【作者】 裴泽光；

【导师】 郁崇文；

【作者基本信息】 东华大学 ， 纺织工程， 2011， 博士

【摘要】 气流在纺织加工过程中得到了广泛的应用。目前最新型的纺纱技术—喷气涡流纺(Vortex Spinning)就是借助在喷嘴中产生的高速气流对纤维进行加捻继而成纱的。喷嘴是喷气涡流纺加捻成纱的关键部件,具有极大柔性和极高长径比的纤维在加捻过程中与喷嘴高速气流场之间存在耦合作用,并与喷嘴壁面之间存在接触和摩擦作用,因此,喷气涡流纺的成纱过程涉及到非常复杂的力学问题。本文旨在对纤维在喷气涡流纺喷嘴气流场中的动力学和运动学特性进行深入而全面的研究,探究其成纱机理,为涉及纤维/气流耦合作用的纺织工艺提供理论参考。目前针对纺织加工中涉及的两相流动问题所建立的纤维模型多为多刚体链式纤维模型,难以体现纺织纤维巨大的长径比,此外也多只考虑了纤维与气流的单向耦合。本文采用计算流体力学与纤维有限元理论方法,建立了高速涡流与纤维耦合相互作用的动力学模型,采用完全拉格朗日法表述纤维在气流场中运动时产生的非线性大变形以体现纤维的柔性,将纤维的物理特性参数如长度、细度、杨氏模量等纳入纤维模型中以体现纤维的几何特征与弹性,选取任意拉格朗日—欧拉法并结合动网格技术对纤维在喷嘴气流场中的耦合作用与运动变形进行求解,同时采用约束函数法求解纤维与喷嘴壁面的接触作用,有效地解决了大长径比柔弹性纤维在喷气涡流纺喷嘴高速涡流作用下的动力学与运动学问题,数值模拟了纤维的非线性大变形。通过系列数值模拟与分析,获得了喷气涡流纺喷嘴中气流流动特征和柔弹性纤维在喷嘴高速涡流中的运动与变形特性。数值模拟结果表明：高速旋转气流形成于贴近喷嘴壁面的薄层区域内,并向下游旋转流动,构成气流场的外部区域。从喷嘴外部分别沿喷嘴入口和空心锭内的纱线通道进入加捻腔的两股弱气流发生碰撞而导致加捻腔中回流和涡的产生,构成气流场的内部区域。纤维在喷气涡流纺喷嘴中发生弯曲变形与螺旋形回转,呈波形运动,在不同位置处与空心锭内壁发生接触。纤维尾端在气流作用下成为自由分离状态,随后通过周期性的螺旋回转,包缠在纤维须条上形成纱线。为定量地表征纤维在纱线中的包缠效果,引入了纤维分离程度、包缠周期数和回转幅度等三个纤维运动规律参数,成纱的性能是三个参数综合影响的结果。纤维分离程度越高,包缠周期数越多,回转幅度越大,则成纱强力越高。喷嘴中气流流动特征的数值模拟结果表明：当喷嘴气压由4×105Pa增加至6×105Pa时,气流的切向速度随之增大,但分布规律保持不变,当喷嘴气压为5×105Pa时,气流的径向速度值最大；当喷孔倾角由60°增大至80°时,气流切向速度的最大值以及径向速度均随之增大；螺旋曲面角度由30°增大到90°对气流分布特征的影响很小；喷嘴入口到空心锭距离由12 mm增加至16 mm有利于外部区域中旋转气流顺利向下游运动,但加捻腔中形成的涡的尺度将明显增加。纤维与气流耦合作用的数值模拟结果显示：喷嘴气压对纯棉喷气涡流纱强力的影响不显著,但不宜超过5×105Pa；随着纺纱速度的提高,纯棉喷气涡流纱强力降低。对于喷嘴结构参数,喷孔倾角为70°,喷孔直径为0.4 mm,喷嘴入口到空心锭距离为14 mm,空心锭锥角为15°～20°时,纯棉喷气涡流纱可获得最佳强力。纤维的运动规律随纤维类型的不同而发生变化,其中棉纤维的分离程度最高,粘胶纤维与lyocell纤维表现出相似的运动规律,涤纶纤维的回转幅度在四种类型的纤维中最大,粘胶纤维、lyocell纤维和涤纶纤维具有相同的包缠周期。采用高速摄影的实验方法捕捉了纤维在喷气涡流纺喷嘴气流场中的运动形态,并与数值模拟结果进行了对比分析。实验中根据雷诺相似准则对喷嘴进行放大以保证喷嘴模型与原型中的气流流动特征相似,使用一根纱线代替单纤维。实验研究发现：纤维在喷嘴中高速旋转气流的作用下发生弯曲并绕喷嘴轴线进行螺旋回转,随后在空心锭锥段产生了“倒伏”现象,并贴服在空心锭外表面进行周期性包缠运动。基于数值模拟结果,对喷气涡流纺的喷嘴结构参数进行了设计,对成纱质量进行预测,使喷嘴中气流场与纤维运动特性更加合理,与纺纱实验的研究结果较为吻合。对苎麻纱毛羽在络筒机上减少毛羽的气流喷嘴中的运动规律进行数值模拟,与高速摄影和工艺实验结果较为一致,表明了喷嘴在络筒机上减少毛羽的机理是利用旋转气流的作用使毛羽包缠到纱体上,同时验证了本文所建立的纤维／气流耦合动力学模型具有较好的普适性。综上所述,本文采用理论与实验的方法,对柔性纤维在喷气涡流纺喷嘴高速气流场中的耦合作用特性以及运动规律进行研究,并对理论研究结果进行实际应用。论文的研究结果可对揭示基于高速旋转气流对柔性纤维体进行加捻的机理提供理论依据,为涉及纤维与气流耦合作用的纺织工艺及设备的研制提供可借鉴的理论研究方法。更多还原

【Abstract】 The application of airflow to the textile processing is gaining more and more popularity. Vortex spinning, which is the most advanced staple-yarn spinning technology nowadays, utilizes high-speed airflow generated in a nozzle to insert twist into the yarn. The nozzle is the key part for the twist insertion of vortex spinning. The fiber which possesses high flexibility and large aspect ratio interacts with the high-speed airflow field inside the nozzle. As well, the fiber makes contact with the nozzle wall during the twist insertion process. Therefore, the vortex spinning process involves very complex mechanical problems. The goal of this work is to take a thorough and comprehensive study on the dynamics and kinematics of the fiber in the airflow field inside the vortex spinning nozzle to obtain the vortex spinning principle in theory and to provide theoretical reference to the textile processes involving fiber-airflow interaction.Up till now, most of the fiber models constructed for the two-phase flow problems involved in the textile processes are multi-rigid-body fiber models, which are hard to reflect the large aspect ratio