Nonuniformity Effects On Transport Phenomena In Porous Media With Liquid-vapor Phase Change

Porous media widely exist in the natural world, including soil, ore and animal/plant tissues, and inside transport with liquid-vapor phase change is often encountered. With the presence of temperature gradient, water changes its phase and migrates together with convection, heat and mass transfer and phase change, which is much more complex than normal single-phase momentum, energy or mass transfer processes. There are many fundamental phenomena and scientific issues to understand, which substantially influences practical applications. The transport processes inside porous media with liquid-vapor phase change are far from being truly well-known, including the basic macro thermal properties and micro transport characteristics. Also, the currently wide accepted assumptions and theories are eagerly expected to be supported by more solid physical foundations and experiments.In the present study a new idea, nonuniformity of mass distribution, was proposed to explore and understand the fundamentals of complex transport processes. From the phenomological equations, macro and pore-scale nonuniformity were mathematically defined in an identical way to describe the characteristics of the nonuniformity, induced phenomena and its effects on inner transport processes. The nonuniformity effects of water distribution, solute distribution and pore structure were theoretically and experimentally explored at both the macro and micro scales during condensing, evaporating, and a special emphasis was addressed on investigating the transport phenomena, induced by the local coexistence of both condensation and evaporation happening in a same pore, which is one of important nonuniformity effects associated pore structure.For condensation occurring in porous media, liquid spreading induces significant nonuniformity in water distribution and a condensation front with complex fluid dynamic phenomena and phase interface. Near the condensation front, the effective area for vapor/gas diffusion, fluid flow and heat conduction varies greatly, causing significant fluctuation of mass and energy transfer there, with spatial sharp changes of condensation rate and temperature gradient.In an evaporation process of solution in porous media, both directional mass transfer and structure of porous media make the solute accumulate, precipitate and cluster near/on the surface. Such nonuniform distribution brings an additional short first falling-drying-rate period, compared with the drying of pure fluids. The precipitation enlarges evaporation surface in earlier stage, while the cluster layer may resist the evaporation in the later stage. Particularly, the growth and morphology of the precipited solute play important roles in the evaporation process.From pore-scale view, macroscopically uniform pores have more delicate sub-structures. The complex curved phase interface between solid, liquid and gas contributes to the pore-scale nonuniformity, which alters the distribution of physical quantities and has significant influence on transport processes. The nonuniformity places the contribution of different phases to transport (e.g., heat conduction) in asymmetrical positions and results in irregular local transport phenomena (e.g., local coexistence of condensation and evaporation). As a result, the influences of pore structure become notoriously complex. A simple parameter was introduced to quantitively describe the pore-scale nonuniformity for concise and effective clarifying the associated mechanisms. This actually provides a new idea and/or technical way to understand complicated phenomena.