Abstract:

The construction of traditional mesoscale drag models is usually based on fine-grid simulation databases in full-periodic domains, without considering the effect of the wall. The wall conditions in a real fluidized bed reactor can affect the inhomogeneous structure in near-wall regions, which further affects the drag force on particles in these regions. Therefore, it’s necessary to study the wall effect influence on the mesoscale drag model. By comparing the radial profile of the relative error for mesoscale drag correction coefficient in the quasi-two-dimensional full-periodic system and downers with different bed widths, the regression phenomenon of the mesoscale drag force toward the full-periodic system drag force in the periodic downer fluidized bed was explored. Here, the mesoscale drag correction coefficient was the ratio between the mesoscale drag coefficient and the corresponding microscopic drag coefficient. The influence factors of the radial profiles for mesoscale drag correction coefficient were analyzed. The results showed that the ratio of the distance affected by boundary walls to the entire bed width changes little under different bed widths, and the relative error for drag coefficient was affected by factors such as solid holdup, solid holdup gradient, solid shear rate and granular temperature distribution. The formula of drag coefficient relative error with respect to the radial position was developed. The influence of the filter scale on the drag coefficient relative error was discussed. The study also showed that the wall effect was not implicitly included in drag modes with markers such as gas-solid slip velocity and pressure gradient of the gas phase. Therefore the developed formula was also suitable for correcting these models when the wall effect was considered.

Key words: fluidization, gas-solid flow, wall effect, meso-scale structure, drag force

摘要：

传统介尺度曳力模型的构建通常基于全周期域的细网格模拟数据集，未考虑壁面的影响，而真实流化床反应器系统中的壁面条件会影响壁面附近的非均匀结构，进而影响到壁面附近颗粒所受的曳力，因此研究壁面效应对介尺度曳力模型的影响很有必要。本文通过对拟二维全周期系统和不同床宽的周期下行床系统的介尺度曳力修正系数相对误差径向分布进行比较，探究了周期下行床中的介尺度曳力回归全周期系统曳力的现象，分析了曳力修正系数径向分布的影响因素。发现不同床宽下受壁面影响区域占整个床的比例改变较小，且曳力系数相对误差的值受该位置的固含率、固含率梯度、固相剪切率及粒化温度分布等因素影响，最后拟合了曳力系数相对误差关于径向位置的公式，并对过滤尺度对曳力系数相对误差的影响和一些变量关联曳力时是否已经隐性包含了壁面的影响进行了探讨。

关键词: 流态化, 气固两相流, 壁面效应, 介尺度结构, 曳力