高密度下行床颗粒浓度与颗粒速度分布特性

Distribution of Solid Holdup and Particle Velocity in a High Density Gas-solids Circulating Fluidized Bed Downer

为了考察下行床内颗粒浓度和速度的分布特性以确定高密度操作条件,在自建的气固下行床冷模装置(总高度18 m,下行床高8 m,直径80 mm)中,测定了不同表观气速(U_g)和颗粒循环速率(G_s)下床内颗粒浓度和颗粒速度的轴径向分布。结果表明,在G_s为700 kg/(m^(2)·s)时,下行床内平均颗粒浓度达0.12,整个下行床内均实现了高密度操作。下行床内颗粒浓度径向分布受入口结构影响显著,近下行床入口处(z为0.08 m),颗粒浓度"中间高边壁低",下行床内颗粒浓度轴向分布总体上呈现"上浓下稀"的形式。截面平均颗粒速度随轴向位置的增加而不断降低,表明下行床内气固流动结构的发展特性。当颗粒循环速率较小时,如G_s为100 kg/(m^(2)·s),颗粒在入口区先加速,随后进入恒速度区,截面平均颗粒速度随表观气速的增加而显著增加。当颗粒循环速率较大时,如G_s为500 kg/(m^(2)·s),实验范围内表观气速对平均颗粒速度的影响很小。下行床内颗粒速度的径向均匀性在入口处更强,说明下行床入口结构对其内气固流动的影响明显。

In order to investigate the distribution characteristics of solid holdup and particle velocity in the downer to determine the conditions for high-density operation,the axial and radial distributions of particle concentration and velocity were measured under different apparent gas velocity(U_g)and particle circulation rate(G_S)in a self-built setup of gas-solid downer(total height 18 m,downer height 8 m,diameter 80 mm).The results showed that the entire downer was operated with high density conditions with solid holdup up to0.12 when solids flux up to 700 kg/(m^(2)·s).Radial distribution of solid holdup in the downer was affected significantly by inlet structure.Near the downer inlet,solid holdup was higher in the center and decreases towards the wall.For axial distribution of solid holdup,it is higher in the bottom and gradually decreases along the downer,which showed development of the gas-solid flow structure.For particle velocity distribution it is found that particles were accelerated near the downer inlet and then flow downward with constant velocity when particle circulation rate was smaller[100 kg/(m^(2)·s)],and the cross-sectional averaged particle velocity increases obviously with the increase of superficial gas velocity.While solids flux increases up to 500 kg/(m^(2)·s),changes of particle velocity was sight with the changes of superficial gas velocity,and radial distribution of particle velocity was more uniform near the downer inlet than that at other axial locations,it was showed that downer inlet has significant effects on gas-solids flow.