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Evaluation of measurement data for laser-induced incandescence (LII) is a complex process, which involves many processing steps starting with import of data in various formats from the oscilloscope, signal processing for converting the raw signals to calibrated signals, application of models for spectroscopy/heat transfer and finally visualization,...
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... The major difficulty with such an approach stems from the implementation of a proper LII model. Although a wide variety of simulation tools have been proposed in the literature (see [12,20,24,51,58,59] and references therein), their predictive capabilities still vary significantly, depending on the nature of the energy fluxes integrated within the energy and mass balance equations accounting for the temporal evolution of the soot temperature and diameter during the laser heating and cooling stages, as well as on the governing equations related to these fluxes. ...
This work represents a first attempt at proposing a refined LII model suitable for simulating signals collected with excitation wavelengths (EWs) of 266, 355, 532 and 1064 nm. In this context, we implemented a comprehensive version of laser-irradiated soot heat and mass balance equations integrating terms representing the saturation of linear, single- and multi-photon absorption processes, cooling by sublimation, conduction, radiation and thermionic emission, as well as mechanisms depicting soot oxidation and annealing, non-thermal photodesorption of carbon clusters and corrective factors accounting for the shielding effect and multiple scattering (MS) within aggregates. This simulation tool was fully parameterized, by coupling design of experiments with a genetic algorithm-based solver, against data collected at different heights above the burner (HAB) in a diesel spray flame. This allowed to assess values of different model parameters involved in absorption and sublimation terms, which, to date, have never been reported in LII modeling studies for UV EWs (e.g., multi-photon absorption cross sections for C 2 photodesorption and saturation coefficients for linear- and multi-photon absorption). The simulation work proposed herein then enabled to infer information regarding the evolution of the absorption function of soot ( E m , λ ). E m , 266 , E(m,355) E ( m , 355 ) and E(m,532) E ( m , 532 ) ranging from 0.25 to 0.51, 0.20 to 0.38 and 0.18 to 0.30, respectively, were notably estimated as a function of the HAB. Alternatively, values ranging from 0.31 to 0.53, 0.26 to 0.44 and 0.22 to 0.38 were assessed while neglecting the effect of MS and provided constant E(m,266) E ( m , 266 ) / E(m,1064) E ( m , 1064 ) , E(m,355) E ( m , 355 ) / E(m,1064) E ( m , 1064 ) and E(m,532) E ( m , 532 ) / E(m,1064) E ( m , 1064 ) ratios of 1.4, 1.2 and 1.0, regardless of the HAB. In addition, the obtained results showed that the wavelength dependence of the soot absorption function was quite negligible for EWs higher than 532 nm, irrespective of whether the effect of MS was considered or neglected. On the other hand, aggregate properties were proven to substantially influence the E ( m , λ ) / E(m,1064) E ( m , 1064 ) ratios for decreasing λ and increasing HAB, thus illustrating the significant effect of aggregation on the optical properties of soot. Finally, the results issued from the different analyses we conducted on the diesel flame investigated in this paper led to the conclusion that values of E m , λ , falling within the 0.38 ± 0.15 and 0.29 ± 0.11 range at 266 and 355 nm, versus 0.25 ± 0.09 at 532 and 1064 nm could be considered as suitable for simulating LII signals of progressively aging aggregated particles.
... The measured instantaneous LII decay traces were first filtered to include only those pulses with sufficient signal level. Data evaluation was done using LIISim for the time-resolved LII signal [40]. Temperature traces were then calculated for each laser pulse. ...
... The background temperature was determined by averaging 200 ns of the temperature trace before the LII laser pulse. The parameter implementation for fitting the LII signal followed that by Kock, detailed in [33,41], i.e. heat transfer model and material constants were used as reviewed in Appendix B of [40]. Variation of the gas composition only showed minor effect on the derived particle size. ...
... Our reference size for the wellcharacterized Gülder flame (standard location on-axis at 42 mm above the pipe exit) we derive roughly 18 nm particle size, which is lower than the value of 28.3 nm determined by TEM in [47]. This value served to refine the parameters used in F. Liu's model, as referenced in [40]. It is known that the parameterization by Kock yields somewhat smaller primary Content courtesy of Springer Nature, terms of use apply. ...