of the textile fiber. Besides, only the one-way coupling between the fiber and airflow has been taken into consideration. This thesis adopted the computational fluid dynamics and finite element methods to construct the dynamics model for the interaction between the fiber and high-speed airflow. The total Lagrangian description was used to describe the nonlinear large deformation of the fiber moving in the airflow field to reflect the flexibility of the fiber. The physical property parameters of the fiber, such as length, fineness, Young’s modulus, etc. were incorporated into the fiber model to reflect the geometrical characteristics and elasticity of the fiber. The arbitrary Lagrangian-Eulerian method combined with the moving mesh technique was adopted to solve the fiber-airflow interaction and the fiber motion. The contact between the fiber and nozzle wall was solved using constraint function method. The dynamics and kinematics problems of the flexible and elastic fiber with large aspect ratio under the action of the high-speed swirling airflow inside the vortex spinning nozzle were effectively solved and the nonlinear large deformation of the fiber was numerically simulated.Through a series of numerical simulations and analyses, the airflow characteristics and the motional and deformational characteristics of the elastic and flexible fiber inside the vortex spinning nozzle were obtained. The numerical results show that high-speed swirling airflow is generated in the thin region next to the nozzle wall and whirls downstream to form the outer region of the airflow field. Two air currents driven to enter the twisting chamber from the nozzle entrance and the yarn passage through the hollow spindle collide to generate a reverse flow and a vortex in the twisting chamber and the inner region of the airflow field is formed. The fiber shows bending deformation and helical rotation with wave shape inside the nozzle. It contacts the inner wall of the hollow spindle at different locations. The trailing end of the fiber splays out and becomes open-ended with the action of the airflow. Subsequently the splayed trailing end rotates periodically and wraps to form the yarn. The wrapping effect and yarn property are related to the motional characteristics of the fiber, including the splay degree, the number of wrapping period and the rotational amplitude. The higher number of wrapper fibers and twists along with tighter wrapping lead to higher tenacity value of the vortex yarn.The numerical results of the airflow characteristics in the nozzle show that when the nozzle pressure increases from 4×105 Pa to 6×105 Pa, the tangential components of the airflow velocity increase accordingly, but the distribution rule keeps the same. The radial components of the airflow velocity reach the maximum values when the nozzle pressure is 5×105 Pa. With the increasing of the jet orifice angle from 60°to 80°, the maximum values of the tangential components together with the radial components of the airflow velocity both increase. The influence of the twisting surface angle on the airflow characteristics is insignificant. When