Etude expérimental sur la formation des suies dans des flammes laminaires prémélangées d’hydrocarbure d’intérêt automobile et aéronautique : accent sur le processus de nucléation des particules de suie

Les particules de suies émissent lors de la combustion incomplète de carburants fossiles et biosourcés sont reconnues comme étant un problème environnemental et sanitaire majeur. Il est essentiel d'acquérir une compréhension fondamentale de leur formation, dont l'étape de nucléation donnant naissance aux premières particules de suie, les nucléis, afin de développer des modèles capable de prédire leur formation et d'aider à la conception de dispositifs de combustion plus efficaces et plus propres. Ce travail démontre en combinant différentes techniques expérimentales: incandescence induite par laser (LII), granulométres SMPS et de microscopie à faisceau d'ions d’hélium (HIM) que ces nucléis ont une taille comprise entre 2 et 4 nm et sont capable d’émettre un rayonnement de type corps noir. Ces nucléis ont été étudiés dans des flammes prémélangées dites de nucléation dans lesquelles les particules de suie sont formées par nucléation, sans croissance de surface et standard où ces nucléis subissent les processus de croissance. Les résultats obtenus offrent une base de données expérimentale utile à l’amélioration des modèles cinétique de formation des suies notamment sur la phase de nucléation dans diffèrent carburants: n-butane et un mélange de n-butane et n-propylbenzene. Pour chaque carburant une flamme de nucléation et une flamme standard sont étudiées. La base de donnée comprend des profils d’espèces obtenus par chromatographie, profils de température mesurés par fluorescence induite par laser sur NO, les profils de fractions volumique de suies mesurés par LII et calibrés par extinction multi passage et les distributions de taille des particules de suies obtenus par SMPS et HIM.Emission of soot formed from incomplete combustion of fossil fuels, biofuels and biomass is a serious concern due to soot’s harmful impact on human health, environment and its radiative forcing on climate. Gaining fundamental understanding of soot formation, particularly the nucleation step leading to the formation of the nascent soot particles, is critical to develop reliable predictive soot models and to help the design of more efficient and cleaner combustion devices. This work demonstrates that nascent soot particles in the size range of 2-4 nm are able to emit a black body radiation by combined Laser Induced Incandescence (LII), scanning mobility particle sizer (SMPS) and helium-ion microscopy (HIM). These nascent soot particles are investigated in nucleation premixed flames in which soot particles are essentially formed by nucleation, without growth by soot surface processes and in standard sooting premixed flames in which growth processes occur. This work provides an extensive database for improvement of kinetics modelling of sooting flames with a focus on the soot nucleation in flames. Two kinds of fuels have been selected: n-butane and mixture of n-butane and n-propylbenzene. For each fuels two flames have been studied: a nucleation and a standard sooting flames. The database consist of species profiles obtained by online gas chromatography, temperature profiles measured by Laser induced fluorescence thermometry, soot volume fraction profiles obtained by LII calibrated by cavity ring-down extinction and particles size distributions obtained in n-butane flames by SMPS and HIM. From this database effect of equivalence ratio and fuel composition is analyzed

Experimental study of soot formation in laminar premixed flames of fuels of interest for automobile and aeronautics: a focus on the soot nucleation process.

Thèse, Discipline: Energétique, thermique, combustio

Evolution of soot maturity in a laminar diffusion CH 4 /air flame using the two-separated pulses LII technique

International audienceA new experimental technique is presented to measure the soot particle absorption function and the gas temperature. The approach is based only on LII and requires measuring soot particles' peak temperature at two different laser fluences. A theoretical investigation of the technique indicates the necessity to work at low fluences with two fluence values as spread as possible. Also, it shows that the underestimation of the absorption function grows with decreasing particle size. The technique has then been applied to a weakly sooting methane/air laminar diffusion flame. A coherent estimation of the absorption function is found, proving the approach's feasibility

Development of highly sensitive quantitative measurements of nascent soot particles in flames by coupling cavity-ring-down extinction and laser induced incandescence for improving the understanding of soot nucleation process

International audienc

Estimation of soot absorption function via the two separated pulses laser-induced incandescence technique

