Antonio D'Alessio

In situ evaluation of the soot refractive index in the UV-visible from the measurement of the scattering and extinction coefficients in rich flames

Abstract An in situ chemical and physical characterization of soot particles during the separate stages of formation and oxidation appears to be an important step forward in the understanding of the heterogeneous aspects of the kinetics. To this end the knowledge of the optical properties of soot particles inside rich flames is of great relevance and categorizes “soot” among the large family of carbonaceous materials. This article proposes a new method for the evaluation of the dispersion of the optical properties of absorbing submicronic aerosols, starting from the simultaneous measurements of the scattering and extinction coefficients in the near UV and visible. The experiments were on premixed flat flames at atmospheric pressure with such nonaromatic fuels as CH 4 , C 2 H 4 , and C 2 H 2 and different CO ratios and flow rates. A quantitative determination of the spectral behavior of the real and imaginary parts of the complex refractive index of soot was obtained in conditions where the molecular contribution could be neglected and where the particles were not agglomerated and behaved as Rayleigh scatterers. The optical properties of soot were compared with those obtained for graphite (250–550 nm), different amorphous carbons, and coals in the same wavelength range. The real part of the refractive index exhibits a strong dispersion in the visible whereas the imaginary part has a hump near 300 nm indicating that the resonance of π electrons, typical of graphite, is much enlarged and shifted toward the red.

UV-visible spectroscopy of organic carbon particulate sampled from ethylene/air flames

A systematic comparison of spectra obtained with extra and in situ diagnostics in the soot preinception region of rich, premixed ethylene air flames suggests that combustion generated organic carbon (OC) particulate can be extracted from flames and isolated from other flame material for further chemical analysis. Both the trend with height above the burner and the form of UV fluorescence and absorption spectra from extra situ sampled material captured in water agree with those measured in situ. These results show that the OC particulate formed in flames is partially water soluble. However, the collection efficiency can be increased using less polar solvents, like acetonitrile and dichloromethane. The fluorescence spectra from the water samples are comprised both a naphthalene-like component and a broad band UV fluorescence component similar to that observed in situ which is attributed to flame generated OC particulate. The broad band UV fluorescence centered around 320 nm is also observed very early in flames and does not change considerably with increasing flame residence time. These results support previous hypotheses that the UV broad band fluorescence is from carbonaceous material comprised two-ring aromatics, formed earlier than soot in the flame, and is still present along with soot at higher heights or flame residence times.

U.V. and visible laser excited fluorescence from rich premixed and diffusion flames

Abstract Fluorescence spectra were obtained in laminar, gaseous, rich premixed and diffusion flames employing different harmonics of a pulsed Nd: YAG laser. Spectra obtained with the fourth harmonic at λ0 = 266 nm exhibit a peak in the u.v. at λ = 320 nm and a broader one in the visible with a maximum beyond 400 nm. The relative role of these two features has been followed in rich premixed flames with different C: O ratios and heights above the burner and along the radius of a cylindrical, coflowing diffusion flame. The u.v. fluorescence detected in our flames should be attributed to structures containing no more than two aromatic rings, while the fluorescence in the visible is attributed to larger aromatic structures. Fluorescence and absorption spectra obtained from the different fractions of the material sampled into the flames showed qualitative agreement with the “in silu” spectra. Fluorescence spectra in the visible, originating from structures heavier than 300a.m.u., has been also detected in the s...

Coagulation and carbonization processes in slightly sooting premixed flames

UV absorption spectra in the 200–400 nm range and transient thermocouple measurements are used tocharacterize the process of nanoparticle formation and their carbonization to soot particles in slightly sooting premixed flames of ethylene. The UV absorption technique allows a quantitative determination of the concentration of both soot and nanoparticles. In the examined flame, the total particulate is initially present as nanoparticles, but even in the sooting region, where the yellow luminosity is dominant, soot particles represent less than 50% of the total mass of particulate. This technique can be easily used for the determination of soot and nanoparticles at the exhaust of real combustion devices and in urban polluted areas. Time-resolved thermocouple measurements are used to follow the variation along the flame axis of the emissivity of the particulate deposited by thermophoresis on the thermocouple junction. Thesevalues are used for the evaluation of an apparent rate of carbonization, which has been compared with the coagulation rate of the particles evaluated by scattering and extinction measurements. Both processes, carbonization and coagulation, show a second-order kinetic with a constant of the order of 5×10 12 cm 3 /s. This technique has shown to be a very simple method for detecting nanoparticles, which can be applied also in diffusion flames and other practical systems. In addition, it furnishes emissivity values, which might be useful for a more realistic assessment of the radiative heat transfer of slightly luminous flames.

