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Resolving Effects of particle Shape and Inertia in Scalar Transport

Periodic Reporting for period 1 - RESIST (Resolving Effects of particle Shape and Inertia in Scalar Transport)

Reporting period: 2019-07-01 to 2021-06-30

Engineering and nature are filled with examples of small particles exchanging material with a turbulent environment, including: the dissolution of fine solids in industrial processes, the preparation of crystalline products used in pharmaceuticals, the absorption of nutrients by phytoplankton and the encounter rates of bacteria with marine viruses in our oceans. These “particles”, as we call them here, come in a huge variety of different shapes, sizes and densities relative to the turbulent fluid in which they are suspended.

To predict and understand how these systems behave, engineers and scientists need physical insights and predictive models for the exchange of material (“mass transfer” or “scalar transport”) between particles and the fluid. It is well understood that properties like size, shape and density affect how a particle moves through and perceives the surrounding flow. Yet, current models of mass transfer focus on spherical particles and there been no systematic study of how average mass transfer depends upon particle and fluid properties when the flow is turbulent and particles cannot be idealised as spherical. Secondly, due to a lack of available tools and knowledge, no study has been able to establish cause-and-effect relationships between the flow field near the particle’s surface (where mass transfer occurs) and the average mass transfer rate for freely suspended non-spherical particles in turbulence. This information is crucial to developing and validating a modelling approach.

This project has aimed to address these two gaps in our scientific knowledge and contribute to the training-through-research of the Fellow to develop their skills as an independent researcher. The scientific objectives of the project have been to: develop ways to measure mass transfer to small particles in turbulent flows at the particle-scale and at an aggregate scale; to quantify the role of particle shape and inertia in the mass transfer process; to apply these methods to resolve mass transfer at the particle scale in turbulent suspensions; and to identify and quantify the mechanisms by which mass transfer is enhanced. The project has mostly achieved these objectives.
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.
The research goes beyond the state-of-the-art by providing new means to study particle-fluid mass transfer in the laboratory. Specifically, it enables future researchers to study and quantify mass transfer to small, non-spherical particles, and to visualise mass transfer at the particle scale in turbulent flows. This will allow researchers to address mass transfer problems in particle laden flows which were previously not possible to study in the laboratory and difficult or impossible to study using numerical simulations. The research provides a particle-scale model for the mass transfer rate which explicitly incorporates particle shape for weakly inertial (i.e. very small) particles. This model can be directly incorporated by scientists and engineers into numerical simulations of particle-laden turbulent flows to predict the behaviour of these systems. Furthermore, using our unique experimental datasets and modelling techniques, we expect to be able to quantify and model the influence of the slip-strain transition upon mass transfer rates for spherical and non-spherical particles in turbulent flows.

The project results will be of utility to scientists and engineers working across a broad range of disciplines, including chemical engineering, process engineering, and oceanography. In particular, we envisage our research will: have applications in process engineering, to help design better manufacturing processes for chemical and pharmaceutical products; help us understand better the diversity of life in our oceans; and enable us to study the impacts of climate change upon the biological carbon pump.
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