Study On The Characteristics And Application Of The Fiber-Airflow Interaction In Vortex Spinning

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.