Experimental Study Of Droplet And Spray Impingement

In the engine with the in-cylinder direct injection, the spray impingement process plays an important role in the mixture formation, because the time is too limited for spray droplets to fully vaporize. The liquid droplets hit the cylinder walls and the piston walls, leaving deposits on the surface. From the fuel injection to the ignition, due to the short vaporization time, the attached fuel can form a rich fuel/air area, which is the major source of the soot emissions. Thus, investigating the spray impingement process is essential to improve fuel efficiency and reduce emissions for engines.First, a test bench was constructed for the investigation of the interaction and vaporization processes of a single droplet on a heated wall. The vaporization process of the droplet was analyzed using high speed photography. The results indicate that, under the conditions of low wall temperatures, the vaporization time of the droplet gradually reduces with the increasing wall temperature, and the vaporization speed become faster at higher wall temperature. When the wall temperature is kept constant and Weber is varied in a wide range, the droplet evaporation time is nearly same. The evaporation time of the droplet on the wall with larger surface roughness is slightly shorter than that on the smaller surface roughness. However, the overall droplet evaporation behavior is identical. It is also found that the evaporation speed of the ethanol droplet is much shorter than the water droplet. At moderate wall temperatures, with wall temperature rising, the heat transfer between the droplet and the wall increases significantly. Big bubbles are generated in the initial stage and bubbles breaks up quickly. However, the formation of the small and medium-sized bubble becomes less obvious. At high wall temperatures, as the wall temperature increases, the gas film increases gradually. The upward lift force of the droplet due to the generation of the spherical droplets gradually strengthens. Due to the rapid gasification in the horizontal direction, the spherical droplets in the horizontal plane always has strong movement.Then, experimental study of the spray impingement was carried on. Keeping all the other conditions same, reducing the distance between the fuel injector and the plate results that the film attached to the wall decreases significantly and the atomization of the fuel spray is enhanced. With the increase of the impingement angle, the entrainment height after droplet impinging the wall increases, and the wall impingement height increases greatly, which is helpful for the fuel vaporization. Increasing the wall temperature directly promotes the film evaporation and reduces the liquid film mass.