Numerical study on soot evolution of wall-impinging flames

Soot formation in diesel-like spray combustion remains a critical issue in the design of low-emission engines. In particular, wall-impinging sprays, which are common in modern high-pressure fuel injection systems, introduce complex interactions between flame, wall, and flow field that significantly affect soot evolution. This study employs numerical simulations to investigate soot formation and oxidation in wall-impinging diesel sprays, with variations in wall distance, wall temperature, injection pressure, ambient temperature, and ambient oxygen concentration. Virtual Particle Tracking (VPT) method is used to analyze the thermochemical history of soot parcels. Results reveal that the increase in total soot mass in wall-impinging cases is primarily caused by soot accumulation near the wall while the influence of low wall temperature on the formation of soot is limited. Shorter wall distance impingement case exhibits suppressed soot formation and oxidation throughout the entire process due to strong thermal and spatial constraints imposed by the wall. Lower injection pressure shortens lift-off length, resulting in a locally higher equivalence ratio that favors soot formation. Additionally, the reduced spray momentum prolongs the residence time in regions favorable for soot formation. Elevated ambient temperature and oxygen concentration both promote soot formation and oxidation. The influence of elevated ambient temperature on the formation of soot is more significant than oxidation of soot, which widens the gap between formation and oxidation rates and leads to greater soot accumulation. In contrast, under high ambient oxygen concentration, the effect on soot formation is relatively weak, resulting in lower overall soot mass. These findings highlight the key role of wall effects in shaping soot behavior, providing useful insights for optimizing injection and wall design to control soot emissions.