Large-eddy simulation of thin film evaporation and condensation from a hot plate in enclosure: First order statistics

Abstract

Numerical investigation of water evaporation and condensation in buoyancy driven flow, along with the thermal coupling between fluid and surrounding solids, is interesting for many industrial applications. The physical complexity of the evaporation and condensation processes, the mutual thermal influence of water change of phase and solid–fluid heat transfer, the anisotropy of turbulence quantities are challenging problems from numerical and theoretical side. The archetypal case of a vertical hot plate inside a cold square enclosure, filled with humid air, is studied. The solid surfaces are wetted by a thin water film. The plate cooling process due to film evaporation is analysed. Numerical simulation adopts the large-eddy methodology along with the dynamic Lagrangian sub-grid scale model. The conjugate heat transfer technique accounts for the solid–fluid thermal coupling, while the water phase is modelled under the thin film assumption. First, a preliminary case with isothermal solid boundaries and fixed film thickness is reproduced at Ra=5×108. The absence of surface heat transfer leads to analogous distribution of temperature and humidity. The cavity is characterised by strong stratification that confines the motion in the upper part. The same setting is used to simulate the case of dry air. It is found that the presence of vapour increases the velocity by a maximum of 20%. Also, the heat transfer generated by the water change of phase, in case of humid air, overcomes the other heat transfer modes. Successively, conjugate heat transfer and water film model are activated, and three cases are studied changing the plate material. The specific heat ρCp of materials is the parameter controlling the plate cooling process, that is mainly due to evaporation and the evolution of the thermodynamic field within the enclosure. An analysis of the dew-point temperature suggests that recondensation onto the plate surface cannot occur, even for materials that are rapidly cooled.