High-fidelity boiling simulations on unstructured grids with geometric VOF method

Capturing complex non-canonical geometries with computational fluid dynamics (CFD) simulations requires unstructured meshes. Despite significant developments in CFD, most high-fidelity phase-change models have traditionally focused on structured meshes. Here, we report the development and validation of a CFD framework designed explicitly for boiling simulations on unstructured meshes using the in-house T-Flows code. This framework employs a geometric Volume-of-Fluid (gVOF) module utilizing Piecewise Linear Interface Construction (PLIC) for accurate reconstruction of liquid–vapor interfaces. The reconstructed interfaces allow evaluation of local heat and mass fluxes at each timestep by extrapolation of temperature gradients along the interface normal direction.

Validation studies include canonical phase-change benchmark problems (Stefan, Sucking, and Scriven problems), which demonstrate good agreement with analytical results. Additional simulations illustrate boiling flows around grid spacers and fuel rods representative of pressurized and boiling water reactor cores. Finally, we outline our approach to integrate a thin liquid film dryout sub-grid scale model in annular flow regimes, which enables accurate simulation of post-critical heat flux phenomena. This comprehensive framework aims to facilitate high-fidelity, two-phase CFD simulations of reactor components to support reactor safety analyses and design improvements.