Sergio Benecchi

Determination of the Soot Volume Fraction in an Ethylene Diffusion Flame by Multiwavelength Analysis of Soot Radiation

Abstract 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.

Time-delayed detection of laser-induced incandescence for the two-dimensional visualization of soot in flames

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.

Determination of soot parameters by a two-angle scattering–extinction technique in an ethylene diffusion flame

We used a two-angle scattering technique to investigate the soot distribution in an ethylene diffusion flame in conjunction with extinction measurements. In the framework of a fractal description, we introduced a modified structure factor to interpret the scattering intensity from polydisperse aggregates. The connection between a mean value of a structural radius of gyration, R(gm1), and the quantities experimentally measured was then established. Soot parameters (volume fraction, particle size, and number densities) were determined along three radial sections of a 8-cm high-diffusion flame. The stability of the results with respect to the parameters of the distribution function was studied.

Simultaneous one-dimensional visualization of OH, polycyclic aromatic hydrocarbons, and soot in a laminar diffusion flame

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 (PAHs) could be detected. Linear (one-dimensional) visualizations of OH, PAHs, 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.

A Complete Treatment of a Steady-State Four-Level Model for the Interpretation of OH Laser-Induced Fluorescence Measurements in Atmospheric Pressure Flames

In order to obtain OH concentration measurements in atmospheric pressure flames from saturated fluorescence signal, we have developed a stationary four-level model. A general relationship between the measured upper state population and the total number density has been derived. The effects of the imbalance between the rotational transfer rates in the upper and ground states is analyzed. We show that in the case of comparable rotational rates, the results of our treatment are similar to the ones obtained with the balanced cross rate model. A set of complementary data which should facilitate absolute OH concentration measurements is also pointed out.

Investigation of the similarity of soot parameters in ethylene diffusion flames with different heights by extinction/scattering technique

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.

Local Flame Temperature Precision by the Two-Line Atomic Fluorescence Method and the Photon-Counting Technique

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.

Spatially resolved saturated absorption measurements of OH in methane-air flames

A cross-beam saturated absorption spectroscopy technique, utilizing a single pulsed dye laser, has been developed for local concentration measurements in flames. With a differential detection of the probe and the reference laser beam intensities, a significant improvement of the technique has been achieved. In this work the basic theory of the method is discussed. Its use in combustion studies is demonstrated by presenting OH concentration profiles in two premixed laminar methane-air flames.

Double pulse technique for the evaluation of the saturation parameter in single-shot laser induced fluorescence measurements: an application to lead in a flame

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.

Use of an intensified photodiode array detector for the acquisition of rotationally resolved laser-induced fluorescence spectra of OH in flames

Abstract The possibility of recording rotationally resolved fluorescence spectra in atmospheric pressure flames, with an intensified diode array detector (PDA), is described. Measurements have been performed with a medium resolution spectrograph coupled with a 1024 diodes array detector and with an IBM-PC for data acquisition and processing. The signal-to-noise ratio achievable with this detection system has been investigated, and the best experimental conditions were determined. The rotational population as resulting from a computer code data elaboration is also presented. This work confirms the capabilities of PDA detectors for acquiring large amounts of data in experiments involving fluorescence measurements.