An original in-situ experimental approach is proposed to estimate the absorption function of soot particles in flames: the two Separated Pulses Laser-Induced Incandescence technique (SP-LII). The SP-LII technique is based on measuring the peak temperature of soot particles heated by laser pulses at two different fluences. From these two temperature measurements, the absorption function is estimated by solving the energy equation applied to soot particles during laser energy absorption once the product of soot density and specific heat is known. In order to solve the energy equation, two methods are considered here. The first method, called the "absorption model" (AM), solves the energy equation when all loss terms are neglected during absorption. The second method uses a look-up table (LUT) generated with an LII code in which the main loss terms are modelled. Both methods also provide information on the gas temperature T 0 , assuming that gas and solid phases are at equilibrium. First, the SP-LII technique's accuracy and limits are theoretically explored using peak temperatures from simulations done with an LII code. Overall, the AM method is efficient but is restricted to soot primary particles diameter >∼ 10 nm and low fluences. By contrast, the LUT method has an extended operational range, but it requires more information than the AM method, and its accuracy depends on the validity of the power loss models used to generate the look-up table. It is then concluded that the AM method represents the best compromise between the complexity of the methods and the expected accuracy of the results. Then, the feasibility of the SP-LII technique is proven by performing measurements in a laminar diffusion methane/air sooting flame and post-processing them with the AM method. Results for the absolute value and for the spatial evolution of E(m λ) are coherent with the literature. Finally, a possible extension of the SP-LII technique to turbulent flames is discussed

Interpretation of the Laser-Induced Emissions for flame-synthesized TiO 2 nanoparticles

International audienceAmong various techniques for monitoring of particle formation in flames, laser-induced incandescence (LII) has generally been developed for soot particles. Recent works have shown the feasibility of LII for flame-synthesized TiO 2. However, extensive work is still needed to use this technique to quantitatively characterize TiO 2 production in flames. In this work, first attempts towards the characterization of TiO 2 synthesis in flames are provided in terms of normalized volume fraction and primary particle diameter. To achieve this, TiO 2 nanoparticles are generated in a laminar diffusion flame of argon-diluted hydrogen and air with pre-vaporized Ti-precursor. A 355 nm laser is used to irradiate the flame-generated particles, and spectral, temporal and spatial measurements are performed. First, laser-induced emission at prompt is investigated for different laser fluences to identify the operating conditions that ensure the LII-like nature of the measured signals and infer the composition of the produced nanoparticles. Then, it is shown that it is possible to obtain information from the LII signal on the spatial evolution of the normalized volume fraction and the primary particle size

Quantitative measurement of volume fraction profiles of soot of different maturities in premixed flames by extinction-calibrated laser-induced incandescence

International audienc

Characterization of Laser-Induced Emission of high-purity TiO2 nanoparticles: Feasibility of Laser-Induced Incandescence

International audienceThe development of Laser-Induced Incandescence (LII) approach for characterizing the production of high-purity non-carbonaceous metaloxides produced in flame synthesis systems is in progress. This work aims to prove the feasibility of LII measurement for titanium dioxide (TiO2). In previous works, laser-induced emission (LIE) was investigated for flame-synthetized TiO2 particles. However, the presence of carbon materials was detected. As this calls into question the nature of the signal, we consider in this work LIE of high-purity engineered TiO2 nanoparticles to circumvent the carbon issue. Specifically, we investigate the spectral and temporal laser-induced emissions of pure TiO2 nanoparticles dispersed in a non-reactive environment. In parallel, LIE from carbon black is examined to validate the strategy and to highlight differences between carbon black and TiO2. The TiO2 results indicate that depending on the laser fluence, different interferences appear at prompt. The literature suggests that these non-thermal emissions are likely to be from fluorescence or phase-selective laser-induced breakdown spectroscopy, both characterized by a short lifetime. To avoid these parasitics signals, measurement acquisition time is delayed. A spectral Characterization of Laser-Induced Emission of high-purity TiO 2 nanoparticles red-shift is observed with time as a result of decreasing particle temperature. This proves the LII nature of delayed emission from pure TiO2, which is confirmed by the LII-like nature of the temporal signals