Abstract
We investigated the morphology and size distribution of satellite droplets formed when a water droplet falls freely and interacts with a swirling airstream of varying strengths. We used shadowgraphy and deep-learning-based digital in-line holography techniques to analyse this phenomenon. Our findings indicate that the behavior of the droplet differs based on the strength of the swirling motion, leading to vibrational motion, retracting bag formation, and normal breakup phenomena. These effects occur within the same aerodynamic field, with no swirl, low swirl, and high swirl scenarios. In the high swirl scenario, the disintegration of nodes, rim, and bag film plays a significant role in determining the number mean diameter of the satellite droplets. This results in smaller droplets being produced. Conversely, in the low swirl case, only the breakup of the rim and nodes contributes to the size distribution, leading to the formation of larger droplets. We observed that the temporal variation of the Sauter mean diameter indicates that a high swirl strength generates more surface area and surface energy than a low swirl strength under a given aerodynamic force. Theoretical predictions of the number-mean probability density for tiny satellite droplets under swirl conditions align well with experimental data. However, these predictions differ from the experimental results in the case of low swirl, mainly due to the presence of large satellite droplets. Our research reveals that the volume-weighted droplet size distribution exhibits two (bi-modal) and three (multi-modal) peaks for low and high swirl strengths, respectively. To accurately predict the shape and characteristic sizes of each mode in the case of high swirl strength, we developed an analytical model that considers various mechanisms, including the breakup of nodes, rims, and bags. The analytical model accurately predicts the shape and characteristic sizes of each mode in the case of high swirl strength. Overall, our findings shed light on the intricate dynamics of droplet interaction with swirling airstreams, highlighting the influence of swirl strength on droplet morphology and size distribution.
