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The auto-compensating laser-induced incandescence (AC-LII) technique has been frequently used to measure soot volume fraction (f v) in flames. AC-LII relies on an assumption about the spectral variation of soot absorption function E (m , λ) at the two detection wavelengths for soot temperature determination. It has been a common practice to assume a constant E (m , λ) based on the believe that LII measures mature soot only. Since soot maturity in flames is a continuous phenomenon from young to mature, it is unclear how soot maturity affects the AC-LII measured f v. In this study, a correction to the AC-LII measured f v with the conventional hypothesis of mature soot is proposed based on the soot maturity coefficient β and absorption function E (m , λ) in a laminar coflow ethylene diffusion flame. The application of this correction is first demonstrated for synthetic time-resolved LII signals based on the spatial distribution of soot properties in the ethylene diffusion flame predicted by the CoFlame code. A significant decrease of the simulated incandescence signals S LII and soot temperature T eff is observed when the maturity coefficient is increased from 0 for fully mature soot to 1.2 for highly immature soot. Then the proposed correction is applied to AC-LII measured f v in the same flame with the help of spatial distributions of β and ratio of spectrally-resolved E (m , λ) by multiwavelength line-of-sight attenuation and emission (MW-LOSA/EMI) measurements. The uncorrected AC-LII measured f v is in general significantly lower than that measured by MW-LOSA/EMI, which inherently considers soot maturity. The corrected AC-LII f v results are in good agreement, within the experimental error, with those measured by MW-LOSA/EMI. Therefore, the proposed methodology can quantify the effect of soot maturity on AC-LII measured f v and largely eliminate the difference in the measurements of f v by AC-LII and MW-LOSA/EMI. [ABSTRACT FROM AUTHOR]

To understand soot maturation mechanism, we investigate the size-evolution of soot maturity in a series of laminar premixed burner-stabilized stagnation ethylene flames, with the maximum flame temperature T f,max ranging from 1630 to 1869 K and the equivalence ratio φ ranging from 2.0 to 2.3. The soot particle size distribution is obtained by using the orifice sampling technique together with a scanning mobility particle sizer, while the maturity of soot is evaluated based on its oxidation characteristics and Raman spectra. The results show that at a fixed φ (say 2.2), soot particles grow bigger and become more mature concurrently in higher T f,max flames (1809 and 1869 K). In contrast, although particle size increases obviously with time in lower T f,max flames (1630 and 1724 K), soot maturity remains almost unchanged, indicating that particle growth is mainly controlled by particle coagulation or PAH condensation, instead of surface reactions in these flames. At a fixed T f,max (say ∼ 1725 K), soot size growth is faster at higher φ as expected. However, when comparing soot particles of similar sizes, the particles from lower φ flames are more mature than those from higher φ flames, where the particle growth rate is too high to allow sufficient residence time for particle maturation. [ABSTRACT FROM AUTHOR]

A novel diagnostic is proposed to characterize the maturity of soot particles in a laminar axisymmetric coflow ethylene diffusion flame in terms of the spectral dependence of soot absorption function. The method relies on the combination of line-of-sight attenuation (LOSA) and emission measurements at four wavelengths (500, 532, 660 and 810 nm). The analysis of the measured signals enables the determination of soot temperature, soot volume fraction, soot maturity and the contribution of soot scattering to extinction. The analysis of extinction and emission measurements considers the spatial variation of soot optical properties. The introduction of a maturity index allows the evaluation of soot maturity based on the spectral variation of the soot absorption function. The maturity index is correlated with the organic or the mature soot content and finally in terms of the absolute value of absorption function at 810 nm. The methodology is validated using a set of synthetic spectral LOSA and emission signals representing experimental measurements based on numerical results obtained using the CoFlame code. A sensitivity analysis of the Abel inversion is also performed to properly address the effect of deconvolution procedure. Finally, the proposed method is applied to analyze the experimental data of spectrally-resolved LOSA and emission acquired in a laminar axisymmetric coflow ethylene diffusion flame established on a Gülder burner. The two-dimensional distributions of soot temperature, soot volume fraction, soot maturity, and the ratio of total scattering to absorption are determined. Mature soot particles are found on the top of the flame in the centerline region and also in the outer edge of the flame wing displaying strong gradients. [ABSTRACT FROM AUTHOR]

