The evolution of soot morphology and nanostructure along axial direction in diesel spray jet flames

This study aims at providing new insights into the soot formation and oxidation processes in diesel spray jet flames under the diesel-like conditions. Soot particles are sampled within a reacting diesel jet spray flame in a constant-volume combustion chamber under diesel-like high-temperature and pressure ambient conditions. The experiments are conducted with an injector hole of 0.28 mm and injection mass of 30 mg per shot. Soot particles at different locations along the axis of the diesel jet flame are sampled by a thermophoretic probe, and the variation of soot morphology and nanostructure of the sampled soot particles along the axial location in the flame is analyzed by using the transmission electron microscopy (TEM). At upstream of the jet flame, large “liquid-like” particles are observed in the TEM images. As these particles are delivered to the downstream, (1) there are many immature primary particles formed on the surface of large “liquid-like” particles, and (2) they are carbonized into the larger soot aggregates with the matured structure, and then at last, (3) the larger soot aggregates gradually break up into the smaller particles with relatively compact structure. Both the diameter of primary particles and radius of gyration of the soot aggregates decrease as the distance between the injector nozzle tip and the center of TEM grid is increased, while the opposite trend is found in the behavior of the fractal dimensions of soot aggregates. The high resolution TEM images show that the soot primary particles exhibit the turbostratic state at the upstream of the jet flame, while more matured structures with the graphitic crystallite in outer shell and several fine particles in the inner core are observed as the sampling distance from orifice is increased. The average fringe length increases while the average tortuosity and separation decrease with the distance from the nozzle. These results suggest that the mechanism of mature soot aggregates generated directly from the large “liquid-like” particles should be one of the ways of soot particle formation along the axis of diesel jet flames.