Rotating Thin Films: the Vortex Fluidic Device
Creators: name: Leivadarou, Evgenia
Publisher: name: Apollo - University of Cambridge Repository
Year: 2019
Subjects: fluid mechanics
rotating flow
thin films
chemical reactions
Vortex Fluidic Device
laminar flow
chemical reactivity
vorticity
pressure gradients
density variations

description: The motivation of this thesis, is the understanding of the role of fluid mechanics in the performance of the Vortex Fluidic Device (VFD), developed by the Raston laboratory at Flinders University, Australia. The VFD is a novel technology in the synthesis of organic chemicals and in material processing, and is considered a green technology device, achieving enhanced chemical reactivity without temperature variations or the use of additional catalysts. This is the first research project focused in the fluid mechanics of the VFD, thus relevant literature is limited to the performance of the device with respect to chemical reactivity. The methodology employed to pursue this understanding is based on the two processing methods, i.e. confined mode (CM) and continuous flow (CF). In CM, a fixed volume of liquid is placed at the base of the VFD before rotation is initiated, with the characteristics of the film at steady state describing the fluid mechanics environment for most of the processing time. In CF, droplets fall periodically in a rotating tube which sustains a liquid thin film developed along the walls. To explore the dynamics of droplet landing and spreading, experiments were conducted using flat and hemispherical disks. In every spinning disk experiment the radial evolution of single droplets and the onset of instabilities, at the contact line and along the free surface of the film, were explored for a range of parameters. The parameters under investigation were the rotation rate, the height of release, the droplet volume, the smoothness and the geometry of the disk. During the spreading, the droplet was divided in two different areas, the film and the ring, with the ring volume being an important component for the onset of instabilities. The fluid motion related to ring volume variations (initially growing and later decreasing in volume, leaving liquid behind), described in this thesis, has not been found in existing literature. Perturbations at the contact line lead to fingering instability, with the fingers unfolding retrograde to the rotating disk. In parallel, spiral waves appear at the free surface of the film, unfolding prograde to the rotating disk. In the VFD, experiments were conducted under CM, exploring the film development over time, the film thickness and the reaction rate at a steady state of rotation. In CF, the entrainment of droplets in an already rotating film was explored. The film thickness tends to be the most critical parameter for the performance of the device as it defines whether the reaction occurs under turbulent or laminar conditions. Neutron imaging experiments, conducted at the Australian Nuclear Science and Technology Organisation (ANSTO), gave an insight to the film thickness at steady state in CM. The technique is more appropriate for stationary objects, thus a new methodology was developed in this thesis for rotating objects. Knowing the film thickness, pressure gradients along the film were determined. Keeping all the non-fluid related parameters the same, increased rotation rate leads to decreased thickness, which works in favour of homogeneous distribution of the reactants and increased pressure differences, directly related to variations in diffusion and advection. Overall, mathematical approximations were developed to describe the experimental observations both in the rotating disks and in the VFD. A new spreading pattern, was observed in the rotating disks, while mechanisms for enhanced diffusion and advection were observed in the VFD.
description: John S. Latsis Foundation Tima Foundation Minas Kulukundis Trust Muscular Dystrophy Association Hellas Department of Applied Mathematics and Theoretical Physics Queens' college, Cambridge Disability Resource Centre, Cambridge Engineering and Physical Sciences Research Council (EPSRC)