Measurement and Comparison of Melt-Blowing Airflow Fields: Nozzle Modifications to Reduce Turbulence and Fibre Whipping

Abstract

In the melt-blowing process, micro/nanofibrous nonwovens are attenuated and formed through aerodynamic force in a turbulent airflow field. In this work, two types of airflow-directors were added under a common melt-blowing slot-die nozzle to obtain modified airflow fields. The effect of airflow-directors on time-averaged characteristics, turbulence intensity, and temperature fluctuation intensity are achieved through the simultaneous measurement of fluctuating velocity and fluctuating temperature using a two-wire probe hot-wire anemometer. Moreover, the influence of airflow-directors on fibre oscillations are also investigated through high-speed photography. The distribution of turbulence intensity and temperature fluctuation intensity reveals the characteristics of fluctuating airflow fields formed by different melt-blowing slot-die nozzles. Through the analyses of airflow characteristics and fibre oscillations, we can find that the arrangement of airflow-directors has a great impact on both turbulence distribution and fibre oscillation.

Advances in Melt Blowing Process Simulations

“…8,59 However, after exiting the orifice, although the fiber diameter is too large to undergo any bending motion, it is still susceptible to the growth of minor oscillations along its surface. 42,55,60 An interesting study by Sun et al found that for high air velocities (280 m/s), the whipping amplitude increases further from the die, but low air velocities (110 m/s) show a rapid increase in whipping amplitude near the die exit, which then decays rapidly further downstream. 61 An approach to minimize fiber whipping is to minimize the spread of the air flow field through the use of a solid boundary to guide the fiber and air flow.…”

“…58 Adding an outerdirector to a slot die (marked red in Figure 21) can decrease air velocity fluctuations, which can suppress fiber oscillations and minimize whipping. 60 If the turbulent nature of fiber whipping can be understood and controlled, it could potentially be used to further attenuate the fibers to smaller diameters. 61 Verifying Fiber Simulations with Experiments.…”

“…Moreover, the high inlet stream perturbations, strong longitudinal stresses along the fiber axis, decreasing drag forces along the fiber length, and a decreasing centerline velocity further from the inlet play a critical role in the onset of whipping after it exits the die. Fiber whipping typically occurs after the region of fiber attenuation. , However, after exiting the orifice, although the fiber diameter is too large to undergo any bending motion, it is still susceptible to the growth of minor oscillations along its surface. ,, …”

“…However, care must be taken to avoid fiber contact with the boundary wall which could lead to clogging. ,, The use of an annular die can lead to less fiber whipping with a more stable air flow field . Adding an outer-director to a slot die (marked red in Figure ) can decrease air velocity fluctuations, which can suppress fiber oscillations and minimize whipping . If the turbulent nature of fiber whipping can be understood and controlled, it could potentially be used to further attenuate the fibers to smaller diameters …”

“…Nozzle (blue) with outer-director (red). Reproduced with permission from ref . Copyright 2021 The Authors under Creative Commons CC-BY 4.0, published by MDPI.…”