Role of Baffles on Flow Fields Inside Wet-Lay Mixing Tanks and Their Potential Influence on Fiber Dispersion

Abstract

Dispersion and separation of fiber bundles requires exposing them to a shear stress field to overcome interfiber frictional forces. To this end, fiber-mixing tanks are equipped with baffles to enhance shear and agitation in the water to help disperse the fiber bundles. The time and agitation required to separate and disperse the fibers depends on the fibers being used. It is well known, however. that excessive agitation will give rise to the formation of rope defects in the output because of the high-energy vortices that form behind these baffles. Optimizing the baffle geometry and position is therefore critically important in the wet-lay process. This paper presents some possible ways of eliminating the regions in the water velocity field where strong vortices may be present: in particular, the motivation for this paper is that minimizing vortex formation will lower the probability of rope forma tion. In this regard, we present a series of numerical simulations to model fluid-flow behavior inside wet-lay mixing tanks. A turbulent flow field is obtained by solving the Navier-Stokes equations in a two-dimensional geometry. The turbulent features of the flow are captured using the RNG k-e model. Fibers, modeled as spherical rigid particles with the same volume as the fibers, are introduced into the solution domain and their trajectories are tracked inside the mixing tank. The effects of the baffles and their orientation with respect to flow streamlines are simulated. We report the simulation results for different baffle configurations and show that aligning the baffles with the streamlines and increasing their surface area can eliminate the formation of vortices while still keeping the shear field at a satisfactory level. We hypothesize that eliminating the vortices in the mixing tank reduces the probability of rope formation, but this hypothesis needs to be verified experimentally.