Nonlocal models for complex, cohesive granular flows – MoNoCoCo

Granular materials (powders, grains) are ubiquitous in industrial processes but often problematic to handle as they easily agglomerate and jam : this yields excessive energy and resource demands. The processes associated to flows of these materials (as feed or product) often involve phenomena like agglomeration and particle breakage. Then, many industrial and natural flows may be close to the solid-liquid transition (jamming), which is characterized by hysteretic behavior with the development of stagnant or creep zones. In addition, the materials treated in these processes are intrinsically polydisperse, and the mechanisms cited above may be accompanied by spatial transport according to particle size (e.g. segregation). Whether the occurrence of such phenomena is wanted (as in granulation or milling processes) or not (attrition during mixing, caking of powders, blocking of silos), there is a strong need for understanding the complex mechanics of such particulate flows.
This considered, in order to sustainably optimize processes involving the complex flow of powders and grains, reliable numerical models are needed. It is evident that critical aspects for the modeling are (1) the generality of the constitutive laws, i.e. their validity in realistic geometries (and in particular where shear localization is triggered), the treatment (2) of polydispersity and consequent segregative transport, (3) of interparticle cohesion (which is higher for fine or wet particles), and (4) of the possibility of particle size evolution for example through breakage.
This project aims to address these issues by developing a continuum, non-local constitutive framework applicable to flows of granular materials undergoing particle breakage taking into account the possibility of interparticle cohesion, the effect of the polydispersity and of particle breakage on the rheology. The working hypothesis will be the two-way coupling of (1) a nonlocal rheology adapted to cohesive systems and supplied with correct boundary conditions (in order to simulate complex flow configurations), and (2) a population balance model to describe the spatio-temporal evolution of the particle size distribution (due to segregation and particle breakage).
In order to face the challenges implied by the aim of the research, this bilateral France/Taiwan project puts together the complementary expertise of two research laboratories on experimental characterization, numerical simulation and modeling of granular flow. In particular, for each task the project will mobilize (1) microscale analyses (discrete element simulation, photoelastic techniques), (2) macroscale investigations (shear cells, rheometers), and (3) theoretical (mesoscale) modeling.
Nowadays, processes treating granular materials are often designed according to empirical knowledge alone. In this perspective, existing processes are often poorly efficient and bear high environmental costs, and new technologies may take a long time to impose themselves. Our ambition is to significantly advance the development of constitutive laws which can be used in continuum modeling of industrial processes involving granular materials subjected to boundary effects, segregation and crushing. The availability of models for the behavior of cohesive and brittle granular materials in complex flow configurations will contribute to design better unit operations and optimize processes with respect to environmental and resource constraints.