燃油喷雾碰壁数值模拟分析

Numerical simulation analysis of fuel spray impingement

中小型直喷式柴油发动机难以避免燃油喷雾碰壁现象,直接影响柴油机的燃烧和排放。采用大涡模拟的方法对喷雾与壁面的相互作用特性进行数值模拟分析,对比了不同喷射压力、环境压力、碰壁距离条件下的喷雾碰壁过程,发现增大喷射压力,碰壁喷雾的发展半径和卷吸高度均有所增加;增大环境压力,碰壁喷雾的发展半径及卷吸高度均显著减小;减小碰壁距离,碰壁喷雾的卷吸高度明显增大,而发展半径只在碰壁后初始阶段有所增大。此外,从液滴索特平均直径(SMD)角度直观分析了不同碰壁条件对燃油液滴碰壁破碎的影响,发现增大喷射压力,碰壁喷雾SMD减小;增大环境压力,减弱了喷雾碰壁时的SMD减小趋势;碰壁距离改变只会影响碰壁发生的快慢,并不影响喷雾碰壁后最终稳定的SMD大小。为后续完善燃烧策略、提高燃烧效率提供了参考。

Small and medium-sized direct injection diesel engines have an unavoidable phenomenon of fuel spray impingement,and fuel spray impingement has a direct impact on the combustion and emissions of diesel engines.The interaction characteristics between spray and wall surface were numerically simulated and analyzed by means of large eddy simulation.The process of spray wall collision under different injection pressure,environmental pressure and wall collision distance was compared.It was found that the development radius and entrainment height of wall spray increased with increasing injection pressure;With the increase of ambient pressure,the development radius and entrainment height of wall impinging spray decreased significantly;The entrainment height of impinging spray increased significantly when the impinging distance was reduced,while the development radius only increased at the initial stage after impinging.In addition,the influence of different wall impingement conditions on the fuel droplet wall impingement and fragmentation was analyzed intuitively from the view of droplet Sauter mean diameter(SMD).It was found that the SMD of wall impingement spray decreased with the increase of injection pressure;Increasing the ambient pressure weakened the decreasing trend of SMD when the spray hit the wall;The change of wall collision distance only affected the speed of wall collision,and did not affect the final stable SMD size of spray after wall collision.It provided a theoretical reference for improving combustion strategy and improving combustion efficiency.