Abstract:

Objective To gain an in-depth understanding of the fiber motion inside the air-jet vortex spinning nozzle under the action of high-speed airflow， a fluid-structure interaction （FSI） approach was adopted， taking into account the frictional effects between the fiber and the nozzle wall.

Method The study investigates the synergistic effects between nozzle parameters and fiber dynamics. A fiber-airflow FSI model is firstly established， and a two-way coupled FSI simulation platform is constructed in ANSYS Workbench. Then， the fundamental spinning mechanism is elucidated through the analysis of flow field characteristics under a benchmark nozzle configuration. Finally， based on the developed FSI platform， the effects of friction and nozzle parameters on the fiber trajectory and motion stability are examined.

Results Fiber motion patterns and trajectories are significantly altered by friction， which is considered a prerequisite for successful yarn twisting. The influence of nozzle structural parameters on fiber motion was found substantial， and fiber movement were effectively optimized through rational design of these parameters， thereby enhancing yarn quality. When the fiber-wall friction effect was taken into account， more pronounced wall-adhering fiber motion was observed， and fiber movement was decelerated by frictional resistance， resulting in a noticeable lag at the same time points. When the friction coefficient is 0.3， the fiber's displacement at 0.002 5 seconds is significantly lagging compared to cases with friction coefficients of 0 and 0.1. The optimal nozzle inclination was found to be 70°， at which a balanced vortex allowed sufficient fiber twisting and maximized yarn strength， whereas a 60° nozzle inclination caused insufficiently fiber twist due to dominant axial flow， and a 80° nozzle inclination led to excessive radial flow producing a less concentrated vortex. As the nozzle diameter increased from 0.3 mm to 0.5 mm， the jet flow rate was significantly enhanced， and the aerodynamic traction on fibers increased. Consequently， the moving velocities of both axial and radial fibers were accelerated， and fibers exhibited a more regular sinusoidal motion. With a twisting chamber diameter of 7 mm， fibers moved the fastest and entered the twisting zone quickly， with a concentrated and strong vortex flow that ensures sufficient twisting， resulting in high yarn strength， low hairiness， and a relatively firm texture. When the nuzzle chamber diameter became 8 mm， the vortex region was enlarged and the vertex weakened， leading to slower fiber motion and more regular and stable sinusoidal motion and yielding an overall favorable fiber state. Further， with a twisting chamber diameter of 9 mm， the vortex region was enlarged more and the vertex weakened more， the fiber wavelike motion was most regular， but twisting efficiency became the lowest， producing yarn with low strength， higher hairiness， yet a softer hand feel. Extending the distance between the guide pin and hollow spindle from 1 mm to 1.5 mm lengthened the free-motion zone， allowing more time for vortex-induced dispersion and twisting， which enhanced yarn strength， reduces hairiness， and improves overall yarn quality.

Conclusion The frictional interaction between the fibers and the nozzle wall induces both rolling and sliding of the fibers along the wall surface， thereby decelerating the motion of the fiber tail and preventing it from being directly entrained into the hollow spindle. This process provides sufficient time and spatial conditions for the airflow to twist and entangle the fiber tail， ultimately facilitating its wrapping around the surface of the core fibers， i.e.， the realization of twisting. It is therefore evident that the frictional effect constitutes a prerequisite for the successful twisting and

Key words: jet vortex spinning, fiber motion, fluid-structure interaction, swirling flow, nozzle structure, friction effect