Study On Compat Spinning Principle And Yarn Structure In Airflow

It has been one of the most significant directions in textile field by utilizing the airflow technique to improve the speed of textile processing and decrease the cost of production.For instance,the wide application of air-jet spinning and air-jet weaving bring a qualitative leap for development of textile industry and progress of textile technology.However,scholars and researchers have lack systematic studies on the specific application of airflow in textile industry,because of the complexity of airflow and the particularity of fiber structure.So there are greater limitations lead to the airflow in actual application process can not play the airflow should have the advantage,and unconsciously lead to waste of energy consumption and increase of production costs.Compact spinning uses the airflow to compact the strand of fibers which has been drafted,making the final yarn structure and yarn properties greatly optimized.With more than 10 years of development,our country already has certain of industrialization basic.According to statistics of China Cotton Textile Association,the domestic compact spinning equipments have more than 7 million spindles by 2010.But for domestic textile machinery enterprises,most products come from technology transfer,mapping and imitation,a few products come from the introduction of technology and secondary development of digestion and absorption,and a serious lack of products with independent intellectual property rights.The reason is essentially due to the lack of basic theoretical research,has not been able to establish a complete theoretical system of compact spinning:study the characteristics of flow field in the compact zone,and quantitative expound the compact effect of fibers,to provide the theory basis for optimizing the compact flow field;study the kinematics and dynamics models of single fiber and strand in the compact zone,and quantitative expound the compact mechanical process of fibers,the relationship between different fibers and different parts,to provide the theory basis for optimizing the institutions;study the influence of strand structure on yarn forming mechanism in twisting triangle,and quantitative expound the twisting mechanics difference of fiber movement between compact spinning and ring spinning,to provide theory basis for optimizing yarn structure and properties.In response to above problems,the thesis makes a system study of compact spinning mechanism from the mechanical point of view,and mainly breakthrough from the following two aspects:firstly,use the computational fluid dynamics method to take a numerical simulation of compact fluid field,study the relationship between compact effect and fluid field of different shape suction slots(straight,oblique and deformed),and analyze the trajectories of fiber or strand in compact zone by establishing single fiber model to provide theory basis for improving and optimizing the compact mechanism;secondly,study yarn forming mechanism and yarn structure in twisting process by establishing the fiber model,explain the yarn characteristics of less hairiness and higher strength from the mechanical perspective,and provide the theoretical support for expansion application of compact spinning technique and product development.The primary contents and conclusions are as follows:(1)Through establishing the computational fluid dynamics model of compact spinning with lattice apron,analyze the flow characteristics of compact zone with deformed slot under specific negative pressure,then compare the influences of flow characteristics under different negative pressures.The results show that:under specific negative pressure,the distribution of airflow static pressure in compact zone presents low at the center and high at both ends;the airflow velocity distribution in compact direction and in vertical direction of fiber strand presents high at the center and low at both ends,only the airflow velocity in the transport direction of fiber strand agrees with the airflow static pressure;with the increasing of negative pressure,the airflow velocity in compact direction of fiber strand increases,which is benefit to the compact effect,but it also brings negative impact on the compact effect with the over large of airflow velocity both in transport direction and in vertical direction.