Numerical Simulation of Coalescence Growth of Charged Droplet Population Based on PIC Method

Accelerating the collision and coalescence process between cloud droplets is essential in promoting the catalytic rainfall, which has the potential to solve the shortage of water resources. The charged droplet seeding rainfall can accelerate droplet collision and coalescence due to the attraction of electricity. Meanwhile, the charged droplet seeding rainfall has a more complex collision and coalescence behavior than traditional rainfall using catalysts. In this article, 2-D particle-in-cell (PIC) simulation is used to study the effect of droplet population (10 000 droplets) charging characteristics (including charge amount, charged ratio, and different charge polarities) and external electric field on the growth law of droplet population coalescence. The results show that at 120 s, the effective diameters of droplet populations with charging 0%, 5%, and 10% <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${q} _{\max }$ </tex-math></inline-formula> increased by 10.2%, 191.1%, and 476.8% compared with the initial value at 0 s, respectively. A suitable proportion of bipolar droplet populations, such as 75% negatively charged droplets and 25% positively charged droplets, has a better effect on promoting droplet coalescence in the early stages compared with the unipolar droplet population with 100% negatively charged droplets. The proportion of large droplets (diameter <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\ge 25~\mu $ </tex-math></inline-formula>m) increases with the charge amount, charged ratios, and external electric field strength. The research results of PIC-simulated droplet coalescence provide theoretical guidance for understanding the temporal evolution of droplet size in droplet population collision and the efficient promotion of large droplets to some extent.