Investigation Of Soot Properties And Soot Oxidation-induced Fragmentation In The Soot-containing Laminar Flames

In modern diesel engines,multiple injection strategy is comprehensively accepted to reduce engine noise and pollutant emissions.In this way,the soot formed from the previous combustion is involved in the combustion of the subsequent injection and inevitably affects soot processes.The goal of this study is to provide a better understanding of how the presence of soot in the combustion affects the soot properties and oxidation-induced fragmentation.Because diesel combustion course is very complicated,the study was conducted on the more easily controlled laminar flames.An aerosol generator was used to homogeneously disperse diesel soot into the laminar flames.This work is divided into two phases.In the first phase,the effect of the diesel soot addition on soot properties and reactivity was carried out in the soot-containing diffusion flame.Visual inspection of the soot particles generated in the diffusion flame with and without diesel soot doping was obtained by means of high-resolution transmission electron microscopy(HRTEM).The results show that the soot formed in the pure diffusion flame only displays external burning during soot oxidation course.When diesel soot is present in the flame,however,partial soot undergoes a unique oxidation process at the later stages of oxidation course.That is,the soot exhibits dual burning mode:external burning and internal burning.This oxidation process accompanies fringe rearrangement to form eventually graphene ribbon structures.Moreover,the diffusion flame with diesel soot facilitates the collision and aggregation between the newly formed aggregates in flame and aged aggregates of diesel soot,which in turn results in the aggregates with more chain-like structure.In order to examine the effect of the diesel soot doping on nanostructure,graphitization degree and surface functional groups of the resulting soot,the soot samples were subjected to comprehensive characterization.The HRTEM results show that the dispersion of diesel soot into the diffusion flame leads to an in fringe length(L_f)and decreases in fringe tortuosity(T_f)and separation distance(S_f),indicating the soot with more highly ordered structure.This statement is confirmed by the data derived from X-ray diffraction(XRD)and Raman spectroscopy(RS).Additionally,the data of Fourier transform infrared spectroscopy(FT-IR)and X-ray photoelectric spectroscopy(XPS)imply that the concentrations of aliphatic C–H,C=O and C–OH groups reduce and the relative amounts of aromatic C–H groups increase due to the addition of diesel soot.The results derived from atomic force microscopy(AFM)show that the electrical properties are modified by the addition of diesel soot into the diffusion flame.The electrical conductivity and work function increase as a consequence of diesel soot doping.The electrical conductivity of soot particles follows percolation theory.Additionally,the crystallite width has a positive correlation with the logarithm of the electrical conductivity and work function for the soot particles,whereas the interlayer spacing has a negative correlation with the logarithm of electrical conductivity and work function.These results suggest that the electrical conductivity and work function can serve as indicators of ordering degree of soot particles.The increases in activation energy of soot derived from the thermogravimetric analysis(TGA)suggests a reduction in soot reactivity as a result of diesel soot inclusion into the diffusion flame combustion process.The data of the characteristic oxidation temperature and active sites provide a further evidence for the reduced oxidation reactivity due to diesel soot addition.Soot nanostructure and aliphatic C–H groups have the definite correlation with the oxidation reactivity of soot and serve as the more important factors governing the soot oxidation reactivity than the soot morphology and oxygenated surface groups.In the second phase of this research,soot oxidation-induced fragmentation was evaluated in the soot-containing fuel-lean premixed flame.No soot was formed in this flame itself,which helps to isolate the soot oxidation process.The results derived from scanning mobility particle sizer(SMPS)show that the oxygen concentration in flame may be more important factor affecting the soot fragmentation than the temperature.Under the similar oxygen concentration and flame temperature,an increase in pore size in soot sample facilitates the occurrence of soot fragmentation.In addition,the bridge sites between the primary particles possess a smaller L_fvalue and larger values of S_f and T_f,which in turn exhibits a higher reactivity and a faster burning rate than bulk soot sites.This behavior is responsible for the oxidation-induced aggregate fragmentation.The effectiveness factor was used to evaluate the feasibility of primary particle breakup by O₂ internal burning.The calculated results indicate that the increases in oxygen concentration in flame and pore size in soot favor the occurrence of primary particle breakup.