Abstract:

Computational particle fluid dynamics(CPFD) approach was employed to investigate the gas-solid flow behavior in the recirculation system of the dense transport bed. The influences of the drag model and the particle close packing limit on the simulation results were analyzed, and the appropriate model parameters were determined. Through the comparison of the simulation results under three operation conditions, the variation trends of the pressure distribution and solids circulating rate with the aeration flowrate were obtained, which was in agreement with experimental observation. The pressure gradient distribution, the gas flow direction and the solids concentration distribution were analyzed. The absolute value of the pressure gradient at the aeration inlet was the local maximum. When the aeration flowrate was zero, the pressure gradient was close to zero in a big area above the inlet. When the aeration flowrate was big enough, there was bubble around the inlet. A high pressure gradient would be formed in the lower area of the standpipe due to the lack of aeration at higher elevation, which would slow down the solids moving. The role of aeration gas on loosening particles just existed in a limited region near the inlet and its role on uniform pressure gradient distribution was more important to enable uniform gas-solid flow in the standpipe. In the design of aeration conditions, the standpipe pressure drop was determined by the solids circulating flowrate, and it needed appropriate aeration to keep constant gas velocity, leading to more uniform pressure gradient in the standpipe.

Key words: recirculation system, moving bed, numerical simulation, gas-solid two-phase flow

摘要：

采用计算颗粒流体力学对密相输运床返料系统内的气固流动行为进行了数值模拟，分析了曳力模型和颗粒最大堆积浓度等参数对模拟结果的影响，确定了合适的模型参数。通过对比3组工况的模拟结果，获得了与实验结果基本一致的立管压力分布和固体循环流率随充气条件的变化规律，并分析了立管内压力梯度分布、气体流动方向、颗粒浓度分布等。结果表明立管充气口处压力梯度绝对值为局部最大值；当立管充气口气量为零时，会使充气口上方一段距离的压力梯度绝对值较小；充气量增大到一定值时会在充气口附近形成明显的气泡。当缺少立管高位充气时，会导致立管下部区域形成大的压力梯度，增加颗粒下落阻力。充气松动颗粒的作用仅对充气口附近区域有一定影响，更大的作用是在立管内形成均匀的压力梯度分布，使立管内气固流动状态保持上下一致。在制定充气方案时，应根据固体循环流率确定立管压降，补充合适气体量以维持气体下行速度均衡，使得各段的平均压力梯度相同。

关键词: 返料系统, 移动床, 数值模拟, 气固两相流