接触式粘性力对颗粒流态化的影响机理研究

发布时间：2018-05-17 13:35

本文选题：接触式粘性力 + 流化床　；参考：《东南大学》2016年博士论文

【摘要】：流化床反应器因其高效的传热传质与连续处理大量颗粒的能力,在不同工业领域中得到了广泛的应用。伴随着流化床处理对象与手段的多样化,颗粒性质经常发生改变,其中较为常见的是颗粒间出现以液桥力和固桥力为代表的接触式粘性力。粘性力的存在降低了颗粒的流动性,导致其流态化行为有别于传统的无粘颗粒,从而影响了流化床内的反应效率以及反应器的稳定运行。因此,揭示颗粒间接触式粘性力对流化特性的影响规律和机理,对此类流化床反应器的设计和运行具有重要的指导意义。但是,相较于无粘颗粒流态化丰富且逐渐完善的理论体系,对于接触式粘性力的研究仍然较为零散,缺乏系统深入的认知。本文基于实验手段,在颗粒层面和颗粒体系层面上开展了接触式粘性力对流态化的影响机理研究,旨在加深对于此类颗粒体系运动特性的认知。全文主要的研究内容和成果如下：(1)颗粒层面上,设计并搭建颗粒-平板含液膜碰撞系统,其中颗粒与平板间的粘性力通过调节液膜的粘度与厚度实现：提出采用横向扰动风赋予颗粒水平运动速度,实现颗粒与覆盖液膜的有机玻璃平板进行斜向碰撞的目标,克服了传统的倾斜平板法只能应用高粘度、低厚度液膜的缺点；借助布置在平板底部的透射光源,清晰地捕捉液膜和液桥几何形态的变化,同时基于图像处理技术准确获取了颗粒的运动轨迹和速度等动力学参数,为颗粒碰撞模型的构建奠定了坚实的基础。与准静态过程不同,碰撞中液桥的形成和发展呈现滞后性,而且液体惯性在碰撞中所占的比重越大,滞后效果越明显。液桥力对于反弹颗粒在法向方向上的动能损耗影响较大,对切向方向的影响较小。通过比较颗粒在碰撞全过程中发生的能量损耗可以发现,液桥力造成的动能耗损远小于其他阻力。在法向方向上,颗粒的能量耗损随液膜粘度和厚度的增加而增加,随碰撞速度的增加而减小；在切向方向上,能量耗损过程主要由液膜厚度主导：厚度较大时,液体主要起阻力作用,而当厚度较小时,液体主要起润滑作用。鉴于液膜厚度在碰撞中的重要作用,本文将其引入,对传统的表征碰撞过程的Stokes数进行修正,并结合液体弹性动力学理论和经典的流体力学理论首次提出了用于判断颗粒是否反弹的临界Stokes数模型以及用于衡量颗粒动能耗损的碰撞恢复系数模型,获得了较为准确的预测结果。(2)颗粒体系层面上,利用“多聚物涂层”法引入接触式粘性力,通过调节涂层表面温度,改变多聚物粘性,实现控制颗粒间粘性力的目标,克服了传统引入方法粘性力分布不均且复现性差的缺点,同时基于图像处理技术提取不同粘性力作用下二维可视化鼓泡流化床中气泡的静态和动态特征参数。与无粘颗粒体系相比,粘性力的存在抑制了气泡在床中段的合并过程,促进了气泡在床层顶部区域的分裂过程。粘性力对床内整体的流化特性呈阶段式影响：在粘性力增加的初始阶段,乳化相的持气能力提高,气泡通过频繁的纵向合并从近似圆形过渡到竖椭圆形,减弱了床两侧颗粒的运动强度,导致当粘性力继续增大时,颗粒率先向床两侧粘结,床层膨胀比迅速降低,并最终以沟流的形式造成流化失效,而且当床内布置有埋管时,流化失效的进程加快。埋管的存在削弱了粘性力对气泡参数的影响,埋管周围的局部气泡分布特性同时受到颗粒间粘性力及埋管位置的制约。(3)利用荷兰代尔夫特理工大学搭建的多源复合X射线断层扫描系统,首次重构出粘性颗粒在三维鼓泡流化床中的气泡形态,同时结合压力波动分析技术获取了丰富的流态化信息。实验结果表明,粘性力的存在促进了气泡合并,导致气泡尺寸上升,频率下降,最终在高粘性力作用下引发节涌,导致流化失效。与无粘颗粒流化不同的是,节涌产生的料栓在粘性力作用下能够完成“自我生长”,使得流化失效区域逐渐向布风板方向扩张。伴随着料栓的破碎,节涌与正常流化在床内交替产生,而且相较于自由气泡,节涌产生的气栓对粘性力更加敏感。由于气泡尺寸是触发节涌的关键参数,因此高粘性力、高流化风速等促进气泡生长的因素都将直接导致节涌持续时间的延长。与之相伴的气泡破裂、床层波动等相关性现象所激发的压力波主导了床内的压力波动,而且高粘性力下压力波沿高度方向的耗散规律与流化风速紧密相关。通过比较二维床与三维床的实验结果可以发现,粘性力的存在降低了颗粒的流动性,使流化质量发生恶化。两种床型下,发生流化失效的粘性力区间基本一致。但是,流化床不同的几何特征直接导致了流化参数变化规律以及流化失效方式的差异,说明反应器几何尺寸对粘性颗粒体系流态化特性具有重要影响。
[Abstract]:Fluidized bed reactor has been widely used in different industrial fields because of its high efficiency of heat and mass transfer and continuous treatment of large quantities of particles. With the diversification of the object and means of fluidized bed treatment, the properties of particles often change, and the more common is the contact type between the particles, which is represented by the bridge force and the solid bridge force. Viscous force. The existence of viscous force reduces the fluidity of the particles, resulting in its fluidization behavior different from the traditional cohesive particles, which affects the reaction efficiency in the fluidized bed and the stable operation of the reactor. Therefore, it reveals the regularity and mechanism of the influence of the indirect viscous force on the fluidization characteristics of the particles, and the setting of this kind of fluidized bed reactor. However, compared with the rich and gradually perfected theoretical system of cohesive particles, the study of contact viscous force is still scattered and lack of systematic understanding. Based on the experimental means, the contact viscous force is carried out on the particle and particle level. The main research contents and achievements of the full text are as follows: (1) the particle and plate liquid film collision system is designed and built on the particle level, in which the viscosity and thickness between the particles and the plate are realized by adjusting the viscosity and thickness of the liquid film. The wind gives the velocity of the particles horizontal movement and realizes the oblique collision between the particles and the organic glass plate covering the liquid film. It overcomes the shortcomings of the traditional inclined flat plate method, which can only apply high viscosity and low thickness liquid film. With the help of the transmission light source arranged at the bottom of the flat plate, the changes of the liquid film and the geometric shape of the liquid bridge are clearly captured and the results are based on the change of the liquid film and the liquid bridge. The image processing technology accurately obtains the kinetic parameters such as the motion trajectory and velocity of the particles, which lays a solid foundation for the construction of the particle collision model. Unlike the quasi static process, the formation and development of the liquid bridge in the collision is lagging, and the larger the proportion of the liquid inertia in the collision, the more obvious the lag effect is. The effect of the kinetic energy loss on the normal direction is larger and the influence on the tangential direction is smaller. The energy loss caused by the bridge force is far less than that of the other resistance. The energy loss of the