Simultaneous application of multi-channel laser-induced incandescence (LII) and shifted vibrational coherent anti-Stokes Raman scattering (SV-CARS) to study sooting flames is demonstrated for the first time. The potential of this diagnostics combination is evaluated on the basis of characterization of soot particles and correlation of soot presence with temperature. For that purpose, a sooting swirl flame operated at three bars has been employed with ethylene as fuel. The novel combination of CARS and time-resolved LII (TiRe LII) enables the estimation of particle size and correlation of this quantity with local gas temperature; simultaneously acquired 2D LII images provide information on the soot distribution in the ambience of the measurement volume which is used by CARS and TiRe LII. Even if the used LII model is approximative in some respect, the detected LII decay times indicate very small particle size throughout the flame relative to an atmospheric laminar diffusion flame which was used for comparison. In most instances, soot presence relates to local gas temperatures in a range between 1600 and 2400 K. Rare soot events at cooler temperatures occur near the nozzle exit and are attributed to transported soot. Comparison of the peak soot temperatures during the LII process shows a significant decrease in the turbulent pressurized flame relative to the laminar atmospheric reference flame. This is attributed to a less-efficient LII heat-up process at turbulent pressurized conditions due to beam steering. The background blackbody temperature, which can be derived by evaluating the signal captured in the different color channels of the LII system towards the end of the LII process, has been identified to be mostly controlled by hotter soot filaments between the laser plane and the detector. Thus, the LII signal tail is not a good measure of the local gas temperature in the measurement volume for this type of configuration.
... Nevertheless, existing models are capable of describing general trends of the LII process under a variety of experimental conditions. Among the various in-house codes, two versions are publicly accessible: LIIsim [168], [169] facilitates the access to modelling and analyzing the LII signal without developing and implementing a dedicated model, and another publicly available model, CLiiME, was recently made available [170]. The following brief description of the sub-processes which are commonly accepted to be present in LII is widely based on [9], which is the most comprehensive review to date, demonstrating the progress after several updates since 2003 [160], [161], [171]. ...
... It is noteworthy that the LIISim.com web-interface by Hofmann has recently been updated with a dynamic evaluation concept using a graphical user interface and modular tools for processing and visualization to get a deeper insight into the data [169]. In any model however, the evaluation of the LII decay curve requires knowledge of the peak temperature, which can be either calculated based on the absorption properties and known laser pulse energy, or inferred from twocolor pyrometry measurements, as will be described in the subsequent two-color LII section. ...
... Comprehensive details can be found in the review of Michelsen et al. [473]. Note also that there are several freely-available software packages for analyzing LII signals, including LIISim [548] and CLiiME [549]. ...
... The size of soot particles throughout a flame is another important indicator that can inform soot formation models. In pointwise and planar tests, two-color time-resolved LII signals are used to calculate the particle temperature via pyrometry; the temperature trace is then used to determine p through the use of heating and cooling models [548,576,577]. However, the number of projections is already severely limited in time-resolved VLII due to the appreciable cost of ultra-high-speed cameras. ...
... The effects of laser slab steering or attenuation, polydisperse primary particles, and self-extinction are likely significant in this context and should be addressed in future work. Moreover, while Hall et al. [67] mentioned that values of p were insensitive to slight changes in temperature and s , most studies report a significant dependence of p on these properties, e.g., [548,561]. ...
This is a comprehensive, critical, and pedagogical review of volumetric emission tomography for combustion processes. Many flames that are of interest to scientists and engineers are turbulent and thus inherently three-dimensional, especially in practical combustors, which often contain multiple interacting flames. Fortunately, combustion leads to the emission of light, both spontaneously and in response to laser-based stimulation. Therefore, images of a flame convey path-integrated information about the source of light, and a tomography algorithm can be used to reconstruct the spatial distribution of the light source, called emission tomography. In a carefully designed experiment, reconstructions can be post-processed using chemical kinetic, spectroscopic, and/or transport models to extract quantitative information. This information can be invaluable for benchmarking numerical solutions, and volumetric emission tomography is increasingly relied upon to paint a more complete picture of combustion than point, linear, or planar tools. Steady reductions in the cost of optical equipment and computing power, improvements in imaging technology, and advances in reconstruction algorithms have enabled a suite of three-dimensional sensors that are regularly used to characterize combustion. Four emission modalities are considered in this review: chemiluminescence, laser-induced fluorescence, passive incandescence, and laser-induced incandescence. The review covers the reconstruction algorithms, imaging models, camera calibration techniques, signal physics, instrumentation, and post-processing methods needed to conduct volumetric emission tomography and interpret the results. Limitations of each method are discussed and a survey of key applications is presented. The future of volumetric combustion diagnostics is considered, with special attention paid to the advent and promise of machine learning as well as spectrally-resolved volumetric measurement techniques.
