1,720,995 research outputs found
Diagnostic for the characterization of nanometric structures in high temperature reactive systems
No full textIt is now well know that the fraction of particulate mater in the ambient air defined as ultrafine particles can be considered the most critical for adverse human health effects because of their chemical composition and the ability of these particles to penetrate deeply into the respiratory tract. Moreover combustion has been recognised as the major source of harmful fine and ultrafine particles.
The aim of the present thesis work is to investigate carbonaceous nanoparticles formation by combustion processes. An experimental procedure based on the use of the fifth harmonic of a Nd:YAG laser at 213 nm as exiting source and on an accurate signals acquisition has been
realized. In-situ spectral optical measurements based on a combination of: Laser Induced Fluorescence (LIF), Laser Induced Incandescence (LII), Light Extinction (Kext) and Laser Light Scattering (Qvv) techniques have allowed to follow particles formation and their evolution directly in combustion environments with high spatial and temporal resolution. Laminar premixed and laminar and turbulent diffusion flames have been investigated burning ethylene,
methane and benzene as fuels. Optical results are then compared with Particle Size Distribution Function (PSDF) obtained by Scanning Mobility Particles Sizer (SMPS) measurements in same flame conditions. An experimental investigation of the particulate emissions from commercial
burners for home appliances fueled with natural gas has been also included.
The experimental evidences, in according to literature in laminar premixed conditions, allow to conclude that two classes of nanoparticles are formed in flame:Nanoparticles of Organic Carbon (NOC) with sizes smaller than three nanometers and “primary” soot particles with sizes larger
than ten nanometers that lead to the formation of soot aggregates. Moreover, the thesis work shows that these combustion-generated nanoparticles strongly depend on the type of fuel, type of combustion system and eventual exhaust treatment systems
Particle Inception in a Laminar Premixed Benzene Flame
No full textSpectral optical techniques, including light extinction and laser induce fluorescence and incandescence measurements, are combined to characterize large-molecule soot precursors and soot in a slightly sooting flame of benzene at atmospheric pressure. Light absorption coupled to in-situ light scattering measurements and ex-situ Atomic Force Microscopy also allowed the evaluation of particle sizes. In the benzene flame high molecular mass structures with typical sizes of 3–4nm are formed in the main oxidation region of the flame. The radical-rich flame environment in which these compounds are formed promotes their dehydrogenation increasing the level of their aromaticity. As a result, nanoparticles with typical sizes of about 5 nm, absorbing and fluorescing in the visible are formed. These compounds reach a maximum concentration just before the appearance of incandescent soot particles
Particle Formation in Opposed-Flow Diffusion Flames of Ethylene: an Experimental and Numerical Study
No full textAn experimental and numerical study on particles inception and growth is performed in opposed-flow diffusion flames of ethylene and air characterized by different sooting tendencies. Spectrally resolved UV-visible laser induced fluorescence, laser induced incandescence and laser light scattering measurements are used to characterize different classes of combustion-generated compounds. A detailed kinetic model accounting for both gas phase and particle formation is used. Comparison between experimental results and numerical predictions gives a qualitative view of the mechanism of particle formation in opposed-flow flames. Particle inception is the result of both chemical growth and coagulation of aromatic compounds. In the region close to the flame front where the temperature is relatively high and radicals are abundant, the particle inception is due to a chemical growth mechanism by which aromatic molecules add aromatic radicals leading to the formation of biphenyl-like structures. The growth process continues as high-molecular mass aromatics are moved away from the flame zone towards the stagnation plane by the addition of acetylene and other aromatics forming particles of increasing sizes. Graphitization of these particles and thermal annealing lead to the formation of soot particles. At relatively lower temperatures, found across the stagnation plane, particles growth still occurs and it is mainly due to a process of physical coagulation of PAHs. The experimental and numerical results obtained in this work demonstrate and explain the sensibility of inception and growth of particles to radical concentration and temperature in opposed-flow flame configurations
An experimental and modelling study of particulate formation in premixed flames burning methane
