Development of a new numerical scheme, based on T-coercivity, for discretizing the Navier-Stokes equations.
In the TrioCFD code, the discretization of the Navier-Stokes equations leads to a three-step algorithm (see Chorin'67, Temam'68): velocity prediction, pressure solution, velocity correction. If an implicit time discretization scheme is to be used, the pressure solution step is particularly costly. Thus, most simulations are performed using an explicit time scheme, for which the time step depends on the mesh size, which can be very restrictive. We would like to develop an implicit time discretization scheme using a stabilized formulation of the Navier-Stokes problem based on explicit T-coercivity (see Ciarlet-Jamelot'25). It would then be possible to solve an implicit scheme directly without a correction step, which could significantly improve the performance of the calculations. This would also allow the use of the P1-P0 finite element pair, which is frugal in terms of degrees of freedom but unstable for a classical formulation.
development of a NET (Negative Emission Technologie) process combining CO2 capture and hydrogenation into synthetic fuel
Until recently, CO2 capture technologies were developed separately from CO2 utilization technologies, even though coupling the CO2 desorption stage with the chemical transformation of CO2, which is generally exothermic, would yield significant energy savings.
The first coupled solutions have recently been proposed, but they are mainly at moderate temperatures (100-180°C) [1], or even recently close to 225°C [2].
The objective of this doctoral thesis is to study, both experimentally and theoretically, a coupled system in the 250-325°C temperature range that allows via Fischer-Tropsch-type catalytic hydrogenation the direct production of higher value-added products
Fluid-structure interaction in a network of slender solids in a confined environment
As part of its study of progressive deformations in fuel assemblies within PWR cores, the CEA has developed two simulation tools. The first, Phorcys [1], calculates the flow of coolant in and around slightly deformed assemblies using a network of parametric pressure drops, then deduces the fluid forces acting on the structures. The second, DACC [2], uses finite element simulation to analyze thermomechanical behavior under irradiation and the interaction between assemblies during power cycles. Finally, fluid-structure interaction is analyzed using numerical coupling of these two tools, within which uncertainties can be propagated and analyzed [3].
The nuclear revival program (SMR, 4th generation reactors, PN, etc.) is providing new technologies and new core and fuel assembly topologies that need to be analyzed in terms of the risks associated with quasi-static deformations of core assemblies. With a view to both capitalizing on and extending the possibilities of simulation, the aim is to enable these two tools to handle the flow and deformation of slender structures in a more generic way in order to cover a wide range of nuclear technologies efficiently and quickly.
To do this, it will be necessary to identify, classify, and then model in a reduced but predictive manner the main flow structures that may occur within a fluid volume cluttered with slender structures with a large exchange surface area. The complete hydraulic model of the core will thus be created by concatenating elementary models that comply with strict interfacing conditions. A method for analyzing the overall flow obtained will then enable the quantification of the force field contributing to the deformations. A similar logic of classification and scaling would also be implemented with regard to the evaluation of reversible and irreversible deformations of a slender structure subjected to external stresses and severe irradiation. One difficulty is that the fine topology of a fuel assembly can exhibit nonlinearities at small scales that propagate in part to the macroscopic scale. Ultimately, a robust, cost-effective partitioned coupling will have to be implemented between the coolant flow and these individual structures, which deform and interact in a constrained environment.
The modeling framework thus constructed will make it possible to study the progressive deformations of assemblies and the associated risks for a wide range of nuclear reactor technologies.
Detailed Numerical investigations on highly-concentrated bubbly flows
To assess the safety of industrial facilities, the CEA develops, validates, and uses thermohydraulic simulation tools. Its research focuses on modelling two-phase flows using various approaches, from the most detailed to the largest system-scale. In order to better understand two-phase flows, Service of Thermal-hydraulic and Fluid Mechanics (STMF) is working on implementing a multi-scale approach in which high-fidelity simulations (DNS, Direct Numerical Simulation of two-phase flows) are used as “numerical experiments” to produce reference data. This data is then averaged to be compared with models used on a larger scale. This approach is applied to high-pressure flows where the bubbly flow regime is maintained even at very high void fractions. The Laboratory of Development at Local Scales (LDEL) belonging to STMF has developed a DNS method (Front-Tracking) implemented in its open-source thermo-hydraulics code: TRUST/TrioCFD [1] (object-oriented code, C++). In several PhDs, it has been used to perform massively parallel simulations to describe interfaces in detail without resorting to models, for example in groups of bubbles (called swarms) [2][3][4].