The impact of organic carbon (OC) on the light absorption of soot is determined by discrete element modeling coupled with the discrete dipole approximation for computing the scattering of radiation by soot particles. The mass absorption cross-section (MAC) of soot is used widely to determine its light absorption. Typically MAC is obtained from the mass average refractive index of OC and elemental carbon (EC) with large C/H that make up mature soot. As such, MAC can be overestimated by a factor of 3 in fuel-rich flames where newly-formed young soot contains EC with small C/H and OC that predominantly scatters light reducing its absorption by soot. Here a relation for the soot refractive index is derived accounting for soot morphology, maturity and OC content through its band gap at wavelength, λ = 266–1064 nm. Using this relation, the MAC of soot containing OC (up to 50 wt%) is in excellent agreement with carbon black, graphene and soot data. This confirms that soot morphology, maturity and OC content greatly influence light absorption during characterization of in-flame and freshly-emitted soot by laser induced incandescence and light extinction, especially in fuel-rich flames, and need to be properly accounted for in the soot refractive index. Image 1 [ABSTRACT FROM AUTHOR]

Anisole fuel has gained increased attention, since it is considered a biofuel, and its addition to gasoline increases the octane number, improving the engine performance. This study focuses on the comprehensive characterization of soot maturity and sooting propensity of a well-controlled laminar coflow diffusion flame fueled with anisole and using methane as the carrier gas. Four laminar non-premixed flames were established below the smoke point. The oxidizer composition ranged from a molar fraction of oxygen (oxygen index, OI) of 21% to 35% (doped with pure oxygen), while keeping constant the total volumetric flow rate of oxidizer stream. Multi-wavelength line-of-sight attenuation experiments were performed from visible to infrared wavelengths, and the measurements of the extinction coefficient and flame temperature were used to retrieve two dimensional fields of soot temperature, volume fraction, radiative intensity and degree of soot maturity. In general, the sooting propensity and radiative behavior of the anisole flames follows a similar trend than typical hydrocarbon fuels when increasing the OI. However, the spatial distribution of soot volume fraction is enhanced, particularly near the flame centerline. In contrast, the soot production is promoted by the OI, near the flame wings. The same effect can be observed for the soot temperature. Indeed, temperature increases, promoting formation and oxidation processes, specially at the locations near the maximum soot volume fraction. In general, the maturity of soot particles is affected as the OI is changed. Also, it is observed that the degree of maturity is enhanced by flame temperature. In order to support these local observations, a statistical analysis was carried out. Finally, the results obtained yield a curated database for validating detailed soot production models of oxygenated fuels, and to gain further insights on the evolution of soot maturity and propensity of these particular fuels under different oxidizer conditions. • Spatially-resolved characterization of anisole sooting and radiative properties. • Oxygen boosts local soot temperature, volume fraction, and radiation. • Increased oxygen content enhances soot particle's maturation. • Increased temperature correlates with increased soot maturity. • These effects are more pronounced near the flame wings. [ABSTRACT FROM AUTHOR]

Abstract Optical characterization of soot (e.g., Laser Induced Incandescence, LII) and its impact on climate largely depend on light absorption. As soot grows, its morphology changes affecting its optical or radiative properties. Here, the impact of soot maturity on its light absorption is investigated by coupling Discrete Element Modeling (DEM) with Discrete Dipole Approximation (DDA) during soot surface growth and agglomeration. The Mass Absorption Cross-section, MAC , of nascent and mature soot agglomerates is estimated by DDA and validated against atomistic point dipole interactions and mesoscale DDA calculations. Using a refractive index, RI , for mature soot yields constant average < MA C> and absorption function < E > overestimating the nascent soot light absorption up to 75%. The RI is interpolated between those of nascent and mature soot for wavelengths, λ = 532 and 1064 nm to account for quantum confinement and evolving number of clustered sp2-bonded rings that affect the optical band gap, E g. This results in excellent agreement of the DEM-derived evolutions of < MAC > , < E > and ratio R = < E (λ = 532 nm) > < E (λ = 1064 nm) > with the corresponding LII measurements in methane and ethylene premixed flames. The nascent soot E g decreases during aging and agglomeration, increasing < MA C> and < E > by 65%. The R decreases from 1.34 to 0.95 by aging and coagulation and slowly converges to the asymptotic 0.89 of mature soot measured in diffusion flames. The good agreement between DEM and LII data confirms that soot dynamics by surface growth and agglomeration strongly correlate with soot maturity, composition and RI that are essential for quantifying soot light absorption and scattering. [ABSTRACT FROM AUTHOR]

