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Although the two-color laser-induced incandescence technique (2C-LII) has proved to be a significant tool for soot diagnostics, many efforts are still required to gain a whole understanding of the chemical and physical processes involved. Time-resolved two-color LII measurements are carried out in a rich ethylene/air premixed flame at different heights above the burner and by changing the laser fluence. The prompt LII at two wavelengths and the corresponding soot incandescence temperature are obtained at different stages of the soot growth and under different laser irradiations. The decay rate of the LII signals, as a method for soot sizing, is investigated at different laser fluence. The time-resolved LII curves, obtained in the low laser fluence regime, are analyzed by a numerical simulation, available on the web. By considering the gas/particle initial temperature obtained with thermocouple measurements and by knowing soot particle diameter with previous TEM and extinction/scattering measurements, information about soot parameters, such as absorption function and thermal accommodation coefficient are obtained. The presence of the so-called young or mature soot along the flame height is strictly related to different optical and heat-exchange properties necessary to fit all the experimental data available.

Particle size is a crucial parameter in nanopowder use and production. A method to obtain information about this during the particle synthesis would be very valuable in order to optimize the process. In the field of nanoparticles flame synthesis, an extension of the techniques used in soot diagnostics sounds rather obvious, but it is far from being easily accomplished. In this paper investigations on the application of the laser induced incandescence to TiO 2 nanoparticles in a flame reactor are reported. The work basically concerns the effects of laser fluence on the spectral structure of the laser-excited radiation emitted from the particles. It will be shown that in many cases such a radiation cannot be plainly assumed as an incandescence signal. Measurements of the signal time decay are reported and interpreted in the light of the spectral indications. Finally, the dependence of such a time decay on the particle size is experimentally demonstrated.

The two-color laser-induced incandescence (2C-LII) technique can be used for soot volume fraction measurements with no use of a reference flame of known soot concentration. In this work we have exploited the 2C-LII technique with the development of a two-dimensional imaging arrangement. By utilizing an IR pulsed laser, a laser sheet with sharp edges, and controlled fluence, the maximum soot temperature was verified to be around 4000 K in all the investigated regions of an ethylene diffusion flame. The calibration-free 2D 2C-LII arrangement was then feasible and applied for soot imaging. Verification of the soot volume fraction measurements was carried out by comparison with other techniques such as “point” 2C-LII, laser extinction, 2D extinction, and 2D two-color emission techniques. Agreements and some discrepancies are discussed.

Soot production of diesel oils, characterised by a well-defined aromatic content, was investigated in rich premixed flames by laser extinction technique. By analysing the soot plume, a criterion to assign a ranking in sooting tendency was found, based on the connection between the maximum soot volume fraction detected along the flame axis, f v M , and the equivalence ratio. A relationship between the total soot load and f v M was established, which allows to determine a correlation between the soot load and the aromatic composition. The resulting linear dependence is discussed.

The features of the multiwavelength emission technique for the measurement of soot volume fraction and temperature in an ethylene diffusion flame have been investigated. For this purpose we have exploited the emission spectra from 300 nm to 800 nm and a mathematical Abel inversion procedure. To interpret the measurements, the quantity fv/Kabs = λ/(36πF(λ)), here called natural length for absorption, l abs, is modeled as a continuous function according to literature data. Soot temperature profiles were also obtained showing small variations in the investigated region of the flame. Extinction measurements on the same flame were compared, and good agreement was found between the two techniques. With respect to the more-common two-color emission technique, the use of a wide set of spectral data reduces the uncertainties due to the determination of the index of refraction of soot. Different sets of refractive index have been compared and the results are discussed.

The scattering/extinction technique was applied to measure the distribution of soot and the features of the aggregates in four ethylene diffusion flames of different total height. Flames were explored radially at three levels above the burner (45%, 60%, and 75% of the total flame height). Scattering measurements were performed at two angles (45° and 135°) to obtain the radius of gyration of soot aggregates. With the combination of extinction data, all other soot parameters (primary particle diameter, number density of particles, aggregates, and number of particles/aggregates) were calculated. The complete set of results extends previous findings and allows a reasonable and self-consistent description of soot aggregation and oxidation steps. Soot aggregates maintain their ramified structure during oxidation, which basically involves a shrinking mechanism of the monomers. One of the major aims of this work was to ascertain the degree of self-similarity of the flames. Results indicate a rather good repeatability of soot volume fraction distributions both axially and radially. The self-similarity holds to a lower extent for soot parameters such as aggregate size and number density, especially for the shortest flame investigated (5 cm). This fact suggests normalization of the heights starting from a characteristic distance to take into account a fixed time necessary for soot nucleation. This allows an almost perfect superimposition of the soot volume fraction axial profiles.

