Direct numerical simulation of monodisperse fluid-particle flows - Methodology, validation, and application to fixed and fluidized beds by Baptiste HARDY

Pour l’obtention du grade de Docteur en sciences de l’ingénieur et technologie

Flows involving the transport of a dispersed solid phase by a carrier fluid are encountered in many environmental phenomena and industrial applications. Fluidized beds are now used in a wide variety of processes in the petrochemical and pharmaceutical sectors, in biomass conversion or in the food industry, among others. Those systems are characterized by a very large range of length and time scales, leading to a multiscale modeling strategy. At the smallest scale, particle-resolved direct numerical simulation (PR-DNS) is a first-principles based approach, providing the complete details of the flow. With growing computational resources, PR-DNS has become a tool of choice to develop closure models for the mass, momentum and energy transfer terms that cannot be captured in large-scale Euler-Lagrange and Euler-Euler approaches.

In this thesis, we first develop and validate a direct-forcing immersed boundary method to perform particle-resolved simulations of two- and three-dimensional fluid-particle flows. The numerical method is subsequently applied to quantify the interfacial momentum and heat transfer terms in random particle arrays. While current drag laws only provide the mean value of the fluid-particle force, we propose a microstructure-based model to estimate the distribution of the hydrodynamic force using a few quantities characterizing the local organization of the solid phase. The inhomogeneity of the thermal problem and the impact of thermal saturation on the distribution of the fluid-particle heat flux are also discussed. Then, we perform a detailed comparison of the dynamics of small-scale fluidized beds in different regimes. In liquid-solid fluidization, the small inertia of the particles leads to a homogeneous suspension and the motion of the particles is mainly driven by anisotropic self-diffusion. In gas-solid fluidization, the higher inertia of the solid particles leads to the emergence of heterogeneities in the bed (bubbles and clusters) and to a strong vertical oscillatory motion. Finally, we investigate the influence of different physical and computational parameters on the bed dynamics.

Jury members :

• Prof.  Juray DE WILDE (UCLouvain), supervisor
• Prof. Grégoire WINCKELMANS (UCLouvain), supervisor
• Prof. Olivier SIMONIN (IMFT)
• Prof. Simon SCHNEIDERBAUER (JKU)
• Prof. Yann BARTOSIEWICZ (UCLouvain)
• Prof. Laurent BRICTEUX (UMons)

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