Characterization of Spray-Drying Nozzles at Industrially Relevant Conditions

This project was a continuation of our previous work on developing atomization model. The goals of this work were to understand the breakup of ligament of highly viscous, rheologically complex fluid under given stretch rate and to predict the droplet size distribution from the ligament breakup. We divided the work in three stages: i) breakup of ligament, ii) breakup of ligament in air crossflow and iii) application of ligament breakup model to spray. The current report is on the first stage of the work. 

We have completed a set of experiments on the breakup of polymeric ligaments by generating a liquid bridge between two rods. We found that the evolution of the liquid bridge can be studied in two stages, each with certain substages: 

• Formation of a beads-on-string structure at equilibrium o First-generation beads form due to surface wave described by Rayleigh-Plateau instability. 
  • Second-generation and higher generation beads form due to the pressure difference between the beads and string region. 
  • Beads will slide along the string until they reach their equilibrium positions. 
  • Unequal end drop sizes can create a liquid flow from small end drop to large one. This will generate a fresh, undisturbed ligament near small end drop. Later, a reverse liquid flow will be generated after the stopper bead formed at the large end drop side. 
    • The initial liquid flow (from small to large end drop) delays the formation of firstgeneration beads. 
    • The reverse liquid flow (from large to small end drop) will let a segment of the liquid bridge be absorbed by small end drop, while the rest  is non-uniformly stretched. 
  • Beads have multiple sizes at the equilibrium state. 
• Breakup of liquid bridge from the equilibrium stage o Tension in strings starts to relax. This makes the string thinner and longer. The tension force in the string reduces. This process starts with two end strings. 
  • Distances between the beads get closer, causing the smaller beads to move towards locally large beads and merge together. 
  • Number of beads largely decreases at the time string breaks, generating multiple large beads.