A computational fluid dynamics technique was developed for the simulation of airflow through an annular jet. The technique used a commercial simulation package with a Reynolds stress model for the simulation of turbulent flows. The model parameters were calibrated using available experimental data for circular and annular jets. It was found that, after this calibration, the computational results agreed well with experimental data (specifically, with the velocity magnitude, velocity decay rate, and the velocity spreading rate). The jet geometry studied was based on industrial melt-blowing nozzles. The velocities studied varied from the low subsonic incompressible range to nearly sonic conditions. Based on both the computational and experimental results, a correlation was proposed that predicts the centerline velocity profiles in both the near-and far-field regions.
Single-walled carbon nanotubes were added to two different grades of polypropylene to produce composites. The composites were then melt-spun into fibers, and the fibers were tested with both a conventional tensile pull tester and dynamic mechanical analysis. The changes in tensile properties were related to the grade of polypropylene used. In addition to fibers being made from the mixes, coarse extrudates (i.e., undrawn, gravity-spun filaments) were also produced. Density measurements on these extrudatesshowed that the addition of nanotubes increased the composite density in a highly nonlinear manner, which suggested interaction between the polypropylene and the carbon nanotubes.
Numerous measurements were taken during the operation of a practical melt blowing slot die. On-line measurements were taken of the mean velocity and temperature of the air jets. Also, on-line measurements of fiber vibration amplitude were done. Off-line measurements were taken to determine fiber diameter distributions in the nonwoven webs. The light absorbance of these nonwoven mats was measured and related to fiber diameter distribution and mat basis weight. Process conditions were varied across the operating range of the die to produce a variety of finished mats. It was found that the mean air velocity and temperature decayed in a manner similar to that observed in both laboratory-scale melt blowing dies and (more generally) in rectangular jets. Fiber vibrations were found to be strongly dependent on operating temperature and air flow rate. The fiber light absorbance correlated well with the projected area of the fibers present in the mat.
Online measurements of the fiber diameter distribution during a melt blowing process were taken using a new laser diffraction technique. This technique measured both the attenuation of the fibers as well as entanglement of the fibers into bundles at large distances from the die. A pilot scale unit with a 20.3 cm (8 inch) slot die was used for the studies. Commercial polypropylene polymer was used. Both the spin-line attenuation and fiber bundling were measured as a function of position both below and across the die face.
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