大孔隙多孔介质内自由表面流的流动结构及输运特性研究

Gas-liquid two phase flow in porous media has been throughly analyzed by many scholars in many research area all over the world. The studies mostly focus on this phenomenon in porous media with small pore size but not the large pore size. What distinguish the large pore size porous media with the small pore size porous media is that for the large one there will be free surface flow involving droplet and liquid film in the large pore void. Most of the literature neglects the structure characteristic of the solid matrix, and also based on the assumption that there is only the liquid film or droplet in the two-phase system and the effect of the collision between the liquid and solid matrix is negligible, therefore, the effect of this flow pattern in the large pore void, especially the influence of collision behavior between the droplet, liquid film and solid matrix on the transport process in porous media is underestimated. Using theoretical derivation, numerical simulation and experimental investigation, the project intends to explore the three dimensional dynamical characteristic of the collision-splash-fusion process in the droplet-liquid film-solid matrix system. The mechanism of the influence of gas-liquid flow rate ratio, porosity on the flow pattern and transport phenomenon will be revealed. The typical flow patterns and its shape parameters such as liquid film thickness and droplet diameter etc. will be concluded, and its effect on the heat and mass transfer and also the pressure drop characteristic will be revealed. For the purpose of heat and mass transfer enhancement between liquid and gas in the porous media with large pore size, flow pattern optimization model will be presented with the reconstruction topology of the solid matrix. Transport theory and its application technology in porous media will be developed.

多孔介质内气液两相流动问题已得到国内外多领域学者的高度关注和深入研究，但其主要针对的是小孔隙介质，对大孔隙介质相关问题的探索尚处起步阶段。两种介质内两相流动特征的显著差异是：在大孔隙介质的孔隙内液相存在液膜-液滴形态的自由表面流动。目前针对该特殊流动的研究多忽略孔隙结构特征、假设孔隙内液相均为液膜或液滴，这大大低估了液体与骨架间的碰撞对多孔介质内气液输运特性的影响。本项目拟经理论建模、实验观测和数值分析，探索大孔隙内液滴-液膜-骨架间碰撞-飞溅-融合的三维流动结构和动态特征；揭示孔隙率、气水比等关键参数的变化对多孔介质内部流动结构的影响规律；归纳出几种典型流动结构及相应的液滴当量直径、液膜当量厚度等特征参数，揭示其对整体传热传质性能和阻力特性的影响关系和作用机理；提出多孔介质内以强化气液相间传热传质为目的以重构骨架拓扑结构为手段的流动结构优化模型，构建大孔隙多孔介质内气液两相流动基础理论。

本项目拟通过理论分析、数值模拟和实验观测并结合模型实验，研究液滴碰撞-飞溅-融合的流动结构（细观尺度）及大孔隙多孔介质传热传质特性（表观尺度）两方面的工作，具体如下：.细观尺度研究：.1）开展了液滴碰撞壁面过程基本动力学行为的实验和模拟研究。将模拟得出的铺展半径与实验观测结果进行了对比，液滴碰撞壁面全过程中接触角取前进接触角、回缩时取后退接触角得出的模拟结果更加贴近实验值。在前进接触角和后退接触角的变化范围内比对不同的定接触角的模拟结果，发现随着接触角的増大使液滴振荡半径逐渐减小，对后续多液滴碰撞、破碎及融合等现象的研究提供了理论基础。.2）提出针对液滴碰撞-飞溅-融合基本流动结构的VOF-DPM方法。水体与壁面的碰撞初期，采用VOF模型描述碰撞初期以表面张力为主要控制因素水体结构；飞溅-融合过程采用DPM模型描述，液滴的破碎离散动态过程中，对较小水体采用离散颗粒进行描述，通过速度更快、计算效率更高的拉格朗日框架跟踪小液滴的运动轨迹，实现了对飞溅-破碎过程模拟的资源最小化，使模拟细孔隙尺度中含小液滴的自由表面流动成为可能，对细化孔隙尺度内气液两相流动与传质传热强化有着重要指导意义。.表观输运特性研究：.3）搭建了基于多孔介质孔隙结构参数（孔隙率、孔径，孔密度，迂回度等基本性质）的表观（阻力）特性的测式平台。提出了适用于孔隙多孔介质的单相输运模型，即适用于低孔隙率低流速下的达西模型、适用于大孔隙率较高流速下的非线性渗透模型。开展一系列不同孔隙结构参数的大孔隙多孔介质的阻力特性测试。实验表明，在低流速下，压降随流速的变化呈线性关系，较符合经典的达西渗流模型范畴。同孔径（孔密度PPI也相同），压降随着孔隙率增大而减小；同孔隙率，压降随着孔径的增大而减小，随着孔密度的增大而减小。.4）揭示了影响逆流湿式薄膜填料传热传质性能的主要因素及其影响规律。搭建了逆流湿式填料试验台，实验研究了气水比等重要参数对薄膜填料传热传质性能的影响规律，给出了传热传质性能最佳运行气水比的范围。建立填料表面水膜与空气之间传热传质的（四变量）数学模型，采用四阶的龙格库塔法可求解获得任意高度上的温湿度信息。气水比较小时，增加扰流孔可显著提高冷却效率、冷却特性数。开发了一种基于SiC的泡沫陶瓷薄膜填料；冷、热态实验表明：新型泡沫陶瓷薄膜填料干板压降大于增加扰流孔填料，但冷却性能优异。