibiting a more challenging geometry than existing validation experiments.

The performance of dry, low NOx gas turbines, which employ lean premixed (or partially premixed) combustors, is often limited by static and dynamic combustor stability, power density limitations and expensive premixing hardware. To overcome these issues, a novel design, referred to as the Stagnation Point Reverse Flow (SPRF) combustor, has recently been developed. Various optical diagnostic techniques are employed here to elucidate the combustion processes in this novel combustor. These include simultaneous planar laser-induced fluorescence (PLIF) imaging of OH radicals and chemiluminescence imaging, and separate experiments with particle image velocimetry and elastic laser sheet scattering from liquid particles seeded into the fuel. The SPRF combustor achieves internal exhaust gas recirculation and efficient mixing, which eliminates local peaks in temperature. This results in low NOx emissions, limited by flame zone (prompt) production, for both premixed and non-premixed modes of operation. The flame is anchored in a region of reduced velocity and high turbulent intensities, which promotes mixing of hot products into the reactants, thus enabling stable operation of the combustor even at very lean equivalence ratios. Also, the flame structure and flow characteristics were found to remain invariant at high loadings, i.e., mass flow rates. Combustion in the non-premixed mode of operation is found to be similar to the premixed case, with the OH PLIF measurements indicating that nonpremixed flame burns at an equivalence ratio that is close to the overall combustor equivalence ratio. Similarities in emission levels between premixed and non-premixed modes are thus attributable to efficient fuel-air mixing in the nonpremixed mode, and entrainment of hot products into the reactant stream before burning occurs.

In the present paper a computational analysis of a confined premixed turbulent methane/air jet flame is presented. In this scope, chemistry is reduced by the use of the Flamelet Generated Manifold (FGM) method [1, 2], and the fluid flow is modeled in a RANS context. In the FGM technique the reaction progress of the flame is generally described by a few control variables, for which a transport equation is solved during runtime. The flamelet system is computed in a pre-processing stage, and a manifold with all the information about combustion is stored in a tabulated form. In the present implementation the reaction evolution is described by the reaction progress variable, the heat loss is described by the enthalpy and the turbulence effect on the reaction is represented by the progress variable variance. The turbulence-chemistry interaction is considered through the use of a presumed pdf approach. A generic lab scale burner for high-velocity preheated jets is used for validation [3, 4]. It consists of a rectangular confinement, and an off-center positioning of the jet nozzle enables flame stabilization by recirculation of hot combustion products. The inlet speed is appropriately high, in order to be close to the blow out limit. Flame structures were visualized by OH* chemiluminescence imaging and planar laser-induced fluorescence of the OH radical. Laser Raman scattering was used to determine concentrations of the major species and the temperature. Velocity fields were measured with particle image velocimetry. The important effect of conductive heat loss to the walls is included in the FGM chemistry reduction method in a RANS context, in order to predict the evolution and description of a turbulent jet flame in high Reynolds number flow conditions. Comparisons of various mean fields (velocities, temperatures) with RANS results are shown. The use of FGM as a combustion model shows that combustion features at gas turbine conditions can be satisfactorily reproduced with a reasonable computational effort.

Abstract Future rich-burn/quick-quench/lean-burn (RQL) burners for aero engines face the challenge to further reduce the emission of soot. Alternative ways of fuel injection are therefore in the focus of modern RQL combustion systems. This contribution aims to investigate experimentally the influence of fuel injection on the reacting flow field, with the emphasis on soot production in the primary zone. For the test, a Rolls-Royce prototype burner was used in two different configurations which differ only in the axial location of jet in cross flow fuel injection and thereby provoke different ways of fuel atomization. In the upstream configuration the burner features characteristics of a pre-filming airblast atomizer. Whereas with the fuel tip in downstream position solely Jet-in-Cross-Flow fuel atomisation is expected. The burner was tested at realistic aero engine combustor conditions (p30 = 9.28 bar, T30 = 603 K, AFR = 7.6). Several optical measurement techniques were used to characterise the reacting flow field. Their difficult application in a rich burn environment is described briefly. The structure of the reacting flow field is illustrated by Particle-Image-Velocimetry (PIV). Planar Mie scattering and Planar Laser-Induced Fluorescence (PLIF) are used to characterise the placement of liquid and gaseous fuel respectively. The location and structure of heat release zones are captured in terms of OH* and CO2* chemiluminescence. Finally Laser-Induced-Incandescence (LII) is used to obtain three dimensional soot distributions in the primary zone. On this basis 20% less soot was measured for the upstream configuration at the axial location of maximal soot concentration. This remarkable difference could be attributed to the different placement of liquid fuel and the resulting better mixing.

