Soot Oxidation in Periphery of Diesel Spray Flame via High-Speed Sampling and HR-TEM Observation

In order to better understand in-flame diesel soot oxidation processes, soot particles at the oxidation-dominant periphery of diesel spray flame were sampled by a newly developed “suck” type soot sampler employing a high-speed solenoid valve and their morphology and nanostructure were observed via high-resolution transmission electron microscopy (HR-TEM). A single-shot diesel spray flame for the soot sampling experiment was achieved in a constant-volume vessel under a diesel-like condition. The sampler instantaneously sucks out a small portion of soot laden gases from the flame. A TEM grid holds inside the flow passage close to its entrance is immediately exposed to the gas flow induced by the suction at the upstream of the solenoid valve, so that the quick thermophoretic soot deposition onto the grid surface can effectively freeze morphology variation of soot particles during the sampling processes. The morphology of soot particles sampled by the novel suck sampler and a conventionally used “skim” sampler directly exposing a TEM grid to the flame was compared and did not show notable qualitative differences. The on-grid concentration of soot sampled by the suck sampler was lower than the ones by the skim sampler, while the repeatability of the sampled on-grid soot concentration was improved especially at downstream in-flame sampling locations. The HR-TEM observations of soot particles sampled from the oxidation-dominant periphery of diesel spray flame showed that the outer layers of the soot particles exhibit lumpy surfaces which is possibly due to stripping and disintegration of the layers by rapid oxidation and makes it difficult to identify boundaries of primary particles within the aggregates. High-resolution images of the whole aggregates produced by stitching of multiple HR-TEM images showed that these nano-structural features are only partially seen in the aggregates sampled from the core regions of diesel spray flame especially at the perimeters of the aggregates, while the aggregates sampled from the flame periphery mostly exhibits wholly collapsed outer layers.