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Ind. Eng. Chem. Res. 1998, 37, 3772-3779
Modified Dual Rectangular Jets for Fiber Production Brian D. Tate and Robert L. Shambaugh* Department of Chemical Engineering and Materials Science, University of Oklahoma, Norman, Oklahoma 73019
Velocity fields were measured below two parallel, rectangular air nozzles. The following five die (nozzle) types were tested: (1) a die where the jets met at a 60° angle and the die tip was blunt, (2) a die with a 60° angle and a sharp tip, (3) a die with a 70° angle and a sharp tip, (4) a sharp 60° die with an inset, and (5) a sharp 60° die with an outset. Since the dies had a large length-to-width ratio, the velocity fields approximated that of a two-dimensional jet. Correlations were developed to predict the velocity fields below these die types. Introduction Recent papers by Harpham and Shambaugh1,2 describe the air flow field below a pair of dual rectangular jets. This type of jet arrangement is commonly used in industry to produce nonwoven fibers via the meltblowing process. In melt blowing (see Shambaugh ref 3), heated gas streams impinge upon molten filaments as the filaments emerge from a heated die; see Figures 1 and 2. The force of the gas upon the filaments causes rapid attenuation of the filaments to fine diameters. The attenuated fibers are cooled and captured upon a mesh screen placed some distance away from the die. The resulting nonwoven mass of fibers can be used as a filter, an absorbent medium, reinforcement, or numerous other uses. As Harpham and Shambaugh state, the most common type of melt-blowing die, the slot die, involves a pair of air slots. The plane of each slot is usually at about a 60° angle relative to the face of the die; see Figure 2. The slot length (in a direction perpendicular to the plane of Figure 2) may run from 7 to 100 cm or more. Molten polymer enters the air field through a series of fine holes drilled along the length of the nosepiece. Figure 2 shows a blunt die where the nosepiece has a flat which runs along its length. Figure 3 shows a sharp nosepiece that is also a 60° die. Machining polymer orifices into such a die is difficult since the holes must be placed extremely accurately along the edge. Furthermore, the edge dulls during service due to oxidation and damage, which occurs when the die is cleaned. Nonetheless, the sharp nosepiece represents a limiting configuration for a 60° die. The angle at which the air exits the die can also be varied. Figure 4 shows a 70° die with a sharp nosepiece. An angle of 90° would result in a purely parallel, rather than partially impinging, flow of the air streams relative to the flow of polymer from the polymer orifices. Such parallel drag flow is what is used in the Schwarz-type of die (see Shambaugh3 and Schwarz4). (Also, the Schwarz die is an annular die, not a slot die.) Zero inset is the situation wherein the tip of the nosepiece is in the same plane as the exposed faces of the air plates. The dies of Figures 2-4 have zero inset. Figure 5 is an example of an inset die wherein the nosepiece tip is withdrawn a distance “a” into the body of the die. If the inset amount is too great, fibers spun from an inset nosepiece may frequently stick against * To whom correspondence should be addressed.
Figure 1. The experimental setup.
Figure 2. A cross-sectional view of the 60° blunt air die. This is the same die used by Harpham and Shambaugh.1 The y-axis (not shown) is perpendicular to the plane of this drawing.
the air plates. Dies can also have outset nosepieces: Figure 6 shows such a die. As mentioned by Harpham and Shambaugh,1 single rectangular nozzles have been studied by a number of investigators, including Miller and Comings,5 Van der Hegge Zijnen,6 Heskestad,7 Sforza et al.,8 Trentacoste and Sforza,9 Jenkins and Goldschmidt,10 Kotsovinos,11 and Sfier.12 Mohammed and Shambaugh13 reference past work on linear arrays of rectangular nozzles. Except for the recent work of Harpham and Shambaugh,1,2 none of the previous work involved dual jets with sharp-edged air plates and with the geometry shown in Figures 2-6.
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Ind. Eng. Chem. Res., Vol. 37, No. 9, 1998 3773
Figure 3. A cross-sectional view of the 60° sharp die.
Figure 4. The 70° sharp die.
Figure 5. The inset die.
This paper examines the flow fields below a group of dies of the types illustrated in Figures 2-6. A primary goal was the determination of the optimum die geometry for producing melt-blown fibers.
Figure 6. The outset die.
mm, an inner diameter of 0.45 mm, and a conical nose shape with a cone angle of 25°. The Pitot tube was 6.35cm long and was connected with 3.2-mm tubing to an oil-filled manometer. The formula used to convert pressure to velocity is discussed by Uyttendaele and Shambaugh (1989). The Pitot tube pressure was referenced to ambient static pressure, and the tube was oriented vertically during the measurements. The Pitot tube was positioned with a Velmex three-dimensional traverse system which permitted x-, y-, and z-motions in 0.01-mm increments. A Pitot tube gives an error of
