Study On The Airflow Characteristics And Fiber Motion In The Transfer Channel In Rotor Spinning

The airflow plays a significant role in rotor spinning.It is used to remove impurities and transport fibers,and therefore its characteristic has a close relationship with fiber configuration,spinning efficiency and the yarn properties.Although the rotor spinning unit is the key component of the rotor spinning machine,so far on its design and application,rarely studies have been reported on the rotor spinning mechanism from the perspective of fiber motion in the airflow,which therefore leads to a lack of theoretical basis on the optimization of the spinning unit structure and process parameters.In this thesis,a numerical model is developed to study the airflow characteristic,the relationship between the airflow and fiber motion in the Air-Suction rotor spinning channel,and further to optimize the spinning unit structure and the spinning parameters.It can not only provide a theoretical basis for machine design,but also enrich the basic theory of the rotor spinning.The work of the thesis covers three parts.The first part is to develop,calculate and analyze the airflow field in the rotor spinning channel.The second part is to simulate fiber motion and verify the simulation results.The third part is to improve the transfer channel structure and conduct the spinning test.The first part of the thesis is presented on Chapters 3,4 and 5,including the development of the numerical model of the airflow in the rotor spinning channel,the analysis of the airflow characteristic,the effects of the spinning parameters and the spinning unit structure on the airflow,and the verification of the simulation results.In Chapter 3,the governing equations are firstly developed.The Realizable 6)-? is adopted due to its good adaptability for the rotational flow.The wall-function method is used to calculate the airflow in the near wall area.The discrete governing equations obtained by the second-order upwind scheme are calculated using the SIMPLE algorithm,thus obtaining the airflow field in the spinning channel.In Chapter 4,Simulation Model I(without considering the opening roller)is developed to analyze the airflow characteristics inside the rotor and the effects of the spinning structure and the spinning parameters on the airflow.In Chapter 5,Simulation Model II which has taken the opening roller into consideration is established to discuss the airflow characteristics in the transfer channel.Meanwhile,the calculated results of Simulation Model I and II are compared.By conducting the pressure measurement inside the rotor,the calculated results of the two models are respectively compared with the experimental data,which show a good consistency.Compared with Simulation Model I,the computational accuracy of Simulation Model II is slightly higher.The numerical results of the airflow characteristics in the rotor show that the air is accelerating along the transfer channel,which can draft the fibers.The airstream from the transfer channel collides with the rotor wall and then divides into two,rotating along the rotor wall in anticlockwise and clockwise direction respectively.The velocity and mass flow rate of the airstream rotating in the same direction with the rotor are larger.These two airstreams then collide with each other in the vicinity of the transfer channel exit,thus forming vortices.The confluence is slowing down on the approach to the rotor center.Together with the airstream from the yarn guiding mouth,it leads to the formation of vortices inside the rotor.The distribution of the static pressure and velocity inside the rotor is uneven,which mainly attributes to the airstream from the transfer channel.The air velocity fluctuates obviously in the vicinity of the transfer channel exit.Along the rotor groove,the lowest velocity occurs in the position against the transfer channel exit.The rotor speed,the rotor outlet pressure and the rotor diameter have a significant influence on the overall airflow characteristics in the rotor.The simulation results show that by increasing the rotor speed to 160000 rpm,decreasing rotor outlet pressure to-5000 Pa,or increasing the rotor diameter from 30 mm to 44 mm,the vortices decrease and the velocity distribution along the rotor groove turn to be more stable.The air velocity at the transfer channel inlet,the positional relation of the transfer channel and the rotor in space and the rotor slide wall angle,mainly influence the airflow characteristics near the transfer channel exit,which affects the process of fiber transportation.When increasing the air velocity at the transfer channel inlet from 12 m/s to 20 m/s,the air velocity near the transfer channel exit increases,while the fluctuation of the velocity along the rotor groove also increases.For the long transfer channel with a small angle,although it can improve the air velocity near the transfer channel exit,the vortices near the rotor wall also increase.The short transfer channel with a large angle is opposite.The air velocity near the transfer channel exit increases as the rotor