Study On Compact Mechanism Of Pneumatic Compact Spinning With Lattice Apron

Compact spinning is an unconventional spinning technology developed from the conventional ring spinning in the recent decade. Not only has the compact yarn kept the existing advantages of the ring spun yarn, but has also greatly improved the yarn quality and grade, especially in yarn hairiness and tenacity. Therefore, this spinning technology has been spread rapidly all over the world. The early studies on compact spinning were basically focused on the comparison of the yarn structure, measurement of the yarn properties, optimization of the spinning process, and the migration of the fibers, etc, while the studies on the compact mechanism were relatively scarce. However, further theoretical elaboration on the compact mechanism of the fiber aggregation in the condensing zone based on constructing kinematic and dynamic models and analyzing the compacting process in the condensing zone in depth is not only of both theoretical and practical values but can also provide a basis for the design and optimization of the compact devices.The main work of this thesis, which was based on the pneumatic compact spinning system with lattice apron, covered the following five main aspects:(1) Study on the additional twist inserted into the fiber strand in the condensing zone with linear suction slotFirstly, based on the production practice and the earlier stage's research of this research group, the definition of the additional twist in the condensing zone was given. Afterwards, the images of the fiber strand moving in the condensing zone were captured using high-speed photography, which revealed that the fiber strand in the condensing zone with linear suction slot showed a wrap twisting characteristic. Next, spinning experiments were carried out to prove that twists had definitely been inserted into the fiber strand and these twists were real twists. Based on these results, a completely new'semi-open-end twisting'model was proposed to explain the mechanism of the additional twist formation. Then the additional twist model for the fiber strand in the condensing zone was constructed through analyzing the kinematics of the fiber strand in the condensing zone with linear suction slot. Finally, spinning experiments were performed to verify the validity of the model.(2) Study on the twist propagation and twist propagation length models for the fiber strand in the condensing zone with linear suction slotAt first, the twist propagation for the linear suction slot was investigated and the method of characterizing the condensing effect using the propagation length of the additional twist was proposed. Next, the differential equation group model for the propagation length of the additional twist was constructed based on mechanical analysis. Subsequently, the effects of the condensed fiber strand radius, friction coefficient between the fiber strand and lattice apron, curvature radius of the profile tube in the condensing zone, inclined angle of the suction slot, and negative pressure on the propagation length of the additional twist were numerically calculated via Matlab programming. Finally, the basis for the theoretical design of the suction slot length was investigated.(3) Theoretical design and practice of the arc suction slotFirst, the problems of the compact spinning with linear suction slot existed were analyzed. Next, a completely new suction slot shape was designed and the arc suction slot was used instead of the linear suction slot to overcome the sudden variation of the yarn velocity direction existed in the nip of delivery roller. The differential equation group model for the additional twist was constructed for the arc suction slot based on mechanical analysis. Subsequently, the model was numerically solved and analyzed using the 4th order Runge-Kutta method and the effects of condensed fiber strand radius, friction coefficient between the fiber strand and lattice apron, curvature radius of the profile tube in the condensing zone, and curvature radius of the arc suction slot on the additional twist were discussed. Finally, the arc suction slot was designed and manufactured to conduct experimental validation.(4) Construction and analysis of the airflow field distribution model in the condensing zoneThe computational fluid dynamics model of the airflow field distribution in the condensing zone was constructed. The computational fluid dynamics software FLUENT was used to solve the airflow field distribution model and the airflow characteristics in the condensing zone were obtained. The influence of the airflow characteristics in the condensing zone on its action on the condensed fiber strand was investigated.(5) Analysis on the twist insertion degree of the compact yarn and determination of the twist factorConsidering the distinct difference between the diameters of the compact yarn and ring yarn with the same linear density, the differences of their twist angles, twist levels and their influence on the twist insertion into the yarns were discussed. Based on these results, the selection of the twist factor of the compact yarns was suggested.Through the model construction, solution, experiments and analysis, the following conclusions can be drawn:(1) Study on the additional twist inserted into the fiber strand in the condensing zone with linear suction slotThrough the high-speed photography and spinning experiment, the existence of the additional twist of the fiber strand in the condensing zone during the compact spinning process was approved. A completely new semi-open-end spinning model which could effectively explain the mechanism of the additional twist formation was proposed.Through kinematic analysis on the fiber strand in the condensing zone, the mathematical model of the additional twist in the condition of linear suction slot was obtained:t=sinβ/ That is to say, the additional twist was related to the radius of the condensed fiber strand and the inclined angle of the suction slot. Through spinning yarns with different linear density and twist direction, the validity of the model was verified and the reason why the suction slot was inclined toward top right was revealed.Furthermore, the results of the research indicated that a certain degree of the inclination of the linear suction slot was one of the necessary conditions for inserting additional twists into the fiber strand in the condensing zone.