Letteratura scientifica selezionata sul tema "Planar Laser-induced Incandescence Imaging (PLII)"

The processes of ignition and formation of soot precursor and soot particles in a diesel spray flame achieved in a rapid compression machine (RCM) were imaged two-dimensionally using the laser sheet techniques. For the two-dimensional imaging of time and of location where ignition first occurs in a diesel spray, planar laser-induced fluorescence (PLIF) of formaldehyde was applied to a diesel spray in an RCM. Formaldehyde has been hypothesized to be one of the stable intermediate species marking the start of oxidation reactions in a transient spray under compression ignition conditions. In this study, the laser-induced fluorescence (LIF) images of the formaldehyde formed in a diesel fuel spray during the ignition process have been obtained by exciting formaldehyde with the third harmonic of a neodymium-doped yttrium aluminium garnet (Nd:YAG) laser. The LIF images of formaldehyde in a spray revealed that the time when the first fluorescence is detected is almost identical with the time when the total heat release due to low-temperature oxidation reactions equals the heat absorption by fuel vaporization in the spray. The formaldehyde level rose steadily until the high-temperature reaction phase of diesel spray ignition. At the start of this ‘hot-ignition’ phase, the formaldehyde concentration fell rapidly, thus signalling the end of the low-temperature ignition phase. Increases in the initial ambient gas temperatures advanced the hot-ignition starting time. The first hot ignition occurred in the periphery of spray head at initial ambient gas temperatures between 580 and 660 K. When the ambient gas temperature was increased to 790 K, the position of the first ignition moved to the central region of the spray head. For the investigation of soot formation processes in a diesel spray flame, simultaneous imaging of the soot precursor and soot particles in a transient spray flame in an RCM was conducted by PLIF and by planar laser-induced incandescence (PLII) techniques. The third harmonic (355 nm) and the fundamental (1064 nm) laser pulses from an Nd:YAG laser, between which a delay of 44 ns was imposed by 13.3 m of optical path difference, were used to excite LIF from the soot precursor and laser-induced incandescence (LII) from soot particles in the spray flame. The LIF and the LII were separately imaged by two image-intensified charge-coupled device cameras with identical detection wavelengths of 400 nm and bandwidths of 80 nm. The LIF from the soot precursor was mainly located in the central region of the spray flame between 40 and 55 mm (between 270 and 370 times the nozzle orifice diameter d°) from the nozzle orifice. The LII from soot particles was observed to surround the soot precursor LIF region and to extend downstream. The first appearance of the LIF from the soot precursor in the spray flame preceded the appearance of the LII from soot particles. The intensity of the LIF from the soot precursor reached its maximum immediately after rich premixed combustion. In contrast, the intensity of the LII from soot particles increased gradually and reached its maximum after the end of injection. Measured LIF spectra, of the soot precursor in the spray flame, were very broad with the peak between 430 and 460 nm.