particles increases with the increase of the viscosity and thickness of the liquid film in the direction of the normal direction. In the tangential direction, the energy loss process is mainly dominated by the thickness of the liquid film in the direction of the tangential direction: when the thickness is large, the liquid mainly plays the drag effect, and when the thickness is small, the liquid is mainly lubricated. In view of the important role of the thickness of the liquid film in the collision, the paper introduces it to the traditional Sto of the collision process. The kes number is modified and the critical Stokes number model used to judge whether the particles rebound or not, and the model of the collision recovery coefficient used to measure the kinetic energy loss of particles is first proposed in combination with the liquid elastodynamics theory and the classical fluid mechanics theory. (2) the use of "polymer" on the particle system level. The coating method introduces contact viscous force. By adjusting the surface temperature of the coating and changing the viscosity of the polymer, the viscosity of the particles is controlled, and the shortcomings of the traditional viscous force distribution are overcome, and the two dimensional visualization of the bubbling fluidized bed under the action of different viscous forces is extracted based on the image processing technology. The static and dynamic characteristic parameters of the bubble. Compared with the inviscid particle system, the existence of viscous force inhibits the merging process of the bubble in the bed, and promotes the splitting process of the bubble in the top of the bed. The viscous force has a stage effect on the fluidization characteristics of the whole bed. The gas holding capacity of the emulsified phase is improved at the initial stage of adding viscous force. By the frequent longitudinal merging of the bubbles from the approximate circle to the vertical ellipse, the motion intensity of the particles on both sides of the bed is weakened. When the viscous force continues to increase, the particles are first bonded to the two sides of the bed and the bed expansion ratio decreases rapidly. Finally, the fluidization failure is caused by the form of the furrow flow, and the fluidization failure when the bed is buried in the bed. The process quickens. The existence of buried tube weakens the effect of viscous force on the bubble parameters. The distribution characteristics of local bubbles around the buried pipe are restricted by the viscous force between particles and the position of the buried tube. (3) using the multi source composite X ray tomography system built by the Technische Universiteit Delft in Holland, the viscous particles are reconstructed for the first time in the three-dimensional bubbling flow. The shape of the bubbles in the bed and the pressure fluctuation analysis techniques have been used to obtain abundant fluidization information. The experimental results show that the existence of viscous force promotes bubble consolidation, resulting in the increase of bubble size and the decrease of the frequency of the bubbles. The plug is able to complete "self growth" under the action of viscous force, making the flow failure region gradually expanding to the direction of the cloth wind plate. With the breakage of the bolt, the surge and normal fluidization are alternately produced in the bed, and the gas plugs produced by the gushing are more sensitive to the viscous force than the free bubbles. The factors such as high viscosity, high fluidization wind speed and so on will directly lead to the prolongation of the duration of the bubble growth. The pressure waves induced by the correlation phenomena such as bubble rupture, bed fluctuation and so on dominate the pressure fluctuation in the bed, and the dissipation law and fluidization along the high direction of the high viscosity pressure wave along the height direction The wind velocity is closely related. By comparing the experimental results of the two dimensional bed and the three-dimensional bed, it is found that the existence of viscous force reduces the fluidity of the particles and makes the fluidization quality worse. Under the two types of bed, the viscous force interval of the fluidization failure is basically consistent. However, the different characteristics of the fluidized bed directly lead to the change of the flow parameters. The difference of fluidization failure modes indicates that reactor geometry has important influence on fluidization characteristics of viscous particle system.
【学位授予单位】：东南大学
【学位级别】：博士
【学位授予年份】：2016
【分类号】：TQ021

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