关键词: 熔喷, 气体湍流, 狭槽型喷嘴, 气体摆动, 纤维运动

Abstract: In melt blown process, the airflow plays a crucial role in the process of superfine fiber formation. Numerous previous theoretical models have emerged on fiber stretch mechanism. However, these models are based on the steady air flow. In contrast, the melt blown airflow is characterized by unsteady turbulence with a high Reynolds number. To further explore the stretching mechanism of the polymer fiber during melt blown process, the unsteady characteristics of the air turbulence in melt blowing were explored.
The characteristics of melt blown air turbulence were numerically simulated by using the detached eddy simulation (DES) model. Subsequently, the melt blown air turbulence was measured experimentally by particle imaging velocimetry (PIV) and the experimental results were compared and verified against the numerical simulation results. Finally, the fiber path in the spinning process of melt blowing was captured by high-speed photography technology, and the fiber path was compared with air turbulence. 
The results showed that, as the inlet air velocity gradually increased, the instability of air turbulence appeared closer to the face of melt blown die. When the inlet velocity reached a critical value, a regular and obvious turbulent oscillation phenomenon appeared in the triangle area of air re-circulation. The characteristic of air oscillation can be described as that air oscillates from left side of the limit position to the right side limit position and then turns back to the left limit position, and form a repeating cycle. It showed that the oscillation still maintained even at inlet velocity conditions higher than the critical level. In addition, the frequency of the alternating positive and negative values of lateral velocity increased with the increase of the inlet air velocity. The particle image velocimetry showed that the "S"-shaped air turbulence profile emerged below the air slots, which was consistent with the characteristics of turbulent fluctuation discovered by the numerical simulation. Moreover, the fiber paths during melt blown process under different inlet flow conditions showed that, when the inlet air flow rate gradually increased, the fiber gradually losed stability and developed into a regular "whipping" state trajectory. At a higher inlet flow rate, the fiber still had the characteristics of "whipping", but the whipping became more violent. The results obtained by high-speed photography were similar with the law of air turbulence oscillation simulated by DES model, and the frequency of turbulent oscillation was the same order of magnitude as that of fiber whipping.
This work explored the air turbulence and fiber motion in blunt die melt bowing by using approaches of numerical simulation and experiment. It verified that the air flow in the melt blowing is obvious turbulent. The characteristics of air turbulence and fiber motion were similar. It showed that both air turbulence and fiber have obvious "whipping" pattern movements. This work illustrates the potential relationship between air turbulence and fiber movement, and reveals that the further exploring of fiber stretching mechanism should take the characteristic of air turbulence into the spinning process. 

Key words: melt blowing, air turbulence, blunt spinneret, gas whipping, fiber movement