An investigation on the influence of droplet deformation and internal circulation on drag coefficient

In this work, we use numerical approaches to investigate the effect of droplet deformation and internal circulation on droplet drag coefficient. In applications like spray combustion, droplets can be modeled as Lagrangian particles to reduce computational cost in simulations. A common starting assumption for the Lagrangian model is that droplets are spherical and have no internal flow. However, in spray, droplets are generated in a wide range of sizes, and the largest ones tend to have significant deformation which can fundamentally affect their behavior. Droplets are also subjected to high temperature and pressure, which enhances the internal circulation. Therefore, we need to improve the physical understanding of droplets to better predict the dynamics of droplets represented by Lagrangian particles. This work performs a numerical study on how the droplet drag coefficient is dependent on relevant parameters. We use DNS to simulate a non-evaporating droplet falling at terminal velocity in high pressure air. The drag coefficient is calculated, and the results are consistent with existing literature. Our study shows that droplet drag is increased by deformation and internal circulation. In addition, the acceleration effect of droplet lowers the actual drag compared to steady state.