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![Optical setup for laser-induced incandescence measurements. 1: Nd:YAG laser; 2: half-wave plate combined with polarization filter; 3: mirror; 4: pinhole; 5: cylindrical lens (f=-80\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$f = - 80$$\end{document} mm); 6: vertical knife edge; 7: spherical lens (f=500\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$f = 500$$\end{document} mm); 8: horizontal knife edge; 9: cylindrical lens (f=100\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$f = 100$$\end{document} mm); 10: counterflow burner; 11: beam splitter; 12: beam profiler; 13: band pass filter (400 nm, FWHM 5 nm); 14: ICCD camera; 15: beam dump](publication/328344685/figure/fig8/AS:960077607497734@1605911747122/Optical-setup-for-laser-induced-incandescence-measurements-1-NdYAG-laser-2-half-wave.png)
Optical setup for laser-induced incandescence measurements. 1: Nd:YAG laser; 2: half-wave plate combined with polarization filter; 3: mirror; 4: pinhole; 5: cylindrical lens (f=-80\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$f = - 80$$\end{document} mm); 6: vertical knife edge; 7: spherical lens (f=500\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$f = 500$$\end{document} mm); 8: horizontal knife edge; 9: cylindrical lens (f=100\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$f = 100$$\end{document} mm); 10: counterflow burner; 11: beam splitter; 12: beam profiler; 13: band pass filter (400 nm, FWHM 5 nm); 14: ICCD camera; 15: beam dump
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Beam steering is often encountered in laser diagnostic measurements, especially in flame environments, due to changes in refractive index caused by thermal and species gradients. It can negatively affect the accuracy of the results. In this work, the effects of beam steering on laser-induced incandescence (LII) measurements of pre-vaporized-liquid...
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... Taking into account the impact of beam steering, the LII technique may offer more advantages in soot diagnostics of laminar flames compared to turbulent flames. It's worth noting that even when applied to laminar flames, the LII technique can be affected by beam steering, with more pronounced and unstable steering away from the flame region (Kruse et al. 2018). Several potential correction methods have been explored, including Rayleigh imaging and ray tracing, which have been demonstrated in non-sooting flames (Kaiser, Frank, and Long 2005). ...
Laser-Induced Incandescence (LII) has been widely acknowledged as a highly effective method for measuring soot in co-flow laminar flames. Conversely, quantitative assessments of soot volume fraction (SVF) are achievable through the Laser Extinction Method (LEM). This comprehensive review centers on the optical systems and experimental parameter configurations employed in LII and LEM measurements. Subsequently, several pivotal parameters specific to LII and LEM, such as laser fluence and detection wavelength, are individually discussed. Furthermore, this review showcases current experimental arrangements utilized for LEM measurements. In addition, recommendations are provided for the execution of planar LII and LEM measurements. Lastly, a summarized analysis and discussion pertain to an optical system capable of jointly measuring LII and LEM. ARTICLE HISTORY
... These asymmetries can be mistakenly attributed to the laser, therefore, to minimize these discrepancies the half of the flame first exposed to the laser sheet is investigated. As the laser sheet passes through the flame beam steering effects become more pronounced especially in highly sooting flames such as the ones studied here [74,141,142]. Beam steering affects the laser fluence, and is more pronounced with a Gaussian laser profile as is used here and can lead to variations of 30% from incident laser fluences as was shown by Zerbs et al. and can lead to asymmetries in an otherwise symmetric flame [141]. Therefore, limiting investigations to the one half of the flame closest to the incident laser sheet, and using a high enough fluence combats this effect in both the gaseous and liquid fuels investigated. ...
