Design Simulation of Glass-Fiber-Loaded Flow in an Internally Spout-Fluidized Bed for Processing of Thermoplastic Composites. I. Flow Characterization

Abstract

A novel fiber dispersion process suitable for the manufacture of composites consisting of long glass fibers and thermoplastic is discussed. An internally spout-fluidized bed with a draft tube and disk baffle has been designed to disperse fiber bundles and perform preimpregnation, which can improve the hydrodynamic behavior inside the spout-fluidized bed for more effective dispersion of fiber bundles and impregnation with resin powder when applying such beds in the manufacture of fiber-reinforced thermoplastics. Features of the flow field in the spout-fluidized bed are important for fiber dispersion. Therefore, this article discusses the design results in terms of numerical simulations [large-eddy simulations (LESs)], focusing on the likely hydrodynamic impact on fiber dispersion in this particular spout-fluidized bed. Because of a strong interaction between the flow in the spout-fluidized bed and the fibers in the actual process and the limitations on direct measurements of fiber floc breakup in the spout-fluidized bed, the strength of the fiber floc was obtained by strength measurements with a rheometer. In the LES runs, the motion of the fibers was neglected because of their low volume fraction (lower than 0.001%). The turbulence kinetic energy, Reynolds stress, and strain rate were determined by statistical analysis. The LES results clearly indicate that the addition of internals significantly alters the flow patterns in the spout-fluidized bed, in particular changing the characteristics of the round turbulent jet and confined impinging jet. The hydrodynamics analysis showed that the introduction of internals into a spout-fluidized bed improves its capacity and efficiency for dispersing fiber flocs. These results provide important information on the flow fields within spout-fluidized beds for process design and scaleup.