Currently applied to low-concentration two-phase bubbly flows (volume fraction less than 12%), the objective of this thesis will be to evaluate and use the method at higher void fractions. Reference HPC simulations of bubble swarms will be conducted on national supercomputers up to gas fractions of 40%. The quality of the results will be evaluated before extracting physical models of bubble interactions under these conditions. The objective of these models is to recover the overall dynamics of the bubble swarm at much lower resolutions, thereby enabling the study of larger systems in disequilibrium (external forcing of imposed turbulence generation, imposed average velocity gradient, etc.).
This work is funded by the French ANR, in collaboration with IMFT and LMFL, in parallel with two other theses with which there will be strong interactions. It will be performed at CEA-Saclay, in the STMF/LDEL laboratory. It includes numerical aspects (validation), computer developments (C++), and a physical analysis of the flows obtained.
Adjoint sensitivity method applied to industrial modeling of nuclear reactor cores
The objective of this thesis is to lay the foundations for applying the adjoint sensitivity method to industrial modeling of solid fuel nuclear reactor cores. The main topic will be the consideration of the coupling between neutronics, thermohydraulics, heat diffusion in fuel rods, and evolution.
Modeling of Wall Condensation Phenomena and Liquid Film Interactions
In this thesis, we focus on modeling mass and energy transfer associated with wall condensation in a turbulent flow of a vapor–noncondensable gas mixture. The flow is two-phase and turbulent, where forced, mixed, and natural convection modes may occur. The framework of this work relies on the RANS approach applied to the compressible Navier–Stokes equations, in which wall condensation is described using semi-analytical wall functions developed in a previous doctoral study cite{iziquel2023}. These functions account for the different convection modes as well as suction and species interdiffusion effects, but neglect the presence of a liquid film.
In the literature, the influence of film formation and flow on mass and heat transfer is often neglected, since it is generally assumed that, in the presence of noncondensable gases, the resistance of the gaseous layer to vapor diffusion is much greater than the thermal resistance of the liquid film.
The objective of this thesis is to improve the prediction of heat and mass transfer by investigating, beyond the thermal resistance of the condensate, the dynamic effect of the liquid and its interaction with the gaseous diffusion layer during wall condensation. The study will first consider laminar film flow, and then attempt to extend the analysis to the turbulent regime.
In the gas phase, the wall-function model developed in cite{iziquel2023} for a binary mixture of vapor and a single noncondensable gas will be extended to mixtures of vapor and $n>1$ noncondensable gases (N2, H2, …), in order to address hydrogen risk issues.
The validation of the implemented models will be carried out using results from separate-effect (SET) and coupled-effect (CET) experiments available in the literature (Huhtiniemi cite{huhti89}, COPAIN, ISP47-MISTRA, ISP47-TOSQAN, RIVA). Comparisons at the CFD scale, using wall functions for condensation neglecting the film, will be performed on benchmark cases from the literature and condensation experiments (COPAIN) to assess the impact of this assumption as well as the improvement provided by the new model in terms of accuracy and computational cost.