Soot are microscopic airborne particles that impact human health and global climate, and are released into the atmosphere as a result of incomplete combustion. Reducing soot formation from burners improves both their efficiency and emissions standards. Modern burners utilize the Rich-Quench-Lean (RQL) technology that employs turbulent swirling flames with downstream dilution air. These turbulent flames exhibit spatial and temporal fluctuations not present in laminar flames. This adds a level of complexity to turbulent soot studies that is yet to be fully understood. In this study, a comprehensive analysis of soot formation in swirl-stabilized non-premixed turbulent flames at atmospheric pressure is performed using both Ethylene and an alternative Gevo alcohol-to-jet kerosene fuel referred to as Jet C-1. The goal is to extend the understanding presently available in the literature on dilution air by investigating the impact of systematically varying the amount and location of downstream radially injected dilution air on soot formation and oxidation for the first time. Firstly, Cold flow PIV was used to study the flow field and data revealed an increase in velocities and turbulence within the central recirculation zone (CRZ) as a result of dilution air. The impact of the dilution jets shifts downstream leaving the CRZ and shear layer unaltered as they are moved farther downstream, making them less effective, even if the dilution momentum through them is increased. Next, OH-PLIF, PAH-PLIF and soot LII experiments were used to study the Ethylene flames. Past work has looked at dilution jets positioned at a fixed downstream location. In this thesis, the location and amount of dilution air were systematically varied. Results revealed a strong correlation between OH, PAH and soot distributions, which are affected by the location of the dilution jets relative to the CRZ, where soot primarily forms. Dilution jets positioned up to two bluff body diameters from the burner inlet have a greater influence on residence times, mixing, and OH dispersion within the CRZ, and thus have a higher impact on soot reduction. Sampling experiments were performed for the first time on an RQL type burner to link in situ and ex situ diagnostics and measure soot concentrations at the exhaust of the burner. Particle size distributions (PaSDs) revealed that all the Ethylene flames produced a higher concentration of small particles under 4 nm. PaSDs also revealed that the flames that are not visibly sooty still produced soot volume fractions in the ppb range at the exhaust of the burner. A combination of sampling, extinction and LII measurements showed that while dilution is effective at reducing soot formation within the burner, LII data on its own misrepresents the degree of efficacy of dilution air as smaller particles fall outside the detection range of this technique. Finally, to mimic spray patterns in industrial applications, novel investigations into the influence of dilution air on a liquid swirling spray flame were performed. OH* visualization, Mie scattering and LII of soot experiments on Jet C-1 swirling spray flames showed that soot forms on either side of the spray cone. Understanding these spray-flame interactions is of the utmost importance and are dynamics not captured in gaseous swirling flames. In these flames, dilution air did not always reduce soot concentrations, which highlights the importance of choosing the correct position and amount of dilution air relative to the primary combustion zone. In all the tested Jet C-1 flames soot concentrations were in the low ppt range and were lower than in the Ethylene flames. This alternative kerosene fuel may prove to be a viable low soot alternative to the conventional jet fuels available today.
... The dependence of the LII signal on the local laser fluence makes LII measurements particularly susceptible for beam steering that is caused by local changes in the refractive index due to density gradients [42,43] . Therefore, beam steering is substantial in counterflow flames where large gradients in the gas density along the burner centerline are inherently present [44] . As the level of beam steering is fuel dependent [44] , fluence curves for each fuel were generated. ...
... Therefore, beam steering is substantial in counterflow flames where large gradients in the gas density along the burner centerline are inherently present [44] . As the level of beam steering is fuel dependent [44] , fluence curves for each fuel were generated. For ethylene and toluene, the LII signal peak is achieved for similar laser fluences, whilst the maximum signal intensity is shifted towards higher laser fluences for iso-octane and n-heptane. ...
... The shift of the fluence curve peak is attributed to the superimposing effects of beam steering and light absorption that occur to different levels for the fuels. For each fuel, measurements were performed with the fluence that yields the LII signal peak as this particular fluence balances the effect of sublimation, beam steering, and absorption [44] . In a detailed study on the impact of beam steering in counterflow flames, it was observed that even with correction, beam steering introduces an uncertainty in the soot volume fraction, f v , of ± 10% [44] . ...
Soot formation is experimentally and numerically investigated in laminar counterflow diffusion flames burning ethylene and three typical gasoline surrogate components; n-heptane, iso-octane, and toluene. Laser-induced incandescence and a light extinction technique are employed to determine the soot volume fraction within the well-controlled region of the burner. The experiments are performed across a wide range of strain rates and stoichiometric mixture fractions. From the experimental data, sensitivities of soot formation on strain rate and stoichiometric mixture fraction are derived for each fuel. The fuels show significantly different sensitivities. For iso-octane and n-heptane, a higher sensitivity of soot production on the strain rate is observed as compared to ethylene and toluene. Moreover, the sensitivities of soot formation on the strain rate increase with increasing stoichiometric mixture fraction. One-dimensional simulations of the flames investigated experimentally were performed using two different detailed chemical kinetic mechanisms, detailed chemical soot models, and the hybrid method of moments as well as a discrete sectional method to describe soot dynamics. The models are capable of predicting the soot volume fraction of the ethylene flames with remarkable accuracy, whereas for the gasoline surrogate components, the overall soot volume fractions are overpredicted for all tested models. In iso-octane flames, soot nucleation and PAH condensation rates are particularly enhanced. A reaction pathway analysis shows that in ethylene flames, the formation of benzene mostly originates from acetylene, while for iso-octane, large amounts of iso-butenyl form propyne, propargyl, and then benzene.
... These profiles may be approximated as Gaussian, triangular, or "top-hats" (Gordon et al., 2008;, but are often significantly more complex due to imperfections in mirrors and sheet-forming-optics, and vary between individual laser pulses (i.e., shot-to-shot). Variations in beam profile may also be caused by variations in refractive index, inherent in flames, which result in refraction of the beam termed "beamsteering" (Kruse et al., 2018). As such, beam profiles may also be measured for each individual shot. ...