Liquid oxygenated biofuels such as anisole have been used as additives, since they increase the octane number and improve spark-ignition engines' performance. In this study, the sooting propensity of a gasoline surrogate, anisole, and gasoline/anisole mixture is quantified by measuring bidimensional fields of soot volume fraction, temperature and radiative heat flux in a coflow laminar flame using well-established non-intrusive optical diagnostics. The maturity of soot particles is also measured using innovative multiwavelength line-of-sight attenuation and emission measurements. Experimental measurements shows that increasing anisole content in fuel mixture results in a greatly enhanced soot volume fraction, while overall temperature is slightly decreased. In addition, maps of soot maturity index are reported based on the spectral dependence of soot optical properties with the amount of immature soot. Mature soot is found near the outer edge of the flame for the three flames, and mature soot content was found to decrease with increased anisole addition to the fuel mixture. In addition, high temperature zones were found to correlate with the presence of more graphitic compounds within the soot particles for the investigated fuels. The comprehensive dataset reported in this study provides rich information for a better understanding of soot formation processes, and for the development and validation of soot models in coflow laminar diffusion flames using complex fuels of interest for practical applications. • Multi-wavelength extinction/emission measurements are used to assess soot maturity. • Soot production along the flame centerline is enhanced by the addition of anisole. • The addition of anisole reduces the overall soot temperature of the flame. • Maturity of soot particles is reduced by adding anisole to gasoline flames. • Overall, maturity is reduced with increasing temperature. [ABSTRACT FROM AUTHOR]

• Temperature-dependent Raman spectra measurements for structural characterization of just-nucleated and aged/mature soot. • Determination of the thermal coefficients (χ G) of the Raman G band position. • χ G is a proxy of in-plane thermal expansion coefficient within the layers of polycondensed rings and depends on disorder. • χ G correlates to soot density and can be employed for the analysis of soot maturity. Flame-formed soot nanoparticles are known to possess different nanostructures as a function of a large variety of combustion operative parameters, including particle residence time in flames, flame stoichiometry, fuel chemical composition, temperature and pressure. This implies that important properties (e.g. , soot oxidation reactivity, optical and electronic gap) may be different depending on the way soot particles are generated. Hence, it is beneficial to develop diagnostic methods that are sensitive to such differences among the various kinds of soot particles. In this work we investigate by off-line variable-temperature Raman spectroscopy two types of soot particles, namely just-nucleated and aged/mature particles, with a different size and maturity degree. The results obtained for soot particles have been compared and discussed with those found for other related carbon materials. As the Raman spectra are recorded at increasing sample temperature, from 300 to 525 K, both soots display a linear downshift of the position of the two main Raman modes (D and G bands). Just-nucleated soot particles exhibit the strongest temperature dependence of the position of the G band. Based on results from density functional theory calculations, interpreted by a simple empirical model, the observed temperature coefficient differences can be ascribed to differences in the nanostructural order of the two soot samples, which causes a different thermal expansion behavior. [ABSTRACT FROM AUTHOR]

In-flame optical characterization of soot is of vital importance to understand soot formation mechanisms as well as to develop and validate accurate soot models. The present work introduces an unconventional methodology adapted to laminar axisymmetric flames that avoids the issue of variable measurement volume with varying scattering angle in the existing light scattering techniques and thus enables the determination of aggregate size with a higher spatial resolution. Coupled with multi-wavelength line-of-sight attenuation measurements, the proposed Horizontal Planar Angular Light Scattering at 532 nm was found able to provide radial profiles of aggregate size, number and diameter of primary spheres, soot volume fraction, and number density in a laminar axisymmetric coflow ethylene/air diffusion flame established over a Gülder burner. The spatial variation of soot optical properties associated with soot maturity was considered in data analysis. [ABSTRACT FROM AUTHOR]

Incomplete combustion is the main source of airborne soot, which has negative impacts on public health and the environment. Understanding the morphological and chemical evolution of soot is important for assessing and mitigating the impact of soot emissions. Morphological and chemical structures of soot are commonly studied using microscopy or spectroscopy, and the best technique depends on the parameter of interest and the stage of soot formation considered (i.e., maturity). For the earliest stages of soot formation, particles exhibit simple morphology yet complex and reactive chemical composition, which is best studied by spectroscopic techniques sensitive to the large number of soot precursor species. The only microscope that can offer some morphological information at this stage is the scanning probe microscopy, which can image single polycyclic aromatic hydrocarbons, the precursors of soot. A broader range of types of spectrometers and microscopes can be used by increasing the soot maturity. Mature soot is primarily carbon, and exhibits complex fractal-like morphology best studied with electron microscopy and techniques sensitive to thin oxide or organic coatings. Each characterization technique can target different morphological and chemical properties of soot, from the early to the late stage of its formation. Thus, a guideline for the selection of the appropriate technique can facilitates studies on environmental samples involving the presence of soot. • Microscopes and spectrometers to evaluate soot maturity. • STM and FTIR are, respectively, to use for image and determine PAH in incipient soot. • HIM measures morphological parameters in young soot minimizing sample damage. • Raman spectrometer analyzes changes in the chemistry of soot through its formation. [ABSTRACT FROM AUTHOR]