Francesca Migliorini

Application of the Hough transform for the automatic determination of soot aggregate morphology.

We report a new method for automated identification and measurement of primary particles within soot aggregates as well as the sizes of the aggregates and discuss its application to high-resolution transmission electron microscope (TEM) images of the aggregates. The image processing algorithm is based on an optimized Hough transform, applied to the external border of the aggregate. This achieves a significant data reduction by decomposing the particle border into fragments, which are assumed to be spheres in the present application, consistent with the known morphology of soot aggregates. Unlike traditional techniques, which are ultimately reliant on manual (human) measurement of a small sample of primary particles from a subset of aggregates, this method gives a direct measurement of the sizes of the aggregates and the size distributions of the primary particles of which they are composed. The current version of the algorithm allows processing of high-resolution TEM images by a conventional laptop computer at a rate of 1-2 ms per aggregate. The results were validated by comparison with manual image processing, and excellent agreement was found.

Absorption correction of two-color laser-induced incandescence signals for soot volume fraction measurements

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.

Probing the equivalence ratio in partially premixed flames by combining optical techniques and modeling results

ABSTRACT Chemiluminescence and laser-induced breakdown spectroscopy (LIBS) measurements have been carried out in laminar ethylene/air partially premixed flames for real-time local equivalence ratio monitoring. OH* and CH* chemiluminescence measurements have been performed in the region where combustion processes mainly occur. Using LIBS measurements, the hydrogen to oxygen emission signal ratio has been obtained as an indicator of the fuel to air ratio and consequently of the mixing processes. Through a calibration procedure performed in nitrogen-diluted ethylene/air premixed flames of known equivalence ratio, the local values of the equivalence ratio have been derived in the flames under analysis. Target flames, proposed in the framework of the International Sooting Flame (ISF) workshop, have been investigated along the radius at different heights above the burner. Correspondingly, chemical kinetic modeling analysis has been performed and the results were compared with the experimental data in order to cross-check the reliability of both experimental techniques and modeling approach.

Laser-Induced Breakdown Spectroscopy Analysis of Lead Aerosol in Nitrogen and Air Atmosphere

In this work, laser-induced breakdown spectroscopy (LIBS) is applied for quantitative measurements of Pb in aerosols. In order to investigate the carrier gas role and, in particular, the effect of O2 addition to the gas itself, measurements are carried out in nitrogen and air atmosphere. Aerosol particles are produced by nebulizing Pb(CH3COO)2 * 3H2O aqueous solutions of known concentration and the atomic line of 405.8 nm is detected as Pb signature. The plasma generated with the laser pulse is characterized in terms of plasma temperature and electron density, showing no substantial differences with the two carrier gases used. The behavior of the LIBS signal as a function of the delay time with respect to the laser pulse is investigated changing the environmental conditions and, in particular, the Pb concentration values. The different trends registered in the case of relatively short (up to 20 μs) and long delay time, resulting to be the same whatever the Pb concentration value, could have a significant effect on the calibration curve performed in different experimental conditions.

Effects of Hydrogen and Helium Addition to Fuel on Soot Formation in Axisymmetric Coflow Laminar Methane-Air Diffusion Flame

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