引 言固定床作为反应器、分离器、换热器等单元设备广泛应用于化工、能源、环境等许多领域.因用途的不同固定床的管径-粒径比(D/d)的范围很宽(1～1000的数量级).大热负荷的填充床设备,如强放热的化学反应器、核反应堆的冷却壁管列等,常采用较小的管径-粒径比,以利于热量通过壁面冷却快速移除.固定床内部由于颗粒分布都可能存在流动不均匀性,例如壁面效应、沟流现象.而对于小直径比固定床其壁效应的显著性和流动的不均匀性会更加严重.流动不均匀性会严重地影响固定床内部的反应和传递过程.%Strong wall effects exist in packed beds with small tube-to-sphere diameter ratios because of the large void fraction near the wall, and the flow characteristics are of fundamental interest and valuable for engineering applications. CFD simulation was conducted to analyze the complex flow fields in three different packed bed models. Detailed investigation was carried out into the non-homogeneity resulting from wall effects. Based on two geometric models containing same number of spheres but packed randomly and in structure respectively, flow fields and especially near wall local flows were studied. Flow rate distribution on the transversal sectional planes determined by the large void fraction near wall was quantitatively investigated. It was found that flow rate through an annular area of 1/4 sphere radius wide near the wall was as high as 40% of the total flow rate, in a random packed bed with D/d = 7. By intentionally increasing the near wall gap of the random bed and structured bed to half of sphere radius both containing 244 spheres, wall effects were analyzed quantitatively and the near wall flow rate was found to be as high as 60% of the total flow rate (structured bed 61% and random bed 74%). The pressure drop in the two packed beds was found obviously higher than predicted with Ergun correlation, which was believed to be the contribution of viscous energy loss resulting from wall effects. The results might imply that wall effects in a packed bed with excessively large near-wall void fraction deteriorated the transport performance and reaction efficiency since no adequate fluid flowed through the core region in the bed.
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