The Diesel Exhaust Aftertreatment (DEXA) Cluster: A Systematic Approach to Diesel Particulate Emission Control in Europe

The DEXA Cluster consisted of three closely interlinked projects. In 2003 the DEXA Cluster concluded by demonstrating the successful development of critical technologies for Diesel exhaust particulate after-treatment, without adverse effects on NO x emissions and maintaining the fuel economy advantages of the Diesel engine well beyond the EURO IV (2000) emission standards horizon. In the present paper the most important results of the DEXA Cluster projects in the demonstration of advanced particulate control technologies, the development of a simulation toolkit for the design of diesel exhaust after-treatment systems and the development of novel particulate characterization methodologies, are presented. The motivation for the DEXA Cluster research was to increase the market competitiveness of diesel engine powertrains for passenger cars worldwide, and to accelerate the adoption of particulate control technology.

Evidence and characterization of nanoparticles produced in nonsooting premixed flames

The size distribution of nanoparticles generated in nonsooting ethylene/air premixed flames and collected in water samples has been evaluated using two particle-sizing techniques that show high sensitivity to particles in the nanometer size range: dynamic light scattering and electrospray-differential mobilty analysis. A d 63 of 2.85 nm has been estimated. By combining the results of ex situ size determination and in situ scattering and extinction measurements in UV, the complex refractive index of nanoparticles has been determined. The result is in good agreement with the value we previously used for the elaboration of in situ measurements and is also consistent with the conceptual model that nanoparticles formed in flames contain 2-, 3-ring polycyclic aromatic hydrocarbon functionalities as their building blocks. This work shows that nanoparticles produced in combustion systems are partially soluble in water, which is particularly relevant for determining their possible effects on human health and climate.

Optical and Spectroscopic Characterization of Rich Premixed Flames across the Soot Formation Threshold

ABSTRACT Phenomenological aspects of formation, destruction, and coagulation of high molecular mass structures formed in the main oxidation zone of rich premixed flames and in rich flames well below the soot threshold limit have been examined. High molecular mass structures transparent to the visible radiation, previously detected in the preinception region of soot forming flames, are also present in flames below the soot formation limit. The onset of ultra-violet fluorescence within the main oxidation zone implies that the formation of these species is a very fast process and can be considered as a “polymerization” of small aromatic groups activated by the presence of oxidizing agents. The final concentration of this material falls down as the C/O ratio is decreased and below C/O = 0.35 it is not anymore present. It appears that rich premixed flames present two critical C/O ratios: a first one for soot formation and a lower second one for high molecular mass structure formation. Ultra-violet scattering/e...

Temperature and size of single transparent droplets by light scattering in the forward and rainbow regions

A light-scattering anaysis based on Lorenz-Mie theory shows that the size and refractive index of transparent droplets can be determined by measuring the polarized components of the scattered light at two angles in the forward direction. The horizontally polarized cross section C(HH)(33°) depends exclusively on the droplet diameter, whereas the ratio C(HH)(33°)/C(HH)(60°) is a sensitive function of the refractive index and hence of the temperature. On this basis, a new optical system for measuring the temperature, size, and velocity of transparent droplets has been developed. This system can make possible the determination of droplet temperature within a few degrees centigrade. In addition, a critical review of the rainbow method to determine droplet temperature is also presented. These techniques have been applied to vaporizing tetradecane droplets (D(0) = 72 µm), which are heated up in a tube furnace with a temperature range of 20°-200 °C.

SOOT AND NANOPARTICLE FORMATION IN LAMINAR AND TURBULENT FLAMES

A new optical diagnostic method has been developed based on the interaction of a pulsed UV laser source with combustion-generated aerosols. This method allows characterization of nanoparticles of organic carbon (NOC) and soot by point measurements. Fluorescence and incandescence measurements induced by the fifth harmonic of a Nd-YAG laser at 213 nm are used for the determination of the volume fractions of particulates in a laminar premixed flame and in a turbulent non-premixed flame of ethylene/air. The selected light source enhances the fluorescence of NOC, which exhibit a large absorption band between 200 and 250 nm and also heats up soot particles to give incandescent emission. Ultraviolet emission signals are correlated with NOC extinction coefficients, while LII signals are correlated with extinction coefficients in the visible region. Laser light scattering measurements are used to estimate the mean sizes of both classes of particles.

Single droplet size, velocity, and optical characteristics by the polarization properties of scattered light.

A method is described for obtaining size, velocity, and optical properties of transparent spherical droplets employing the polarization characteristics of scattered light. A preliminary analysis of the Lorenz-Mie solution, in comparison with geometrical optics, points out the importance of surface waves in the side scattering region between theta = 85 degrees and theta = 120 degrees . Here the horizontal component of scattered light due to surface waves prevails over that due to external reflection for droplets smaller than 100 microm and the polarization ratiogamma = C(HH)/C(VV) can be employed for determining the particle size. A dual-beam system is made of two equal intensity circularly counterotating polarized laser beams which generate a polarized fringe pattern in the interference volume. The polarization ratio of scattered light, at a fixed scattering angle theta, and velocity is obtained by analysis of the bursts produced by individual droplets. The method was tested by determining the size and velocity distribution functions of droplet arrays produced by a Berglund-Liu atomizer operated either in monodisperse or in bidisperse regimes. The angular pattern of the polarization ratio was determined on calibrated streams of transparent droplets with different refractive indices, and the influence of this parameter on the role of surface waves in different angular scattering regions is discussed.