Abstract:

Circulating fluidized bed unit is widely used in energy, chemical engineering, and other fields, featuring complex structures such as risers, non-mechanical sealing chamber, cyclone, and dipleg. Under varying operating conditions, on the one hand, there are the characteristics of the coexistence of multiple fluidization regimes in the system. On the other hand, there are also non-uniform mesoscale structures such as granular clusters or bubbles in the system. Reasonable quantification of the flow/transport characteristics in the circulating fluidized bed full-loop system is essential to optimize the operating conditions and reactor structure and find the scale-up law of the full-loop system. Previous work has preliminarily proved that the mesoscience-based structural model based on the idea of multi-scale decomposition in mesoscience can take the influence of non-uniform mesoscale structures into account from the perspective of governing equations and constitutive relations, and it has the ability to accurately simulate complex heterogeneous gas-solid systems under different operation conditions. Due to some critical difficulties such as balancing the pressure drop, mesoscale structures and continuous operation, the fast and accurate simulation of the three-dimensional full-loop system has been restricted. Therefore, this work attempts to use the mesoscience-based structural model to predicted the hydrodynamics of the gas-solid flow in two different types of circulating fluidized bed full-loop systems. The simulation results show that compared with experimental data, the mesoscience-based structural model can accurately predict the hydrodynamics of full looping system such as the axial solid concentration, radial solid concentration, radial velocity, and pressure drop distribution with the high computational efficiency. The model has good applicability and robustness under a wide range of operating conditions with high computational efficiency, which is more suitable to predicting the dense gas-solid flows, and it lays a solid foundation for the optimal design and scale-up of circulating fluidized beds on an industrial scale.

Key words: computational fluid dynamics, multiphase flow, fluidized-bed, numerical simulation, full-loop simulation

摘要：

循环流化床全回路系统广泛应用于能源、化工等领域，系统中普遍包含提升管、非机械密封仓、旋风分离器、料腿等复杂结构。随着操作条件变化，一方面系统中出现跨流域流化特性，另一方面系统中也普遍存在颗粒聚团、气泡等非均匀介尺度结构。合理量化循环流化床全回路系统中的流动/传递特性，对优化操作条件及反应器结构、寻找全回路系统的放大规律至关重要。本文作者课题组前期工作已初步证明基于介科学中多尺度分解思想构建的结构双流体模型可从模型架构上兼顾控制方程及本构关系均考虑非均匀介尺度结构影响，它具有针对气固复杂系统多流域情况下的准确模拟能力。由于循环流化床全回路系统平衡压降困难、构体结构复杂、需要满足连续操作等条件，三维全回路系统快速准确模拟一直备受制约。因此，本文尝试利用结构双流体模型刻画两类无机械阀控制的循环流化床全回路系统中的气固流动特性。模拟结果显示：与实验数据相比，结构双流体模型可准确预测床内气固复杂流动特性，如轴向固体浓度、径向固体浓度、径向速度与压降分布等；该模型在多种操作条件下的适用性和鲁棒性良好，且计算效率较高，更适用于预测稠密气固两相流系统，因此为工业尺度的循环流化床系统优化设计与放大奠定坚实的基础。

关键词: 计算流体力学, 多相流, 流化床, 数值模拟, 全回路模拟