湍流床稠密气固流中介尺度流动结构的识别与表征

The turbulent fluidized bed reactors have been widely used in chemical and petrochemical processes. The lack of knowledge on meso-scale flow structure in these reactors has considerably restricted the mass and heat transfer and reduced the yield. In a dense gas-solid flow of a turbulent fluidized bed, a stable flow structure is achieved resulting from the compromise of minimal input energy per unit mass of the particles and smallest potential energy of the particles. Exploratory experiment result shows that there exist a lot of particle agglomerates in the dense phase and particles inside the agglomerate are unfluidized. Such phenomenon suggests that the stable flow structure can be described by a physical model with respect to three meso-scale flow structures: bubbles with particles inside, the emulsion phase and particle agglomerates. This project aims for resolution of the second key scientific problem of the guide by using multi methods, such as decoupling optical fiber fluctuation signals and pressure fluctuation, suggesting an accurate approach to identify the bubble and the dense phase in signals, proposing the definition and the distinguish basis of the agglomerates. By using these methods, the project is designed to resolve the key scientific problems with respect to identification and quantitive characterization of meso-scale flow structures and prediction model of the equivalent diameter of agglomerates. The influence of operating condition and particle physical property on solid holdup, size distribution as well as immigration velocity of bubbles and afflomerates is evaluated and related. Results of this project will facilitate the revelation of the influence of meso-scale flow structure on mass and heat transfer and reaction inside the turbulent fluidized bed reactors.

湍流床反应器在石油化工中具有广泛的用途，对介尺度流动结构认识不清是反应器内传递受限和产率低下的主要原因。湍流床内稠密气固流的介尺度流动结构是在气体输入能耗最低和颗粒势能最低妥协机制控制下而形成的稳定状态，申请者前期的探索性研究表明，稠密气固流中存在大量颗粒聚团，聚团内部处于未流化状态，稳定状态可以用一个含有颗粒的气泡、乳化相和颗粒聚团三结构模型来描述。本课题主要针对重大研究计划中第2个核心科学问题而展开，拟通过多种方法对高度耦合的光纤和压力脉动信号分别进行解耦，找到准确的气泡—密相信号识别方法，提出颗粒聚团的定义和判别依据，最终解决介尺度结构的识别、定量表征和颗粒聚团当量直径预测的关键科学问题，并对操作条件和颗粒性质对气泡和颗粒聚团的固含率、尺寸分布、运动速度等参数的影响进行分析和关联。研究结果有利于揭示湍流床稠密气固流反应器中介尺度结构对流动、传递与反应的影响规律。

湍流床反应器内普遍存在传递受限和产率低下等问题，对其中的介尺度流动结构认识不清是造成这一状况的主要原因。稠密气固流中采集的光纤信号具有高度复杂、高度耦合的特性，由于缺乏科学的判别依据，人们无法获得准确的介尺度流动结构的信息。本项目提出稠密气固流中的介尺度结构可以用一个含有颗粒的气泡、乳化相和颗粒聚团三结构模型来描述。基于统计矩一致性原理提出了一种气泡阈值的确定方法，通过理论推导得出多阶统计矩与气泡体积分率等三个介尺度流动参数的关系，以此为基准通过遍历法得出准确的气泡阈值，为复杂信号中气泡的识别提供了科学的方法，大大提高了气泡识别的准确度。提出了具有鲜明物理意义的颗粒聚团定义，基于固含率概率密度分布提出了稠密气固流中颗粒聚团的判别依据。编译了MATLAB软件，获得了气泡出现频率、体积分率、气泡内固含率、尺寸分布、聚团体积分率、出现频率、直径分布、固含率分布等介尺度结构流动特性。通过大量实验，成功实现了对典型A类、B类粒子湍流床内稠密气固流介尺度流动结构的定量表征。通过对常用的基于力平衡和能量平衡模型进行分析，发现这两个模型仅仅考虑了聚团碰撞时的不同情况，而且均是针对C类颗粒形成的聚团所建立，对于A类粒子形成的相对较为松散的聚团并不适用，因而无法准确预测A类或B类粒子流化床中形成的聚团。得出了改进的聚团直径模型和聚团运动速度、频率、体积分率等流动参数的关联模型。本项目的结果有利于进一步揭示湍流床稠密气固流反应器中介尺度结构对流动、传递与反应的影响规律。