HPC two-phase simulations with lattice Boltzmann methods and adaptative mesh refinement
CEA/STMF develops computational fluid dynamics (CFD) codes in thermohydraulics that aim to quantify mass and energy transfers in nuclear cycle systems such as reactors and management devices of radioactive wastes. This thesis focuses on Lattice Boltzmann Methods (LBM) adapted to Adaptive Mesh Refinement (AMR) inside a generic computing environment based on Kokkos and executable on multi-GPU supercomputers. The proposed work consists in developing LB methods in the Kalypsso-lbm code to simulate coupled partial differential equations (PDEs) modelling incompressible two-phase and multi-component flows such as those encountered in downstream cycle devices. Once the developments have been completed, they will be validated with reference solutions. They will allow a comparison of various interpolation methods between blocks of different sizes in the AMR mesh. A discussion will be held on the refinement and de-refinement criteria that will be generalized for these new PDEs. Finally, benchamrks of performance will quantify the contribution of AMR for 3D simulations when the reference simulation is performed on a static and uniform mesh. This work will use supercomputers which are already operational (e.g., Topaze-A100 from CEA-CCRT), as well as the future exascale supercomputer Alice Recoque depending on the progress of its installation.
Modelling of Thermo-Fluid Phenomena in the Plasma Nozzle of the ELIPSE Process
The ELIPSE process (Elimination of Liquids by Plasma Under Water) is an innovative technology dedicated to the mineralization of organic effluents. It is based on the generation of a thermal plasma fully immersed in a water-filled reactor vessel, enabling extremely high temperatures and reactive conditions that promote the complete decomposition of organic compounds.
The proposed PhD research aims to develop a multiphysics numerical model describing the behavior of the process, particularly within the plasma nozzle, a key zone where the high-temperature gas jet from the torch interacts with the injected liquids.
The approach will rely on coupled thermo-aerodynamic modeling, integrating fluid dynamics, heat transfer, phase change phenomena, and turbulence effects. Using Computational Fluid Dynamics (CFD) tools, the study will characterize plasma–liquid interaction mechanisms and optimize the geometry and operating conditions of the process. This modeling will be compared and validated against complementary experimental data obtained from the ELIPSE setup, providing the necessary input for model calibration and validation.
This work will build upon previous research that has led to the development of thermal and hydraulic models of both the plasma torch and the reactor vessel. Integrating the new model within this framework will yield a comprehensive and coherent representation of the ELIPSE process. Such an approach represents a decisive step toward process optimization and industrial scale-up.
The ideal candidate will be a Master’s or final-year engineering student with a background in process engineering and/or numerical simulation, demonstrating a strong interest in physical modeling and computational approaches.
During this PhD, the candidate will develop and strengthen skills in multiphysics numerical modeling, advanced CFD simulation, and thermo-aerodynamic analysis of complex processes. They will also acquire solid experience in waste treatment, a rapidly expanding field with significant industrial and environmental relevance. These skills will provide strong career opportunities in applied research, process engineering, energy, and environmental sectors.
Simulation of flow in centrifugal extractors: the impact of viscous solvents on operation
Within the framework of nuclear spent fuel reprocessing, the CEA co-developed with ROUSSELET-ROBATEL liquid/liquid extraction (ELL) devices aimed at bringing two immiscible liquids into contact, one of which contains the valuable metals to be recovered and the other an extractant molecule. The multi-stage Centrifugal Extractor is one of the devices used to perform ELL at the La Hague plant. The future use of solvents potentially more viscous than current industrial standards may pose performance issues that need to be studied in advance in the laboratory to provide the necessary recommendations to restore the expected performance levels for the plant. The nuclear environment in which these devices operate makes in situ studies nearly impossible, thus depriving R&D of valuable information that is nevertheless essential for a deep understanding of the physicochemical mechanisms at the heart of the issues involved. To address this, the proposed study will rely on a numerical approach that will have been previously validated by comparison with either historical experimental data or data acquired from more recent ad hoc pilot systems. Thus, following a phase of literature review and capitalization of recent measurements, it is proposed to first create test cases that will be used to validate the numerical models. Based on this validation and in light of the knowledge acquired from previous theses concerning the effect of viscosity on flows, it is proposed to numerically explore the impact of an increase in solvent viscosity on centrifugal extractors. This will pave the way for a better understanding of the operation of the devices as well as operational or geometric improvements. The student will work at CEA Marcoule, in a research environment at the crossroads between a team of experimentalists and a team of numerical simulators. This experience will enable the student to acquire important skills in modeling liquid-liquid flows as well as solid knowledge on the development of liquid-liquid contactors.