【作者】 张扬；

【导师】 彭晓峰；

【作者基本信息】 清华大学 ， 动力工程及工程热物理， 2008， 博士

【摘要】 多孔介质广泛存在于自然界,如土壤、矿石、动植物组织等,其中汽液相变传递过程更是常见的物理现象。温差作用下,水分发生相变和迁移,同时包括流动、传热、传质和相变,比单一动量、能量或质量传递复杂得多,又本质性地影响着实际应用。多孔介质内汽液相变传递过程的研究还远未完善,各种机理的认识和描述均不够清晰,包括基本宏观物性和微细观过程特征,所采用的一些假设和理论也还强烈地期盼有更为坚实的物理基础与实验事实支撑。本文提出从物质分布非均匀性的新视角重新审视和认识这一复杂传递过程。试图由唯象方程出发,按统一思路在数学上定义宏观和孔隙尺度非均匀性,阐明非均匀性产生、所引发的效应及它们作用于传递过程的机理。具体从宏观和孔隙尺度两个层次切入,探讨冷凝、蒸发和孔隙局部冷凝蒸发共存三种现象中水分分布、溶质分布和结构非均匀性效应。冷凝过程中,液体在孔隙内部铺展特性引发非均匀水分分布,形成特征复杂的冷凝前沿。在其附近,蒸汽扩散和导热有效通道面积都快速变化,引起物质和能量传递在此处明显起伏,表现出冷凝速率和温度梯度在空间上迅速变化。多孔介质内溶液表面蒸发过程的定向特点和多孔介质本身结构特点引起溶质向表面富集、析出并团簇状生长。这种非均匀分布使得干燥过程与纯净液体的干燥明显异同,干燥曲线上附加产生一个短暂的第一减速干燥段。溶质析出初期扩展了干燥表面积,后期形成溶质层阻碍干燥进行,尤其析出溶质的生长及形态对干燥过程产生重要影响。从孔隙尺度来看,宏观上的均匀孔隙有更细致的微观结构。孔隙中固液气复杂弯曲界面构成孔隙结构非均匀性,扭曲了物理量的分布,极大影响传递过程。非均匀性使得各相对传递(如导热)的贡献处在非对称位置上,引发一些非常规局部传递现象(如局部蒸发冷凝共存),结构对传递过程影响变得异常繁杂。本文引入定量描述孔隙尺度非均匀性的参数,简明、有效地分析阐明物理过程和机理,提供了认识复杂问题的新思路和技术手段。更多还原

【Abstract】 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.更多还原