A numerical investigation on segregation in gas-fluidized beds of short, flexible fibers

Abstract

Numerical simulations of gas-fluidized beds of short, flexible fibers are performed using a coupled approach of discrete element method (DEM) and computational fluid dynamics (CFD), and segregation of bidisperse fibers is investigated. A gas critical velocity exists for achieving the maximum degree of segregation, characterized by a segregation index SI. and it is about 1.5 times minimum fluidization velocity for present particulate systems. The segregation is determined by two mechanisms: air entrainment mechanism, attributed to the difference in air effects on fiber motion, and sifting mechanism, due to the difference in fiber sizes. Smaller and lighter fibers tend to move to upper region of a fiber bed due to the air entrainment mechanism, while the smaller fibers more likely migrate to lower region due to the sifting mechanism. The interlinked effects of the two mechanisms are analyzed, and a phase diagram to determine SI is consequently created based on an air-effect ratio and a fiber diameter ratio. When the air-effect ratio is higher than 1.3, significant segregation occurs due to the enhancement from both the mechanisms; while when the air-effect ratio is below 0.8, weaker segregation is observed as the air entrainment and sifting effects counterbalance each other.