Predicting Free-Falling Droplet Drying Behaviours at High Temperatures

Boiling within a falling droplet is a special and important phenomenon, which is poorly understood among the drying industry and is not well developed within the literature. Droplet drying at the boiling regime is explored and investigated in this research, of which the outcomes are of value to the spray drying community. The thesis presents a high-resolution model with innovative features, which predicts the behaviour of a free-falling droplet drying at high temperatures. The mathematical framework of the model includes coupling the conduction, convection and diffusion within the droplet to the phase change happening at the interface, whilst solving for the free-surface model simultaneously. The Finite Element Method (FEM) is used to solve this Multiphysics system, and the droplet moving interface is tracked by the Arbitrary Lagrangian-Eulerian (ALE) algorithm. The computed drying information, such as the droplet averaged temperature or weight profiles, agrees closely with the experimental data for a sucrose droplet. The detailed insights into the distribution of the temperature and moisture content within the droplet are achievable thanks to the 2D axis-symmetrical model. The description of the droplet internal flow field suggests the asymmetrical formation of vortices, which is impossible to predict using currently available models. The correlation of the species diffusion coefficient is a critical variable, as it determines the size of the vortices and the solid shell thickness. At the boiling point, bubble expansion drives the droplet shape and significantly decreases the droplet heat and mass transfer coefficients. As the bubble is offset from the droplet centre, it recentralises itself while growing due to the non-uniform pressure field within the droplet. The bubble behaviour is highly sensitive to the conditions set at boiling, and is mainly driven by the heat transfer, which is a function of the solute concentration.