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chapter 5 turbulent diffusion flames - FedOA

chapter 5 turbulent diffusion flames - FedOA

in non-sooting flame

in non-sooting flame conditions is probably due to these transparent high molecular mass compounds. In addition, the experimental results obtained in the works of D’Alessio et al. [20 – 22] and in other further publications [25 – 27] suggested that there is an initial fast polymerization process building bricks of which are aromatic compounds with few condensed rings (no more than 2-3 rings). The authors concluded that soot inception is primarily controlled by the internal arrangement of these polymers of aromatics leading to structures with more condensed aromatic rings and more compact three dimensional shape thus forming the first soot nuclei. The optical results obtained from D’Alessio and co-workers were in very good agreement with those obtained from Dobbins and Subramaniasivam [28] by particles thermophoretic sampling in diffusion flames, burning ethane, methane and acetylene. They found by electron microscopy analysis the presence of small polydisperse singlet particles which were more transparent than soot particles and with typical size around 3 nm. However a different chemical interpretation of these structures was given. Soot precursors were supposed in the work of Dobbins and Subramaniasivam [28] to be very large PAH while different was the interpretation of D’Alessio et al. [25] which, based on their optical data, concluded that these particles are composed by structures which have non more than two or three aromatic rings connected by aliphatic bonds. Similar results was also obtained by Vander Wal [29] performing both optical (LIF and LII measurements) and TEM analysis in a normal and inverse diffusion flame. Both techniques revealed a so-called “dark region” between the PAH and soot-containing regions that the author attributed to nascent molecular particles. All these results are, then, confirmed more recently by using Differential Mobility Analysis (DMA) [23, 30 - 32] and Atomic Force Microscopy (AFM) [24, 33]. Independently Sgrò et al. [30] and Zhao et al. [31] used DMA technique to measure the particles size distributions functions (PSDFs) in flames to follow the evolution of the particles formed in combustion. PSDF images showed that at low residence times just downstream the flames front 20

the first formed particles have a diameter of about 3 nm while at longer time the PSDFs became bimodal with a first mode that remains unchanged and shows a pick at 3 nm and a second mode characteristic of primary soot particles with mean diameter of about 10 – 20 nm. Fig. 1.2 PSDFs measured at different height above the burner in a laminar premixed ethylene-argon-oxygen flame (Zhao et al. [31]). Barone et al. [33], instead, measured PSDFs in flames using AFM technique. Particles were collected on mica substrates by thermophoretic deposition and then analysed by AFM. The results of Barone et al. [33] were in according with those obtained by Sgrò et al. [30] and Zhao et al. [31] using DMA measurements on the presence of a bimodal PSDF indicative of the two cited classis of combustion formed particles: NOC and soot. Moreover, AFM measurements, as shown by Barone et al. [33] can also been used to obtain a three dimensional topological characterization of the particles, fig. 1.3. The analysis of these images produced the important result on the different Sphericity Ratio (SR=Particles Height/Diameter) that characterize the two particles classes: 0.02 – 0.05 for NOC and 0.2 – 0.5 for soot particles. 21

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