Thèse de doctorat en Mécanique, Energétique, Génie Civil et Acoustique
Soutenue le 02-12-2016
à Lyon , dans le cadre de Ecole Doctorale Mecanique, Energetique, Genie Civil, Acoustique (MEGA) (Villeurbanne) , en partenariat avec École centrale de Lyon (établissement opérateur d'inscription) , Laboratoire de mécanique des fluides et acoustique (Rhône) (laboratoire) et de Institut Français du Pétrole. Energies Nouvelles (Lyon site) (laboratoire) .
Le président du jury était Éric Climent.
Multi-scale numerical simulation of fluidized beds by high performance computing
Fluidized beds are a particular hydrodynamic configuration in which a pack (either dense or loose) of particles laid inside a container is re-suspended as a result of an upward oriented imposed flow at the bottom of the pack. This kind of system is widely used in the chemical engineering industry where catalytic cracking or polymerization processes involve chemical reactions between the catalyst particles and the surrounding fluid and fluidizing the bed is admittedly beneficial to the efficiency of the process. Due to the wide range of spatial scales and complex features of solid/solid and solid/fluid interactions in a dense fluidized bed, the system can be studied at different length scales, namely micro, meso and macro. In this work we focus on micro/meso simulations of fluidized beds. The workflow we use is based on home made high-fidelity numerical tools: GRAINS3D (Pow. Tech., 224:374-389, 2012) for granular dynamics of convex particles and PeliGRIFF (Parallel Efficient LIbrary for GRains In Fluid Flows, Comp. Fluids, 38(8):1608-1628,2009) for reactive fluid/solid flows. The objectives of our micro/meso simulations of such systems are two-fold: (i) to understand the multi-scale features of the system from a hydrodynamic standpoint and (ii) to analyze the performance of our meso-scale numerical model and to improve it accordingly. To this end, we first perform Particle Resolved Simulations (PRS) of liquid/solid and gas/solid fluidization of a 2000 particle system. The accuracy of the numerical results is examined by assessing the space convergence of the computed solution in order to guarantee that our PRS results can be reliably considered as a reference solution for this problem. The computational challenge for our PRS is a combination of a fine mesh to properly resolve all flow length scales to a long enough physical simulation time in order to extract time converged statistics. For that task, High Performance Computing and highly parallel codes as GRAINS3D/PeliGRIFF are extremely helpful. Second, we carry out a detailed cross-comparison of PRS results with those of locally averaged Euler- Lagrange simulations. Results show an acceptable agreement between the micro- and meso-scale predictions on the integral measures as pressure drop, bed height, etc. However, particles fluctuations are remarkably underpredicted by the meso-scale model, especially in the direction transverse to the main flow. We explore different directions in the improvement of the meso-scale model, such as (a) improving the inter-phase coupling scheme and (b) introducing a stochastic formulation for the drag law derived from the PRS results. We show that both improvements (a) and (b) are required to yield a satisfactory match of meso-scale results with PRS results. The new stochastic drag law, which incorporates information on the first and second-order moments of the PRS results, shows promises to recover the appropriate level of particles fluctuations. It now deserves to be validated on a wider range of flow regimes.