Modelling powder flows in Agitated Devices: from rheological studies to predictive SIMulation at process scale

Whether behaving as a solid, fluid or gas, powder is a state of matter that is difficult to model on a large scale, specifically in industrial equipment. The sizing of powder agitation devices and the choice of operating parameters (device filling rate, agitation speed and duration) are often carried out empirically, or by analogy with fluids, according to rheological models chosen by default. Rationalising these operations using adapted laws and methods is a real challenge in ensuring product quality and process safety. In addition, reducing the number of trials required for these designs will help to reduce the product-related waste.

To achieve this, engineers can use dimensional analysis and simulation tools. Based on small-scale experiments, dimensional analysis requires establishing a basis for extrapolation based on the principle of geometric similarity and the choice of an invariant. Using this method therefore requires precise knowledge of the phenomena involved. As concerns numerical methods, while methods based on discrete element modelling (DEM) provide accurate access to particle trajectories and velocities, their use requires significant computing time, which makes their direct deployment unrealistic for simulating a pilot-scale or industrial mixer with real powders [1]. Therefore, more macroscopic models need to be developed. CFD-type simulation methods [2] might be developed to that purpose, as they allow simulations to be carried out on a larger scale. These continuum mechanics models nevertheless require the constraint-deformation relationships specific to the product used to be known [3][4]. Whatever the method employed, knowledge of the constitutive laws describing powder rheology is therefore essential for the rational design of equipment.

Most studies on powder rheology have been conducted under quasi-static conditions, as research has often been motivated by storage-related issues. Existing characterisation equipment is often associated with these operating conditions. Recently, Boussoffara et al. [5] have observed dense-phase flow in horizontal powder mixers agitated by blade-equipped devices with pilot sizes ranging from 2 to 9 litres, which can be described by the rheology m(I) [6][7][8], revealing the transition between frictional and pre-collisional regimes. For a given powder, this relationship can be used as an invariant basis for scaling up processes [9]. While this work represents a fundamental advance in understanding the mechanisms at play in agitated devices, it still needs to be consolidated and supplemented in order to become a robust design tool. Furthermore, these physical models have often been studied for model granular media, and their use for industrial applications requires extending them to real powders [10].

The aim of the thesis is to explore rheological approaches and continuum mechanics models in order to simulate powder flows in industrial mixers or stirred devices. The instrumented pilot in which the initial observations were made will be used as a systemic rheometer. Physical and digital experiments will be carried out to refine our understanding of the phenomena involved, in particular to identify the geometry of the shear zone and the physical mechanisms that cause it to form, in relation to the properties of the particles and the geometry of the agitator. Observation of the flow at the wall by visualisation and image analysis will complement the torque measurement. More specifically, discrete element modelling DEM will be used to characterise phenomena localised in a restricted volume close to the agitated zone. It is expected to reveal details that might be impossible to measure in a laboratory environment. The so-called effective viscosity thus identified might “mimic” the microscopic behaviour of a fluid. It can also lead to other type of multi-phase flow models. Finally, the continuous macroscopic mechanical model will be implemented in versatile CFD software used in industry, enabling large-scale simulations. Initially, the continuous model will be applied to simulate the pilot mixer, which will then undergo experimental validation. Looking ahead to this doctoral work, the model could be adapted to different geometries, ultimately resulting in volumes of several hundred or thousand litres. In addition, it is also expected to establish the principles for a prototype rheometer capable of generating data for dense flows in a semi-confined environment, as there are currently no such characterisation devices on the market.

To summarise, this topic proposes to apply recent developments in granular physics to real powders in a situation representative of industrial operations. The aim is to highlight the fundamental mechanical processes involved in the convective agitation of powders, identify an appropriate rheological law and implement it in a continuum mechanics model, while drawing the outlines of laboratory-scale rheometer prototype capable of characterising agitated powders. This will pave the way for an integrated approach to powder characterisation and flow simulation at industrial scale. It will open up a wide range of applications, such as the development of digital twins to optimise the eco-design of equipment and the choice of operating parameters for powder mixing or any industrial operation involving powder agitation.

Supervising team :

Jari HÄMÄLÄINEN, Professor, LUT School of Engineering Sciences, Department of Computational Engineering, Finland

Cendrine GATUMEL, Associate Professor, Henri BERTHIAUX, Professor, Brayan PAREDES-GOYES, Assistant Professor, IMT Mines Albi, RAPSODEE research centre

Sylvain MARTIN, Associate Professor, Guillaume DUMAZER, Assistant Professor, Ecole des Mines de Saint Etienne, SPIN research centre, PMMG team

Why choose this doctoral subject ?

✅ This topic will offer you an opportunity to develop both experimental and numerical skills. The methods proposed for modelling and simulating powder flows will also enable you to acquire fundamental knowledge.

✅ The simulation of granular processes is a major challenge for the development of digital twins of processes in many industrial sectors such as pharmaceuticals and agri-food, but also in civil engineering, energy and materials manufacturing.

✅ Completing this project will allow you to gain international experience. Joint supervision will be required, offering the possibility of obtaining both a French and a Finnish doctorate.

The RAPSODEE laboratory, UMR CNRS 5302, is a recognised player in the field of particulate solids process engineering. It combines a range of scientific expertise in physics, chemistry, chemical engineering, energy, thermal engineering and modelling/simulation.

Profile and skills required

• Engineering degree or Master's degree in chemical or process engineering, applied physics, mechanics, applied mathematics, fluid mechanics, scientific computing.
• Motivated candidates with interest, knowledge and skills in physics and mechanics: modelling and numerical calculation as well as in experimental work
• Candidates should also be able to demonstrate teamwork skills, flexibility, autonomy, initiative, organization and intellectual curiosity.
• English level: B2