... Based on Planck's law, the spectral model can convert the temperature temporal curve into a simulated LII signal temporal curve. By integrating Planck's law on all solid angles, the incandescence signal can be obtained, which yields [50] : ...
To comprehensively understand soot formation in flames of liquid fuel, the soot primary particle size (PPS) in n-heptane\air diffusion flame at pressures up to 5 bar was investigated in this work. A series of experiments were conducted in a high-pressure laminar flame burner system, and the spatial distributions of soot primary particle size in the flames at atmospheric and elevated pressures were evaluated by two-dimensional Time-Resolved Laser-Induced Incandescence (TiRe-LII). The results indicated a tendency that the conunt mean diameter (D p,mean) of soot primary particle increased with enhancing laser fluence, which might be attributed to inaccuracy of the thermal accommodation coefficient at the increasing laser fluences. The spatial distribution of D p,mean varied with pressures. Further, the evolution of D p,mean along the centerline and the radial distribution of D p,mean at the pressures of 1.0-3.0 bar were also presented. The zone of the maximum D p,mean moved from the flame centerline to the flame edge as the pressure increases. The spatial distribution of D p,mean at 3.5-5.0 bar showed the area of maximum D p,mean located at the annular wing region in flames and being floated as the pressure increased. This study complemented the existing theoretical knowledge of spatial primary particle size distribution (PPSD) in n-heptane lami-nar diffusion flames.
... Figure 23 : Décroissance de la température d'une particule de suie pour différents en fonction du temps écoulé depuis l'interaction avec l'impulsion laser . Ces courbes sont calculées avec le logiciel LIISim [186] (modèle de Liu et al. [180,187]). ...
L’augmentation du trafic aérien s’accompagne d’un durcissement de la réglementation visant à limiter les émissions de gaz à effet de serre et de polluants. Parmi ces polluants, on retrouve les particules de suie qui pourraient compter pour un tiers de la contribution de l’aviation au réchauffement climatique. L’OACI (Organisation de l’Aviation Civile Internationale) a adopté en 2020 une nouvelle formulation de la norme d’émissions particulaires autorisant la certification des moteurs aéronautiques. Même si cette nouvelle réglementation entrera en vigueur en 2023 et ne sera pas plus restrictive que l’ancienne, elle témoigne d’un intérêt grandissant pour la réduction des émissions des particules de suie rejetées dans l’environnement. C’est dans ce cadre qu’a été lancé le projet européen SOPRANO dont les activités scientifiques de ma thèse font partie intégrante. L’objectif de ce projet est d’accroître la compréhension de la formation des particules de suie dans les chambres de combustion aéronautiques. La contribution de ma thèse à ce projet européen concerne la caractérisation de la formation et de l’oxydation des particules de suie produites dans des flammes aéronautiques caractérisées par une injection swirlée et stratifiée. Pour appréhender le rôle joué par ces deux paramètres, un brûleur académique, dont l’architecture a été simplifiée par rapport à celle des injecteurs aéronautiques de nouvelle génération (multipoint), a été conçu et fabriqué, puis utilisé dans ma thèse. Plusieurs conditions opératoires, présentant des productions de suie différentes, ont été définies en faisant varier les valeurs des nombres de swirl et la stratification d’une flamme de prémélange éthylène / air à pression atmosphérique. La formation des particules de suie a principalement été étudiée grâce à l’utilisation de diagnostics optiques. Tout d’abord, une caractérisation des grandeurs clés intervenant dans les processus de formation et de croissance des suies a été réalisée : l’aérodynamique des flammes a été étudié par PIV, la position du front de flamme par PLIF-OH, la formation des précurseurs de suie (aromatiques) par PLIF-HAP et la température de flamme par thermocouple. Concernant les particules de suie, la fraction volumique et le diamètre des particules primaires ont été mesurées par LII et la