(2)Through establishing the computational fluid dynamics model of compact spinning with lattice apron,compare and analyze the flow characteristics of compact zone with different slots(straight,oblique and deformed).The results show that:the static pressure contours of three slots present a similar distribution of decreasing from the center to two ends;compared with the straight slot,the airflow static pressure of oblique slot is symmetrical about the slot center line,then the maximum,minimum values and decline law are almost the same,except for tilted 6°;compared with the oblique slot,the airflow static pressure of deformed slot isn't absolutely symmetry because of its asymmetric structure,but the distribution law is similar.Considering the compact effect,it mainly depends on Vy in the compact direction of fibers:along Y direction,the airflow velocity increases from 0 at the center line L to the maximum value at two boundary lines S1 and S2,then decreases;along X direction,like the static pressure,the airflow velocity distribution also presents big at the center and small at both ends.Compare and analyze Vy distribution of straight and oblique slots,which characteristics are the same except for tilted 6°,that is to say they are not significantly different for the function of airflow force,but considering the coupling effect between airflow and fiber and the fiber trajectory in the compact zone,the existence of angle may have an influence on the compact effect;compare and analyze Vv distribution of oblique and deformed slots,which presents generally similar,however the deformed slot doesn't have a strictly center line,which makes a difference of velocity on S1 and S2,so the strand will transport along the zero velocity contour near S1,and the deformed slot may get a better compact effect by the turning of fiber strand than straight and oblique slots.Considering the condensing effect,it mainly depends on Vz in the direction vertical to the fiber transport:along Y direction,the velocity of airflow outside of S1 and S2 is close to zero,and increases to the maximum value at the center line L;along X direction,like the static pressure and Vv,Vz distribution also presents big at the center and small at both ends.Compare and analyze Vz distribution of straight and oblique slots,which difference basically agrees with Vy that is to say they are also not significantly different for the function of airflow force,but considering the coupling effect between airflow and fiber and the fiber trajectory in compact zone,the existence of angle may have an influence on the condensing effect;compare and analyze Vz distribution of oblique and deformed slots,which also have no obvious difference except the increasing of airflow velocity of deformed slot as a whole,so considering the condensing effect,the function of airflow force is to make the strand close enough to the lattice apron while transporting,and maintain it until reach the nip of delivery rollers,which means it is easier for the deformed slot to obtain a more stable condensing effect than the oblique slot.(3)Based on the numerical simulation of compact fluid,analyze the fiber trajectory and compact mechanism in compact zone by choosing rigid fiber model and establishing dynamics equation.The results show that:using the straight slot,the fiber strand finally will transport at the center of slot;using the oblique slot,the strand finally will transport along S1,which is the same as using the deformed slot.Considering the two-way coupling effect of air and fiber,there is a rotating fluid field around the fiber strand,which makes the single fiber could be adsorbed or wrapped,and finally takes an affect on the yarn properties,such as twist or hairiness;considering elastic modulus and end length of fiber,the less of elastic modulus and longer of end length,the more possible fiber will be adsorbed and wrapped,which also explains why the compact yarn has less long hairiness.(4)The fiber movement in twisting triangle could be divided into three phases:the head of fiber is freedom while the end is held,both ends of fiber are held,the head is held and the end is freedom.By establishing the mechanical model of fiber in twisting triangle,the structure and properties of compact yarn are studied.The results show that:as the compact spinning greatly decreases the twisting triangle,the duration of first phase which produce head hairiness and the third phase which produce end hairiness are reduced,while the duration of the second phase is extended.That is to say the duration of fiber freedom is reduced,which decreases the probability of hairiness;the duration of fiber under control is increased,which helps improving the utilization rate of fiber in the final yarn.Meanwhile establish the mechanical model of yarn strength and fiber elongation,and introduce the pre-elongation coefficient ?(0??? 1)to characterize the transfer degree of fiber.Through studying the law of fiber elongation under different ?,it's found that the final strength of compact yam is higher than what of ring yarn because of the lower transfer degree.(5)Design five groups of experiments,then compare and analyze yarn properties of different counts under different negative pressures and the influence of different slots.The results show that:compared with traditional ring spinning yam,the number of short hairiness of compact yarn is reduced 38%to 62%,especially for the harmful hairiness longer than 3mm with a reducing of 65%to 80%;the tensile properties of compact yarn has also improved with a increasing of 20%;for different shape slots,the advantage of deformed slot is the most significant,the harmful hairiness longer than 3mm is reduced about 90%,and the breaking strength and breaking tenacity increase 12%to 25%;the oblique slot comes second,and the straight slot is the last.