Research On Heat And Mass Transfer In The Process Of Phase Change Of Porous Media

The thesis is mainly on the research and analysis of the freezing process in the saturated porous media and solute distribution in that process.Based on volume-averaging theory, saturated porous media is treated as virtual continuous body in macro-scale, Darcy law is referred here, Primary mechanisms of transportation for liquid in freezing process of saturated porous media, and enthalpy method which is popular is applied to present the freezing process. Assuming saturated porous media in local thermodynamics equilibrium and based on the mechanisms of fluid flow, the interactions of mass, momentum and energy conservation for the phases including liquid, solute, solid (ice) and solid (porous media) in the freezing process, solidification quantum of liquid is expressed as mass sink for itself in the liquid mass conservation equation, while it is the mass source for solid (ice). Compared with the single phase-change interface during the freezing process before, the enthalpy-porosity technique is used here for modeling the freezing process. In this technique, the interface is not tracked explicitly. Instead, a quantity called the liquid fraction, which indicates the fraction of the cell volume that is in liquid form, is associated with each cell in the domain. The mushy zone is a region in which the liquid fraction lies between 0 and 1. The mushy zone is modeled as a "pseudo" porous medium in which the porosity decreases from 1 to 0 as the material solidifies. When the material has fully solidified in a cell, the porosity becomes zero and hence the velocities also drop to zero.The CFD software, Fluent, is used to simulate the freezing process, the planar mathematic model is combined with modules in Fluent such as solidification model, transport model and porous condition, some UDF (User-defined functions) are introduced here.Through the simulation, the differences in the freezing process between the isotropic porous media and the anisotropic porous media are presented, furthermore, the changing trends of temperature and saturation of liquid in anisotropic porous media during freezing is analyzed, the freezing processes of speediness and tardiness are analyzed here under different boundary conditions. The distribution of the solute is also presented, and the emphasis is focused on the influences of the freezing temperature and the boundary conditions.