Abstract:

Random packed bed unit operations of mono-sized spheres are widely used in many chemical and biochemical processes. The influence of the confining walls on fluid flow cannot be ignored in packed beds with low tube-to-particle diameter ratios due to its uneven distribution of radial porosity. In present paper, the discrete element method (DEM) and the computational fluid dynamics (CFD) are coupled to investigate the characteristics of fluid flow and heat transfer in the composite packed beds of spheres with two kinds of diameters. Three kinds of composite packed models and a control group (randomly packed bed with mono-sized spheres) were constructed with about 200 spheres, including radially layered model (RLM), axially layered model (ALM) and randomly composite packed model (RCM). Simulation results showed that, not a big difference was observed on radial porosity distribution in the near-wall region (up to half a sphere diameter from the solid wall in the wall-normal direction), and wall effect significantly affects the velocity and temperature distribution in packed beds. Both the distributions of porosity and pore scale hydraulic diameter are critical to the velocity and temperature distribution in packed beds, and smaller difference of temperature in core and near-wall regions would be found in radially layered composite packed model. Furthermore, the overall heat transfer efficiency would be improved with radially layered composite packing form. These simulation results may be useful for the improvement and further optimization of the design of packed beds with low tube-to-particle diameter ratios.

Key words: numerical analysis, flow, porous media, composite packed bed, temperature distribution, uniformity

摘要：

圆球堆积床内孔隙分布影响其内部流场及温度场分布, 且小管径-球径比堆积床由于壁面限制, 内部孔隙率变化剧烈, 其内部流动和传热不均匀现象明显。针对D/d_p为3的圆球无序堆积床构建了3种非等直径圆球复合堆积结构:径向分层复合堆积、轴向分层复合堆积以及随机复合堆积结构, 并采用DEM-CFD方法建模计算, 从径向及整体角度分析比较不同复合堆积床内流动换热特性及其流场和温度场分布的均匀性。结果表明:孔隙率及孔隙大小分布共同影响堆积床内流场和温度场分布;相对于单一等直径圆球堆积, 采用复合堆积结构能使堆积床内部孔隙率分布更均匀, 其内部流场和温度场分布也更为均匀;对于D/d_p为3的堆积通道, 径向分层堆积结构对于提高整体流动换热性能及改善内部流动换热均匀性都有显著效果。

关键词: 数值分析, 流动, 多孔介质, 复合堆积床, 温度分布, 均匀性