A Study on Droplets Evaporation at Diesel Spray Boundary During Ignition Delay Period

摘要

Diesel spray evaporation in a high pressure and high ambient temperature close to actual diesel engine condition was investigated in this study. A nano-spark shadowgraph photography technique and a rapid compression machine were applied in this experiment. By using this method, relatively clear image of liquid phase, vapor phase and droplets was obtained. In order to quantify the spray characteristics in the spray liquid area and vapor phase area, an image analysis method was applied. An algorithm was developed to quantify the droplets size and number of the droplets characteristic in the vapor phase. Experimental results have revealed that the injection pressure and the ambient temperature do not affect the spray penetration length apparently. In the case of ambient temperature T{sub}I=700K, the liquid core is observed in the region near the spray axis. Meanwhile, the vapor exists mostly in the outer region in the middle of the spray. However, at T{sub}I=950K, the vapor phase expands and dominates the spray tip region. This is because high ambient temperature promotes the evaporation, and consequently, instantaneous evaporation on a fuel spray occurs as soon as the fuel injection. According to close reference of vapor phase at the spray boundary region, many dark spots can be visualized inside the vapor region. These dark spots seem to indicate the existence of liquid droplets inside the vapor phase. When the fuel is injected into the spray chamber, droplets at outer region of the spray are flying away and evaporate simultaneously. In the case of the evaporation process at high ambient temperature and high injection pressure, the evaporation speed is expedited exceeding the droplets velocity thus trapped the flying droplets in vapor region.