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Alternative TitleEulerian simulation of gas-solid flow in a countercurrent downer
Thesis Advisor林伟刚 ; 王军武
Degree Grantor中国科学院研究生院
Degree Discipline化学工程
Keyword介尺度结构 曳力关联式 气固逆流 欧拉模拟 流态化
Abstract气固流化床因其传热传质效率高而被广泛应用于工业过程。气固流化床在垂直管中可以有不同的操作状态:气固并流上行,气固并流下行,固体颗粒下行、气体上行。和并流上行的提升管相比,气固并流下行床具有气固反混小、气固接触时间短的优点,但是也存在颗粒浓度低的缺点。气固逆流下行床则在一定程度上可以结合提升管和并流下行床的优点,即:较高的床层颗粒浓度、较少的气固反混和较短的气固接触时间。气固逆流下行床已被用于煤拔头和生物质气化工艺,然而关于此类反应器中流体动力学的基础研究还很少,尤其是还没有关于气固逆流下行床的CFD研究报道。 本文利用商业计算流体力学软件包FLUENT,以双流体模型为基础对2D气固逆流下行床流场进行数值模拟。用一个基于文献中数据的相间曳力经验关联式来描述决定相间有效曳力的介尺度团聚结构,并将该曳力关联式接入双流体模型中。模拟结果显示: 1) 可以较好的预测逆流下行床中的轴向压力分布、径向浓度分布及定性预测颗粒速度分布。本文初步验证了利用欧拉模型模拟气固逆流下行床的可行性。 2) 壁面边界条件,恢复系数和曳力模型对气固逆流下行床中的颗粒浓度有显著影响。 3) 不考虑团聚物影响的Gidaspow曳力模型高估了轴向压力和压降,原因是Gidaspow曳力关联式是均匀流化床和填充床曳力的结合。这也说明了介尺度颗粒团聚结构对相间作用力和CFD结果的重要性。 4) 颗粒循环流率Gs不变时,逆流气速Ug增加,颗粒速度减小。逆流气速Ug不变时,颗粒循环流率Gs增加,颗粒浓度增加。 5) 与逆流气速Ug对颗粒浓度的影响相比,颗粒循环流率Gs对颗粒浓度的影响更大。因此,若要提高逆流下行床中颗粒浓度,增加颗粒循环流率比增加逆流气速有效。
Other AbstractGas-solid fluidization technologies are widely used in process industry due to their excellent heat and mass transfer characteristics. Gas-solid fluidization in vertical pipe can be operated at different modes, such as cocurrent up-flow, cocurrent down-flow and countercurrent flow. Compared to cocurrent risers, gas-solid cocurrent downers usually have the advantages of much less solid backmixing and shorter gas-solid contact time, but suffer the disadvantage of very low solid holdup. Gas-solid countercurrent downer has the potential to combine the merits of cocurrent riser and cocurrent downer, that is, high solid holdup in the bed with small solid backmixing and short contact time. It has been used, for instance, in coal topping process and biomass gasification. However, fundamental studies on the hydrodynamics of such reactors are sparse, especially, there is no computational fluid dynamics study available. The commercial software FLUENT is used to solve the mathematical model in 2D geometry in this work. An empirical inter-phase drag correlation, based on experimental data available in literature, is proposed to address the key role of meso-scale particle clustering structure in determining effective inter-phase drag force. The proposed drag correlation is then integrated into Eulerian model to study the hydrodynamics of gas-solid flow in a countercurrent downer reactor. The results show that: 1) The measured axial pressure distribution and radial solid concentration profiles can be reproduced reasonably well and the radial particle velocity profile can be qualitatively captured as well. Present study offers a preliminary validation of the feasibility of modeling gas-solid in a countercurrent downer using Eulerian model. 2) Wall boundary condition, restitution coefficient and drag correlation have a major effect on the solid holdup in the downer which is the most concerned parameter of present study; numerical scheme has a minor effect on the time-averaged solid holdup in the downer. 3) The results obtained using Gidaspow’s drag correlation significantly over-estimates the pressure drop as well as pressure gradient along the height. The reason is that Gidaspow’s drag correlation is a combination of the drag correlations obtained from homogeneous fluidization and packed bed. This highlights the importance of meso-scale particle clustering structure in determining inter-phase drag force and CFD results. 4) Increasing gas velocity at a given solids flux leads to decrease in solids velocity, increasing solids flux at a given gas velocity leads to increase in solids holdup. 5) Comparing with the effects of superficial gas velocity on the solid holdup, solid flux has a more significantly effect on the solid holdup. Therefore, increasing solid flux is a better way to densify the downer.
Document Type学位论文
Recommended Citation
GB/T 7714
彭果. 欧拉法模拟逆流下行床中气固两相流[D]. 中国科学院研究生院,2013.
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