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.

Converging plane jets are used commercially to produce polymeric fibers in the melt blowing process. The behavior of the air flow below the die face is critical for the production process. The die configuration can affect this flow field. Previous work has studied the effects of changing the angle of convergence of the two jets for blunt and sharp die faces, as well as the case of recess or excess (inset or outset) positioning of the sharp die nose for isothermal flow conditions. This study utilized a computational fluid dynamics approach that was validated through experimental data to investigate the effects of nonisothermal conditions on the air flow. The Reynolds stress model was used to simulate the turbulence, and the model parameters were calibrated with the experimental data. The behavior of alternative die designs was also correlated to the die configuration. It was found that, similarly to the isothermal case, the sharper the angle of convergence, the higher the mean air velocity under the die and the higher the intensity of the turbulent velocity fluctuations. The flow field exhibits a self-similar region beyond the point at which the flow from the two jets merges. It was found that the mean temperature and the mean velocity field beyond the merging distance can each be described by a single correlation. In these correlations, the merging distance was used as the length scale, and the mean flow velocity at the merging point was used as the velocity scale.

Exxon slot melt blowing dies consist of dual, rectangular, converging jets and are used in the industrial melt blowing process to attenuate molten polymer fibers. The air flow creates a drag force that accelerates the polymer and rapidly reduces the fiber diameter. For previous experimental and computational fluid dynamics studies, the effect of the fiber on the air was assumed to be negligible. By including the fiber as a boundary in the computational domain, this assumption was tested. A modified version of the Reynolds stress model was used to simulate the turbulent air flow field. It was determined that the centerline air velocity (located halfway between the fibers) has an increased maximum due to the presence of the fiber. In addition, the turbulence in the flow field is dampened by the presence of the fiber. The jet spreading rate is higher halfway between the fibers than at the center of the domain, where the fiber is located. The air flow around the fiber is nonuniform, leading to varied shear stress at different radial positions on the fiber edge. The temperature of the air flow is related to the ratio of the polymer to air flow rate; as this ratio increases, the hot fiber reduces the cooling rate of the air.

Melt-blowing dies are used industrially to fabricate fine polymer fibers. The Schwarz die is a type of melt-blowing die that uses multiple columns of polymer orifices and associated air jets. This arrangement contrasts with the single column of orifices that is used in the slot, or Exxon, die. The experimental measurements of the air flow field from a Schwarz die can be reproduced using a k-turbulence model. Six different multihole die geometries were simulated in 3D with this model. In comparison with the flow field of a single jet, the velocity maximums occurred closer to the die face for an array of jets. The spreading rates for the center jets of the multihole dies were similar to each other, and close to 0.5, while the spreading rate of a single annular jet has been observed to be close to twice this value. In addition, the differences in the air fields of the multihole geometries lead to observations concerning multiple jet interactions. The distance required for the inside column of jets to affect the outside jets was determined as a function of the jet orifice spacing. Finally, the turbulence intensity of all the simulated flow fields did not vary as significantly as the velocity profiles.

In commercial operation, a melt-blowing die is used to produce fine fibers that are solid, not hollow. In this paper, a model was developed to predict what would happen if hollow fibers were produced with a modified melt-blowing die. The model involved the simultaneous solution of the momentum, energy, and continuity equations. The model equations were solved numerically. Predicted parameters included fiber hollowness, fiber attenuation, vibration frequency, vibration amplitude, temperature, and stress. Hollowness can affect the laydown pattern of the melt-blown fibers.