Effects of Die Geometry on the Flow Field of the Melt-Blowing Process Holly M. Krutka, Robert L. Shambaugh,* and Dimitrios V. Papavassiliou School of Chemical Engineering and Materials Science, The University of Oklahoma, 100 East Boyd Street, SEC T335, Norman, Oklahoma 73019 Sharp dies are often used commercially to produce polymeric fibers in the melt-blowing process. In these sharp dies, the flow field results from two similar converging plane jet nozzles with no space between the nozzles. This study utilizes a computational fluid dynamics approach that is validated through experimental data to investigate the effect of recess or excess (inset or outset) of the die nose on the flow field. The Reynolds Stress Model is used to simulate the turbulence, and the model parameters are calibrated with experimental data. The flow field downstream from the sharp die is found to exhibit (a) a merging region, which includes a maximum in turbulence intensity, and (b) a self-similar region. The behavior of alternative die designs is correlated to the die configuration. The more that the nose piece is recessed, the larger is the mean velocity under the die, but at the same time the turbulence becomes stronger. 1. Introduction Dual rectangular jets are commonly used in industry to fabricate polymer fibers in the melt-blowing pro- cess. 1,2 Air is propelled through two converging jets to rapidly attenuate a molten strand of polymer below the die face. Figure 1 shows a cross-section of a sharp flush die, which is frequently used in this process. 3 The flow field created by the dual rectangular jets strongly affects the size and the strength of the polymer fiber. Therefore, it is important to understand this complicated flow field to improve the conditions for polymer extrusion and to optimize die design. In general, a high mean velocity at the centerline (x ) 0) is desired in order to rapidly attenuate the polymer fibers. In addition, turbulence intensity along this centerline must be minimized, since strong velocity fluctuations in the flow field can cause the polymer fibers to break off and/or stick to the die face. The flow field that results from two parallel plane jets has been studied previously in the experimental inves- tigation of Nasr and Lai 4,5 and the numerical investiga- tions of Anderson and Spall 6 and Lai and Nasr. 7 The flow field that results from two converging plane jetss the configuration that is critical to melt-blowingshas been studied experimentally by Harpham and Sham- baugh 8,9 for blunt and flush sharp die faces and by Tate and Shambaugh 3 for several different jet geometries. Tate and Shambaugh measured the z-component of the mean velocity at different positions in the flow field that exists downstream from a sharp die (both inset and outset). Computational fluid dynamics (CFD) has re- cently offered a complementary, additional method of studying such flow fields. 10 Simulations using CFD can be completed in a fraction of the time required for laboratory experiments, while the cost of running each simulation is much lower than the cost of laboratory experiments. Also, CFD simulations allow for close examination of regions of the flow field that are difficult to test in the laboratory, such as the area very close to the die face or nose piece. However, as shown by Krutka et al., 10 experimental data are necessary to properly calibrate the CFD simulations. In the work described herein, the flow fields of several types of sharp dies (i.e., inset and outset dies) were simulated using the computational fluid dynamics pack- age Fluent 6.0. These CFD results were validated with the laboratory measurements of Tate and Shambaugh. 3 In addition to the mean velocity, the turbulence proper- ties of the flow fieldsincluding the turbulence intensity, the turbulent kinetic energy, and the turbulence dis- sipation rateswere examined. Furthermore, predictive correlations for the effect of the nose piece location on the flow field were developed. 2. Numerical Modeling and Simulation Parameters Several variations of the sharp die are used in industrial melt-blowing. The inset die is characterized by a nose piece that is recessed above the die face. Figure 2 shows the geometry of a standard inset die. Observe that, unlike the situation with the flush die (Figure 1), in the inset die the slot face width b o is not equal to the distance b (which equals h/2). Since actual commercial dies are “large” in the y- direction (the direction perpendicular to the plane of * To whom correspondence should be addressed. Tel.: (405) 325-6070. Fax: (405) 325-5813. E-mail: shambaugh@ou.ed. Figure 1. Melt-blowing die with a flush nose piece. 5541 Ind. Eng. Chem. Res. 2003, 42, 5541-5553 10.1021/ie030457s CCC: $25.00 © 2003 American Chemical Society Published on Web 10/04/2003