(2) Study on the twist propagation and twist propagation length models for the fiber strand in the condensing zone with linear suction slotThe concept of the propagation of the additional twist of the fiber strand in the condensing zone was proposed and the condensing effect was characterized by the propagation length of the additional twist of the fiber strand in the condensing zone. The mechanical analysis method was adopted to construct the model of the propagation length of the additional twist of the condensed fiber strand and analytical solution to the model was: (?).The solution indicated that the propagation length was related to the condensed fiber strand radius r, friction coefficient between the fiber strand and lattice apronμ, curvature radius of the profile tube in the condensing zone R, inclined angle of the suction slotβ, and negative pressure p.Based on these results, the relationship between the condensed fiber strand radius r, friction coefficient between the fiber strand and lattice apronμ, curvature radius of the profile tube in the condensing zone R1, inclined angle of the suction slotβ, and negative pressure p on the propagation length of the additional twist were calculated and discussed via Matlab programming. The numerical results showed that the influence of the curvature radius of the profile tube in the condensing zone R was not significant on the propagation length of the additional twist of the fiber strand in the condensing zone, while the condensed fiber strand radius r, friction coefficient between the fiber strand and lattice apronμ, inclined angle of the suction slotβ, and negative pressure p had significant influences on the propagation length of the additional twist. Thereinto, interactional relationship existed between the condensed fiber strand radius r and negative pressure p. This needed further investigation.The propagation lengths of the additional twist of the cotton compact yarns of 15.3 tex and 9.7 tex were calculated via Matlab programming and the result was around 10～20 mm. The validity of the model of the propagation length of the additional twist of the condensed fiber strand in the condensing zone was verified by experiment. The research provided a theoretical basis for the design of the suction slot length.(3) Theoretical design and practice of the arc suction slotAccording to the problem of the sudden variation of the yarn velocity direction existed in the nip of delivery roller in the compact spinning with linear suction slot, the idea of designing an arc suction slot was proposed. Through mechanical analysis, the differential equation group model for the additional twist of the condensed fiber strand was constructed for the arc suction slot, which indicated the additional twist was related to the condensed fiber strand radius r, friction coefficient between the fiber strand and lattice apronμ, negative pressure p, curvature radius of the profile tube in the condensing zone R, curvature radius of the arc suction slot R1 and the cross-section shape of the profile tube, etc.Based on the numerical solution of the model, the effects of the condensed fiber strand radius, friction coefficient between the fiber strand and lattice apron, negative pressure, curvature radius of the profile tube in the condensing zone, curvature radius of the arc suction slot on the additional twist were discussed. The numerical results indicated that the effect of the curvature radius of the profile tube in the condensing zone was insignificant while effects of the other factors were significant.The arc suction slot was designed and manufactured and spinning experiment was conducted to verify the validity of the additional twist model, indicating the quality of the compact yarn spun using the arc suction slot was better and could be definitely used in the production practice.(4) Construction and analysis of the airflow field distribution model in the condensing zoneIn the condensing zone of compact spinning, the airflow converged toward the suction slot from outside. On the left and right sides of the suction slot, the air flowed toward the suction slot perpendicularly to its edges. Above the profile tube, the airflow converged toward the suction slot and its velocity increased when approaching the suction slot. The airflow characteristics made the cross-section of the fiber strand contract from a flat shape to a round shape to realize the condensing effect. In most regions of the condensing zone, the static pressure value of the airflow was closed to the atmospheric pressure. In the vicinity of the suction slot, the static pressure of the airflow decreased rapidly to generate a negative pressure zone. The static pressure of the airflow became lower when getting closer to the center and interior of the suction slot.The results of the numerical simulation showed that when the inclined angle of the linear suction slot varies in a small range (0°～5.6°), the condensing forces exerted on the left and right sides of the fiber strand were similar, which made the condensed fiber strand to move nearly along the central line of the suction slot. With the further increase of the inclined angle of the suction slot, the fiber strand tended to approach to the right side. When the two ends of the fiber strand were both gripped, this tendency would make the condensed fiber strand rotate around its own axis.The pressure exerted on the fiber strand by the airflow in the condensing zone fluctuated along the length direction of the suction slot. However, the range of the fluctuation was small. This indicated the hypothesis that the pressure exerted on the fiber strand by the airflow was approximately constant in the theoretical models of the additional twist constructed in Chapter 4 and Chapter 5 was rational. The effects of the inclined angle of the linear suction slot and suction slot shape on the pressure exerted on the fiber strand by the airflow were not significant while the increase of the suction slot width and negative pressure led to increased pressure exerted by the airflow.(5) Analysis on the twist insertion degree of the compact yarn and determination of the twist factorThe structures of the compact yarn and ring yarn along with the characterizations of their twist insertion degrees are different. The twist factor of ring yarn is usually not suitable for the compact yarn.The packing of the fibers in the compact yarn is compacter. Compared with the ring yarn with the same linear density, the volumetric density of the compact yarn is increased by 15～30%. Therefore, relationship of the twist angle and twist level to the yarn diameter should be considered comprehensively when evaluating the degree of twist insertion of compact yarn.The selection of the twist factor of the compact yarn can be determined according to the yarn tensile strength. It generally can be decreased by around 10～15% than the twist factor of the ring yarn with the same linear density.For the pneumatic compact spinning technology with lattice apron, the couplings between the fiber aggregation and the lattice apron, the mechanical system, and the negative pressure airflow field are very complex. This thesis can promote the study on the compact mechanism in the forementioned five aspects of research works and can provide guidance to the further optimization of the compact devices, improvement of the compact effects and compact yarn quality, promotion of the development of the compact spinning technology.