The enlargement of the probe volume in saturated laser induced fluorescence measurement is a considerable drawback for absolute concentration measurements. For practical purposes, a double pulse technique was developed allowing determination of the saturation parameter, which has a major influence on the probe volume enlargement. The technique is based on the detection of the fluorescence produced by two time-delayed laser pulses of different intensity. The basic theory of the technique is presented, together with the results of an experimental application to Pb in an acetylene/air flame. The implications for absolute concentration measurements are also discussed.

The effect of hydrogen addition to fuel in soot formation and growth mechanisms is investigated in a rich ethylene/air premixed flame. To this purpose, three-angle scattering and extinction measurements are carried out in flames with different hydrogen contents. By applying the Rayleigh–Debye–Gans theory and the fractal-like description, soot concentration and morphology, with the evaluation of radius of gyration, volume-mean diameter and primary particle diameter are retrieved. To derive fractal parameters such as fractal dimension and fractal prefactor to be used for optical measurements, sampling technique and TEM analysis are performed. In addition, data concerning soot morphology obtained from TEM analysis are compared with the optical results. A good agreement in the value of the primary particle diameter between optical and ex-situ measurements is found. Significant effects of hydrogen addition are detected and presented in this work. In particular, hydrogen addition to fuel is responsible for a reduction in soot concentration, radius of gyration and primary particle diameter.

The effect of multiple laser pulses reaching soot particles before an actual laser-induced incandescence (LII) measurement is investigated in order to gain some insights on soot morphological and fine structure changes due to rapid laser heating. Soot, extracted from a premixed and a quenched diffusion flames, is flowing through a tubular cell and undergoes a variable number of pulses at different fluence. The response of soot is studied by the two-color LII technique. Transmission electron microscopy (TEM) analysis of laser-modified soot aggregates from the diffusion flame is also presented. The results indicate that even at low laser fluences a permanent soot transformation is induced causing an increase in the absorption function E(m). This is interpreted as an induced graphitization of soot particles by the laser pulse heating. At high fluences the vaporization process and a profound restructuring of soot particles affect the morphology of the aggregates. Soot from diffusion and premixed flames behaves in a similar way although this similarity occurs at different fluence levels indicating a different initial fine structure of soot particles.

In this work a three-angle scattering and extinction technique has been applied in order to study soot formation and growth in a rich ethylene/air premixed flame (F = 2.34). The Rayleigh-Debye-Gans theory together with the fractal-like approach has been applied to derive soot parameters, in terms of volume concentration and morphology. A mathematical procedure is presented to obtain the radius of gyration by considering scattering signals collected at two supplementary angles. TEM measurements, carried out at different locations on the flame axis, are used to derive some parameters, such as fractal prefactor, fractal dimension and size distribution, to be entered in the mathematical treatment of optical data. The radius of gyration and the primary particle size as obtained by TEM and by optical measurements are compared. Good agreement has been found in the upper part of the flame investigated. Discrepancies observed low in the flame are discussed.

The time-delayed detection of soot incandescence is demonstrated to discriminate against other laser-induced signals that have shorter decay times. This technique exhibits high sensitivity and no need for any verification of the spectral content of the signal; it is promising for two-dimensional imaging applications in hostile environments, such as in practical flame and combustion chambers, in which it permits an easy visualization of sooty regions.