concentration en nombre et le diamètre des particules par diffusion élastique angulaire ainsi que par prélèvement et analyse SMPS. Une analyse d’un échantillon de particules de suie prélevées directement dans la flamme a également été réalisée par microscopie électronique afin d’obtenir des informations de taille et de microstructure. Plusieurs de ces diagnostics optiques ont bénéficiés de développements importants au cours de ma thèse (mise en oeuvre et prise en compte de la durée d’ouverture de la caméra pour la LII auto-compensée, analyse spectrale de la diffusion, diffusion angulaire 2D résolue en temps). La combinaison de ces techniques de mesure a en outre permis une étude détaillée des zones de formation des précurseurs de suie, de nucléation, de croissance, d’agrégation et enfin d’oxydation des particules de suie. Un schéma descriptif de la formation et de l’oxydation des particules de suie dans les flammes swirlées et stratifiées étudiées a donc finalement été proposé.
... This is because the effective soot temperature becomes lower as the camera gate width increases (Cenker et al. 2015). In order to take into consideration the cooling effect of soot particles over the camera gate width in AC-LII data analysis, we propose a methodology in Section 2 that relies on the evolution of the soot particle temperature, which is predicted numerically using an opensource LII modeling software called LIISim (Mansmann et al. 2018). Our methodology is also extended to the determination of the primary particle diameter by using the two-color time-resolved signal detection features of the AC-LII method and by considering the natural intermittency of the turbulent flame. ...
... Evolution of the effective temperature (solid curve) based on the ratio between the LII signals integrated over the detector gate width Dt d in the two-color method (shown by the red and blue shaded areas) and the instantaneous soot temperature modeled by LIISim (dashed curve). The input parameters are D p ¼ 15 nm, T gas ¼ 1550 K, T peak ¼ 4000 K and Dt d ¼ 30 ns: camera gate width on the effective temperature of the soot is shown in Figure 1 for simulated LII signals based on LIISim (Mansmann et al. 2018), for a primary particle diameter D p of 15nm, a bath gas temperature of 1550K and a peak temperature of 4000K. The LIISim model of the decay of the soot temperature starts with an assumed peak soot temperature, without considering the laser heating phase. ...
Laser-induced incandescence (LII) is an optical technique that is widely used for the in situ measurement of the soot volume fraction in flames. The low intensity of LII signals in low-sooting turbulent flames means that long camera acquisition times are needed to achieve sufficiently high signal-to-noise ratios. The direct application of auto-compensating LII (AC-LII) to infer the soot volume fraction from the measured LII signals can lead to large errors due to the significant decrease in the temperature of the soot particles over the duration of the camera gate. In order to reduce the measurement errors from AC-LII in low-sooting turbulent flames, we propose an improved approach in which the cooling of soot particles during the camera acquisition of LII signals is considered. The proposed methodology is applied to planar AC-LII measurements for the determination of soot volume fraction and primary particle diameter in a low-sooting turbulent flame, with the help of the open source LII software LIISim. This study represents the first application of planar AC-LII to a turbulent flame. A sensitivity analysis is also conducted in order to determine the main factors affecting the uncertainty of the proposed approach.
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... It is observed that peak D p increases with coflow oxygen up to 27%, after which it drops with a further addition of oxygen. A modular signal processing toolbox for time-resolved laser-induced incandesce measurements (LII-Sim) was used for the calculation of D p [33]. The uncertainty in D p calculations is estimated to be 2.9 nm based upon temperature uncertainty, particle size distribution assumptions, and different heat conduction models. ...