There is a wealth of existing experimental data of flames collected using laser diagnostics. The primary objective of this review is to provide context and guidance in interpreting these laser diagnostic data. This educational piece is intended to benefit those new to laser diagnostics or with specialization in other facets of combustion science, such as computational modeling. This review focuses on laser-diagnostics in the context of the commonly used canonical jet-in-hot-coflow (JHC) burner, although the content is applicable to a wide variety of configurations including, but not restricted to, simple jet, bluff body, swirling and stratified flames. The JHC burner configuration has been used for fundamental studies of moderate or intense low oxygen dilution (MILD) combustion, autoignition and flame stabilization in hot environments. These environments emulate sequential combustion or exhaust gas recirculation. The JHC configuration has been applied in several burners for parametric studies of MILD combustion, flame reaction zone structure, behavior of fuels covering a significant range of chemical complexity, and the collection of data for numerical model validation. Studies of unconfined JHC burners using gaseous fuels have employed point-based Rayleigh-Raman or two-dimensional Rayleigh scattering measurements for the temperature field. While the former also provides simultaneous measurements of major species concentrations, the latter has often been used in conjunction with planar laser-induced fluorescence (PLIF) to simultaneously provide quantitative or qualitative measurements of radical and intermediary species. These established scattering-based thermography techniques are not, however, effective in droplet or particle laden flows, or in confined burners with significant background scattering. Techniques including coherent anti-Stokes Raman scattering (CARS) and non-linear excitation regime two-line atomic fluorescence (NTLAF) have, however, been successfully demonstrated in both sooting and spray flames. This review gives an overview of diagnostics techniques undertaken in canonical burners, with the intention of providing an introduction to laser-based measurements in combustion. The efficacy, applicability and accuracy of the experimental techniques are also discussed, with examples from studies of flames in JHC burners. Finally, current and future directions for studies of flames using the JHC configuration including spray flames and studies and elevated pressures are summarized.
... We consider the annealing, polydispersity, background luminosity, and detector time response effects. This is not an exhaustive list; other possible effects include pressure-induced effects (e.g., beam steering [31,32], signal trapping [17], or changes in the dominant cooling mechanism [33]), local gas heating [26,28], and misrepresentation of the effective laser sheet thickness [12]. We note that the effective ISF could also help researchers identify model errors in these instances, providing a more rigorous approach than only considering time-resolved effective temperatures or integrated signals. ...
In many time-resolved laser-induced incandescence (TiRe-LII) experiments, it is common practice to relate the intensity emitted by laser-heated nanoparticles to the detected LII signal through a factor (here called the intensity scaling factor, ISF) that includes the particle volume fraction and other parameters that may not be the focus of the analysis. While, in the absence of evaporation or sublimation, the ISF should theoretically remain constant with respect to time, recent multi-wavelength measurements show that, in reality, it may vary with both time and fluence. We consider four candidate effects that contribute to this behavior: particle annealing; polydispersity in the nanoparticle-size distribution; background luminosity due to emission from nanoparticles in the line-of-sight before and behind the probe volume; and the temporal resolution of the detector. We demonstrate these effects by simulating TiRe-LII data for in-flame soot at atmospheric pressure, using new simplified heat transfer and annealing models. Analysis of experimental signals collected from flame-generated soot at atmospheric pressure reveals trends in the ISF similar to those predicted by simulations. These temporal variations provide important insights that can help to diagnose problems in TiRe-LII experiments and improve TiRe-LII models.
Spontaneous Raman scattering is a conventional in-situ laser-diagnostic method that has been widely used for measurements of temperature and major species. However, utilization of Raman scattering in sooting flames suffers from strong interference including laser-induced fluorescence, laser-induced incandescence, and flame luminosity, which has been a challenge for a long time. This work introduces an easy-to-implement and calibration-free Raman scattering thermometry in sooting flames based on a 355-nm nanosecond-pulsed laser beam. Several strategies were utilized to increase the signal-to-noise ratio and suppress the interference: (1) nanosecond intensified CCD gate width; (2) optimized intensified CCD gate delay; (3) specially designed focused laser beam; (4) ultraviolet polarizer filter. The temperature was obtained by fitting the spectral profile of Stokes-Raman scattering of N 2 molecules without any calibrations. Based on the measured temperature, the mole fraction of major species can be evaluated. This method was applied to measure the temperature and major species profiles in a steady ethylene–air counterflow diffusion flame with a spatial resolution of 1.2 mm × 10.8 mm × 0.13 mm. The experimental results agree well with the simulation results in both sooting and non-sooting regions, demonstrating the feasibility of this method for quantitative diagnostics of temperature and major species in multiphase reacting flows.
