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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.

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.

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.

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.

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.