Microscopic behavior of spray droplets under flat-wall impinging condition

The impingement of fuel spray on the wall in direct-injection spark-ignition (DISI) engines affects the fuel-air mixture formation, combustion and exhaust emissions. A detailed understanding of the fuel spray and impingement is required to reduce the undesirable products of combustion whilst maintaining good fuel economy. In this study, the droplet size was measured by particle image analysis (PIA). The microscopic characteristics of impinging spray were obtained using ultra-high-speed imaging. By changing the injection and ambient pressures, the influences of breakup and coalescence on the droplet behavior were investigated. Before impingement, the region near the center of the spray has larger droplets size and lower droplet number density than the edge, which suggests that spray breakup and atomization is poor in the center region. After impingement, the droplet size decreases along the distance from the wall under low ambient pressure. However, large droplets appear in the region far from the wall under high ambient pressure, indicating the existence of a coalescence effect on the droplets.