Microscopic characteristics of multiple droplets behaviors at the near-wall region during the quasi-steady state

摘要

It is well known that the liquid adhesion by impingement in the direct-injection spark-ignition (DISI) engine hinders the engine combustion efficiency and increases the particular matter (PM) emissions. Although numerous investigations were done on it, it is still blur for the scholars owing to the complicated droplets-wall dynamics. Due to the dense liquid near the wall, it is difficult to observe the impinging droplets in this region, let alone analyzing it on the micro level. Therefore, the "multiple droplets producer" was applied to cut the spray for clear observation in this study. Tslicer was finally determined at 40 mu m to make the multiple droplets impacting on the wall at the near-wall region under various injection pressure from 10 to 30 MPa during the quasi-steady state. Four different locations of (15, 15), (17, 15), (20, 15), (22, 15) were selected along the jet development after impingement. Particle image analysis (PIA) technology was applied to capture the micro behaviors. The diameter and velocity of droplets were calculated and analyzed as well as the Weber number. Results show that droplet behaviors near the wall can be recorded at T-slicer = 40 mu m during the injection. Droplet size becomes larger with spray propagation at the near-wall region. Observations demonstrate that the coalescence phenomenon of the secondary droplets as well as the splashing crown structures occurrence leads to the larger droplet and lower velocity. Moreover, non-dimensional parameter We was used to further prove the splashing transition occurring from (17, 15) to (20, 15). Additionally, the experimental results can provide strong evidence and verification basis for computational fluid dynamics (CFD) simulations.