In modern industrial gas turbines swirling flow is widely used for stabilizing flames at the transition from the burner to the combustor. In premixed combustion systems using highly reactive fuels, flashback due to combustion induced vortex breakdown (CIVB) has been observed frequently when swirl was present. This paper focuses on the effect of low swirl intensities on the flashback propensity and the predominant flashback mechanisms in a hydrogen-air tube burner. An existing test rig with a vertical quartz tube and a generic swirl generator has been used. At the tube exit the flame was stabilized in the free atmosphere. The turbulent flashback limits were measured for hydrogen-air mixtures at atmospheric conditions over a broad range of equivalence ratios for both non-swirling and swirling flow. The upstream flame propagation during flashback was observed through the OH*-chemiluminescence captured by two synchronized intensified high-speed cameras in a 90° arrangement, both looking at the flame from the side. In addition to that, a high-speed particle image velocimetry (PIV) system was used to insert a horizontal laser sheet into the vertical tube in order to investigate the propagation path of the leading flame tip through a time series of Mie-scattering images from the bottom. As expected, it turned out that the flame always flashes back along the wall boundary layer for non-swirling flow. For swirling flow it could be shown that again only boundary layer flashback takes place for equivalence ratios lower than ϕ≈0.75. In this rather lean region, the resistance against flashback is improved compared to non-swirling flow due to higher wall velocity gradients. For higher equivalence ratios, flashback is initiated through CIVB. That is, the flame enters the tube on the burner centerline until its tail gets in touch with the burner walls. Subsequently, there is a shift in flashback mechanism and the flame propagates further upstream along the wall boundary layer. For the given setup and these near-stoichiometric mixture compositions, this resulted in a significantly increased flashback propensity when compared with non-swirling flames. The present studies showed that imposing low swirl upon the burner flow can improve the resistance against boundary layer flashback for low and moderate equivalence ratios, whereas the change to the CIVB mechanism deteriorates the performance for high equivalence ratios.

In this contribution, comprehensive optical and laser based measurements in a generic multi-jet combustor at gas turbine relevant conditions are presented. The flame position and shape, flow field, temperatures and species concentrations of turbulent premixed natural gas and hydrogen flames were investigated in a high-pressure test rig with optical access. The needs of modern highly efficient gas turbine combustion systems, i.e., fuel flexibility, load flexibility with increased part load capability, and high turbine inlet temperatures, have to be addressed by novel or improved burner concepts. One promising design is the enhanced FLOX® burner, which can achieve low pollutant emissions in a very wide range of operating conditions. In principle, this kind of gas turbine combustor consists of several nozzles without swirl, which discharge axial high momentum jets through orifices arranged on a circle. The geometry provides a pronounced inner recirculation zone in the combustion chamber. Flame stabilization takes place in a shear layer around the jet flow, where fresh gas is mixed with hot exhaust gas. Flashback resistance is obtained through the absence of low velocity zones, which favors this concept for multi-fuel applications, e.g. fuels with medium to high hydrogen content. The understanding of flame stabilization mechanisms of jet flames for different fuels is the key to identify and control the main parameters in the design process of combustors based on an enhanced FLOX® burner concept. Both experimental analysis and numerical simulations can contribute and complement each other in this task. They need a detailed and relevant data base, with well-known boundary conditions. For this purpose, a high-pressure burner assembly was designed with a generic 3-nozzle combustor in a rectangular combustion chamber with optical access. The nozzles are linearly arranged in z direction to allow for jet-jet interaction of the middle jet. This line is off-centered in y direction to develop a distinct recirculation zone. This arrangement approximates a sector of a full FLOX® gas turbine burner. The experiments were conducted at a pressure of 8 bar with preheated and premixed natural gas/air and hydrogen/air flows and jet velocities of 120 m/s. For the visualization of the flame, OH* chemiluminescence imaging was performed. 1D laser Raman scattering was applied and evaluated on an average and single shot basis in order to simultaneously and quantitatively determine the major species concentrations, the mixture fraction and the temperature. Flow velocities were measured using particle image velocimetry at different section planes through the combustion chamber.