Soot oxidation and the mechanisms of oxidation-induced fragmentation in a two-stage burner for ethylene and surrogate fuels.

摘要

Studies on the evolution of soot particle size distributions during the process of soot oxidation were carried out in the two-stage burner by using a Scanning Mobility Particle Sizer (SMPS) for n-butanol/n-dodecane, methyl decanoate/n-dodecane, and ethylene flames. This experimental technique, along with measurements of flame temperature, gas-phase composition, surface functional groups, and soot nanostructure and morphology, allowed for identifying effective parameters during soot oxidation and the mechanisms associated with soot oxidation-induced fragmentation.;The results of increasing n-butanol and methyl decanoate in n-dodecane showed a reduced sooting propensity; however, it did not enhance soot oxidative reactivities. The result of image analysis technique demonstrated a strong dependence of soot oxidation rate on the initial soot nanostructure, whereas oxygen functionalities did not matter as much. The highest soot oxidative reactivity was found for the soot nanostructure with the minimum degree of orderliness. On the other hand, the lowest oxidative reactivity was observed for the soot with the nanostructure composed of large layer planes with either low or zero curvatures.;Soot oxidation-induced fragmentation was studied by using ethylene fuel. The mechanisms of soot oxidation-induced fragmentation were explored by following changes in mobility size, number and concentration, flame temperature, and gas-phase compositions. Results showed that the rate of fragmentation was inversely proportional to the peak temperature, and the onset of fragmentation depended on the presence of aggregates. In addition, two main mechanisms suggested in the literature, (i) aggregate break-up by burning bridges; (ii) primary particle break-up by O2 diffusion, were tested with the aid of an image analysis technique. The results demonstrated that bridge sites were formed by less-ordered nanostructure, resulting in a faster burning rate, suggesting aggregate fragmentation by this mechanism. The effectiveness factor calculation was used to evaluate the feasibility of primary particle breakup by O2 internal burning. It was shown the primary particle breakup for particles smaller than 10 nm becomes more probable by decreasing temperature and increasing O2 partial pressure.