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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.