Numerical investigation of two hollow cylindrical droplets vertically impacting on dry flat surface simultaneously

In this paper, the effect of two hollow droplets' impact on a solid substrate is numerically studied. A coupled level set and volume of fraction method is used to investigate the fluid dynamics and heat transfer characteristics of two hollow cylindrical droplets vertically impacting on a dry flat surface simultaneously. Numerical results show that, different from two continuous dense droplets, counter-jet at impact point (CJIP) is observed as a distinguished feature during the two hollow droplets' impact process. However, counter-jet at symmetric point (CJSP) is formed in the vicinity of the symmetric point for both two hollow and dense cylindrical droplets. The analysis of pressure and velocity distribution is performed. It is shown that the formation of CJSP and CJIP is mainly caused by the pressure gradient. Upon further analysis of average heat flux, the formation of CJIP and the liquid shell rupture are the two main factors determining that the hollow droplet has a lower heat transfer capacity with the flat solid wall than that of the dense droplet. Through the investigation about the effect of impact velocity on fluid flow and heat transfer characteristics, the spread factor, the height of CJSP and CJIP, and the average heat flux will all increase with higher impact velocity. These results will provide a better understanding of hollow droplet impingement and heat transfer on flat surfaces.