流化超细颗粒相间作用与颗粒动理学的研究

Both comprehensive physical model describing the agglomeration behavior in fluidization of ultrafine powders and experiments are studied on the basis of kinetic theory of gases in a circulating fluidized bed. The key issue is that both hydrodynamic and interparticle forces between particles due to the collision of agglomeration are of importance. The collision dynamics of ultrafine powders is investigated to consider the energy dissipation caused by agglomeration of ultrafine powders. The momentum and energy transfer and exchange during collisions are analyzed. The quadrature method of moments (QMOM) is proposed to treat the variation of number density of particles using weighted quadrature points (abscissas) in phase space. The influence of surface cohesion of particle-particle bindings and gas turbulence on the change of number density due to agglomeration is analyzed. By defining a new distribution function of the instantaneous particle velocity relative to the average velocity based on the volume fraction of the particulate phase that was conserved upon agglomeration, the kinetic theory of ultrafine powders flow is proposed by the Chapman-Enskog expansion approach used in the kinetic theory of gases. The constitutive equations for pressure, viscosity and energy dissipation of ultrafine powders are presented. The distributions of concentrations, velocity and agglomerate diameter of cohesive particles are predicted by solving the derived conservation equations of mass, momentum, fluctuation energy, and particle number density. Three different kinds of ultrafine powders, SiO2, TiO2 and Al2O3, are used for bed materials in a circulating fluidized bed. The high-resolution microimaging/measuring system (CCD) is used to measure instantaneous velocity of particles and agglomerates. The Electrical Capacitance Tomography (ECT) is used to measure the concentration of powders in a riser. The distributions of velocity, fluctuating velocity and concentration of particles are obtained at the different superficial gas velocity and solids mass fluxes. The flow structure and patterns are analyzed from measurements. Incorporating the large eddy simulation for gas turbulence into the kinetic theory of ultrafine powders, the two-phase Eulerian approach is proposed. The hydrodynamics of gas and solids phases are simulated in fluidized beds. The validation is performed from experimental data. A comprehensive parametric analysis will be investigated to determine the effect of agglomeration of ultrafine particles on fluidization. Present study will provide a foundation for investigation of flow, heat and mass transfer and reaction of ultrafine powders in fluidized beds.

本项目将以气体-超细颗粒两相流态化过程为研究对象，以稠密气体分子运动论为基础，建立超细颗粒碰撞动力学，研究超细颗粒相互作用规律。基于矩量法，建立超细颗粒团聚粒径分布密度函数，分析超细颗粒表面能和气体湍流作用对流化过程中超细颗粒团聚的影响。基于Chapman-Enskog逐级渐近法，建立超细颗粒动理学模型。分析超细颗粒能量传递和耗散的变化规律，研究超细颗粒粘附聚团对颗粒相宏观流动特性的影响。采用SiO2、TiO2和Al2O3 超细颗粒为流化颗粒，应用高速摄像技术和电容层析成像技术，进行气体-超细颗粒流化特性的实验研究，研究气体-超细颗粒流态变化规律。结合考虑超细颗粒脉动流动效应的气相大涡模拟和超细颗粒动理学，建立流化床气体-超细颗粒气固两相欧拉-欧拉双流体模型，研究气体-超细颗粒两相流动流体动力特性。为正确预测气体-超细颗粒两相流动、传热传质和反应过程和工程应用奠定理论基础。

本项目以流体超细颗粒流动过程为研究对象，建立超细颗粒动理学模型，基于Chapman-Cowling的稠密气体动理学方法，推导应用于超细颗粒的固相粘度系数、固相压力等参数的计算模型，提出了颗粒相本构方程构建了超细颗粒的颗粒动理学模型和气固两相流动模型。基于颗粒数量平衡方程，分析在超细颗粒黏性力作用下的颗粒数密度的变化，揭示流化过程中颗粒数量和颗粒聚团直径等的变化规律。. 数值模拟循环流化床内流体动力特性，研究时均颗粒速度、聚团大小和浓度的分布特性以及颗粒聚团温度随颗粒聚团浓度的变化规律。研究发现在床层底部和边壁区域易出现大超细颗粒聚团。瞬时颗粒聚团浓度的快速傅立叶变换显示颗粒浓度波动主频为0.03到1.26Hz。.建立了喷动床超细颗粒气固两相流流动模型，模拟结果表明喷动床内超细颗粒气固两相流动仅有喷射区和环隙区，无喷泉区。采用傅立叶变换和小波多分辨分析对颗粒聚团瞬时浓度信号分析表明喷动床内气固两相流动具有非线性特性，小波多尺度预测了瞬时颗粒聚团浓度的脉动频率特性。研究结果表明超细颗粒在喷动床内流化的Shannon信息熵值在1-3之间。. 数值模拟鼓泡床内超细颗粒流化特性。计算结果表明，流化聚团直径沿床高分布可以分为三个区：底部大尺寸聚团聚集区、上部小尺寸聚团分布区和中间过渡区。并联鼓泡床中超细颗粒的聚并和破碎过程数值模拟表明不同分床中进行聚团和破碎过程，发现聚团直径的差异导致不同床体间形成压力差，该压力差是形成颗粒内循环的主要动力。. 超细TiO2颗粒和碳酸钙超细颗粒流化特性进行了实验研究，测量获得TiO2超细颗粒和碳酸钙超细颗粒的临界流化速度分布，实验结果表明临界流化速度与超细颗粒粒度无关，取决于超细颗粒聚团分布。电容层析成像浓度测试技术进行三维鼓泡流化床内气体和超细颗粒流化研究，实验获得不同操作速度下鼓泡床内超细颗粒浓度的分布规律，揭示了气体超细颗粒流化特性的变化规律。