the distance between the nozzle inlet and the hollow spindle increases from 12 mm to 16 mm, the size of the vortex in the twisting chamber significantly increases.The numerical results of the fiber-airflow interaction show that the effect of the nozzle pressure is not significant on the tenacity of the cotton vortex yarn. However, a nozzle pressure not higher than 5×105 Pa is suggested. The increased yarn delivery speed leads to decreased cotton vortex yarn tenacity. The nozzle structure parameters for obtaining the optimized yarn tenacity are: jet orifice angle:70°, jet orifice diameter:0.4 mm, distance between the nozzle inlet and the hollow spindle:14 mm, hollow spindle cone angle:15°-20°. The cotton fiber shows the highest splay degree. The dynamic behavior of the lyocell fiber is quite similar to the viscose rayon fiber. The polyester fiber exhibits the largest rotational amplitudes among all types of fibers.The high-speed photography was used to capture the images of the fiber motion in the vortex spinning nozzle. The experimental results were compared to the numerical results. The nozzle was magnified based on the Reynolds number similarity to ensure the similarity of the airflow patterns in the nozzle model and prototype. A cotton yarn was employed instead of the single fiber in the experiment. The experimental results show that the fiber bends under the action of the airflow and rotates around the nozzle axis. Subsequently the trailing end of the fiber turns reversed onto the hollow spindle cone and rotates periodically while appressed to the outer surface of the hollow spindle to insert twist into the yarn.The dynamics model of the fiber-airflow interaction was applied to the design of vortex spinning nozzle structure and the prediction of vortex yarn tenacity. The airflow field and fiber dynamic behavior are made to be more rational and the numerical simulation results agree well with the yarn spinning experimental results. In addition, the motional characteristics of the ramie yarn hairiness in the air-jet nozzle for hairiness-reduction on the winder were investigated. The numerical simulation results of the motional characteristics of the ramie yarn hairiness in the air-jet nozzle for hairiness-reduction on the winder are in accordance with the experimental results, showing the principle for reducing yarn hairiness is to make the hairiness wrap onto the yarn body under the action of the airflow. Meanwhile, the universality of numerical model was validated.In summary, in this thesis, the interaction between the fiber and airflow, and the dynamic behaviors of the fiber in the high-speed airflow inside the nozzle of vortex spinning were investigated using theoretical and experimental methods. In addition, the theoretical results were applied to the yarn spinning process. The results of this study are dedicated to provide a theoretical basis for the twist insertion principle of the flexible fibers using high-speed swirling airflow. The methodology provided in this work can also be extended to the development of the textile processes and equipments involving fiber-airflow interaction.更多还原