内构件鼓泡流化床中流动结构及其计算机模拟研究

发布时间：2019-05-24 14:59

【摘要】：气固内构件鼓泡流化床具有气固接触效率高、颗粒返混小的特点,在化学工业生产中得到了广泛的应用。内构件鼓泡流化床的工程放大主要采用传统实验方法进行逐级放大,所需周期长、费用高,而目前计算流体力学(CFD)方法日趋成熟,与传统实验方法相比省时省力,因此采用CFD方法对内构件鼓泡流化床进行工程放大研究成为新的趋势。在CFD模拟中,曳力模型对流化床内气固流动状态的准确预测十分关键。传统的曳力模型采用气固流动均匀化假设而高估计了床内曳力,导致计算结果与实际偏差较大。添加内构件后,气泡及聚团尺寸减小,床层空隙率增加,流化床内气固流动结构发生改变。对于准确度较高的基于介尺度结构的曳力模型,内构件的添加使无内构件鼓泡流化床中曳力模型内的力平衡方程、经验关联式等在局部床层不适用,因此无内构件鼓泡流化床中的曳力模型需要进行修正才能应用于内构件鼓泡流化床中。由于目前尚无普适性的修正方法,因此本文选用横向内构件作为研究对象,根据添加横向内构件后气固流动特性的改变,修正了课题组开发的基于气固不均匀流动结构的曳力模型,开发了基于气固不均匀结构的适用于横向内构件鼓泡流化床的模拟方法,对内构件鼓泡流化床气固流动行为进行模拟研究。在横向内构件中选取了多孔挡板和单旋导向挡板作为研究对象,通过实验方法测量了不同表观气速下内构件鼓泡流化床中A类颗粒浓度径向分布、轴向分布、压降分布和单旋导向挡板上方气泡的平均直径。实验发现横向内构件的添加将整个流化床分成了多个流化区域,气泡在上升过程中被内构件破碎后气体进行重新分布,横向内构件起到了类似气体分布器的作用。气泡通过单旋导向挡板后产生的新气泡直径与单旋导向挡板中相邻两叶片的间距相同。轴向固含率分布在横向挡板下方的稀相区降低至最低值,且降低的程度随气速的增加而增加,气速越高,横向挡板对固相返混的抑制作用增强。上述实验结果为后续的CFD模拟提供了基础。基于横向内构件将鼓泡流化床分成多个流化区及气泡在上升过程中被内构件重新分布的事实,提出以横向内构件为气体分布器、每一流化区作为无内构件鼓泡流化床的内构件流化床模拟思路,将整个床层处理为多个无内构件鼓泡流化床的串联。将基于此建立的结构曳力模型耦合到双流体模型中,使用商业软件Fluent对添加了上述两种横向内构件的鼓泡流化床进行了三维数值模拟,模拟结果与Gidaspow曳力模型的模拟结果相比具有更高的精度,与实验结果吻合较好,能够较准确的预测横向内构件鼓泡床内床层膨胀、轴向及径向固含率分布、轴向压力(强)分布等气固流动特性。将修正的结构曳力模型耦合到离散颗粒模型中,使用商业软件Barracuda对横向内构件鼓泡流化床进行了三维数值模拟,并与Wen Yu-Ergun曳力模型和Parker曳力模型的模拟结果进行对比,发现本论文发展的结构曳力模型比其他曳力模型模拟精度更高。横向内构件鼓泡流化床CFD模拟方法的开发,合理的预测了流化床内气固流动,为该反应器在工业生产中的优化操作及工程放大提供一定的理论指导。
[Abstract]:The gas-solid inner component bubbling fluidized bed has the characteristics of high gas-solid contact efficiency and small particle back-mixing, and is widely applied in the production of the chemical industry. the engineering amplification of the inner member bubbling fluidized bed is mainly carried out by the conventional experimental method, the required period is long, the cost is high, and the current computational fluid dynamics (CFD) method is becoming more mature, and compared with the conventional experimental method, time and labor are saved, Therefore, the CFD method is used to study the internal component bubbling fluidized bed, which is a new trend. In the CFD simulation, the drag model is critical to the accurate prediction of gas-solid flow in the fluidized bed. In the traditional drag model, the gas-solid flow homogenization assumption is used to estimate the drag force in the bed, which leads to a large deviation of the calculation result and the actual deviation. After the inner component is added, the air bubble and the size of the air-solid flow in the fluidized bed are reduced, and the air-solid flow structure in the fluidized bed is changed. for a high-accuracy drag model based on a medium-scale structure, the addition of the inner member results in a force balance equation in the drag model in the non-inner member bubbling fluidized bed, empirical correlation, and the like that are not applicable to the local bed, So that the drag model in the bubble-free fluidized bed of the inner member needs to be modified to be applied to the inner member bubbling fluidized bed. Because there is no universal correction method at present, the transverse inner component is used as the research object, and the drag force model based on the gas-solid non-uniform flow structure developed by the research group is modified according to the change of the gas-solid flow characteristic after the transverse inner component is added. The simulation of the gas-solid flow behavior of the internal component bubbling fluidized bed was developed based on the gas-solid non-uniform structure and applied to the lateral inner component bubbling fluidized bed. The radial distribution, the axial distribution, the pressure drop distribution and the average diameter of the air bubbles above the single-rotation guide baffle were measured by the experimental method. The experiment shows that the addition of the transverse inner component divides the whole fluidized bed into a plurality of fluidization regions, and the air bubbles are re-distributed by the gas after being crushed by the inner member during the rising process, and the transverse inner member functions as a gas distributor. The diameter of the new bubble generated by the bubble passing through the single-rotation guide baffle is the same as that of the adjacent two blades in the single-rotation guide baffle. The axial fixation rate is reduced to the lowest value in the dilute phase area under the transverse baffle, and the degree of reduction is increased with the increase of the gas speed, the higher the gas speed, and the effect of the transverse baffle plate on the solid phase back mixing is enhanced. The above experimental results provide a basis for subsequent CFD simulations. based on the fact that the transverse inner member divides the bubbling fluidized bed into a plurality of fluidized areas and the bubbles are re-distributed in the ascending process, the transverse inner member is a gas distributor, The entire bed is treated as a series of a plurality of non-inner member bubbling fluidized beds. The structure drag model based on this is coupled to a two-fluid model, and a commercial software Fluent is used to simulate the three-dimensional numerical simulation of the bubbling fluidized bed with the two transverse inner components, and the simulation result has higher accuracy compared with the simulation result of the Gidasow drag model. The results agree well with the experimental results, and can accurately predict the gas-solid flow characteristics such as the expansion, axial and radial solid-content distribution and axial pressure (strong) distribution in the lateral inner-component bubble bed. The modified structural drag model was coupled to the discrete particle model, and the three-dimensional numerical simulation was carried out using Barracuda commercial software Barracua and compared with the simulation results of the Wen Yu-Ergun drag model and the Parker drag model. It is found that the structural drag model developed in this paper is more accurate than other drag models. The development of the CFD simulation method of the horizontal inner-component bubbling fluidized bed is a reasonable method to predict the gas-solid flow in the fluidized bed, and provide some theoretical guidance for the optimization operation of the reactor in the industrial production and the engineering amplification.
【学位授予单位】：中国科学院研究生院(过程工程研究所)
【学位级别】：博士
【学位授予年份】：2016
【分类号】：TQ051.13

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