柴油液滴撞击热壁面的动态行为特性

Dynamic Behavior of Diesel Droplets Impact on a Heated Surface

为了分析柴油机内喷雾液滴撞击活塞表面的现象,采用高速相机对柴油液滴撞击加热壁面的动态行为特性进行了研究。结果表明:在壁面温度为25~550℃和韦伯数(We)为25~174的条件下,柴油液滴撞击加热壁面会出现四种行为模式:粘附模式,回弹模式,破碎模式和破碎回弹模式。在粘附模式下,随着壁面温度提高,液滴粘性的降低会使液滴的铺展系数和动态接触角的变化幅度增大。随着We数增大,液滴的惯性力增强,进而形成更大的铺展系数。液滴的回弹行为会在壁面温度超过动态Leidenfrost温度时发生,且动态Leidenfrost温度随We数的增加约从425℃提高至450℃。在回弹模式下,随着We数增大,液滴最大铺展系数和无量纲接触时间显著增大,并结合文献分别总结出适用于多种工质的预测关系式。

In order to analyze the phenomenon of spray droplets impacting on the surface of the piston in diesel engines, a high-speed camera was used to study the dynamic behavior of diesel droplets impact on a heated surface. The results show that when a diesel droplet impacts on a heated surface with different surface temperatures(25~550℃) and different We(25~174), there are four behavioral mode which are the stick mode, the rebound mode, the breakup mode and the breakup-rebound mode. In the stick mode, with the increase of surface temperature, the decrease of droplet viscosity will increase the variation range of droplet spreading factor and dynamic contact angle. With the increase of We, the inertial force of the droplet increase, which in turn forms a larger spreading factor. The rebound behavior of the droplet will occur when the surface temperature exceeds the dynamic Leidenfrost temperature, and the value of dynamic Leidenfrost temperature will increase from 425 to 450℃ as We increases. In the rebound mode, as We increase, the maximum spreading factor and the dimensionless resident time of the droplet increase significantly. Combined with the literature, the prediction equations applicable to multiple working fluids are summarized respectively.