【作者】 王启立；

【导师】 胡亚非；

【作者基本信息】 中国矿业大学 ， 流体力学， 2011， 博士

【摘要】 石墨材料是典型的多孔介质,微观结构中存在大量微小孔隙,必须通过浸渍密实或者过滤吸附才能成为性能优异的工程材料,但是对石墨坯料无序孔隙的盲浸很难获得高品质材料,制约了高端石墨材料制备工艺的发展。本文从多孔介质流体力学角度出发,研究沥青粘结剂在焙烧过程中的流动与热质输运情况,分析焙烧后多孔介质孔隙结构特征,探讨孔隙结构形成过程的逾渗机理以及孔隙结构对渗透率的影响规律。本文首先从理论上分析了石墨多孔介质孔隙形成过程中的热质输运现象,将整个过程分为沥青软化、液相迁移、热解挥发、缩聚固化四个阶段。沥青液相迁移阶段表现为饱和多孔介质热质传递过程,孔隙空间完全由液相沥青占据;沥青软化、热解、缩聚阶段为非饱和多孔介质热质传递过程,孔隙空间由液态沥青、水蒸汽和挥发分气体共同占据。利用Whitaker体积平均法,推导了各阶段的热质输运方程,阐述了方程中各参数的物理意义。为了研究石墨多孔介质孔隙结构形成过程的影响因素,在差异性工艺条件下制备了多组石墨生坯试样,进行了焙烧实验,通过金相实验及压汞实验测量多孔介质孔隙结构,重点考查了不同升温曲线对孔隙结构的影响。结果表明,升温速度过快促使沥青热解过程不充分,结焦率降低,形成的孔隙较为松散,不具备长程联结性。降低升温速度和延长保温时间有利于沥青充分进行热解缩聚反应,形成分布合理,具有长程联结性的孔隙结构。焙烧升温曲线按照“两头稍快,中间缓慢,延长保温时间”的原则,热解阶段的升温速度以5℃/h左右为宜,保温时间不低于36h。本文应用分形理论描述了石墨多孔介质不规则的孔隙结构,并指出了各分形维数的物理意义。研究表明:石墨多孔介质具有典型的分形特征,其孔隙分形维数在1.683~1.821之间,骨架分形维数在1.709~1.852之间,体积分形维数在2.883~2.958之间,骨料颗粒边界分形维数在1.09~1.21之间,迂曲度分形维数在1.146~1.169之间。可以应用Menger海绵模型构造某孔隙度、比表面积的石墨多孔介质,需要根据其分形维数D,选取合适m、n、i进行构造。石墨多孔介质形成过程表现出典型的逾渗特征,焙烧过程就是“孔隙产生-孔隙局部联通-形成有限大集团-各局部联通的孔隙长程联结,形成无限大集团”的逾渗化过程。对孔隙结构图像进行了重整化群计算,并用压汞实验进行验证。结果表明:过滤用多孔介质面孔隙度在0.508~0.575之间,体孔隙度在0.273~0.324之间,具有逾渗结构;浸渍用多孔介质的面孔隙度在0.189~0.314,体孔隙度在0.107~0.155,小于正方形和简单立方体的键逾渗阈值,不具有逾渗结构。本文利用分形理论对石墨多孔介质的渗透率进行研究,研究结果表明:(1)多孔介质渗透率与其孔隙结构参数紧密相关,并由孔隙结构参数共同决定,并不是某一个孔隙结构参数的单值函数,也没有随某个参数呈现简单的递增或递减关系。(2)预测石墨多孔介质的渗透率必须考虑迂曲度的影响,考虑迂曲度的渗透率更接近实验值,相对误差在8.60%~13.56%。(3)渗透率的实验测量值受到诸如毛细压力、流动速度、流体性质等外部条件影响。更多还原