Soot particles are one of the main causes of today's pollution because of their negative contribution to global warming and human health. Aviation is one of the domains dealing with soot reduction as the new engine concepts are developed to reduce fuel consumption and global emissions. Despite the fact that most combustion devices used for air transportation operate at high pressure (e.g., aircraft gas turbines up to 40 bar), our understanding of soot formation and oxidation in such conditions is not yet at an appropriate level, as there is still a fundamental lack of experimental data and corresponding predictive models in the literature. Thus, the objective of the current study is to evaluate soot formation and oxidation processes in stratified, swirled, premixed ethylene/air flames examined with a variety of laser diagnostics designed to simultaneously measure soot particle and soot precursor 2D-distributions, as well as the flame structure and the aerodynamic field. For that, the SIRIUS burner was selected because of its ability to produce flames with topologies similar to those encountered in aircraft combustors. Soot particle distributions are measured by Planar Laser-induced incandescence (PLII) diagnostic. The flame structure is obtained by detecting the hydroxyl radicals (OH) with Planar laser-induced fluorescence (PLIF). A second PLIF diagnostic is also used to investigate the production of polycyclic aromatic hydrocarbons (PAH) with the detection of two benzene rings molecules which are recognized as good precursors of soot nucleation and growth. Finally, the particle Image Velocimetry (PIV) diagnostic is used for measuring the velocity distributions. These laser diagnostics are coupled together in order to obtain cross-correlations between several scalar parameters playing a determining role in the soot formation/consumption processes. The experimental results collected at atmospheric pressure are reviewed and critically assessed. A scenario describing the link between the soot inception, growth, aggregation and oxidation processes is proposed by analyzing velocity, OH, PAHs and soot distributions. In particular, the data reveal the presence of distinct regions for these processes. Incipient soot production zone is strongly function of specific local conditions of velocity, PAH concentration, and strain rate encountered at the interface of the internal recirculation zone and the fuel/air jet. The central part of the inner recirculation zone in which large structures move at low velocities provides suitable conditions for the aggregation of nascent soot particles while an oxidation region located in the upper zone of the internal recirculation zone favors the consumption of soot.

Snapshot multispectral imaging of chemical species in the flame is essential for improved understanding of the combustion process. In this article, we investigate the different configurations of a structured laser sheet-based multispectral imaging approach called the Frequency Recognition Algorithm for Multiple Exposures (FRAME). Using FRAME, a snapshot of Laser-Induced Fluorescence (LIF) of Polycyclic Aromatic Hydrocarbons (PAH) excited by 283.5 nm laser and Laser-Induced Incandescence (LII) of soot particles excited by 532 nm laser are acquired simultaneously on a single FRAME image. A laminar diffusion flame of acetylene produced by a Gülder burner is used for the experiments. The standard FRAME approach is based on creating two spatially modulated laser sheets and arranging them in a cross-patterned configuration (X). However, the effect of using different configurations (angles) of the two laser sheets on the multispectral planar imaging of the flame has not yet been studied. Therefore, we have compared the FRAME approach in four different configurations while keeping the same flame conditions. First, we have compared the relation between laser fluence and LII signals with and without spatial modulation of the 532 nm laser sheet and found that both detections follow the same curve. When comparing the maps of flame species reconstructed from the standard FRAME configuration and other configurations, there are some dissimilarities. These differences are attributed to minor changes in the imaging plane, optical alignment, laser path length, different modulation frequencies of the laser sheet, laser extinction, laser fluence, etc.

Soot formation in the region downstream of the lift-off length of diesel fuel jets was investigated in an optically accessible constant-volume combustion vessel under quiescent-type diesel engine conditions. Planar laser-induced incandescence and line-of-sight laser extinction were used to determine the location of the first soot formation during mixing-controlled combustion. OH chemiluminescence imaging was used to determine the location of high-heat-release reactions relative to the soot-forming region. The primary parameters varied in the experiments were the sooting propensity of the fuel and the amount of fuel-air premixing that occurs upstream of the lift-off length. The fuels considered in order of increasing sooting propensity were: an oxygenated fuel blend (T70), a blend of diesel cetane-number reference fuels (CN80), and a #2 diesel fuel (D2). Fuel-air mixing upstream of the lift-off length was varied by changing ambient gas and injector conditions, which varied either the lift-off length or the air entrainment rate into the fuel jet relative to the fuel injection rate. Results show that soot formation starts at a finite distance downstream of the lift-off length and that the spatial location of soot formation depends on the fuel type and operating conditions. The distance from the lift-off length to the location of the first soot formation increases as the fuel sooting propensity decreases (i.e. in the order D2 < CN80 < T70). At the baseline operating conditions, the most upstream soot formation occurs at the edges of the jet for D2 and CN80, while for T70 the soot formation is confined to the jet central region. When conditions are varied to produce enhanced fuel-air mixing upstream of the lift-off length in D2 fuel jets, the initial soot formation shifts towards the fuel jet centre and eventually no soot is formed. For all experimental conditions, the observed location of soot formation relative to the heat-release location (lift-off) suggests that soot formation occurs in a mixture of combustion products originating from partially premixed reactions and a diffusion flame. The results also imply that soot precursor formation rates depend strongly on fuel type in the region between the lift-off length and the first soot formation.

