Difference between the dynamics of bubble and droplet formation
Difference between the dynamics of bubble and droplet formation lead image
The generation of bubbles or droplets from an orifice plays an important role in many applications in industrial processes such as ink-jet printing and food processing. However, previous attempts to understand this phenomenon often study bubbles and droplets separately, and differences in their dynamic mechanisms have not been fully explored.
Zhang et al. present a numerical study that compares the dynamics of bubble and droplet formation in quiescent flow. By considering bubble/droplet formation as an incompressible, isothermal, and immiscible two-phase flow problem, they employed an improved Cahn-Hilliard model to capture the interfacial evolution and obtained the flow field by solving the Navier-Stokes equations. Their calculated results matched well with values from previous experiments.
Their results reveal that vorticity is distributed principally in the lighter fluid, i.e. the air surrounding the droplet and the air inside the bubble, meaning that the interface deformation of bubbles and droplets is different even under the same governing parameters. For a rising bubble, the vorticity is mainly distributed at the ends of the tail, which stretches it into a skirt-like shape. On the other hand, the outer vorticity of the droplet leads to a heavy head and pointed tail.
They discovered that for a submerged orifice (a hole in the wall), bubbles rise without affecting one another, while droplets impact and coalesce with each other. For an inserted orifice (a spout jutting out of the wall) the detached bubble size is greatly reduced, while droplet deformation remains the same.
In terms of future work, the authors hope to further explore bubble/droplet formation from inserted orifices of different shapes, as well as the effect of bubble/droplet dynamics on heat and mass transfer characteristics.
Source: “Numerical investigation on formation and motion of bubble or droplet in quiescent flow,” by Tongwei Zhang (张童伟), Jie Wu (吴杰), and Xingjian Lin (林星箭), Physics of Fluids (2020). The article can be accessed at https://doi.org/10.1063/1.5143098 .