slide wall angle decreases from 80° to 62°.The simulation results of the transfer channel airflow field show that the two airstreams from the open roller chambers collide with each other,leading to the formation of two symmetrical vortices at the transfer channel inlet.The vortices decrease the effective fiber transfer area,and may also deteriorate fiber configuration.The air velocity in the transfer channel center is higher than that near the wall,that is the air speed difference along the same cross section.The closer to the transfer channel exit,the smaller the air speed difference is.The rotor speed has little impact on the transfer channel airflow characteristic,while the rotor outlet pressure,the opening roller speed and the transfer channel structure have a great influence on it.When the rotor outlet pressure increases to-9000 Pa,the vortices amplify,but meanwhile the air velocity at the transfer channel outlet increases.With the decreasing of the opening roller speed from 8400 rpm to 4800 rpm,the air velocity at the transfer channel inlet decreases,while its distribution is more uniform.Increasing the area of the transfer channel inlet from 132 mm2 to 176 mm2,the vortices increase obviously and the air velocity at the transfer channel inlet decreases.Increasing the transfer channel length from 37.5 mm to 50 mm is conducive to eliminate the vortices.The second part of the thesis is conducted based on the first part.It includes the following two aspects:(1)To present the fiber motion in the rotor spinning channel visually,the dynamic equations are established by analyzing the forces applied on the fiber by the airflow.Based on the simulation results in Chapter 5,the fiber trajectories can be obtained.The simulation results show that straight fibers can keep their configuration along the transfer channel.With the aid of the accelerating air,the hooked fibers can be straightened gradually.When the hook degree increases from 9.1 % to 27.3 %,it takes more time to straighten the hook,and the straightening process becomes more complicated.However,for the same hook degree,the trailing hook fibers need less time to finish the straightening.The fibers tend to move along the transfer channel center rather than near the wall.The area of the transfer channel inlet,the rotor speed and the rotor outlet pressure influence the movement of the leading hooked fibers with the hook degree of 18.2 % along the transfer channel.As the area of the transfer channel inlet decreases from 176 mm2 to 132 mm2,the time consumed by straightening the hook is reduced by 38.1 %,and the complete time is decreased by 7.41 %.The larger the rotor speed,the shorter the time.It is noted that when the rotor outlet pressure is-5000 Pa,until the fiber enters the rotor,the hook has not been eliminated.As the rotor outlet pressure increases to-9000 Pa,the hook is gradually straightened.The total fiber moving time is also shorter,which reduces the possibility of the fiber collision with the wall,thus improving fiber configuration.(2)To verify the simulation results,the spinning tests are conducted by designing the rotor speed,the rotor outlet pressure and the opening roller speed.The simulation results agree well with the experimental data.The third part of the thesis is to improve the transfer channel structure to eliminate the vortices at the transfer channel inlet.A air-supplement channel,located on the long side of the transfer channel and having its flow direction tangent to the transfer channel wall is created.The simulation results show that the external air from the air-supplement channel can eliminate the vortices.Moreover,it also increases the air velocity in the fiber separation area,increasing the peeling ratio,which is helpful for fiber removal from the opening roller pins and the fiber straightening.By conducting the spinning test and the fiber straightness test,the effect of the geometric modification of the transfer channel is analyzed.The spinning test results show that modifying the transfer channel improves the yarn tenacities significantly,while the yarn hairiness and the yarn evenness and imperfections vary with the yarn linear density.The results of the fiber straightness test reveal that the increase of the yarn tenacity mainly attributes to the decrease in the number of fibers with leading and trailing hooks.The proportions of the leading and trailing hooks decrease by 48.37 % and 41.96 % respectively,and the straight fiber increases by 25.55 %.This indicates that the airflow inside the modified transfer channel is helpful for the fiber straightness and hence the yarn properties.In summary,in this thesis,the numerical model is developed to study the airflow characteristics in the rotor spinning channel.The airflow characteristics and the fiber motion in the airflow are investigated using the numerical and experimental methods.The spinning parameters are then optimized and the transfer channel structure is modified according to the simulation results.The study of the thesis provides a theoretical guidance for rotor spinning unit design and applications.