埋管流化床内湿颗粒流动及混合特性的CFD-DEM数值模拟

CFD-DEM simulation of wet particles flow and mixing behavior in fluidized bed with immersed tubes

掺液滴的气固流化床,越来越多地应用于颗粒材料合成和改性工程,但是液滴使颗粒具有一定的湿度。湿颗粒具有黏性,与干燥颗粒流化特性有很大差异。本文应用迟滞碰撞模型,分别在颗粒碰撞的加载和卸载阶段考虑液体黏弹阻力(lubrication force)和液桥力引起的动能耗损,建立湿颗粒流化床CFD-DEM模型。对含有埋管的湿颗粒流化床进行数值模拟研究,比较了不同黏性条件下颗粒的流动及混合特性。发现随着颗粒黏性的增大,颗粒聚团加剧,气泡边界变得粗糙并逐渐转变为气体沟流,最终导致流化失效,埋管在床中起骨架作用,促进了沟流的形成。随着颗粒黏性的增大,颗粒的相对运动被阻碍,颗粒速度降低,床层压降逐渐减小,颗粒混合速度减慢。

Gas-solid fluidized beds with the presence of some liquid droplets are applied widely in industry. Due to the existence of droplets, particles become wet and viscous, which makes fluidization significantly different with dry particles. In this paper, a hysteresis contact model is applied to consider energy dissipation of lubrication force and liquid bridge force during wet particle collision, which is coupled with CFD-DEM model. The fluidization and mixing characteristics of wet particles under different conditions in a 2D fluidized bed with immersed tubes are compared. Generally, it is found that with the presence of liquid, the particles begin to agglomerate and the bubbles become gas channels. The immersed tubes play the part of frameworks, promoting the formation of gas channels. With increasing cohesiveness, the particle velocity decreases generally and the internal recirculation of wet particles is restrained. It is also found that with an increase in the cohesiveness, the pressure drop of the fluidized bed and the mixing speed of particles decrease.