柴油液滴撞击倾斜壁面碰撞速度效应数值分析

Numerical Analysis of Effect of Impacting Velocity on Diesel Droplet Impacting on Inclined Surface

采用复合水平集和流体体积法并综合考虑传热及接触热阻的作用建立了液滴碰撞倾斜壁面的数值模型,并验证了模型的可靠性。通过分析计算,获得了柴油液滴撞壁动力学形态的变化规律,揭示了液滴撞壁流动破碎机理,探索了液滴前、后铺展系数和铺展速度、总铺展系数以及壁面平均热流密度的碰撞速度效应规律。研究表明:液滴碰撞倾斜壁面射流和飞溅特征仅出现在前铺展边缘,且射流存在颈部破碎和根部破碎;液滴前铺展边缘射流区域内压力梯度是射流形成、发展及其颈部、根部断裂的主要原因;毛细波的作用是射流颈部和根部破碎的关键因素;碰撞速度越大,液滴的前、后铺展系数和铺展速度、总铺展系数以及壁面平均热流密度均越大;随着碰撞速度的增加,液滴撞壁特性的碰撞速度效应逐渐减小。

In order to explore the process of liquid film formed by fuel droplet impact on the surface in small-size combustor of diesel engine,a numerical model was developed using coupled level set and volume of fluid method,including heat transfer and contact resistance,to explore the mechanism of a droplet impacting on inclined surface. The dynamic motion and mechanism of flow and breakup of droplet were obtained according to results analysis. The effect of impacting velocity on spreading factor,average wall heat flux,forward and backward spreading factors and spreading velocities were also explored. The results showed that the liquid jet was observed only at the front spreading edge during the oblique impact,which was different from the horizontal impact. The tangential forces for the oblique impact promoted the forward spreading and the formation of liquid jet at the front spreading edge. The breakup of liquid jet was observed at both neck and root areas. The pressure gradient inside the liquid jet was the main factor resulting in the formation,development and breakup of the liquid jet; capillary wave resulted in the breakup at the neck and root of liquid jet. The impacting characteristics were closely related to impacting velocity,the spreading factor,average wall heat flux,forward and backward spreading factors and spreading velocities obviously increased with the increase of impacting velocity; the effect of impacting velocity on impacting characteristics decreased as impacting velocity increase.