A late-injection, high exhaust-gas recirculation rate, low-temperature combustion strategy is investigated in a heavy-duty diesel engine using a suite of optical diagnostics: chemiluminescence for visualization of ignition and combustion, laser Mie scattering for liquid-fuel imaging, planar laser-induced fluorescence (PLIF) for both OH and vapor-fuel imagings, and laser-induced incandescence for soot imaging. Fuel is injected at top dead center when the in-cylinder gases are hot and dense. Consequently, the maximum liquid-fuel penetration is 27 mm, which is short enough to avoid wall impingement. The cool flame starts 4.5 crank angle degrees (CAD) after the start of injection (ASI), midway between the injector and bowl rim, and likely helps fuel to vaporize. Within a few CAD, the cool-flame combustion reaches the bowl rim. A large premixed combustion occurs near 9 CAD ASI, close to the bowl rim. Soot is visible shortly afterward, along the walls, typically between two adjacent jets. OH PLIF indicates that premixed combustion first occurs within the jet and then spreads along the bowl rim in a thin layer, surrounding soot pockets at the start of the mixing-controlled combustion phase near 17 CAD ASI. During the mixing-controlled phase, soot is not fully oxidized and is still present near the bowl rim late in the cycle. At the end of combustion near 27 CAD ASI, averaged PLIF images indicate two separate zones. OH PLIF appears near the bowl rim, while broadband PLIF persists late in the cycle near the injector. The most likely source of broadband PLIF is unburned fuel, which indicates that the near-injector region is a potential source of unburned hydrocarbons.

This study shows the in-cylinder soot reduction mechanism associated with injection timing variation in a small-bore optical diesel engine. For the three selected injection timings, three optical-/laser-based imaging diagnostics were performed to show the development of high-temperature reaction and soot within the cylinder, which include OH* chemiluminescence, planar laser–induced fluorescence of hydroxyl and planar laser–induced incandescence. In addition, detailed soot morphology analysis was conducted using thermophoresis-based soot particle sampling from two locations within the piston bowl, and the subsequent analysis of transmission electron microscope (TEM) images of the sampled soot aggregates was also conducted. The results suggest that when fuel injection timing is varied, ambient gas temperature makes a predominant effect on soot formation and oxidation. This is primarily combustion phasing effect as the advanced fuel injection moved the start of combustion closer to the top dead centre, and therefore, soot formation and oxidation occurred at elevated ambient gas temperature. There was an overall development pattern of in-cylinder soot consistently found for three injection timings of this study. The planar laser–induced incandescence images showed that a few small soot pockets first appear around the jet axis, which promptly grow into large soot regions behind the head of the flame marked planar laser–induced fluorescence of hydroxyl. The soot signals disappear due to significant oxidation induced by surrounding OH radicals. When the injection timing is advanced, the soot formation becomes higher as indicated by higher total laser–induced incandescence coverage, increased sampled particle counts and larger and more stretched soot aggregate structures. However, soot oxidation is also enhanced under this elevated ambient temperature environment. At the most advanced injection timing of this study, the enhanced soot oxidation outperformed the increased soot formation with both peak laser–induced incandescence signal coverage and late-cycle coverage showing lower values than those of more retarded injection timings.

