A HOLISTIC FRAMEWORK FOR HYBRID MODELLING OF SOLID-LIQUID FLOWS

The movement of solid-liquid suspensions in pipes and vessels is a generic complex problem which is commercially challenging and technically important. Industrial applications are numerous, e.g. chemicals, consumer goods, food, pharmaceuticals, oil, mining, river engineering, construction, power generation, biotechnology and biomedical. Despite such large markets, industrial practice and processes are neither efficient nor optimal because of a severe lack of fundamental understanding of these flows. Such flows involve complex phenomena on a wide range of scales as flow conduits generally vary from the micron scale to the centimetre scale, and vessels vary from the millilitre scale to the cubic metre scale. Flows may be turbulent or viscous and the carrier fluid may exhibit complex non-Newtonian rheology. Particles occur in various shapes, sizes, densities, bulk and surface properties which exacerbates the complexity of the problem.

The design of processes for conveying or processing solid-liquid suspensions requires information about particle behaviour such as particle trajectory, radial migration across streamlines, particle velocity distribution, and solids distribution. There are, however, huge practical difficulties in imaging solid-liquid flows and measuring local fluid and solid velocities, since little of the available instrumentation is applicable. Mixtures of practical interest are often concentrated and opaque so that flow visualisation is impossible, and particles may be deformable, breakable or prone to aggregation. Such complex phenomena are presently difficult to predict. They have hampered fundamental research and the development of rigorous holistic modelling strategies and, as a result, work has generally followed a piecemeal empirical approach.

This proposal will use a multiscale approach to study the flow of solid-liquid suspensions including fluids of complex non-Newtonian rheology and particles with complex properties: (i) experimentally via a unique and accurate Lagrangian technique of positron emission particle tracking, which can measure local 3-D phase velocities as well as phase distribution in opaque systems; and (ii) by developing and validating novel modelling approaches to predict such flows including detailed interactions between particles, fluid and walls. A number of advanced modelling techniques will be used including principally the Discrete Element Method (DEM), Computational Fluid Dynamics (CFD), Smooth Particle Hydrodynamics (SPH), Lattice Boltzmann Method (LBM) and Coarse-Grained Molecular Dynamics (CGMD).

None of these methodologies on its own, however, is able to effectively model these complex flows as they all enjoy strengths as well as weaknesses. We will, therefore, exploit the strengths of each technique by assembling these methods in an efficient hybrid fashion to produce an integrated multiscale modular framework to be made available free of charge within the unique and well-known open source code DL_MESO. Thus, we will evaluate the best hybrid approaches and develop a paradigm for modelling these complex flows by mapping the model hybrids against flow characteristics.

The use of a hybrid modelling methodology and a multiscale approach to include concentrated turbulent flows, fluids of non-Newtonian rheology, particles of complex shapes and properties will produce a quantum leap advance in the modelling of these complex flows. In the medium to long-term, the findings from this work should improve the competitiveness of the UK solid-liquid processing technologies. Our industrial and academic partners, however, will be able to draw immediate benefits through engagement with the project.

Complex solid-liquid flows challenge our understanding of multiphase physics. The complex fluid-particle-wall interactions and particle morphological transformations which tend to accompany such flows require further modelling innovations to improve existing multiphase fluid science as well as industrial practice. The issues at stake engage, on the one hand, academic researchers looking to probe, understand and model these complex multiphase fluids, and, on the other, industrialists seeking to exploit their properties to develop new applications or enhance existing ones based on understanding rather than trial and error.

This project brings together complementary experimental, theoretical and computational expertise from Chemical Engineers and Mathematicians/Computational Modellers at three UK institutions. Birmingham has a strong track record in experimental research in fluid dynamics, multiphase systems and formulation engineering, as well as in modelling multiphase flows using particle techniques such as SPH and DEM. Surrey has a long track record in developing DEM and CFD-DEM models for granular flow and powder technology, whilst Daresbury Laboratory has been a key international player in high performance computing as well as computational modelling of engineering processes, being authors of the unique open-source softwares: DL_POLY for molecular dynamics, and DL_MESO for Lattice Boltzmann and Dissipative Particle Dynamics methods. The experimental and theoretical methodologies developed here are mostly generic and therefore applicable to other types of two-phase systems, e.g. emulsions, gas-solid, blood, aerosols. We expect that the combination of our strengths will result in new interdisciplinary views and tools for the study of solid-liquid flows and multiphase flows in general, delivering high impact fundamental research across disciplinary boundaries between several EPSRC areas, including: Complex Fluids and Rheology, Soft Matter Physics, Fluid Dynamics, Process Engineering, Computational Physical Sciences, and Innovative Production Processes, which are related to several EPSRC themes including Engineering, Manufacturing the future, Physical Sciences and Healthcare Technologies.

The project is pre-competitive, designed to develop a modelling framework that can be used to address real problems across a wide range of industries. More specifically, this research is supported by world-leading companies: Unilever (food, personal care); Imerys (mineral-based specialties for consumer goods, industrial equipment and construction); Briggs (processing equipment for food and pharmaceuticals); P&G (consumer products); and CD-adapco (engineering simulation software); as well as two international academic research centres: The State Key Laboratory of Hydraulics and Mountain River Engineering in China; and The Biomechanics and Bioengineering Laboratory - CNRS in France. During the project, we will share results and ideas with all of them, and we will also engage, as outlined in their letters of support: (i) with the industrial partners to help them evaluate and further enhance/develop industrial applications specific to their own businesses; and (ii) with the academic partners to help them with their research interests and activities.

We will train 4 postdoctoral research fellows and at least 3-4 PhD students, who will become the next scientific leaders in this area. At the same time, we will use our results to enthuse school children and the general public about the value of scientific research and the science of complex fluids and computational modelling to the UK, through the generation of outreach materials (e.g. short videos, interactive demonstration tool) and a project website. Computer simulations/animations of complex multiphase flows are a particularly appropriate vehicle for this sort of outreach, since these flows are familiar, fascinating, surprisingly ubiquitous and of great benefit to UK industry.