【作者】 王天宇；

【导师】 易红亮； 何玉荣；

【作者基本信息】 哈尔滨工业大学 ， 热能工程， 2016， 博士

【摘要】 气固两相颗粒系统广泛存在于自然界和工业过程中。而在当前针对流化床内气固两相流动的研究中,通常基于球形颗粒的假设。尽管这样能够降低研究难度,但是不可避免的引入了误差,尤其是造成了颗粒运动及脉动的各向异性特性缺失,导致了气固两相颗粒系统内颗粒动力学特性的不同。在近期逐渐开展起来的非球形颗粒系统研究也主要应用离散单元模型(Discrete Element Method,DEM)中的软球模型,使用硬球模型的研究还未见报道,相关机理与模型研究仍需进一步的探索。本文基于非球形颗粒间动量和能量守恒原理,应用DEM硬球模型研究了非球形颗粒稠密气固两相流动中的颗粒行为。以组合颗粒模型为基础,采用四元数理论描述非球形颗粒的运动。对于颗粒碰撞搜寻算法,提出了几何代数两步搜寻方法,大大减少了非球形颗粒搜寻算法的计算需求,并保证了计算精度。基于Routh的图解法,结合Poisson恢复系数假设和Coulumb定律,建立了适用于非球形颗粒的三维碰撞模型。基于PIV技术搭建了喷动床实验台,开展了非球形颗粒流态化行为的实验研究,并应用本文建立的非球形颗粒DEM硬球模型进行了相同操作条件下喷动床内颗粒行为的数值模拟研究。与球形颗粒相比,非球形颗粒喷动床内颗粒的运动更为剧烈,颗粒的扬析现象更为明显,床内气泡的边界较为模糊,并且气泡内包含较多分散颗粒。对比数值模拟与实验测量结果,可以看出应用非球形颗粒硬球模型得到的结果与实验结果吻合较好,速度分布趋势基本相同。应用建立的非球形颗粒离散颗粒硬球模型,对鼓泡流化床内气固两相流体动力学特性进行了数值模拟研究。鼓泡流化床内首先进行了球形颗粒动力特性的数值模拟研究,并得到了非常规重力加速度条件下颗粒的鼓泡行为。随后,对比了非球形颗粒和球形颗粒的速度、颗粒分离情况、气泡行为和颗粒的脉动运动等,获得了不同弹性恢复系数和重力加速度条件下非球形颗粒的动力学特性。对单组份鼓泡流化床,非球形颗粒系统的气固分布周期性规律较为复杂,并且在系统内气泡的边界并不清晰,内部存在大量分散的颗粒,气泡的脉动运动趋势不明显,而颗粒的微观脉动运动与气泡的行为有着密切的联系。对于双组份鼓泡流化床,非球形颗粒双组份鼓泡流化床中颗粒的混合较为均匀,其中气泡的脉动运动要强于球形颗粒系统。应用建立的非球形颗粒离散颗粒硬球模型,对喷动流化床内气固两相流体动力学特性进行了数值模拟研究。应用区域相关的颗粒行为分析方法,获得了非球形颗粒与球形颗粒喷动流化床流态化特性在空间上的关联。研究表明球形颗粒喷动流化床中的分区方式是以床内中部为中心,不同的流化区域呈环形分布。而在非球形颗粒系统中,其分区呈现分层的形式。不同的分区特性表明球形颗粒与非球形颗粒的流态化行为存在较大的差异。应用建立的非球形颗粒离散颗粒硬球模型,对喷动流化床内气固两相流体动力学特性进行了数值模拟研究。开展了非球形颗粒和球形颗粒系统内颗粒聚团微观脉动特性的研究,提出了广义的聚团颗粒温度的概念,用于考察颗粒系统中聚团的脉动运动。对球形颗粒系统,不同曳力模型下的聚团颗粒温度分布差别较大。大颗粒的分布更集中,小颗粒分布更分散。对非球形颗粒系统,提升管中的颗粒位置分布较为均匀,聚团的微观脉动运动也相对较弱。与球形颗粒系统类似,小颗粒的广义聚团颗粒温度比大颗粒的数值更大,说明了小颗粒及其颗粒聚团的脉动运动更剧烈。更多还原

【Abstract】 Gas fluidized bed with non-spherical particles is widely used in various industrial processes. However, researchers usually used the spherical hypothesis to simplify the model. Inevitably, deviations will be introduced, especially for the anisotropic characteristics of particles. It is of great significance to understand the micro mechanisms and macro dynamic characteristics of the behaviors of non-spherical particles. Related researches have been conducted with the soft-sphere approach of Discrete Element Method(DEM), but the mechanism of non-spherical particle system still needs further investigations.In this work, a complete numerical framework has been built to deal with the problem of non-spherical particles in gas-solid two-phase particle systems based on the hard-sphere approach of DEM. Based on a multi-element model, the kinematic model, a contact detection method, a collision model and a mechanical model are proposed. Quaternions and classic rigid body dynamics are employed and improved in this work, and new algorithms including a two-segment contact detection approach are established. The Current work takes into consideration efficiency along with accuracy, which makes the simulations at an experimental scale reliable. The collision model is based on Routh graphical method, coupling with the Possion restitution coefficient hypothesis and the Coulumb friction law.Based on the PIV system, the measurement on a spout fluid bed is conducted, and the DEM numerical simulation is carried out on the gas-solid two-phase flow of the non-spherical particles under corresponding conditions. The results show that in non-spherical particle systems, the mixing of particles is enhanced, and the elutriation is obvious. In addition, individual particles are in and around the bubbles. By comparing the experimental data and the numerical results, it is found the DEM numerical simulation predicts PIV experimental results accurately. The established DEM hard-sphere model for non-spherical particles is of favorable applicability.The DEM model built for non-spherical particle systems is applied to simulate the hydrodynamics behavior in a bubbling fluidized bed, spouted fluid bed and riser. For bubbling fluidized bed, as a simple gas-solid particle system, the particle behavior is typical. First, the spherical method is applied to investigate the particle behaviors in the bubbling fluidized bed, and the researches on particle behaviors under different gravities are conducted. Then the bubbling fluidized bed with non-spherical particles is simulated, and the comparison of particle behaviors between spherical and non-spherical particles are studied. For the monocomponent solid case, the periodic rules in the system with non-spherical case are more complex. Individual particles are in and around the bubbles, and the turbulent movement of bubbles is complex. For binary solid, the mixing is enhanced, and the turbulent movement in non-spherical particle systems of bubbles is stronger than the case of spherical particles.The model is also applied to simulate the hydrodynamics behavior of non-spherical particle systems in a spout-fluid bed. By using the region dependent method, the spatial relationship of particle behaviors is obtained. It is found that the partitions are different in the spherical and non-spherical particle systems. For the spherical cases, the center is the middle of bed, and different parts of fluidization behaviors are annular-distributed. But for the non-spherical case, the partitions are layered. The different partitions in spherical and non-spherical spouted bed indicate that there are different particle behaviors in the two beds. Compared the granular temperature distribution of spherical particles with the non-spherical particles, it can be seen that the turbulent movement of non-spherical particles is more violent. In addition, different bubble behaviors are presented.For the riser, the new concept of cluster granular temperature is proposed to investigate the turbulent movement of clusters in the system. For spherical particles, the difference of cluster granular temperatures under different drag models is obvious. The one of large particles is more concentrated, and it is opposite for the small particles. For non-spherical particle systems, the turbulent movement of clusters is weaker, and the cluster granular temperature of small particles is larger than that of large particles. That means the turbulent movement of small particles is stronger.更多还原