Bag breakup of low viscosity drops in the presence of a continuous air jet

TL;DR

The paper addresses how low-viscosity drops breakup into bags when exposed to a continuous horizontal air jet, focusing on the interplay of surface tension and aerodynamic forces captured by $We$ and $Oh$. It develops a coupled one-dimensional aerodynamic–viscous model for drop deformation, deriving the transition condition $We_{cr}=12\left(1+\dfrac{2}{3}Oh^2\right)$ and equations for radial and bag growth, $\phi(T)$ and $\beta(T)$, including viscous corrections. Experimental data across fluids (water, ethanol, glycerol) validate the exponential-like growth of radial extent and bag size with strong $We$ dependence and weak $Oh$ influence, and confirm the predicted onset boundary. The findings enhance understanding of bag breakup in atomization and have practical implications for spray and combustion processes by clarifying when and how secondary atomization initiates.

Abstract

This work examines the breakup of a single drop of various low viscosity fluids as it deforms in the presence of continuous horizontal air jet. Such a fragmentation typically occurs after the bulk liquid has disintegrated upon exiting the atomizer and is in the form of an ensemble of drops which undergo further breakup. The drop deformation and its eventual disintegration is important in evaluating the efficacy of a particular industrial process, be it combustion in automobile engines or pesticide spraying in agricultural applications. The interplay between competing influences of surface tension and aerodynamic disruptive forces is represented by the Weber number, $We$, and Ohnesorge number, $Oh$, and used to describe the breakup morphology. The breakup pattern considered in our study corresponds to that of a bag attached to a toroidal ring which occurs from $12 < We < 16$. We aim to address several issues connected with this breakup process and their dependence on $We$ and $Oh$ which have been hitherto unexplored. The $We$ boundary at which breakup begins is theoretically determined and the expression obtained, $We = 12(1 + 2/3Oh^2)$, is found to match well with experimental data available in literature. An exponential growth in the radial extent of the deformed drop and the streamline dimension of the bag is predicted by a theoretical model and confirmed by experimental findings. These quantities are observed to strongly depend on $We$. However, their dependence on $Oh$ is weak.

Figures (13)

• Figure 1: Sketch showing deformation of drop as it moves through the air flow field.
• Figure 2: Experimental setup.
• Figure 3: Various parameters studied.
• Figure 4: Images showing bag breakup of drops. Top to bottom $Oh = 0.002, 0.007, 0.010, 0.015, 0.034, 0.058$ for $We \sim 14$. $\Delta t$ between successive frames is roughly 2 ms.
• Figure 5: Variation of $\phi\left(T\right)$ vs $T$ for a fixed $We ( = 13)$. Symbols are experimental data while lines represent theoretical results.