摘要
喷气纺纱以其速度快、产量高、用工少等优越性,被普遍认为是最具发展前景的一种新型纺纱方法,它是通过在两喷嘴内所形成的相互反向旋转的气流对纤维须条施加捻度而成纱的。目前,人们对喷气纺纱的研究主要集中在成纱机理、喷气纱结构、各种参数对喷气纱性能的影响上,且这些研究多是基于纺纱实验的,有较大的局限性。事实上,喷气纱的结构及其特性是由纤维在喷嘴内的运动规律所决定的,然而,在这方面仅有曾泳春对纤维在第一喷嘴内的运动进行了二维数值研究。而喷嘴内的高速旋转气流是高度三维的瞬时流场;再者,据两喷嘴的不同作用,第一、二喷嘴被分别做成圆柱管和渐扩管,因而,两喷嘴内的流动特性是不同的,从而影响了纤维在其内的运动。
基于上述原因,本文先用数值与实验方法对喷气纺不同喷嘴(包括有开纤管的喷嘴)内的三维旋转流场进行研究,并讨论了各喷嘴的不同参数对流场和纱特性的影响,因而,对这些喷嘴参数进行了优化。在对流场模拟的基础上,将柔性纤维离散成由不计质量的杆所连结的珠链,并认为纤维的质量以及所受力都分布在珠上,而杆只起到传递内力和变形的作用,在考虑纤维的弯曲和扭转变形情况下,建立其动力学方程,对柔性纤维在高速旋转流中的运动进行模拟。
本文的主要研究内容和结果如下:
1.三维瞬时旋转气流场的模拟
对所有喷嘴来说,由于气流的反喷,在喷孔上游壁面附近会形成回流区,而在喷孔的下游会发生涡破裂。在不同喷嘴中,这些回流和涡破裂将经历复杂的变化。在第一喷嘴内,涡破裂是周期性变化的螺旋型破裂,且在整个周期内,涡破裂位置缓慢地向下游移动,而上游回流会向喷嘴入口方向拉伸;在第二喷嘴内涡破裂由最初的泡型破裂发展为锥形破裂,且在锥形破裂的内部螺旋结构也呈现周期性变化,而上游回流的大小随时间的增加先增长后减小;在有开纤管的第一喷嘴内,随时间的增长,初时泡型涡破裂沿流向拉伸增长,进一步地,泡型破裂转变为螺旋破裂,最后,当在喷孔的上游和沟槽台阶后的两回流区不再变化时,螺旋型破裂呈现周期性衰减。
2.喷嘴压力对旋流场的影响
对所有喷嘴来说,当喷嘴压力增加时,速度增加,但其增长趋势下降。然而,速度分布规律不随喷嘴压力的变化而变化。然而,随着喷嘴压力的增加,涡破裂的发生位置在第一喷嘴(或有开纤管)内向喷嘴出口方向移动,而在第二喷嘴内向喷孔方向移动。
3.第一、二喷嘴几何参数对旋流场的影响
在两喷嘴内,喷孔上游的速度和回流强度都随喷射角的增加而增加;然而,喷孔下游的涡破裂位置在两喷嘴内的移动是相反的,即,在第一喷嘴内向喷嘴出口方向移动,而在第二喷嘴内向喷孔方向移动。当喷孔数或喷孔直径增加时,速度会随之增加但其增长趋势下降;然而,涡破裂的强度和大小在第一喷嘴内减小而在第二喷嘴内却增加。由于捻室管径沿轴向变化不同,喷孔位置对两喷嘴的影响明显不同。对第一喷嘴而言,当喷孔位置变化时,其喷孔附近的速度变化不大;然而,当喷孔位于喷嘴入口附近时,较大的反喷不利于引纱。由于第二喷嘴捻室的扩散,当喷孔位置向下移动时,涡破裂的发生也移向下游。当第一喷嘴的捻室直径增加时,流动变得极为紊乱。然而,在第二喷嘴内,喷嘴出口直径的变化不影响速度分布,然而,随着喷嘴出口直径的增加,速度和涡破裂的强度都减小。
4.开纤管沟槽参数对旋流场的影响
对于所有工况,在沟槽和捻室内的气流旋向是相反的。沟槽高度增加时,台阶后面的角回流区的长度增大,沟槽内的初始涡环减小,并在槽底形成一个新的同向旋转涡。当沟槽宽度增加时,喷孔下游的切向速度不变,然而,喷孔下游的涡破裂以及沟槽内的涡环尺度都会稍微增加,然而,角回流区沿流向的尺度减小。随着沟槽长度的增加,角回流区的大小以及沟槽内的负切向速度和涡环大小都不变。沟槽数的变化对喷孔附近的速度分布影响不大,且四矩形沟槽喷嘴会产生较大速度和较强的涡破裂。
5.第一喷嘴内旋流的PIV实验研究
