【作者】 李明；

【导师】 李小川；

【作者基本信息】 扬州大学 ， 流体机械及工程， 2018， 硕士

【摘要】 多孔介质是一类在自然界中广泛存在,由固体骨架和连通性孔隙组成的,具有复杂内部结构的功能性材料。多孔介质中的流动、传热问题涉及到人类生产生活的方方面面,微观孔隙结构的随机和无规则分布特性,使得多孔介质孔隙结构定量表征和热质输运性能研究未能取得突破性进展,并成为多个学科领域的研究热点和难点。基于多孔介质具有分形结构的特点,本文将随机行走方法应用于多孔介质中流体的渗流性能研究,提出了势差驱动作用下的粒子在孔隙通道中的方向随机行走模型,对孔隙通道结构特征进行定量描述,并在此基础上对多孔介质中渗流的过程和机理进行数值模拟研究。本文的主要研究成果可归纳为以下几个方面:1.多孔介质渗流性能影响因素分析多孔介质具有多种不同的类型和结构,本文以颗粒型多孔介质作为渗流研究对象。采用有限元方法对孔隙率、颗粒的形状、大小、排列方式、连通性和方向性进行数值分析,探讨了孔隙(颗粒)几何形状参数对多孔介质渗流性能的影响规律。研究结果表明,多孔介质渗流性能与多孔介质的结构紧密相关,由于孔隙结构在不同的条件下具有复杂的无规则分布特点,如果用传统的几何方法进行描述将会涉及到大量的几何参数而无法实现,对孔隙结构的简化又存在适用性和较大偏差的问题。基于上述原因,本文将选用对复杂无规则几何对象进行直接描述具有独到优势的分形几何方法来对孔隙结构进行定量描述。2.非均匀分形多孔介质渗流性能研究基于分形理论构造了孔隙率相同、孔隙结构各异的椭圆形颗粒的二维多孔介质分形模型。对颗粒的非均匀自相似分布和多孔介质的各向异性特征的渗流影响规律和机理进行数值分析。研究结果表明,流体在孔隙通道中的流动过程具有自相似性特征,与孔隙通道的分形自相似性一致;非均匀分布的颗粒对流场的影响是不同的,颗粒在垂直于流动方向上的截面积越大,对渗流的影响越大;对于相似结构特征分布的多孔介质,面积分形维数相同,渗透率随着孔隙率的增加而增大;在面积分形维数和孔隙率均相同的条件下椭圆形颗粒长轴的方向垂直于流动方向时渗透率最小,而平行于流动方向时渗透率最大。研究结果表明孔隙率和面积分形维数还不足以实现对孔隙结构特征的完全描述,渗流过程和渗透率与孔隙通道的结构和分布方向具有很大的关联性。3.多孔介质孔隙结构的分形随机行走描述将多孔介质中流体渗流的物理过程看作是流体在外部势差(压差等)作用下的流体质点运动,本文提出了粒子在多孔介质孔隙空间中,具有一定方向性粒子的随机行走方法来模拟流体的真实流动过程;并结合分形理论中分形谱维数的概念,提出了方向随机行走分形谱维数来对孔隙通道结构特征进行定量描述。研究结果表明:在孔隙率和面积分形维数均相同时,多孔介质的孔隙通道中粒子的方向随机行走分形谱维数与渗透率之间具有一致性特征。更多还原

【Abstract】 Porous media is a kind of functional material widely distributed in nature,which is composed of solid skeleton and connected pores.The internal pore structure is very complex.Fluid flow and heat transfer in porous media exist in many aspects of industrial production and human life.The random and irregular distribution of micro pore structure make the quantitative characterization of porous media and the study of heat and mass transport properties fail to make breakthrough.Heat and mass transfer in porous media has become the hot and difficult points in many fields of scientific research.Based on the characteristics of the porous media with fractal structure,random walk method is applied in this paper to study the fluid seepage performance in porous media.The characteristics of the pore channel are described using the new random walk model under the driving force of potential difference proposed in this paper.The process and mechanism of mass flow in porous media are numerically simulated.The main achievements of this paper can be summarized as follows:1.Analysis of factors affecting the seepage performance of porous mediaThere are many different types and structures of porous media.Granular porous media are selected to study the seepage performance in this paper.The finite element method is applied to analyze the porosity,the shape,size,arrangement,connectivity and orientation of the particles.The influence of the geometric parameters of the pore(particle)on the percolation performance of porous media is discussed.The results show that the percolation performance of porous media is closely related to the structure of porous media.Because the pore structure has complex and irregular distribution under different conditions,it can not be realized if a large number of geometric parameters are involved in the description of the porous structure,and the simplification of the pore structure is also applicable.The problem of large deviations.Based on the above reasons,the fractal geometry method,which has a unique advantage to describe the complex irregular geometric objects,is used to describe the pore structure in a quantitative way.2.Study on the seepage performance of non-uniform fractal porous mediaBased on fractal theory,a two-dimensional porous media fractal model with the same porosity and different distribution of pore structure and distribution is constructed.The seepage law and influence mechanism of non-uniform self-similar distribution and anisotropy of porous media are analyzed numerically.The results show that the fluid flow in the pore channel has similar characteristics and is consistent with the fractal self-similarity of pore channels.The influence of non-uniform particles on the flow field is different,and the larger the particle is in the flow direction,the greater the influence on the seepage flow.For porous media with similar structure characteristics,the surface integral dimension is the same,and the permeability increases with the increase of porosity.In the same condition,the direction of the long axis of the elliptic particle is perpendicular to the flow direction and the maximum permeability is parallel to the flow direction.The results show that the porosity and the area fractal dimension of the porous media are not enough to fully describe the characteristics of the pore structure.The process and permeability of the percolation are closely related to the structure and distribution of the pore channels.3.Fractal random walk description of pore structure of porous mediaBased on the seepage process in porous media can be seemed as fluid particles flow under the external potential difference,A new directional random walk model is developed to describe the particles walk in porous space of porous media in this paper.Combined with the fractal geometry theory in the concept of spectrum fractal dimension,the conception of directional random walk spectral dimension is putting forward to describe the structure characteristics of pore channel quantitatively.The results show that there is a consistent characteristic between the number of particles in the porous media and the permeability in the pore channels of porous media at the same time.更多还原