No full textSpectral optical techniques, including light extinction and laser induce fluorescence and incandescence measurements, are combined to characterize large-molecule soot precursors and soot in a slightly sooting flame of benzene at atmospheric pressure. Light absorption coupled to in-situ light scattering measurements and ex-situ Atomic Force Microscopy also allowed the evaluation of particle sizes. In the benzene flame high molecular mass structures with typical sizes of 3–4nm are formed in the main oxidation region of the flame. The radical-rich flame environment in which these compounds are formed promotes their dehydrogenation increasing the level of their aromaticity. As a result, nanoparticles with typical sizes of about 5 nm, absorbing and fluorescing in the visible are formed. These compounds reach a maximum concentration just before the appearance of incandescent soot particles
Emission of fine particles from natural gas domestic burners
No full textAn experimental study of the combustion characteristics of new burners used for home and water heating appliances and domestic cook top is presented with the aim of evaluating the effect of burner configurations and operating conditions on the emissions of gaseous pollutants and organic carbon (OC). Advanced in-situ optical diagnostics, based on laser induced emission spectroscopy (LIE), and ex-situ measurements, based on scanning mobility particle sizer (SMPS), and particles collection by water-based sampling technique, are used in order to evaluate the total particulate concentration and size distribution functions. For the home heating burners three different configurations: two premixed and one diffusive are studied under various operative conditions. Measurements have shown that particulate matter with size in the 1 nm to 10 nm size range is formed in all the examined conditions. The emitted mass concentration of these compounds is very low, of the order of 0.001 ppm. They are formed in large number concentrations in the flame region but are also strongly oxidized in the post-oxidation region of the devices. While domestic cook-top emitted a larger amount of organic carbon since it is not post-oxidized. Soot particles with size larger than 10 nm are not formed in all the examined conditions
Thin film coatings prepared by direct thermophoretic deposition of flame-made nanoparticles
No full textThis study reports the development of a one-step method for the production of thin film coatings made with metal oxide nanoparticles. An aerosol flame synthesis system is used to produce monodisperse, ultra-fine nanoparticles of different metal oxide, by changing the precursor fed to the flame. The flame reactor is a fuellean reactor of ethylene and air. Flame-synthesized nanoparticles are directly deposited by thermophoresis onto different substrate by means of a rotating disc. Substrates were mounted onto the rotating disc that repetitively passes through the flame. Convection due to the rotational motion cooled the substrates, on which particles were deposited as films by thermophoresis. Such a system allowed to obtain submicron coatings of different thickness and porosity, by varying the total time of deposition. Different substrates can be coated using this method, such as aluminum and steel plate. Particle and coating characterization is performed by means of Differential Mobility Analysis, Raman and X-Ray Diffraction spectroscopy, and UV-Vis absorption. A preliminary analysis of the antimicrobial activity of TiO2nanoparticle coatings produced with this method has been performed against Candida Albicans, and compared to that of commercial TiO2nanopowder. The results are promising for using titania films as protective coatings for applications where an antimicrobial activity is required, such as self-cleaning materials able to reduce microbial infections
Antimicrobial activity of TiO2 coatings prepared by direct thermophoretic deposition of flame-synthesized nanoparticles
No full textStructure and size of soot nanoparticles in laminar premixed flames at different equivalence ratios
No full textMonitoring flame soot maturity by variable temperature Raman spectroscopy
Full text linkFlame-formed soot nanoparticles are known to possess different nanostructures as a function of a large variety of combustion operative parameters, including particle residence time in flames, flame stoichiometry, fuel chemical composition, temperature and pressure. This implies that important properties (e.g., soot oxidation reactivity, optical and electronic gap) may be different depending on the way soot particles are generated. Hence, it is beneficial to develop diagnostic methods that are sensitive to such differences among the various kinds of soot particles. In this work we investigate by off-line variable-temperature Raman spectroscopy two types of soot particles, namely just-nucleated and aged/mature particles, with a different size and maturity degree. The results obtained for soot particles have been compared and discussed with those found for other related carbon mate- rials. As the Raman spectra are recorded at increasing sample temperature, from 300 to 525 K, both soots display a linear downshift of the position of the two main Raman modes (D and G bands). Just-nucleated soot particles exhibit the strongest temperature dependence of the position of the G band. Based on results from density functional theory calculations, interpreted by a simple empirical model, the observed temperature coefficient differences can be ascribed to differences in the nanostructural order of the two soot samples, which causes a different thermal expansion behavior
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