To achieve this, we have developed new experimental approaches to quantify and visualise the mass transfer from small particles with controlled properties (shape, size and density) in the laboratory. This has involved creating small rod-like and spherical particles which are loaded with a slow-release fluorescent dye. It has also involved building experimental apparatus to create well-defined turbulent flows and adapting existing measurement techniques to quantify this release of dye. We have conducted laboratory experiments which quantify the mass transfer rate in terms of particle geometry (spheres and rods) and inertia (controlled by size and density) at an aggregate scale and the particle scale. This has generated unprecedented experimental datasets, which will be publicly available. These allow us to quantify the effects these different parameters have upon the overall mass transfer rate and to identify the mechanisms by which mass transfer occurs in turbulent flows. Furthermore, we have generalised existing theory and conducted simplified numerical simulations to model the mass transfer rate of very small particles with arbitrary shape in turbulent flows.
One experimental result is illustrated in the image attached, which compares two exposures taken less than a millisecond apart of soluble dye being released from a slender rod, to a numerical simulation of the material transferred from a rod-like particle. Elongated particles are found to preferentially orient themselves in a turbulent flow with the prevailing fluid strain, in a way that enhances the mass transfer rate. Our experimental results identify, and our simulations and theory confirm, that adopting an elongated, rod-like shape results in an increase in the mass transfer rate per unit area compared to an equivalent spherical shape. Our experiments also demonstrate a transition in the mechanism of mass transfer which occurs as particles begin to slip relative to, rather than follow along with, the turbulent flow. A further significant result is the development of an inexpensive model for the mass transfer rate incorporating particle shape, which extends well-established theory and has been validated against experimental data.
The research has been prepared for publication in four open-access journal articles and, despite the COVID-19 pandemic, presented at one academic conference, with a second scheduled in September 2021. To exploit the project results in an industrial context, the Fellow will prepare future proposals to examine particle-fluid mass transfer in industrially relevant flows such as batch crystallisers and fluidised beds. To exploit the project results in an environmental context, the Fellow has teamed up with academics from National Oceanography Centre, Southampton. One collaboration will aim to explore ecosystem level consequences of varying phytoplankton nutrient uptake by changing shape and turbulence levels, using our models as inputs. Another will examine the aggregation of marine snow in turbulence and its consequences for the biological carbon pump, using apparatus developed and techniques learned from the project.
