Droplet breakup in airflow with strong shear effect

TL;DR

This study investigates how strong shear airflow influences droplet deformation and breakup, revealing unique breakup features, stages, and the role of viscosity, supported by experiments and theoretical analysis.

Contribution

It provides a detailed experimental and theoretical analysis of droplet breakup under shear flow, including a regime map and insights into the breakup mechanisms and effects of viscosity.

Findings

Droplet breakup exhibits three stages: sheet, swing, and rim.

Swing breakup is governed by transverse Rayleigh-Taylor instability.

Viscosity inhibits droplet breakup in strong shear airflow.

Abstract

The deformation and breakup of droplets in airflows is important in spray and atomisation processes, but the shear effect in non-uniform airflow is rarely reported. In this study, the deformation and breakup of droplets in a shear flow of air is investigated experimentally using high-speed imaging, digital image processing, and particle image velocimetry. The results show that in airflow with a strong shear effect, the droplet breakup exhibits unique features due to the uplift and stretching produced by the interaction between the deformed droplet and the shear layer. The breakup process can be divided into three stages according to the droplet morphology and the breakup mechanism, namely the sheet breakup, the swing breakup, and the rim breakup stages. Theoretical analysis reveals that the swing breakup is governed by the transverse Rayleigh-Taylor instability. A regime map of the droplet breakup is produced, and the transitions between different regimes are obtained theoretically. The stretching liquid film during the droplet deformation and the fragment size distribution after droplet breakup are analysed quantitatively, and the results show that they are determined by the competition of breakup at different stages affected by the shear. Finally, the effect of the droplet viscosity is investigated, and the viscosity inhibits the droplet breakup in a strong shear airflow.