Bio-hybrid fuels are a promising solution to accomplish a carbon-neutral and low-emission future for the transportation sector. Two potential candidates are the heterocyclic acetals 1,3-dioxane (C4H8O2) and 1,3-dioxolane (C3H6O2), which can be produced from the combination of biobased feedstocks, carbon dioxide, and renewable electricity. In this work, comprehensive experimental and numerical investigations of 1,3-dioxane and 1,3-dioxolane were performed to support their application in internal combustion engines. Ignition delay times and laminar flame speeds were measured to reveal the combustion chemistry on the macroscale, while speciation measurements in a jet-stirred reactor and ethylene-based counterflow diffusion flames provided insights into combustion chemistry and pollutant formation on the microscale. Comparing the experimental and numerical data using either available or proposed kinetic models revealed that the combustion chemistry and pollutant formation differ substantially between 1,3-dioxane and 1,3-dioxolane, although their molecular structures are similar. For example, 1,3-dioxane showed higher reactivity in the low-temperature regime (500-800 K), while 1,3-dioxolane addition to ethylene increased polycyclic aromatic hydrocarbons and soot formation in high-temperature (>800 K) counterflow diffusion flames. Reaction pathway analyses were performed to examine and explain the differences between these two bio-hybrid fuels, which originate from the chemical bond dissociation energies in their molecular structures.
Soot formation of n-heptane is experimentally and numerically studied in low-strain rate (K = 50 s⁻¹) counterflow soot formation (SF) flames. Flame temperatures and soot volume fractions at different oxygen mole fractions on the oxidizer stream (xO2=0.3–0.45) are measured by using thermocouple-calibrated OH two colors laser-induced fluorescence (2C-PLIF) and laser-induced incandescence (LII) calibrated by light extinction methods, respectively. Mono (MAHs) and polycyclic aromatic hydrocarbons (PAHs) are also qualitatively measured by using the PAHs-LIF technique. Good agreement between measured and predicted maximum flame temperature (Tmax) and peak soot volume fraction (fv,peak) is obtained. However, discrepancies in the shape of the entire soot volume fraction profiles along the axial centerline are observed between 1D and 2D simulations. This result is found to be primarily related to the thermophoretic and radial effects in the low strain rate flames investigated, which hamper neglecting the underlying 2D effects. MAH/PAH peak mole fractions and fv,peak increase from xO2=0.3 to xO2=0.45 due to the related increase of flame temperature, which fosters n-heptane pyrolysis near the fuel nozzle leading to higher concentrations of C1C4 hydrocarbons. The latter are found to grow to MAHs/PAHs and finally to soot particles through analogous pathways independently of xO2. However, the higher flame temperature in the sooting region at xO2=0.45 leads to soot particles and aggregates characterized by larger sizes and more dehydrogenated than those produced at xO2=0.3.
Bio-hybrid fuels combine the utilization of bio-based feedstocks and carbon dioxide with renewable electricity to achieve a carbon-neutral and low-emissions future for the transportation sector. A potential candidate is the heterocyclic acetal 1,3-dioxolane (C3H6O2). However, little is known about the mechanism behind the impact of 1,3-dioxolane on soot formation. In this study, we investigated ethylene counterflow diffusion flames with 1,3-dioxolane addition in different amounts. The equilibrium temperature, the stoichiometric mixture fraction, and the strain rate were kept nearly constant to highlight the chemical effect of 1,3-dioxolane addition. Measurements of soot volume fraction with laser-induced incandescence showed that the addition of 1,3-dioxolane up to 30 % leads to a synergistic effect on soot formation with a maximum at 10 %. To reveal the chemistry behind the synergistic effect and its attenuation, speciation measurements were performed on three representative flames (0 %, 10 %, and 30 % fuel mole fraction of 1,3-dioxolane) with gas chromatography-mass spectrometry. The experimental data were compared to kinetic model simulations for interpretation. It was found that the addition of 1,3-dioxolane enhances methyl radical formation, which promotes C3 species formation via C1+C2 pathways. As a consequence, C3 species increase monotonically, whereas some C4 species show a non-monotonic behavior. Pathway analyses revealed that benzene dominantly forms via the C3 route. The main soot precursor naphthalene primarily forms via C4+2C3 pathways through C7 species, and benzene-based pathways, i.e., C6+C4 and H-abstraction-C2H2-addition (HACA), play a minor role. The observed synergistic effect on naphthalene formation is promoted by C3 species for 1,3-dioxolane addition in small amounts. At higher fuel mole fractions of 1,3-dioxolane, C4 species become the bottleneck in naphthalene formation and the synergistic effect subsides. Naphthalene passes this behavior mainly via the HACA pathway to larger polycyclic aromatic hydrocarbons and subsequently to soot.
