Abstract: 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.

Key words: airflow, hollow oil droplet, impact, asymmetric, VOF