This paper reports an experimental investigation of the effects of varying the oxygen concentration in the oxidizer stream on the formation and evolution of soot and precursor nanoparticles in an axisymmetric ethylene laminar coflow diffusion flame. The optical diagnostics employ temporally and spectrally resolved point-measurements of 266 nm excited scattering and laser-induced fluorescence, combined with 1064 nm excited laser-induced incandescence. Previous studies have indicated that oxygen enhancement changes soot volume fraction in sooting flames. However, the effects of coflow oxygen concentration on soot precursors are not well understood, and this aspect is addressed explicitly here. A set of eight flames is studied where the oxygen concentration in the coflow stream is varied from 19 to 40%, by volume, while the chemical composition of the jet stream is kept constant at 60% C 2 H 4 / 40% N 2. Oxygen enhancement in the coflow leads to a change in flame temperatures, luminosity, and flame length. It is found that the concentration of precursor nanoparticles increases with coflow oxygen. A combination of higher temperatures, red-shifting, and increased decay-times suggest structural growth in precursor nanoparticles with increasing coflow oxygen. In all flames, a similarity of particle evolution is noted with axial location, and this includes the presence of an intermediate stage during the transformation of precursors to soot. Moreover, the structural transformation from aliphatic to aromatic features is shifted towards the beginning of the precursor region when coflow oxygen is increased. It is found that structural growth starts at a similar temperature, regardless of coflow oxygen concentration. Soot volume fraction and primary soot particle diameter increase with coflow oxygen enrichment up to a threshold oxygen volume fraction, after which both the soot volume fraction and diameter decrease with further oxygen enrichment.
... where l 1 and l 2 are the bandpass center wavelengths of the two detection channels, and Eðm l Þ is the absorption function. LII signal processing was performed by the open-source software tool LIIsim [47]. In a least-square fitting algorithm the particle diameter is retrieved by minimizing the residual between the simulated and the measured temperature decay curve. ...
... Assuming a monodisperse particle size distribution, the primary particle diameters, d p , were determined from fitting calculated temporal temperature decays (Eq. (6)) to measured two-color effective temperature decay profiles using LIIsim [47]. Fig. 10 presents the primary particle diameters for the 1.25 bar flames using the model briefly described in section 2.4, with an assumed fixed gas temperature of 1600 K for all the cases. ...
Soot formation at lean-threshold conditions referred to as “near-threshold sooting conditions” (i.e., with stoichiometry, ϕ, around 1.90 for ethene as a fuel) are studied in laminar premixed ethylene/air flames at pressure from 1 to 10 bar. Laser extinction is used to measure the soot volume fraction. Time-resolved laser-induced incandescence (TiRe-LII) is used to determine particle diameters from the LII signal temporal decay after pulsed laser heating. Thermophoretic sampling is applied to extract particle samples from the flame and ex situ transmission electron microscopy (TEM) is used to measure particle sizes and morphology. The soot volume fraction scales with pressure in a power-law function with the parameter n as 1.4 to 1.9 for flames at the equivalence ratio (ϕ = 2.1) even at the onset of soot formation. The elevated dependence of soot volume fraction on height above burner is detected with increasing pressure in the near-threshold sooting conditions. The measured soot diameter increases with pressure and equivalence ratio and its sensitivity to the equivalence ratio increases with increasing pressure. The TiRe-LII signal decay varies only little with height above burner and laser fluence in the near-threshold sooting flame (ϕ = 1.90–1.95), which indicates that the soot particle surface growth and oxidation are balanced. For a slightly sooting flame, TEM measurements from thermophoretically-sampled soot agree well with the LIIsim-evaluated particle size, indicating the reliability of TiRe-LII particle diameter determination under near-threshold conditions.
... Lemaire and Mobtil eventually highlighted the need for additional theoretical analyses supported by comparisons between simulated data and experimentally monitored signals obtained in well characterized media. With this in mind and considering that the validity of the database provided by Goulay et al. has been recently somewhat questioned by Mansmann et al. [20], Menanteau and Lemaire proposed a series of works [21,22] in which a set of LII signals measured in a diffusion flame of Diesel has been simulated using one of the most (if not the most) refined LII-model formulation ever implemented so far. By using an original optimization procedure coupling design of experiments and a genetic algorithm-based solver, these authors proposed a fully parameterized LII model formulation allowing obtaining simulated and measured signals merging on a single curve. ...
