The effect of droplet deformation and internal circulation on drag coefficient

Abstract: The current study uses numerical approaches to investigate the effect of droplet deformation and internal circulation on droplet dynamics. Although droplet drag is a classical area of study, there are still theoretical gaps in understanding the motion of large droplets. In applications like spray combustion, droplets of various sizes are generated and move with the flow. Large droplets tend to deform in the flow, and have complex interactions with the flow because of this deformation. To better model spray, the physical understanding of droplets need to be improved. Under spray conditions, droplets are subjected to a high temperature and pressure environment, and the coupling between liquid and gas is enhanced. Therefore, the deformation and internal circulation will affect droplet drag coefficient more significantly than in atmospheric conditions. To study the mechanism on how droplet shape and internal circulation influence droplet dynamics, we will use direct numerical simulation (DNS) to simulate a droplet falling at its terminal velocity in high pressure air. An in-house code developed for interface-capturing DNS of multiphase flows will be employed for the simulation. The drag coefficient is calculated, and the results are consistent with existing literature for slightly deformed droplets. The results show that the drag coefficient is directly related to the droplet deformation and droplet internal circulation. The paper also develops a theory to account the effect of Weber number and liquid/gas properties in droplet deformation.