Numerical Study On Diesel Spray/Wall Impinging Process

Ignition, combustion and emission of direct injecting diesel engines are severely influenced by spray breakup and mixture formation. Especially in small engines liquid reaches the wall of the piston and the cylinder due to the high momentum of the liquid spray. The spray impinging on the surface causes fuel rich regions with low evaporation rates that lead to insufficient combustion and soot formation. Moreover, HCCI is an attractive advanced combustion process that offers the potential for substantial reductions in both NOx and PM. If the influences of spray parameters on the spray/wall interaction are understood in detail the spray impinging process also becomes a combustion design tool. Then carefully used the surface contact can support spray breakup and mixture formation.To further understand fuel/wall impinging process, fuel/plane impingement was studied. Based on this, mixture formation in a new chamber was studied using CFD software STAR-CD. The new chamber is one chamber with a protrusion. It improved fuel atomization, breakup, evaporation and mixture formation through impinging on the protrusion. The protrusion can change the spray's shape, developing direction and increase fuel's diffusion, all these were benefit to mixture formation. The conclusion was that the new chamber was good at homogeneous mixture formation, and realizing HCCI combustion.The results between fuel/plane impingement and fuel/protrusion impingement show that when fuel spray impinging on the plane wall ,wall jet was formed and fuel rich region was found nearby the wall ,while when spray impinging on the protrusion in the new chamber, it was divided into two parts: one produced a secondary space jet, proceeding in different direction from the former jet; the other one proceeded in the same direction with the former jet. Because of repeated fuel/wall impingement and increased air entrainment, it was benefit to fuel breakup and evaporation, and formed more homogeneous mixture. Injection angle was an important factor in mixture formation, it was proved that 38 degree was the best injection angle. In high ambient pressure and temperature circumstance, spray angle was bigger than that in normal atmospheric pressure and temperature, and the spray spreading area was shrunk, but fuel evaporation rate was higher; the parent spray was not divided, but its direction swerved. In conclusion, the new chamber was good at homogeneous mixture formation.