Study On The Air Flow Field And Polymer Drawing Model In The Air Assisted Centrifugal Spinning

The centrifugal force produced by the high speed rotating device is used to fabricate fibers from polymer melts or solution.Recently,along with the emergence of nanofibers and development of science and technology,researchers have found the advantages of the centrifugal spinning in fabricating nanofibers.Compared with the electrospinning commonly used to fabricate nanofibers,the centrifugal spinning has a higher output and is more environment-friendly.Both polymer solution and polymer melt can be processed in the centrifugal spinning.In this dissertation,the assisted air is introduced into the centrifugal spinning to add the air force to the inertia force so as to fabricate finer fibers.The research purpose of this dissertation is to establish the polymer drawing model in the air assisted centrifugal spinning to realize the prediction of fiber diameters.Numerical simulations and experimental tests are employed to study the air flow field and polymer drawing in the air assisted centrifugal spinning.Firstly,the air flow field model is established and simulated numerically.The air velocities are measured with the Hot Wire Anemometer and the simulation results are verified.Then the polymer drawing model is established and simulated numerically and the simulation results are verified.”In this dissertation,the following research work is carried out on the air flow field and polymer drawing in the air assisted centrifugal spinning.(1)The air flow field model of the circumferential arranged nozzle group for the air assisted centrifugal spinning is established and simulated.The air flow field model of the circumferential arranged nozzle group for the air assisted centrifugal spinning is established.The air flow field model consists of the mass conservation equation,momentum conservation equation,turbulence model and boundary conditions.The Reynolds number of the air flow is calculated based on the working conditions of the nozzle group.The standard k-? two-equation is chosen.The Gambit and Fluent software are utilized to simulate the air flow field under different rotating speeds and initial air velocities.The simulation results show that the shape of the air velocity decay curve is not affected by the rotating speed greatly.Increasing air velocity at the nozzle could be achieved by adding air jet to the centrifugal spinning.The air flow field in the whole spinning area is the superposition of the centrifugal rotation and the air jet flow.The centrifugal rotation itself forms an air flow ring near the nozzle.A parabolic air tail forms by the air jet due to the inertia effect.The simulation results of the air streamlines show that the air flows back to the centrifugal rotation center at a certain distance from the nozzle.(2)The air flow field model is verified by measuring the air velocity of the air flow field of the circumferential arranged nozzle group.An IFA300 Hot Wire Anemometer is employed to measure the air velocity of the air flow field of the circumferential arranged nozzle group.Air velocities at some positions along the nozzle axis are measured under different rotating speeds and initial air velocities.The nondimensionalized measured air velocities are compared with the simulated air velocities.The results show that they have the same trend: air velocity near the nozzle is slightly higher than the initial air velocity.The air velocity decreases with the increase of the distance from the nozzle when the rotating speed is higher than 880 r/min,especially in the early stage.The reason could be the energy of the air jet is consumed quickly by the air flow ring around the nozzle.There is a small peak at a short distance from the nozzle,then the air velocity decreases step by step.The position of this small peak is the forming position of the air flow ring.The measured results prove the effectiveness of the established air flow field model.(3)The polymer drawing model of the centrifugal spinning is established and simulated and verified preliminarily.The drawing process of the polymer melt could be regarded as the elongation flow along the threadline.Without considering the influence of the air flow field on the polymer drawing,the polymer drawing model of the centrifugal spinning is established after analyzing the force balance and heat transfer of the particle of polymer melts.The Matlab software is used to simulate the polymer drawing process.The excircle radius,streamline shape,diameter and temperature of polymer melts are obtained.Streamlines of polymer melts under different conditions are analyzed.Simulated fiber diameters under different conditions are compared with measured fiber diameters cited from the reference.The results show that streamline of polymer melts returns to the centrifugal center at a certain distance from the nozzle.The streamline length increases within a return cycle with the decrease of the viscous force.The minimum diameter of the polymer threadline appears at the farthest position from the rotation center.The simulated fiber diameters have the same trend as the measured fiber diameters and good agreement with the latter,which confirms that the polymer drawing model is effective.(4)The polymer drawing model of the air assisted centrifugal spinning is established and simulated.The polymer drawing model of the air assisted centrifugal spinning is established via introducing the air force.The air force is determined by the air drag formula.The air velocities are derived from the simulation result of the air flow field.The regression equations of the air velocity to the corresponding particle coordinates are obtained.Air velocities in the air drag formula come from the simulation results of the air flow field.The Matlab software is utilized to compose programs to calculate diameters of the polymer melt and streamlines of the polymer particle.The simulation results show that the air velocity along the streamline drops to around 25 m/s finally despite changes of the rotating speed and initial air velocity.It's found from the air velocity decay curve that the curve shape is not affected by the rotating speed,and the larger the initial air velocity,the faster the air velocity decays.Simulation results of the streamlines and diameters of the polymer melt show that the assisted air influences the diameter of the polymer melt and radius vector of the polymer particle.Keeping the rotating speed constant,the radius vector of the polymer particle increases and the diameter of the polymer melt decreases with the increase of the initial air velocity.Under the same initial air velocity,the diameter of the polymer melt decreases when the rotating speed increases.Influence of the rotating speed on the radius vector of the polymer particle is not as great as that of the initial air velocity.(5)Fibers are fabricated with the centrifugal spinning device.The polymer drawing models are verified by measuring the fiber diameters.A simple centrifugal spinning device is utilized to fabricate fibers from polypropylene chips under different rotating speeds and initial polymer temperatures.The polymer drawing models are used to simulate the fiber diameters at different conditions.Fiber photos are taken with the scanning electron microscope.Fiber diameters are measured with the image analysis software.The measured fiber diameters are compared with the simulated fiber diameters.The results show that the simulated fiber diameters have the same trend as the measured fiber diameters and good agreement with the latter,which confirms the effectiveness of the polymer drawing models.Fiber diameters can be reduced with the introduction of the assisted air.It is found that both the rotating speed and initial polymer temperature influence the spinning process.The fiber diameter reduces when the rotational speed increases or the initial polymer temperature increases.Qualities of the fiber forming and fiber web become worse when the rotating speed is lower than 900 r/min or the initial polymer temperature is lower than 220?C.In summary,the air flow field model of the circumferential arranged nozzle group for the air assisted centrifugal spinning is established,simulated and verified.The polymer drawing models of the centrifugal spinning and air assisted centrifugal spinning are established and simulated.Fibers are fabricated with the centrifugal spinning device.Fiber diameters are measured with the image analysis software.The simulation results of the polymer drawing models are verified with experiments.The established polymer drawing model of the air assisted centrifugal spinning in this dissertation is helpful to predict the fiber diameter of the centrifugal spinning and favors the optimization and improvement of the equipment and technology of the centrifugal spinning.This dissertation contributes to build the theoretical basis for this new fabrication method of superfine fibers.