The CNRS and Tec21 are co-financing a doctoral research project entitled “Numerical modelling of the mechanical behaviour of cellulose fibers in a fluid flow”
Following on from the Workshop on “Biorefinery, Fluid Mechanics and Modelling” organised in 2013, four partner laboratories within Tec21 have identified a field of research concerning the mechanical behaviour of cellulose fiber suspensions that has not received appropriate attention from the research community. This topic is a key issue with regard to biomass processing.
By associating their expertise in fluid and solid mechanics, modelling and process engineering, the partners of the project wish to link the fundamental processes involved at the micro-scale level to the industrial processing of biomass constituents.
The objective of the proposed thesis is to set up within the Grenoble mechanical and process engineering community, a simulation tool to describe the movement of objects such as flexible and very elongated fibres (millimetre to nanometre size, aspect ratio from 10 to 100) transported by complex flows (very confined areas, turbulence). Direct simulations of elementary processes such as deformation and agglomeration of fibres are targeted. This thesis is part of a project carried out by LabEx Tec21 on biorefinery where the aim is to control the production processes of cellulose micro/nanofibrils (production, separation into micro-channels, jet coating).
As regards the simulation of suspensions of deformable objects, significant progress has recently been made, particularly in the simulation of deformable vesicles, for example by the phase field
approach developed at LIPhy and LJK in Grenoble, and by the "volume of fluid" approach on blood cell flows including interactions with wall adsorbed proteins, where the description of the
membranes involves taking into account their elasticity. Such a description is difficult to transpose to cellulose fibres referred to here. Difficulties concern in particular the large
deformation of slender objects, the management of contacts between fibres, and an appropriate translation of the mechanical behaviour of cellulose fibres, which are by nature inhomogeneous and
difficult to model using 3D equivalent continua.
The choice of the modeling strategy will thus constitute the first step of the thesis. Ideally, it would be a fine description of the fluid field by direct numerical simulation coupled with a curvilinear description of the fibres, i.e. their deformation and interactions. These interactions occur for very small volume fractions of fibers (0.1 to 1% by volume) due to their very high slenderness. Fluid-fibre and fibre-fibre couplings are non-trivial at these scales and for the flow conditions considered. From a numerical point of view, the tool developed will lead to coupling discrete methods, of the molecular dynamic type or discrete elements for the solid part such as those developed at 3SR, and methods of the finite volume type developed at LEGI.
Once the strategy has been defined, the first lock will be to simulate a single homogeneous elongated object in complex flow. The next step will be to treat the case of several interacting fibers, thus introducing contact forces. The ultimate goal is to simulate denser suspensions in order to predict the transport of fibre assemblies and the appearance of agglomerates. In addition, the estimation of the hydrodynamic stresses undergone by these fibres will serve as a guide to better control or make more efficient the processes under consideration.
The PhD follow-up will be ensured more particularly by P. Dumont (LGP2), B. Harthong (3SR) and G. Balarac (LEGI), specialists respectively in biobased materials, numerical simulations of divided media and simulation of complex fluid flows.