非等密度颗粒气固流化床的微观尺度模拟与分析

Micro-scale simulation and analysis of gas-solid fluidized bed with multi-density distribution of particles

采用计算流体力学与离散单元法相耦合的CFD-DEM方法对两种表观气速下三维非等密度颗粒流化床内的气固运动进行了数值模拟研究,对比了两种气速下流化床内颗粒的分层和混合现象,发现在非等密度颗粒流化床内,有不同程度的颗粒分层现象存在。当表观气速较低,处于最小密度颗粒的临界流化速度和最大密度颗粒的临界流化速度之间时,颗粒体系出现了较为明显的分层现象,整体上为重颗粒在下、轻颗粒在上的分层结构;当表观气速较高,大于最大密度颗粒的临界流化速度时,分层现象不再明显。采用Lacey混合指数分析了流化床内颗粒之间的混合状况,发现颗粒密度差越小,混合指数越大,越难分离;颗粒密度差越大,则混合指数越小,分离越完全.

A three-dimensional multi-density fluidized bed was simulated by using computational fluid dynamics-discrete element method （CFD-DEM） at two superficial gas velocities. The mixing and segregation behavior of particles at two different superficial gas velocities was compared. Results showed that different degree of segregation phenomenon existed in the fluidized bed. The obviously segregation phenomenon that heavy particles moved down and light particles moved up occurred when the superficial gas velocity was low, being larger than the critical fluidization velocity of the lightest particles but smaller than that of the heaviest particles. But the segregation phenomenon was not clear when the superficial gas velocity was higher than the maximum critical fluidization velocity. The mixing and segregation behavior of particles at two different superficial gas velocities was described by Lacey indexes. Results show that particles with smaller density difference have larger mixing index and are more difficult to be segregated, and that with larger density difference have smaller mixing index and can be segregated much more completely.