鼓泡流化床曳力模型及气固流动特性数值研究

发布时间：2018-04-20 11:39

本文选题：气固两相流 + 曳力模型　；参考：《南京航空航天大学》2016年硕士论文

【摘要】：流化床因具有优良的传热传质特性而成为石油化工等工业领域的重要设备。近几十年来,人们发展和应用计算流体动力学(CFD)的方法来解决流化床放大设计过程中存在的问题。能否准确地描述颗粒的受力,尤其是曳力,成为数值求解方法的关键。本文针对低颗粒浓度条件下颗粒所受曳力下降的问题,提出改进的曳力模型,并耦合欧拉双流体模型对鼓泡流化床内的流动特性进行数值模拟研究。随后,本文又讨论了颗粒尺寸分布对气固流动的影响。所开展的具体研究内容和获得的相关数值结果如下:首先,构建了改进的曳力模型。改进的曳力模型具有这样的特点:曳力系数kgs在颗粒浓度?s(0.1,0.38)范围内低于Syamlal-O'Brien模型和Gidaspow模型的曳力系数,这与曳力下降的颗粒浓度区间吻合。其次,进行了鼓泡床内气固流动特性的模拟研究。研究结果表明,与Gidaspow和Syamlal-O'Brien模型相比,改进的曳力模型对床层局部压降的预测结果更好;随着表观气速的增加,改进的曳力模型能够更加准确地预测床层膨胀;当表观气速为0.36 m/s时,相比于Gidaspow模型,改进的曳力模型对颗粒时均轴向速度的预测能力得到明显提升;当表观气速为0.46 m/s时,改进的曳力模型对径向颗粒浓度分布的预测结果明显好于Syamlal-O'Brien模型;在气泡当量直径方面,模拟结果与实验结果相符;在床内颗粒流型方面,相比于Gidaspow模型,改进的曳力模型能够准确地预测床内涡的分布。最后,进行了群体平衡模型(PBM)和多粒径模型的研究。结果显示,多粒径假设过高地估计了床层膨胀,而PBM粒径假设所获得的床层膨胀略高于单一粒径假设;PBM粒径假设所预测的气泡直径与Werther计算模型基本一致;随着颗粒粒径范围的扩大,滑移速度的峰值向低颗粒浓度偏移。
[Abstract]:Fluidized bed has become an important equipment in petrochemical industry because of its excellent heat and mass transfer characteristics. In recent decades, the method of computational fluid dynamics (CFD) has been developed and applied to solve the problems in the design of fluidized bed. Whether we can accurately describe the force of particles, especially the drag force, is the key of numerical solution. In this paper, an improved drag model is proposed for the reduction of drag force on particles with low particle concentration, and numerical simulation of flow characteristics in bubbling fluidized bed is carried out by coupling Eulerian two-fluid model. Then, the effect of particle size distribution on gas-solid flow is discussed. The specific research contents and relevant numerical results are as follows: firstly, an improved drag model is constructed. The drag coefficient (kgs) of the improved drag model is lower than that of the Syamlal-O'Brien model and the Gidaspow model in the range of particle concentration (0.1 ~ 0.38), which is consistent with the decreasing particle concentration range. Secondly, the characteristics of gas-solid flow in bubbling bed are simulated. The results show that the improved drag model can predict the local pressure drop of the bed better than the Gidaspow and Syamlal-O'Brien models, and with the increase of the apparent gas velocity, the improved drag model can predict the bed expansion more accurately. When the apparent gas velocity is 0.36 m / s, compared with the Gidaspow model, the improved drag model can significantly improve the prediction ability of the particle mean axial velocity, and when the apparent gas velocity is 0.46 m / s, The improved drag model is better than the Syamlal-O'Brien model in predicting the radial particle concentration distribution, the simulation results are in agreement with the experimental results in the bubble equivalent diameter, and the flow pattern of the particles in the bed is compared with the Gidaspow model. The improved drag model can accurately predict the vortex distribution in the bed. Finally, the population balance model (PBM) and multi-particle size model were studied. The results show that the expansion of the bed is overestimated by the multi-particle size hypothesis, while the bubble diameter predicted by the PBM particle size assumption is slightly higher than that predicted by the single particle size hypothesis, which is basically consistent with the Werther calculation model. With the increase of particle size range, the peak value of slip velocity shifts to low particle concentration.
【学位授予单位】：南京航空航天大学
【学位级别】：硕士
【学位授予年份】：2016
【分类号】：TQ051.13

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