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Abstract: Small Angle X-ray Scattering (SAXS) measurements have been performed on a partially pre-mixed ethylene–air flame with and without the introduction of a water aerosol for the same flame height. Our measurements are sensitive to particles with radii in the range of 6–78nm. It is found that water addition leads to soot suppression below the detection limit at low heights along the flame centerline (<11mm) and to an increased sooting tendency higher up in the flame. There, larger primary particles (Rg ∼20–45nm) shaped as agglomerates with fractal dimension about 2 are observed together with a large number of smaller (Rg ∼10nm) smooth particles, which we identify with sub-primary units observed in previous work on pure diffusion flames. In the case of the un-doped partially pre-mixed flame, however, only relatively (Rg ∼20–35nm) large spherical particles (Porod’s exponent about 4) are observed. An intriguing result is that water addition induces the enhanced formation of sub-primaries, with these having more than 2 orders of magnitude higher number concentration with respect to the spherical particles in the water free flame. [Copyright &y& Elsevier]

Abstract: The high photon flux (1014 photons/s) and high spatial resolution (∼100μm) of synchrotron radiation available at ID09b/ESRF facility has been exploited in an in situ investigation of the early stages of soot formation in an ethylene–air diffusion flame. SAXS data demonstrate in situ evidence that the size distribution of soot nanoparticles within the flame exhibits three distinct modes at different heights above the burner z’s. In particular, at z ∼3mm, small particles (4–6nm size), called sub-primary particles, are observed to come into existence. The corresponding monomodal distribution is observed to evolve to bimodal one at z >5mm, with the sub-primary particle mode being progressively depleted in favor of the growth of a mode corresponding to larger primary particles (10–12nm size). The sub-primary peak vanishes completely at about z ∼20mm where distribution is again single mode about 12nm diameter. Porod’s plots show that the sub-primaries are born as configurational flat entities similar to discs or lamellae with a small aspect ratio (Porod’s exponent about 2), and upon going to higher z’s, they approach a spherical form with a smooth surface (Porod’s exponent about 4). Moreover, the careful use of Kratky plots has allowed to demonstrate the presence within the flame of very small nuclei, sized about 1.5–2nm, which have a nucleation burst at about z ∼5mm, whose number concentration progressively decreases at larger z, finally disappearing around z ∼15mm. The relative increase of primary particles (size larger than 12nm) is found to correspond to the progressive decrease of these very small nuclei (∼2nm) concentration. At heights larger than 15mm a strong ionization signal is observed that increases with height. These findings are in agreement with previous experimental works in the literature performed by Transmission Electron Microscopes and Differential Mobility Analyzers as well as with theoretical studies of dimer-dominated stochastic coagulation. [Copyright &y& Elsevier]