Although swirling flow is known to enhance fuel-air mixing in diesel engines and thereby reducing the engine-out soot emissions, its influence on in-cylinder soot formation is poorly understood. The present study provides a direct comparison of soot particle structures between the wall-jet heads developing on up- and down-swirl sides via the thermophoresis-based soot particles sampling inside the piston bowl of a small-bore diesel engine. The sampled soot particles are imaged using a transmission electron microscope and post-processed to obtain the size of soot primary particles and aggregates as well as the fractal dimension. The TEM images show more significant reduction in soot concentration for the wall-jet head developing from the flame-wall impingement region in the up-swirl direction. The statistical analysis also shows that the increased size of soot aggregates and primary particles as well as the decreased fractal dimension are more apparent on the up-swirl side as the wall-jet travels against the swirl flow.
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