关键词: 固定床, 小直径比, 壁效应, CFD, Chimera网格

Abstract: 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.

Key words: packed bed, small tube-to-sphere diameter ratio, wall effect, CFD, Chimera grid