| Due to advantages in processing speed and cost, air-jet spinning is accepted as one of the most promising technologies. For prevailing Murata jet spinning (MJS) system, the forming yarn is 'twisted' by operating two swirling air currents, in mutually opposite directions in two nozzles. At present, most of the information available in the literature on air-jet spinning, which was based on spinning experiments, was related to the yarn structure, the principle of yarn formation and the effects of various parameters on yarn quality. All these mainly depend on the fiber motion. Only Zeng used numerical method to study fiber motion in two-dimensional airflow field of the first nozzle. However, the swirling flows in the nozzles of air-jet spinning are highly three-dimensional and time dependent in nature. Again, according to the different functions of two nozzles, normally the first and second nozzles are made cylindrical and diverged conical shapes, respectively. It is obvious that the (?)ifferent flow behaviors in the two nozzles will lead to different motion of fiber.Based on the above reason, the 3D swirling flows in different nozzles (including the slotting-tube) of air-jet spinning have been studied using experimental and numerical methods in this paper. The effects of the different nozzle parameters on both the flow and yarn properties are also investigated. Hence, these nozzle parameters are optimized. A flexible fiber is modeled as rigid beads connected by mass-less rods. Only the beads generate and are affected by forces. The rods only serve to transmit forces and maintain the configuration of the fiber. The dynamical equations describing bead motion in a fiber are derived, which the fiber could be bent and twisted in the model. The flexible fiber motion in high swirling flow is calculated. The main contents and conclusions are listed below:1. Simulation of 3D transient swirling airflowFor all the nozzles, a recirculation zone near the upstream wall of the injectors is generated due to reverse jet, and the vortex breakdown (VB) in the injector downstream is also observed. These vortices experience complex flow processes. In the first nozzle, periodic change of spiral-type VB can be observed; the recirculation zone near the upstream wall of the injectors increases in size and moves gradually upstream while the VB shifts slowly towards the nozzle outlet during the whole period. A conical breakdown in the second nozzle can form from the bubble breakdown, and its internal spiral structure shows periodical change; the extent of the recirculation zone in upstream of the injectors first increases, and then reduces with time. In the first nozzle with a slotting-tube, an initial bubble-type VB grows and stretches in axial direction as time is increased. Further, it experiences a transition from bubble- to spiral- breakdown. Finally, the spiral breakdown shows periodic decay as the size of two recirculations near the injector upstream wall and the step retains almost constant.2. Effect of nozzle pressure on swirling flowFor all nozzles, the velocity increases and its increasing trend declines as the nozzle pressure increases. However, the rule of the velocity distribution does not change with increasing nozzle pressure. In the first nozzle (with and without the slotting-tube), VB location shifts "slightly downward the nozzle outlet with the increase of nozzle pressure. The reversed results can be obtained for the second nozzle, which VB moves in upstream direction.3. Effects geometric parameters of the first and second nozzles on swirling flowFor the first and second nozzles, in the upstream of the injectors, both velocity and the strength of the recirculation flow increase with increasing the in ection angle. However, as the injection angle increases, the location of VB moves rapidly downward in the first nozzle, while it moves towards the injectors in the second nozzle. With increase in the injector diameter or injector number, the velocities increase and their increasing tendency declines. However, the strength and area of the VB decrease in the first nozzle, yet they increase in the second nozzle. For the first nozzle, the velocity near the injection location does not change significantly as injector position changes, however, as the injector position is closer to the nozzle inlet, larger reverse flow will be not helpful to draw fibers into nozzle Due to the divergence of the pipe, in the second nozzle, VB moves in downstream direction with the increase of the injection position. As the twisting chamber diameter of the first nozzle increases, the flow is more turbulent. For the second nozzle, as the nozzle outlet diameter increases, the rule of the velocity distribution does not change, however, both the velocity and the strength of the VB decrease.4. Effects of groove parameters of the slotting-tube on swirling flowFor all the cases under study, there are air currents in mutually opposite direction in the grooves and the twisting chamber. With the increasing of the groove height, the length of the corner recirculation zone (CRZ) behind the step increases, and the initial vortex ring in the groove decreases and a same direction-rotating vortex forms in the bottom of the groove. As the groove width is increased, the tangential velocities in downstream of the injectors retain constant. However, the extents of both VB in downstream of the injectors and the vortex ring in the groove increase slightly, while the CRZ lengths in stream-wise direction decrease. Some factors, such as the negative tangential velocities in the grooves, the size of the CRZ and the vortex rings in the grooves keep constant with the increase of the groove length. Near the injectors, the effect of the groove number on the velocity distribution is not larger. A nozzle with four grooves will generate a larger velocity and a stronger VB.5. PIV study of swirling flow in the first nozzleIn PIV experiments, the velocity, which shows a complicated helical shape, decays along the flow path. There is a low velocity zone inside the helix structure, and axial velocity distribution is axisymmetric on the centerline of the helix. As the injector number increases, both the axial and tangential velocities increase on the whole, and the degree of the swirl intensity decay with stream-wise direction decreases. For a nozzle with small number of the injectors, the swirling movement is diminished and changed to a uniform flow near the nozzle outlet. The axial velocity in the region far from the nozzle outlet increases with the increasing of the injector diameter. The larger the injector diameters are, the quicker the decay of axial velocities with stream-wise direction. However, with the increase in the injector diameter, the change of the tangential velocity is complex, and the decay of the tangential velocity is slower along the flow path. Hence, as the injector diameter increases, the swirl intensity is higher near the nozzle outlet.6. Flow visualization study of the wall swirl angle in the second nozzleFor all the cases under study, the wall streamline angle decreases gradually with increasing downstream distance, signaling the decay of the tangential component of the wall shear. At the nozzle outlet cross-section, the droplet moved outward from a circle concentric with the nozzle outlet, and its tracks show a clockwise swirl. This is contrary to that of the first nozzle, which the droplets moves in counter-clockwise. With increase in the injection angle or injector diameter, the wall swirl angle decreases. However, as the injector number or the injection position increases, the wall swirl angle increases. As the total area of the injectors keeps constant, the increasing of the injector number, which means to decrease the injector diameter, will causes an increase in the streamline angle.7. Effects of fiber parameters on fiber motionFor all the fiber categories under study, with increase in the fiber flexural rigidity, both the width and turn number of the helices in the leading end of the fiber will increase slightly. However, on the whole, fiber flexural rigidity has effect little on fiber motion in the nozzles. As the fiber length increases, the extent of flexural deformation of the fiber increases, while the number of turns of the winding decreases.8. Effects of release positions on fiber motionIn the first nozzle, the release position of the fiber is far from the cen:er axis of the nozzle, the flexural deformation of the fiber decreases, and the extent of the winding weaken. The twist difference between edge fibers and the core ones will form. In contrast, in the second nozzle, as the release position of the fiber is close to the wall, the flexible deformation of the fiber increase. The screw-pitch of the winding decreases. Hence, the edge fiber will be more tightly wrapped on the yarn core. The fiber, which is released near the axis center, will rotate as a rigid rod, and the winding will do not occur.9. Effects of geometric parameters of the nozzles on fiber motionIn the first nozzle, with the increase in the injection angle or the injector diameter, the extent of the flexible deformation of the fiber decreases. For smaller injection angle or injector diameter, coiled deformation with/without self-entanglement will form. It is bad to untwist by the second nozzle. However, in the second nozzle, the greater the injection angle or the injector diameter is, the bigger the fiber flexibility is, and the better the winding is. As the injection angle is 90°, a leading hook will forms in the far from the injectors, and yarn evenness will decrease.
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