Numerical simulation of boiling on 3D unstructured grids

Abstract

The objective of this thesis is the numerical simulation of the boiling henomenon on unstructured grids. Boiling is the phase change of fluid particles from the liquid phase to the vapor phase under the action of thermal fluxes at the interface separating the two phases. Boiling is thus encountered in two-phase flows and driven by the mass transfer rate at the interface. This mass transfer rate is computed from the thermal fluxes on both sides of the interface. Consequently, a highly accurate numerical method is needed to locate the interface throughout the simulation. The Navier-Stokes equations are then coupled to the heat equation by means of the mass transfer rate at the interface. Such simulations have been performed by Tanguy et al. (J. Comput.Phys., 2014) on two-dimensional axisymmetric cartesian grids. In this thesis, weextend this methodology to three-dimensional unstructured grids (composed ofirregular tetrahedra, useful to describe complex geometries). We then developed a specific solver in the YALES2 code (finite-volume-based code for simulations of two-phase flows on 3D unstructured grids). The interface motion is captured by the Level Set method. Phase change implies velocity and pressure discontinuities at the interface which especially depend on the mass transfer rate. These discontinuities are taken into account by the Ghost Fluid Method, with two velocity fields and two temperature fields. This methodology being already well established for structured cartesian grids, the contribution of this thesis relies on the ability to simulate phase change by boiling on three-dimensional unstructured grids. The particularities of unstructured grids have demanded numerous developments for the reinitialization of the Level Set function after advection, as well as the use of high-order operators forthe computation of the mass transfer rate at the interface. The proposed developments are finally validated on unstructured grids against the analytical test-case of a 3D bubble expanding inside a superheated quiescent liquid.