Transmission electron microscopy of soot directly sampled at different axial locations in diesel spray flame

Abstract

For further reduction of particulate emission from a diesel vehicle, it is fundamentally important to understand in-flame soot formation and oxidation processes. In order to clarify the processes, the morphology of soot directly sampled at different locations in the spray flame (40 mm to 90 mm from the injector nozzle tip) were investigated using a transmission electron microscope (TEM). The diesel spray flame was achieved in a constant volume combustion chamber under diesel-like conditions (2.5 MPa and 940 K). The concentration, diameter of primary particles and the radius of gyration of soot aggregates increased in the upstream region (40 to 50 mm), exhibited a peak around the mid-stream region (60 to 70 mm), and then decreased in the downstream region (80 to 90 mm), which corresponds to formation, peak concentration and oxidation of soot particles in the spray flame, respectively. The fractal dimension of soot aggregates was also analyzed and was almost constant, with a value of approximately 1.73 over different axial locations in the spray flame, which suggests that the dominant growth process of in-flame soot aggregate is cluster-cluster aggregation in diffusion limited regime.

Keywords

• diesel engine
• diesel spray flame
• tem
• soot
• pm
• morphology