Geometric Factors in the Processing of Fine Fiber by Fluid Shear and Heat Transfer

Abstract

In this article, a synergistic approach encompassing numerical simulation and laboratory experimentation is used to identify the optimal geometry for the creation of fine fiber by the melt-blown process. The problem involves highly complex fluid flow and convective heat transfer. The fine fiber is created by the use of high-velocity, obliquely impinging air jets that stretch a polymer extrudate in the partially fluid state. High-temperature air is used to maintain the fluidity of the polymeric material as it exits the die. Four different geometrical configurations were investigated with regard to their capability of producing high fluid shear and high temperatures in the critical region just downstream of the emergence of the polymer extrudate from the tip of a die. The results of the numerical simulations provided a definitive conclusion about the relative efficacies of the four investigated geometrical configurations. The velocity and temperature profiles of the oblique jets were carefully documented to identify their decay with increasing downstream distance from the die tip. Velocity profile measurements were in excellent agreement with the numerical predictions, thereby validating the simulation model. Another major parameter of the study, in addition to the geometric-configuration issue, was the pressure difference responsible for setting the magnitude of the jet velocity. The accuracy of the results was established by a mesh independence study and by varying the size of the solution domain.

Three-Dimensional Fluid Flow in the Processing of Fine Fibers

“…This positioning, with the die tip recessed with respect to the face, is called setback. Alternative positions of the tip with respect to the face of the die have been studied previously . That investigation was carried out for a simplified, two-dimensional version of the actual fine-fiber production process.…”

“…In particular, the oblique jets converge within a confined space in the present configuration, whereas the convergence of the jets in the configuration of Figure b occurs in an open space. As already noted in a previous paper, the authors have compared the shear stresses acting on the fiber corresponding to the setback position with those corresponding to the flush configuration. It was found that substantially higher shear stresses were obtained using the setback positioning.…”

“…Alternative positions of the tip with respect to the face of the die have been studied previously. 5 That investigation was carried out for a simplified, two-dimensional version of the actual fine- fiber production process. The investigated alternatives included the die tip situated forward of the face and the tip aligned with the face.…”