【Abstract】 As a typical porous media, the graphite performs well after impregnation or filtration because there are lots of micro-pores in the graphite. However, it’s difficult for the graphite with disordered pore to receive high quality by blind impregnation. The development of preparing high quality graphite is restricted. Based on the fluid dynamics in porous media, the transportation phenomenon of heat and mass for binder pitch and the fluid flowing in roasting process are researched. Moreover, the characteristics of micro-structure in porous media after roasting process are analyzed. The percolation theory in the building process of micro-structure and the effect of micro-structure on the permeability rate of the porous media is discussed in the paper.The phenomenon of heat and mass transfers during the building pores process in graphite matrix is analysed firstly. Whole process is divided into four stages: pitch binder soften, liquid phase transfers, pyrolysis and polycondensation. The liquid phase transfer stage displays that heat and mass transfers in saturated porous media and the pores room is occuppied by liquid phase pitch. The other stages exhibit that heat and mass transfers in unsaturated porous media and the pores room is occuppied by liquid pitch, water vapour and volatile gas. Applying Whitaker volumetric average method, the controlling equations of heat and mass transfer in various stages are established and the physical meanings of the parameters are also described.In order to study the influence facors on forming the micro-pore structure in graphite, the graphite matrix are prepared under different technological conditions and the roasting tests are conducted. The micro-structure in the pores is measured with metallurgical test and injecting mercury test. The more attention is paid to the effects of the heating-up curves on pore structure. The results have shown that the faster heating rate leads to insufficient pyrolysis and lower coke yield of the coal tar pitch, which decreases the bonding among aggregate particles and makes the micro-pore structure without long range connection. Nevertheless, it is beneficial to complete the pyrolysis and polycondensation by means of decreasing heating rate and prolonging holding time, which make a reasonable distribution and realize the mutual connection in long range among the pores in the porous medium. Consequently, the roasting temperature curve follow the rules: faster heating at the beginning stage and the ending stage, slower heating at the middle stage, longer holding time. According to the rules, the heating rate is suitable in the range of 5℃/h and the holding time is no less than 36h.In the paper, the fractal theory is used to describe the irregular micro-pore structure in the graphite porous media, and the physical meaning on fractal dimension is researched. The results have shown that micro-pore structure of the graphite porous materials have typical fractal characteristics. The fractal dimension of pore ranges from 1.683 to 1.821, the fractal dimension of matrix ranges from 1.709 to 1.852, the fractal dimension of volume range from 2.833 to 2.958, the fractal dimension of grain boundary range from 1.09 to 1.21, the fractal dimension of tortuosity range from 1.146 to 1.169, and the different fractal dimension have illustrated various physical meanings. By logical equal portions(m), number of subtracted cubes(n), iteration times(i), Menger sponge model can be used to simulate the graphite porous media with given porosity and specific surface area according to the fractal dimension (D).Typical percolation characteristics are demonstrated in the preparation process of graphite porous media. The roasting process is the percolation process including the formation of the pore- the local connectivity of the pore - the formation of limited clusters - the long distance connectivity of various local clusters and the formation of infinity clusters. Metallurgical pictures of graphite porous medium after roasting were managed by renormalization group method and verified by injecting mercury test. The results shown that the graphite porous medium for filtration have percolation structure, the threshold pressure range from 13 to 18 psia, area porosity range from 0.508 to 0.575 and the volume porosity range from 0.273 to 0.324. The graphite porous media for impregnation haven’t percolation structure, threshold pressure range from 86 to 121 psia, area porosity range from 0.189 to 0.314 and the volume porosity range from 0.107 to 0.155.Based on the fractal theory, the permeability of the graphite porous media was studied in this work. The results were obtained that: (1) The permeability of porous media is related to the structural parameters, and the permeability is decided jiontly by all structural parameters, which is neither the single valued function nor the single increasing or diminishing trend. (2) As an important factor, tortuosity must be taken into account in prediction of the porous media. Compared with the result neglected the tortuosity, the permeability considering the tortuosity is closer to the actual value, and the relative errors range from 8.6 to13.56%.(3) The experimental results on the tortuosity is affected by the external factors such as capillary pressure, flow velocity and fluid properties.There are 49 figures, 12 tables and 170 references in this doctoral dissertation.更多还原