LBM (Lattice Boltzmann Methods) are numerical techniques used to simulate transport phenomena in complex systems. They allow modeling fluid behavior in terms of particles moving on a discrete grid (a 'lattice'). Unlike classical methods, which solve the differential equations of fluids directly, LBM simulate the evolution of the fluid particle distribution functions in a discrete space using propagation and collision rules.

The choice of lattice in LBM is a crucial step, as it directly affects the accuracy, efficiency, and stability of the simulations. The lattice determines how fluid particles interact and move through space, as well as how the discretization of space and time is performed.

LBM methods exhibit a natural parallelism because the computations at each grid point are relatively independent. Compared to classical CFD methods, LBM can better capture certain complex phenomena (such as multiphase, turbulent, or porous media flows) because they rely on a mesoscopic modeling of the fluid, directly derived from particle kinetics, rather than on a macroscopic resolution of the Navier–Stokes equations. This approach allows for a finer representation of interfaces, nonlinear effects, and local interactions, which are often difficult to model accurately using classical CFD methods. LBM therefore enables the capture of complex phenomena at a lower computational cost. Recent studies have notably shown that LBM can reproduce the Nukiyama boiling curve (pool boiling) and, consequently, accurately calculate the critical heat flux. This flux corresponds to a bulk boiling, known as a boiling crisis, which results in a sudden degradation of heat transfer.

The critical heat flux is a crucial issue for the experimental devices (DEX) of the Jules Horowitz Reactor, as they are cooled by water either via natural convection (fuel capsule-type devices) or forced convection (loop-type devices). Thus, to ensure the proper cooling of the DEX and reactor safety, it is essential to verify that the critical heat flux is not reached within the studied parameter range. It must therefore be determined with precision. Previous studies conducted on a fuel-capsule-type DEX using the NEPTUNE-CFD code (classical CFD methods) have shown that modeling is limited to regions far from the critical heat flux. In general, flows with high void fractions (greater than 10%) cannot be easily resolved using classical CFD approaches.

The student will first define a lattice to apply LBM to a RJH device under natural convection. They will consolidate the results obtained for the critical heat flux on this configuration by comparing them with available data. Finally, exploratory calculations under forced convection (laminar to turbulent regime) will be conducted.

The student will be hosted at the IRESNE institute.