柴油液滴冲击液膜润湿壁面实验和数值模拟

从实验和数值模拟两方面研究了液滴碰撞液膜润湿壁面的过程。实验采用高速摄像仪观测并分析了液滴撞壁后的铺展、水花形成和液滴破碎飞溅现象,并定量获得了液滴的铺展系数和铺展速度随时间的变化规律。实验结果表明:液滴的铺展系数和铺展速度的碰撞速度效应明显;碰撞速度越大,液滴的铺展系数越大;液滴撞壁后的铺展速度迅速减小,碰撞速度越大铺展速度越大。数值模拟采用流体体积法,综合考虑液滴与壁面间传热及接触热阻的作用,建立了液滴撞壁数值模型。该模型模拟结果和实验数据吻合较好,证明了数值模型的准确性。数值结果表明:液体内部的压力梯度是液滴铺展、产生皇冠水花和以及液滴破碎飞溅的主要原因;毛细波是水花颈部收缩和液滴破碎飞溅的关键因素。

Experiment and Numerical Simulation of Droplet Impacting onto a Wetted Surface

In order to explore the mechanism of fuel droplet impacting on a wetted surface in a small-size combustor of diesel engine, experiments and numerical simulation were carried out. High speed camera was used to observe the phenomenon of spreading, liquid sheet formation, breakup and splashing during droplet impact. The results showed that the spreading factor and velocity were closely related to impact velocity. The higher impact velocity droplet had a larger spreading factor, and the intial spreading velocity was the largest, which could be increased by increasing impact velocity. The numerical simulation was developed using volume of fluid method including heat transfer and contact resistance. The mechanism of droplet fluid and heat transfer as well as breakup during impact were obtained according to analysis of results. The numerical results also showed that the pressure gradient inside liquid was the main factor resulting in droplet spreading, liquid sheet formation, breakup and splashing. The capillary wave resulted in the shrink at the neck of the liquid sheet and breakup and splash from the liquid sheet.