PIV的流速测量表明,流速矢量以螺旋形沿流向衰减,并在螺旋内部存在低速区,且轴向速度以螺旋的中心为轴呈对称分布。当喷孔数增加时,轴向速度和切向速度总体上呈增加趋势,因而,旋度沿流向的衰减程度减小,且在较小的喷孔数下,流动在喷嘴出口附近变为轴流。当喷孔直径增加时,轴向速度在远离喷嘴出口的区域增大,但沿流向喷孔直径越大,轴速的衰减越快;切向速度随喷孔直径的变化较复杂,其沿流向衰减较慢。因而,随喷孔直径的增加,在喷嘴出口附近的旋动越强。
6.第二喷嘴内流线角的流动可视化研究
对所有工况来说,壁面流线角在总体上是沿流向逐渐减小的,这意味着壁面剪切的切向分量的衰减;在喷嘴出口截面上,流线以与管横截面同心的圆向外呈顺时针螺旋状分布,这与第一喷嘴的逆时针旋向是相反的。随喷射角度或喷孔直径的增大,壁面流线角有减小的趋势;但随着喷孔数或喷孔位置的增加,壁面流线角增加,特别是在同样的喷孔总面积下,减小喷孔直径的同时增加喷孔数会使流线角增加。
7.纤维参数对纤维运动轨迹的影响
对本文所研究的纤维来说,随着纤维刚度的增加,纤维的头端所形成的螺旋旋转的程度和捻回数都会稍微增加,但总体而言,纤维刚度对其在喷嘴内的运动轨迹影响不大;纤维长度增加,柔性变形程度增加,但其捻回数减少。
8.纤维释放位置对纤维运动轨迹的影响
在第一喷嘴中,纤维的释放位置离中心越远,纤维的刚性变形越大,其缠绕作用减弱,从而和近轴心处释放的纤维形成捻差。与之相反,在第二喷嘴中,当纤维的释放位置远离中心时,纤维的柔性增加,包缠螺距很小,从而,能更紧的缠绕芯纤维,使纱获得强力;在近轴心处释放的纤维以刚性杆状在喷嘴内旋转,是无缠绕的。
9.喷嘴几何参数对纤维运动轨迹的影响
在第一喷嘴中,随着喷射角或喷孔直径的增加,纤维的柔性变形程度减小,且在较小的喷射角或喷孔直径下,纤维出现有/无自纠缠的卷绕变形,不利于其在第二喷嘴内的解捻,易引起纱条干不匀。然而,在第二喷嘴中,喷射角或喷孔直径越大,纤维的柔性越大,包缠性越好,然而,当喷射角为90°时,在喷孔的远下游,纤维的头端会形成倒弯钩,将影响纱条干均匀度。
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 airflow
For 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 flow
For 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 flow
For 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 flow
For 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 nozzle
In 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 nozzle
For 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 motion
For 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 motion
In 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 motion
In 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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