Experimental Investigations on the Grouping Behaviour in a Mono-Disperse Droplet Stream

Clustering of droplets in a spray, referred to as ‘droplet grouping’, is an important phenomenon which affects droplet vaporization and combustion processes. The factors which govern droplet grouping mechanics are not yet fully understood. In this experimental study we consider a single stream of monodisperse droplets to investigate droplet grouping. A circular cylindrical jet of isopropanol is injected into ambient air through a circular hole under a controlled upstream pressure. The jet is broken into droplets by excitation using a piezo element. A double frequency excitation generates a stream of droplet groups each consisting of two droplets. The droplets are injected into a quiescent ambient air at velocities much greater than their terminal velocity of free fall. From high-resolution images of the droplet stream, the evolution of the intra- and inter-group distances is measured. It is observed that, for the experimental conditions explored here, the droplets within a group approach each other, collide, coalesce, and exhibit droplet shape oscillations. The characteristics of the droplet grouping process are reported.