基于EMMS曳力的湍动床气固流动CFD模拟

CFD simulation of a turbulent fluidized bed using an EMMS-based drag model

湍动流化床已广泛用于费托合成、FCC催化裂化等工业过程。其主要流动特点是由底部密相鼓泡区与顶部稀相分散区两者构成。由于湍动流化的复杂性,导致目前对该类体系内非均匀结构形成与演化的研究相对较少。近年来,快速发展的计算流体力学方法为深入研究此类流化提供了一种有效的手段,然而传统的双流体模型往往不能成功捕获到湍动流化的复杂结构。为了考虑非均匀结构对曳力的影响,本文提出了一个改进的曳力模型,即将湍动流化床分为3个区域:底部鼓泡区、过渡区和顶部稀相区。其中,底部区采用气泡EMMS曳力模型进行修正,过渡区采用Wen-Yu曳力模型,顶部区采用Schiller-Naumann曳力模型。修正的曳力模型与双流体模型耦合后,再通过FLUENT商业软件平台对湍动流化床内的气固两相流动进行CFD模拟。模拟结果显示,修正的曳力模型可以较好的预测湍动流化床内的流体动力学特征。该曳力模型能够模拟底部密相鼓泡区和顶部稀相分散区的两相共存结构,颗粒浓度在径向分布上为"环-核"结构,模拟的轴向颗粒浓度分布与实验数据吻合较好。

Turbulent fluidized bed has been utilized widely in many industrial applications, such as Fischer-Tropsch synthesis, fluidized catalytic cracking and Methanol-to-Gas process. The turbulent fluidized bed is characterized by two different coexisting regions, the bottom bubbling region and the top dilute region. Due to the complexity of fluidized structures, there was little study on their formation and evolution. In the recent years, the computational fluid dynamics （CFD） has become a powerful tool for gas-solid fluidization research. However, the traditional two-fluid model （TFM） could not capture the reasonable heterogeneous structures in turbulent fluidization. To account for the effect of structures on the drag force, this paper presented a modified drag model. In this model, the turbulent fluidized bed consists of the bottom bubbling region, the top dilute region and their transitional region. In the bottom bubbling region, the bubble-based EMMS drag was used. In the transitial region, the Wen-Yu drag model was employed. In the top region, the Schiller-Naumann drag model was applied. By coupling of the modified drag and TFM model, the CFD simulation of turbulent fluidized beds was performed based on the FLUENT platform. The results show that the modified drag can predict the flow characteristic reasonablely. The simulated axial bed density fits well with experimental data.