Modelling of Droplet Drying Mechanisms

This thesis aims to develop an understanding of the drying mechanisms of a liquid droplet containing solid particles by consideration of the heat and mass transfer processes involved and mechanical behaviour of the porous material during drying. The previous studies mostly used methods to solve coupled heat and mass transfer equations using mathematical tools. This study contributes to an understanding of the mechanical behaviour of drying a single droplet containing solids. Modelling of the drying mechanisms involves heat, mass transfer and changing of the droplet size. An inflation model was added to the drying model to describe the particle expansion under internal vapour pressure in the second drying stage when the droplet temperature is higher than the boiling temperature. COMSOL Multiphysics ® finite element software was used to solve the governing equations. The numerical results were validated using analytical solutions, existing experimental data from the literature and published numerical studies. Different model parameters were varied to simulate their effect on droplet temperature, mass profiles and also the effect on the timing of the shell. Higher air velocity results in faster formation of the shell. Increasing the air temperature generates high internal pressure which in turn produces significant inflation of a porous particle. The mechanical expansion is considered using concepts adapted from poroelasticity, whereby the deformation of the porous structure is strongly influenced by the pore pressure arising from heat and mass transfer as well as evaporation of the solvent in the pore space. Qualitative agreement has been obtained with published experimental results, highlighting the need for appropriate characterisation of the mechanical properties of the porous matrix which evolve as functions of porosity and temperature during the drying process.