This research aims at simulating and contributing to understand the generic physics which could occur in nuclear spent fuel pools during loss of cooling accidents. Based on the limitations inherent to the Direct Numerical Simulation approach in terms of Rayleigh number and geometry, the thesis intends to provide relevant reference results for RANS simulations. The latter presents in a stepwise layout the different necessary components to simulate the natural convection of water in a cuboid domain combining vapor bubbles dynamic and evaporation through the free surface resulting into a descent of the water level. 

An Eulerian-Lagrangian approach is implemented and allows to compute the motion and growth/shrinkage of vapor bubbles. This method allows not only bubbles to be influenced by the fluid flow, but also to impact the surrounding fluid through additional source terms, this feature being called two-way coupling. In addition to the classical Rayleigh–Bénard control parameters, the problem therefore involves bubble-related properties such as their number, initial size, latent heat, and thermodynamic quantities associated with the fluid, including the saturation temperature and the degree of superheat.

Heat transfer associated with evaporation is modeled using the approach proposed by Hay et al. (Phys. Fluids 33, 2021) by replacing the classical fixed-temperature upper boundary condition with an evaporating condition, formulated as a dynamic Neumann condition prescribing both convective and evaporative heat fluxes at the interface. The associated mass loss is handled through a dedicated remeshing procedure, in which the domain height is adjusted by redistributing the descent proportionally over all grid cells at each time step. The evaporation model introduces additional control parameters related to the ambient conditions of the cover gas, namely its temperature and relative humidity.

Both models are first studied independently and validated against a range of experimental, analytical, and numerical reference cases. This stepwise approach makes it possible to assess the respective contributions of bubble-induced effects and surface evaporation on convection, while ensuring the reliability of each modeling component. The two mechanisms are then combined within a unified numerical framework, allowing their coupled impact on flow structure, heat transfer, and global transport properties to be investigated. Overall, this work provides a consistent and extensible modeling strategy for multiphase convective flows, with direct relevance to safety analyses of liquid pools under severe thermal conditions.

Membres du jury :

• Prof. Yann Bartosiewicz  (UCLouvain)(Promoteur)

• Dr. Pierre Ruyer (ASNR) (Promoteur)

• Prof. Aude Simar (UCLouvain) (Président)

• Prof. Grégoire Winckelmans  (UCLouvain) (Secrétaire)

• Prof. Diego Angeli (Unimore)

• Prof. Adrien Toutant (UPVD)

• Dr. Mathieu Duponcheel (UCLouvain)

Soutenance publique via le lien (TEAMS) : Défense de thèse - Joauma Marichal | Réunion-Joindre | Microsoft Teams