AbstractUnburnt hydrocarbon flames produce soot, which is the second biggest contributor to global warming and harmful to human health. The state-of-the-art high-speed imaging techniques, developed to study non-repeatable turbulent flames, are limited to million-frames-per-second imaging rates, falling short in capturing the dynamics of critical species. Unfortunately, these techniques do not provide a complete picture of flame-laser interactions, important for understanding soot formation. Furthermore, thermal effects induced by multiple consecutive pulses modify the optical properties of soot nanoparticles, thus making single-pulse imaging essential. Here, we report single-shot laser-sheet compressed ultrafast photography (LS-CUP) for billion-frames-per-second planar imaging of flame-laser dynamics. We observed laser-induced incandescence, elastic light scattering, and fluorescence of soot precursors - polycyclic aromatic hydrocarbons (PAHs) in real-time using a single nanosecond laser pulse. The spatiotemporal maps of the PAHs emission, soot temperature, primary nanoparticle size, soot aggregate size, and the number of monomers, present strong experimental evidence in support of the theory and modeling of soot inception and growth mechanism in flames. LS-CUP represents a generic and indispensable tool that combines a portfolio of ultrafast combustion diagnostic techniques, covering the entire lifecycle of soot nanoparticles, for probing extremely short-lived (picoseconds to nanoseconds) species in the spatiotemporal domain in non-repeatable turbulent environments. Finally, LS-CUP’s unparalleled capability of ultrafast wide-field temperature imaging in real-time is envisioned to unravel mysteries in modern physics such as hot plasma, sonoluminescence, and nuclear fusion.

