Large-eddy simulations of conjugate heat transfer with evaporation-condensation and thermal radiation.

Heat transfer in fluids and between a fluid and a surrounding solid body is often encountered in practical applications. The numerical investigations of such phenomena is of great interest for a number of practical applications, both on industrial and environmental side. Among the others we mention optimization of home appliances (oven, dishwasher, refrigerators), building design (ventilation and heating systems), electronic equipment design (solar collectors, device cooling systems), environmental flow analysis (atmospheric thermal stratification, evaporation from lakes and channels, solar thermal radiation). Despite the wide range of application, the correct numerical simulation of such systems poses big challenges. First, the physics governing wet-surface evaporation/condensation processes and thermal radiation is extremely complex; thus, a mathematical model is derived only under simplification hypotheses. Second, the transient nature of surface heat transfer along with the complex geometry and anisotropic turbulence flow, requires a careful numerical resolution technique of the fluid flow equations. Third, particular attention has to be payed when the different heat transfer modes are coupled, because of their mutual strong interaction. The present work focuses on numerical investigation of the heat transfer mechanisms in fluid dynamics systems, considering different physical processes. Specifically, this topic is declined in three different studies: (i) turbulent buoyant flow in a confined cavity with conjugate heat transfer; (ii) thin film evaporation and condensation process from a vertical plate inside an enclosure; (iii) radiative heat transfer in systems with participating media. All these points are tackled. A complete numerical solver is presented and employed. In all cases we use large-eddy simulation, carried out in conjunction with a dynamic Lagrangian subgrid-scale turbulent model. Also, the thermal interaction between fluid and solid media is considered by means of conjugate heat transfer; the problem with evaporation and condensation over solid surfaces is studied taking advantage of the thin film assumption; a radiative heat transfer model is integrated in the numerical solver. The heat exchange, through fluid and solid interface, is due to conduction, water change of phase and surface thermal radiation. The temperature alteration of conductive solids, can substantially change the system thermodynamic equilibrium and can eventually lead to significant variations on the overall fluid dynamics. An in-house solver, developed within the open-source software package OpenFOAM, is used for the numerical studies and, later, extended to include the effects of thermal radiation and surface radiative heat exchange. The influence of various solid boundary materials on natural convection in closed cavity is investigated, pointing out the different effects induced on the fluid motion by thermal conductors and insulators (this part of the research has been published in Physics of Fluid). The study has shown that thermal conductive boundaries strongly influence the fluid flow; the use of simplify boundary condition (such as the adiabatic condition) instead of conjugate heat transfer, leads to unrealistic results. Successively, the cooling effects of water evaporation from a plate are studied, changing the plate materials, and the different behaviour of each substance is analysed. This study has shown that water change of phase completely rules the interface heat exchange and that material heat capacity governs the cooling process of wet bodies. Finally, the physics of thermal radiation is reviewed, the mathematical derivation of the spherical harmonic approximation model is reported and its accuracy studied, the numerical implementation carefully documented and validated.