An experimental investigation of droplet morphology in swirl flow

Abstract

The interaction of a droplet with a swirling airstream is investigated experimentally using shadowgraphy and particle image velocimetry techniques. In swirl flow, the droplet experiences oppose-flow, cross-flow and co-flow conditions depending on its ejection location, the velocity of the airstream and the swirl strength, which results in distinct droplet morphologies as compared with the straight airflow situation. We observe a new breakup phenomenon, termed as ‘retracting bag breakup’, as the droplet encounters a differential flow field created by the wake of the swirler's vanes and the central recirculation zone in swirl airflow. A regime map demarcating the various modes, such as no breakup, vibrational breakup, retracting bag breakup and bag breakup modes, is presented for different sets of dimensionless parameters influencing the droplet morphology and its trajectory. In contrast to the straight flow, the swirl flow promotes the development of the Rayleigh–Taylor instability, enhancing the stretching factor in the droplet deformation process, resulting in a larger number of fingers on the droplet's surface. In order to gain physical insight, a modified theoretical analysis based on the Rayleigh–Taylor instability is proposed for the swirl flow. The experimental behaviour of droplet deformation phenomena in swirl flow conditions can be determined by modifying the stretching factor in the theoretical model.

Droplet size distribution in a swirl airstream using in-line holography technique

“…To the best of our knowledge, the study by Kirar et al. (2022) is the first to explore the phenomenon of droplet breakup in a swirling airstream. In contrast to the present study, which focuses on droplet size distribution, their aim was to demonstrate a new breakup mode in a swirling flow using Raleigh–Taylor instability.…”

“…These locations are chosen in accordance with the regime map presented by Kirar et al. (2022) (see figure S4 in the supplementary material). The dimensionless time, , is mentioned at the top of each panel in figure 5.…”

“…We have also calculated the flow-based swirl number using the stereo-PIV measurements (Kirar et al. 2022). The flow swirl number is given by where and are the axial and tangential components of the velocity and denotes the radius of the swirler (Candel et al.…”

“…2020; Jackiw & Ashgriz 2021; Kirar et al. 2022; Xu, Wang & Che 2022), there have been very few studies on the size distribution of satellite droplets after fragmentation of a large droplet (Srivastava 1971; Villermaux & Bossa 2009; Gao et al. 2022; Jackiw & Ashgriz 2022).…”

“…Recently, Kirar et al. (2022) experimentally analysed the droplet fragmentation phenomenon under a swirling airstream using shadowgraphy and PIV techniques. They discovered a new breakup mode, termed as ‘retracting bag breakup’, due to the differential flow field created by the wake of the swirler's vanes and the central recirculation zone in swirl airflow.…”