Les effets anthropogéniques sur l’environnement posent un défi majeur pour l’industrie aéronautique. Des réglementations de plus en plus strictes et la nécessité de rendre le transport aérien durable orientent les recherches actuelles vers des systèmes propulsifs innovants. Dans ce contexte, Safran Helicopter Engines développe sa technologie brevetée de combustion giratoire (SCT), visant à améliorer les performances des moteurs d’hélicoptères. Déjà implémentée sur le moteur Arrano, cette technologie est davantage optimisée pour réduire significativement les émissions de NOx et de suies. Dans le cadre du programme européen LOOPS, deux nouveaux systèmes d’injection de carburant sont étudiés : l’un conçu pour un régime riche dans une chambre RQL, et l’autre pour une combustion pauvre. Cette thèse évalue expérimentalement ces systèmes à l’aide de diagnostics laser avancés, adaptés aux environnements réactifs à haute pression. Le banc HERON, développé au CORIA, permet d’analyser leurs performances de combustion et évaluer les émissions dans des conditions représentatives des moteurs d’hélicoptères : pressions de 8 à 14 bar, températures d’entrée d’air de 570 à 750 K, et richesses de 0,6 à 1,67. Des diagrammes de stabilité de flamme sont établis, suivis d’analyses des propriétés du spray liquide par PDPA (Phase Doppler Particle Anemometry). Les champs aérodynamiques sont mesurés en conditions réactive et non-réactive par PIV (Particle Imaging Velocimetry) ultra-rapide à 10 kHz. La structure des flammes est caractérisée par PLIF-OH, tandis que la PLIF-kérosène permet d’étudier l’évaporation du carburant en détectant les mono- et di- aromatiques. Les diagnostics couplés simultanément PLIF-NO, PLIF-OH et PLIF-kérosène corrèlent les structures des flammes, les distributions des phases liquide et vapeur, et les zones de formation de NO. De même manière, la PLII (Planar Laser-Induced Incandescence) couplé avec PLIF-OH, PLIF-kérosène permets d’analyser les mécanismes de formation et d’oxydation des suies. Des méthodes spécifiques déterminent des distributions 2D des concentrations de NO, OH et des fractions volumiques de suies. Les résultats montrent une flamme asymétrique pour l’injecteur riche, avec une efficacité de combustion élevé dans la partie supérieure grâce à une injection liquide augmenté localement. Malgré des richesses élevées, les niveaux de suies restent modérés, tandis que le NO se forme principalement près de la flamme, confirmant le mécanisme thermique de Zeldovich. L’injecteur en régime pauvre présente une structure de flamme typique des flammes swirlées stratifiées, malgré la légère asymétrique. Une meilleure évaporation du carburant y favorise une combustion plus efficace, réduisant la longueur de flamme et les NO, grâce à des températures de flamme plus basses. Cependant, des niveaux modérés de suies sont également observés malgré le régime pauvre. Les conditions opératoires influencent fortement les performances. À haute pression, l’atomisation du spray est accélérée, l’angle d’expansion du spray augmente, et les zones de recirculation interne sont renforcées, modifiant la structure des flammes. L’augmentation des émissions de suies par la haute pression est observée pour l’injecteur en régime riche, gardant une richesse constante sur l’ensemble des conditions testées, tandis que les niveaux de NO restent stables. Pour l’injecteur en régime pauvre, les conditions réactives avec une richesse minimale à haute pression atténuent les effets de la pression, stabilisant la production de suies tout en réduisant les concentrations de NO. Ces résultats mettent en évidence le potentiel des deux systèmes d’injection pour optimiser les performances tout en réduisant les émissions des futurs moteurs d’hélicoptères
Anthropogenic effects on the environment present a major challenge for the aeronautical industry. Increasingly stringent pollution regulations and the necessity for sustainable air transport are driving the nowadays research toward innovative propulsion systems. In this context, Safran Helicopter Engines is advancing its patented Spinning Combustion Technology (SCT), aimed at improving helicopter engine performance. Already implemented in the Arrano engine, SCT is now being refined to significantly reduce NOx and soot emissions. As part of the European LOOPS program, two novel fuel injection systems are under investigation: one operating in a rich combustion regime tailored for an RQL combustion chamber and the other designed for lean combustion. The scientific activity of this thesis focuses on the experimental characterization of these injection systems using state-of-the-art laser diagnostics optimized for high-pressure reactive environments. The HERON combustion facility at CORIA enables the analysis of combustion and pollutant performance under conditions representative of helicopter engines, with pressures from 8 to 14 bar, air inlet temperatures from 570 to 750 K, and equivalence ratios ranging from 0.6 to 1.67. Initial flame stability maps are established, followed by in-depth analyses of liquid spray properties using Phase Doppler Particle Anemometry (PDPA). High-speed Particle Imaging Velocimetry (PIV) captures aerodynamic fields under reactive and non-reactive conditions at 10 kHz. Flame structures are examined via OH-PLIF fluorescence imaging, while kerosene-PLIF evaluates liquid and vapor fuel distributions, particularly probing aromatic components in Jet A-1 kerosene. Furthermore, NO-PLIF imaging, combined with OH-PLIF and kerosene-PLIF, enables spatial correlations between flame structure, fuel distribution, and NO production zones. Soot formation and oxidation mechanisms are explored through Planar Laser-Induced Incandescence Imaging (PLII), integrated with OH-PLIF and kerosene-PLIF. Specific methods are developed to obtain 2D distributions of quantitative concentrations of NO, OH and soot volume fraction. Results reveal that the rich-burn injector produces an asymmetrical flame with enhanced upper-zone combustion efficiency due to locally intensified liquid fuel injection. Moderate soot levels are observed despite high equivalence ratios, while localized NO production, primarily near the flame, is attributed to the Zeldovich thermal mechanism. Conversely, the lean-burn injector forms a flame structure characteristic of stratified swirl flames, despite the minor asymmetry. Improved fuel evaporation leads to higher combustion efficiency, shorter flame lengths, and a reduction in NO formation, attributed to lower flame temperatures. In spite of the lean combustion conditions, moderate soot levels are measured for the second injector. Operating conditions strongly influence performance. Higher pressures accelerate spray atomization, increase spray expansion angles, and strengthen internal recirculation zones, reshaping flame structures. The increase in soot production at higher pressure is particularly demonstrated by the rich-burn injector due to constant equivalence ratios across all test conditions, while NO levels remain stable. For the lean-burn injector, leaner operation at elevated pressures moderates pressure effects, maintaining consistent soot levels and reducing NO concentrations. These findings highlight the potential of both injection systems for optimizing performance and reducing emissions in future helicopter engines

