气流对含气泡油滴撞壁铺展流动过程的影响

Effect of Airflow on the Spreading Process of Hollow Oil Droplets after Wall Impacting

油-气润滑系统工作过程中,有效的润滑油膜是确保摩擦副润滑效果的关键,其形成质量与输送气流作用下含气泡油滴撞壁后的铺展流动过程密切相关。为探索该铺展流动过程,基于VOF(Volume of fluid)方法对输送气流作用下含气泡油滴的撞壁过程进行数值模拟研究。观察了输送气流作用下含气泡油滴撞壁后的铺展流动过程,探讨了气流入射角及碰撞-气流速度比对含气泡油滴铺展特性的影响,分析了铺展过程中的气泡破裂及中心射流机制。发现当气流方向与含气泡油滴碰撞方向(竖直方向)存在差异时,含气泡油滴撞壁后在x方向的铺展呈非对称性。随着气流入射角的减小,非对称铺展现象越明显。碰撞-气流速度比增大时,非对称铺展现象减弱。碰撞速度较低(碰撞-气流速度比小于4)时含气泡油滴撞壁后,上侧油膜厚度不断减小直至达到极限值,最终在气流拖曳力、黏性剪切力及表面张力的共同作用下发生破裂。碰撞速度较高(碰撞-气流速度比等于4)时含气泡油滴撞壁后,其内部产生的速度涡旋致使部分铺展油液向气泡底部汇聚,中心射流得以形成。中心射流逐步发展直至穿透上侧油膜,气泡发生破裂。之后,气泡两侧油膜发生破碎并形成膜油滴,壁面油膜逐渐收缩成油膜层。

In the oil-air lubrication system,the effective lubrication film is the key to ensure the lubrication effect,and its formation quality is closely related to the spreading flow process of the hollow oil droplets impacting the wall in the conveying airflow.To explore the spreading flow process,the VOF method is adapted to numerically simulate the wall impact process of the hollow oil droplet in the conveying airflow.The spreading process is observed,the effects of incident angle and collision-airflow velocity ratio on the spreading characteristics are discussed,and the bubble breakdown and central jet mechanism in the spreading process are analyzed.It is found that when the airflow direction is different from the collision direction(vertical direction)of the hollow oil droplet,the spread of the hollow oil droplet in the x direction is asymmetric.With the decrease of the incidence angle,the asymmetric spreading becomes more obvious.When the velocity ratio of collision-airflow increases,the phenomenon decreases.At lower collision velocity(collision-airflow velocity ratio less than 4),the oil film thickness upper side of the bubble decreases to the limit in the spreading process,and finally the bubble ruptures under the combined action of airflow drag force,viscous shear force and surface tension.At higher collision velocity(collision-airflow velocity ratio equal to 4),the velocity vortices generated inside the droplets after wall impact cause part of the spreading oil to converge to the bottom of the bubble,and the central jet is formed.With the further development of the central jet,it penetrates the oil film on the upper side of the bubble and the bubble bursts.After that,the oil film on both sides of the bubble breaks up and forms film droplets,and the oil film on the wall gradually shrinks to form oil film layer.