Study on the microscopic characteristics of successive droplet clusters impacting on the wall

In Direct Injection Spark Ignition (DISI) internal combustion engines, fuel droplets inevitably impact the cylinder walls, resulting in droplet adhesion and incomplete combustion, thereby increasing pollutant emissions. The injection strategy has been proven to significantly improve the droplet-wall interaction. Initially, the Refractive Index Matching (RIM) method was used to measure fuel adhesion under both single and double injection conditions, with results confirming that the double injection strategy significantly reduced the fuel adhesion mass. To investigate the mechanisms underlying the reduction in fuel adhesion with double injection, Particle image analysis (PIA) and a multiple droplets producer were employed to examine the micro-behavior of successive droplet clusters near the wall surface. Statistical analyses were conducted on droplet size and velocity. Results showed that, when the two successive droplet clusters impact the wall, the Sauter Mean Diameter (SMD) of the second cluster is greater than that of the first cluster. There are two reasons for this. First, this difference is attributed to the coalescence between the leading droplets of the second droplet cluster and the trailing droplets of the first droplet cluster. Second, when the second droplet cluster impacts the wall, the breakup of fuel adhesion can also lead to the formation of larger droplets. Moreover, the second droplet cluster exhibits a significantly higher penetration velocity than that of the first, which can be attributed to the interaction of the velocity fields between the trailing droplets of the first cluster and the leading droplets of the second cluster. Furthermore, analysis of the average minimum droplet distance shows that droplet number density near the wall is relatively high and decreases with increasing distance from the wall. Lastly, the Bai model was used to predict the probabilities of stick, rebound, spread, and splash of successive droplet clusters at various observation points.