Investigation of fluid dynamics in a vibrating mesh nebulizer using the finite volume particle method

Abstract

In Aerogen aerosol drug delivery technology, an aperture plate vibrates at high frequency under a piezoelectric drive. Liquid drug stored on one side of the plate passes through the apertures to form an aerosol of microscopic droplets, which can be inhaled. A computational fluid dynamics (CFD) technique based on the finite volume particle method (FVPM) was developed to capture the physics of droplet formation in a single aperture. Two novel extensions to the modelling capability of FVPM are presented in this thesis. Firstly, a kinematic criterion for free surface extension (KCFSE) was developed to differentiate physical free surfaces from spurious numerical voids. The novel method enables background pressure to be applied at physical free surfaces and throughout the fluid, but not in non-physical voids, facilitating the suppression of spurious voids. The results obtained are in good agreement with benchmark data. Secondly, a surface tension model was developed in the FVPM. We present a model in which the direction of the pairwise surface tension force is approximated by the common tangent of free-surface particle supports. The results are in good agreement with theoretical and experimental data. Since each aperture in Aerogen's nebulizer is approximately axially symmetric, axisymmetric simulations of the flow field were performed for surface-tension-dominated flows. The results demonstrate the accuracy of the surface tension model in axisymmetric FVPM. Finally, FVPM simulations were performed for an oscillating single aperture in microscale and millimetre-scale to investigate the fluid dynamics of droplet formation. The simulations show good agreement with the observations from an experiment on a millimetre-scale single aperture. In the simulation of a microscale single aperture, the effects of the liquid properties and aperture plate operating conditions on liquid column breakup and drop formation were studied. The simulations show that breakup of liquid columns is associated with Rayleigh theory of instability.