Theoretical papers predict that prompt LII signals are weakly dependent on the soot size due to the fact that larger particles reach higher temperatures during the heating process by nanosecond laser pulses. This question is of crucial importance for establishing LII as a practical technique for soot volume fraction measurements. In this work two-color prompt LII measurements have been performed in several locations of diffusion and rich premixed ethylene-air flames. The experimental apparatus was carefully designed with a probe volume of uniform light distribution and sharp edges, a 4 ns integration time around the signal pulse peak and narrow spectral bandwidth. Measurements did not confirm the theoretical predictions concerning an increase of temperature for larger particles. On the contrary, larger particles in richer premixed flames exhibit a lower 400/700 signal ratio. This can probably be attributed to small differences in the refractive index of soot.

McKenna burners are widely used in the combustion community for producing "flat" premixed flames. These flames are considered as standards for the development and calibration of optical techniques. Rich premixed flames produced by McKenna burners are frequently investigated in order to understand soot formation processes both by optical and by sampling techniques. Measurements are normally performed along the axis of the flames, with a uniform distribution of temperature and species concentration assumed in the radial direction. In this work it is shown that the soot radial profiles of rich premixed ethylene-air flames produced by a McKenna burner with a stainless steel porous plug may be far from being "flat." Soot is mainly distributed in an annular region and nonsoot fluorescing species are present in the core of the flames. This surprising result was verified under several working conditions. Furthermore, flames cannot be considered axial-symmetric but present a skewed soot distribution. Another McKenna burner with a bronze porous disk was used to produce flames of the same equivalence ratio and flows. These flames show a completely different soot radial profile, closer to the claimed flat distribution. These results cast doubts about the conclusions drawn in several studies on soot formation performed with a stainless steel McKenna burner.

Laser-induced fluorescence (LIF) is one of the most popular techniques for flames diagnostics. Temperature measurements are in general obtained by scanning the laser wavelength through a molecular absorption band in order to probe the ground-state rotational population. OH is frequently used as thermometric species because of its favorable spectral characteristics and of its recurrence in combustion processes. Minor species concentration measurements are also performed by LIF, owing to its great sensitivity. While excitation spectra for temperature measurements must be carried out with lowintensity probing beams (linear regime), absolute concentration measurements require high-power excitation to overcome quenching problems (saturation regime). Moreover this latter regime allows maximum sensitivity and minimum dependence from laser intensity fluctuations. However, the quantitative interpretation of saturated LIF signals poses some problems not yet completely resolved (see Ref. 5 and references therein). The most typical is the evaluation of the effective probe volume due to nonuniformities in the intensity distribution of focused laser beams.

The uses of webcam as a probe beam analyzer were discussed. The webcam allowed an easy and highly spatially resolved visualization of the beam shape together with standard laboratory equipment and a common personal computer (PC). The webcam was operated at fixed exposure time through the factory control software and images were processed by means of the image pro-plus software. The webcam was also used for visualizing beam shape either pulsed or continuous wave at a low cost.

Numerical and experimental studies were conducted to investigate the effects of hydrogen and helium addition to fuel on soot formation in atmospheric axisymmetric coflow laminar methane-air diffusion flame. Soot temperature and volume fraction distributions were measured using a two-dimensional two-color technique. Numerically the conservation equations of mass, momentum, energy, and species in the limit of low-Mach number were solved. Detailed gas-phase chemistry and thermal and transport properties were accounted for. Radiative heat transfer by CO, CO2 , H2 O, and soot was calculated using the discrete-ordinates method with the radiative properties of the mixture obtained from a wide-band model. Soot was modeled using a two-equation semi-empirical model in which the mechanisms for inception and surface growth are assumed to be PAH coagulation and H-abstraction acetylene addition. Both experimental and numerical results show that helium addition is more efficient than hydrogen addition in reducing soot formation in the methane flame. These results are different from the previous investigations in ethylene flames where the hydrogen addition was found to be more effective in reducing soot formation than helium addition due to the additional chemical suppression of hydrogen on soot. It is suggested here that hydrogen chemically enhances soot formation when added to methane.Copyright © 2007 by National Research Council Canada

A numerical iterative procedure is presented for the evaluation of the effect of signal absorption in two-color laser-induced incandescence measurements. The correction process is applied to our experimental data in an axisymmetric flame [Appl. Opt. 44, 7414 (2005)]. The influence of signal trapping on peak soot temperature and on soot volume fraction has been found to be minimal. Some numerical tests were performed to investigate the effects of soot concentration, flame size, and soot refractive index on the magnitude of the signal absorption correction.

