Partial coalescence is a complicated flow phenomenon. In the present study, the coalescence process is simulated with the volume of fluid (VOF) method. The numerical results reveal that a downward high-velocity region plays a significant role in partial coalescence. The high-velocity region pulls the droplet downward continuously which is an important factor for the droplet turning into a prolate shape and the final pinch-off. The shift from partial coalescence to full coalescence is explained based on the droplet shape before the pinch-off. With the droplet impact velocity increasing, the droplet shape will get close to a sphere before the pinch-off. When the shape gets close enough to a sphere, the partial coalescence shifts to full coalescence. The effect of film thickness on the coalescence process is also investigated. With large film thickness, partial coalescence happens, while with small film thickness, full coalescence happens. In addition, the results indicate that the critical droplet impact velocity increases with the increase of surface tension coefficient but decreases with the increase of viscosity and initial droplet diameter. And there is a maximum critical Weber number with the increase of surface tension coefficient and initial droplet diameter.

部分聚结是一种复杂的流动现象。在本研究中，聚结过程是用流体体积 (VOF) 方法模拟的。数值结果表明，向下的高速区域在部分聚结中起重要作用。高速区域不断地将液滴向下拉动，这是液滴变成扁长形状和最终夹断的重要因素。基于夹断之前的液滴形状解释了从部分聚结到完全聚结的转变。随着液滴撞击速度的增加，液滴形状将在夹断之前接近球体。当形状足够接近球体时，部分聚结转变为完全聚结。还研究了膜厚度对聚结过程的影响。膜厚大，发生部分聚结，而膜厚小，发生完全聚结。此外，结果表明临界液滴撞击速度随着表面张力系数的增加而增加，但随着粘度和初始液滴直径的增加而减小。随着表面张力系数和初始液滴直径的增加，存在一个最大临界韦伯数。