Simultaneous measurements of OH planar laser-induced fluorescence (PLIF) and laser-induced incandescence (LII) are used to characterize the flame structure and soot formation process in the reaction zone of a swirl-stabilized, JP-8-fueled model gas-turbine combustor. Studies are performed at atmospheric pressure with heated inlet air and primary-zone equivalence ratios from 0.55 to 1.3. At low equivalence ratios (φ &lt; 0.9), large-scale structures entrain rich pockets of fuel and air deep into the flame layer; at higher equivalence ratios, these pockets grow in size and prominence, escape the OH-oxidation zone, and serve as sites for soot inception. Data are used to visualize soot development as well as to qualitatively track changes in overall soot volume fraction as a function of fuel-air ratio and fuel composition. The utility of the OH-PLIF and LII measurement system for test rig diagnostics is further demonstrated for the study of soot-mitigating additives.

A late injection, high exhaust-gas recirculation (EGR)-rate, low-temperature combustion strategy was investigated in a heavy-duty diesel engine using a suite of optical diagnostics: chemiluminescence for visualization of ignition and combustion, laser Mie scattering for liquid fuel imaging, planar laser-induced fluorescence (PLIF) for both OH and vapor-fuel imaging, and laser-induced incandescence (LII) for soot imaging. Fuel is injected at top dead center when the in-cylinder gases are hot and dense. Consequently, the maximum liquid fuel penetration is 27 mm, which is short enough to avoid wall impingement. The cool flame starts 4.5 crank angle degrees (CAD) after the start of injection (ASI), midway between the injector and bowl-rim, and likely helps fuel to vaporize. Within a few CAD, the cool-flame combustion reaches the bowl-rim. A large premixed combustion occurs near 9 CAD ASI, close to the bowl rim. Soot is visible shortly afterwards along the walls, typically between two adjacent jets at the head vortex location. OH PLIF indicates that premixed combustion first occurs within the jet and then spreads along the bowl rim in a thin layer, surrounding soot pockets at the start of the mixing-controlled combustion phase near 17 CAD ASI. During the mixing-controlled phase, soot is not fully oxidized and is still present near the bowl-rim late in the cycle. At the end of combustion near 27 CAD ASI, averaged PLIF images indicate two separate zones. OH PLIF appears near the bowl rim, while broadband PLIF persists late in the cycle near the injector. The most likely source of broadband PLIF is unburned fuel, which indicates that the near-injector region is a potential source of unburned hydrocarbons.

Filtered Rayleigh Scattering (FRS) is demonstrated in a premixed, sooting ethylene-air flame. In sooting flames, traditional laser-based temperature-imaging techniques such linear (unfiltered) Rayleigh scatting (LRS) and planar laser-induced fluorescence (PLIF) are rendered intractable due to intense elastic scattering interferences from in-flame soot. FRS partially overcomes this limitation by utilizing a molecular iodine filter in conjunction with an injection-seeded Nd:YAG laser, where the seeded laser output is tuned to line center of a strong iodine absorption transition. A significant portion of the Doppler-broadened molecular Rayleigh signal is then passed while intense soot scattering at the laser line is strongly absorbed. In this paper, we demonstrate the feasibility of FRS for sooting flame thermometry using a premixed, ethylene-air flat flame. We present filtered and unfiltered laser light-scattering images, FRS temperature data, and laser-induced incandescence (LII) measurements of soot volume fraction for fuel-air equivalence ratios of φ = 2.19 and 2.24. FRS-measured product temperatures for these flames are nominally 1500 K. The FRS temperature and image data are discussed in the context of the soot LII results and a preliminary estimate of the upper sooting limit for our FRS system of order 0.1 ppm volume fraction is obtained.