In order to understand the processes involved in the laser-induced incandescence (LII) technique, the value of soot temperature at the peak of the incandescence signal has been studied. To this purpose, an absolute two-color LII technique has been applied on ethylene and methane diffusion flames, based on the comparison with a calibrated tungsten ribbon lamp. The dependence of peak temperature on the fluence has been investigated by using a sharply edged probe beam. Above a certain fluence threshold a value close to 4000 K was obtained for both flames at all locations, that means in largely different soot conditions. At a suitably selected laser fluence, radial and axial profiles of peak soot temperature and volume fraction were performed. Soot volume fraction data have been validated with results from laser extinction technique measurements. The quite low values observed for methane prove the sensitivity of the LII technique. Moreover, a discussion about soot refractive index is presented. In the visible region a test of its influence on both soot volume fraction and soot peak temperature was carried out, while in the infrared the heating process was analyzed.

A two-color version of the laser-induced incandescence (2C-LII) technique was implemented for measuring absolute soot volume fraction in flames. By using a calibrated tungsten ribbon lamp, soot peak temperatures were measured as a function of fluence at several locations in an ethylene diffusion flame by using a steeply edged laser beam profile. Above a certain fluence threshold, peak temperatures were tightly distributed just above 4000 K independent of the particle size and number density. Radial profiles of soot volume fraction were obtained and compared (not calibrated) with results from the laser extinction technique. Good agreement showed the validity of the 2C-LII technique at a controlled fluence.

A two-dimensional soot diagnostic technique has been developed as an extension of the well-known two-color pyrometry. Two flame images are simultaneously collected on a CCD at selected wavelengths through suitable optics. By use of the dependence of soot emissivity on the soot volume fraction and by comparison with images from a calibrated light source, both the temperature field and the soot distribution can be determined. Validation was carried out through data obtained with other soot diagnostic methods on ethylene diffusion and Diesel oil-rich premixed flames. The current technique readily allowed us to obtain a large amount of data for a thorough description of the soot distribution within the flame. As an example of the technique’s potential, data about methane and propane diffusion flames are reported.

The features of the multiwavelength emission technique for the measurement of soot volume fraction and temperature in an ethylene diffusion flame have been investigated. For this purpose we have exploited the emission spectra from 300 nm to 800 nm and a mathematical Abel inversion procedure. To interpret the measurements, the quantity fv/Kabs = ?/(36?F(?)), here called natural length for absorption, abs, is modeled as a continuous function according to literature data. Soot temperature profiles were also obtained showing small variations in the investigated region of the flame. Extinction measurements on the same flame were compared, and good agreement was found between the two techniques. With respect to the more-common two-color emission technique, the use of a wide set of spectral data reduces the uncertainties due to the determination of the index of refraction of soot. Different sets of refractive index have been compared and the results are discussed.

Because of the planned limitations on pollutants it is becoming more and more stringent to reduce IC engines emissions. In particular, for diesel oil engines, the reduction of soot emission is mandatory. It is well known that aromatic compounds play an important role in the mechanisms of soot production. It is possible that the different classes of aromatics naturally present in the Diesel fuel lead to the formation of different amounts of soot. For this work a set of seven fuels with calibrated composition of different blend of mono-, di- and tri-aromatics but with total aromatic content of about 27% was made available by ELF (Solaize, France). Soot and aromatic hydrocarbons profiles have been detected on the axis of rich premixed flames in order to find possible correlations.

Laser-induced fluorescence measurements were performed in several regions of an ethylene-air diffusion flame. With the use of a 290-nm excitation wavelength, the fluorescence emissions from OH molecules and polycyclic aromatic hydrocarbons (PAH's) could be detected. Linear (one-dimensional) visualizations of OH, PAH's, and soot scattering were performed by utilizing an intensified photodiode array detector. Measurements taken at different heights above the burner permitted a two-dimensional reconstruction of the flame structure. Finally the simultaneous visualization of the three species for a rapid determination of the relative distributions was demonstrated.

In recent years the two-line atomic fluorescence method (TLAF) has received increasing attention for performing local temperature measurements in flames with a good spatial resolution even with low power sources.