Abstract:

Objective The quality of yarn produced by airjet spinning is highly dependent on the airflow field characteristics inside the nozzle. However, the nozzle's complex structure and small internal space make direct experimental measurement of the airflow field challenging. In oder to clarify the airflow variation and fiber motion rules in the twisting chamber during spinning, finite element simulation was employed to investigate the airflow and fiber motion during both initial and staple spinning stages.

Method First, a three-dimensional geometric model of the air-jet vortex spinning nozzle was established, followed by mesh division and boundary condition configuration. Numerical simulation of the flow field in the standard nozzle model was then performed, with in-depth analysis of the flow patterns inside the twisting chamber. Second, a discretized flexible fiber model was developed to characterize the fiber's physical and mechanical properties. Finally, coupled simulations were conducted using Rocky DEM 2022R1 and ANSYS Fluent 2022R1 to simulate the fiber's dynamic behavior during the yarn piecing and twisting processes of air-jet vortex spinning, and systematic spinning experiments were carried out accordingly.

Results The results indicate that the rotational suction effect of the high-speed swirling airflow induces negative pressure at the nozzle inlet to draw in fibers. Meanwhile, it drives the airflow in the twisting chamber to move upward and forms a backflow region in the fiber feeding channel. The opposite directions of radial and axial airflow at different radial positions are crucial for ensuring the smooth convergence of fibers into the twisting chamber for entanglement and twisting, thereby forming a core-sheath yarn structure (outer fibers wrapping core fibers). The distribution pattern of the airflow field in the nozzle twisting chamber during the initial spinning stage is similar to that in the steady spinning state. However, affected by the airflow in the hollow tube, the negative pressure and airflow velocity increase as the position approaches the hollow spindle orifice, while the gradient becomes less significant near the guide needle. A distinct pressure and velocity gradient distribution is observed in the guide channel.

Fiber motion within the nozzle in the spinning stage can be divided into four stages, corresponding to the following periods: fiber passage through the inlet and outlet of the fiber feeding channel, the inlet and outlet of the twisting chamber, fiber residence on the hollow spindle surface, and passage through the inlet and outlet of the guide tube. Fiber velocity increases gradually in the fiber feeding channel. Upon entering the twisting chamber, the acceleration increases further, leading to a higher speed. As the fiber moves forward and contacts the hollow spindle surface, its speed decreases gradually before entering the guide tube. After entering the guide tube, the speed increases again and peaks at the guide tube outlet. The fiber velocity variation in the yarn piecing stage follows the same trend.

Conclusion This study conducts an in-depth investigation into the airflow characteristics within the twisting chamber, analyzes the motion characteristics of fibers during both the yarn piecing stage and the spinning stage, and reveals the coupled motion mechanism between the airflow and fibers inside the nozzle. The findings are of great significance for optimizing the spinning process and designing key structural components. Currently, fiber simulations are limited to motion pattern variations. Future research could extend to simulating fiber aggregation, twisting, and yarn piecing processes, which would facilitate a more comprehensive understanding of the laws governing fiber motion and morphological evolution. Owing to computational resource limitations, the current simulations are constrained by the number of fibers and time steps. Future work should focus on expanding the simulation scale (i.e., increasing the number of fibers and prolonging the simulation duration) and simulating continuous fiber feeding to better replicate actual production scenarios.

Key words: air-jet vortex spinning, numerical simulation, twisting chamber, airflow behavior, fiber motion, novel spinning, semi-open-end spinning