| Two spinning processes of Melt blowing and Electrospinning are studied in this dissertation. Based on the modeling and experimental results, the airflow field in melt blowing and electrostatic field in electrospinning are obtained; A unified model which can be used to predict the fiber draw attenuation is established through the analysis of fiber mechanics and thermodynamics in a certain physical field; the geometry optimization of melt blowing dual slot dies are also discussed in this thesis. The following four main parts were included in this dissertation.(1) The air flow field of melt blowing slot die is simulated by three dimensions finite volume method, and the efficiency of the numerical simulation is verified by experiments.(2) Based on the three dimensional model of airflow field in melt blowing slot die, a systematic approach, which combines the application of numerical simulation and Orthogonal Array method or Genetic Algorithm, to optimize the airflow field of melt blowing slot die is proposed. The optimized geometry of the slot die under certain processing condition is obtained.(3) The whipping instability when fibers attenuating in melt blowing process is researched, a numerical approach to model fiber motion during melt blowing process is established. This approach also can predict fiber diameter, temperature, inner stress and so on.(4) Melt blowing and Electrospinning are analogous in the processes of drawing fibers:the polymer jets are both drawn in the external fields. A three dimensional model of whipping motion in the processing of microfibers is established. This model can simulate the fiber motion in air flow field or electrostatic field. After analyzing the fiber motion, several factors that restrict melt blowing further fiber attenuation were concluded.This dissertation included 7 chapters. In Chapter 1, the references relevant to this research field at home and abroad are reviewed; they are mainly focused on the research of airflow field in melt blowing and the fiber drawing model in melt blowing and electrospinning process.In Chapter 2, a three dimensional model of airflow field in melt blowing slot die is established using computational fluid dynamics approach. This model consists of the continuity equation, momentum equations, energy equation and constitutive equation. The k-εmodel is adopted as the turbulence model. The governing equations are discretized by the finite volume method. The first order upwind scheme is chosen as the interpolation method for discretized governing equations. The geometry of melt blowing slot die is the same with the experimental machine described in Chapter 3. The simulation results are obtained after the proper boundary conditions are set up. The results show that the development of the airflow field downstream exhibits three major zones depending on two points, namely merging point and combined point. First, closest to the orifice, is the converging zone, where the jets are still flow separately. The dominant characteristic of this zone is the presence of a recirculation area where flow is traveling in the opposite direction from the main direction of the jets. The merging zone is next; this is a transition between the converging zone and the fully developed region. The dominant characteristics of the merging zone are the lack of a recirculation area and the presence of peak velocities away from the centerline. The final region is the well-developed region, where the velocity maximum is along the centerline, and the velocity is decaying.In Chapter 3, with Hot Wire Anemometer, the distributions of the air velocity and the air temperature of the airflow field of melt blowing slot die are measured. It is efficient for the three dimensional model of airflow field in melt blowing slot die as compared the experimental results with computation results which obtained in Chapter 2. Therefore, the 3D model of airflow field can be used to model fiber motion and die geometry optimization.In Chapter 4, a systematic approach, which combines the application of numerical simulation and Orthogonal Array method or Genetic Algorithm, to optimize the airflow field of melt blowing slot die was proposed. Firstly, the orthogonal array method and CFD technique are integrated to find out the geometry parameters of the slot die which give the optimal air flow field. A parameter, stagnation temperature, which combines the air velocity and air temperature, is proposed to evaluate the air flow field of the melt blowing die. By choosing the slot width e, slot angle a and nose piece width f as the critical parameters, a three-level orthogonal array analysis was performed. The simulation results reveal that the slot width and the slot angle are important factors, while the influence of the nose piece width on the air flow is insignificant. The stagnation temperature increases with increasing slot width and decreasing slot angle.Secondly, Genetic Algorithm (GA) combined the application of numerical simulation is used to optimize the airflow field of melt blowing slot die. The stagnation temperature, is stilled used as the objective function. The slot width e, slot angle a, nose piece width f and setback S are investigated by using GA method. During the GA optimizing process, the coefficient of variation is used as the terminal condition from the time-saving view. It has proved that the systematic approach combining the application of numerical simulation and genetic algorithm is an effective way to optimize the geometry parameters of the melt blowing slot die. The results also show that the smaller slot angle and larger slot width result in the higher stagnation temperature.Finally, multi-objective optimization using genetic algorithms (MOGA) combined the application of numerical simulation is proposed to optimize the airflow field of melt blowing slot die. The geometry parameters researched and terminal condition is the same with before. The optimal results are achieved in the 50th generation with 20 individuals of each generation. The final optimal geometry parameters are:slot width e=1.998 mm, slot angle a= 12.28°, nose piece width f= 1.9384 mm and setback S= 1.393 mm.In Chapter 5, a three dimensional model of fiber motion during melt blowing process was established. The dynamics and heat transfer on the spinning line during the drawing process are discussed in detail. The Maxwel model is adopted as the constitutive equation to describe the rheological properties of polymer melt; this fiber model also considers the changing of the density and specific heat capacity of the polymer melt with the polymer temperature. it describes the character of large aspect ratio, viscoelasticity and flexibility of the fiber. Therefore, it can be used to simulate the fiber formation in melt blowing process. Mathematical model is developed using mixed Euler-Lagrange approach, which treats the air flow by the Euler approach and predicts the fiber motion by the Lagrange approach.The proposed approach is applied to simulate the fiber motion in melt blowing process. The three-dimensional paths of fiber motion are calculated. The fiber path shows a small perturbation developing into the whipping. The results of predicted fiber diameter, fiber temperature, fiber stress, fiber velocity, and fiber whipping amplitude are compared with Shanbaugh and coworkers'simulation and experimental results. The mathematical model provides a reasonable representation of the experimental data.In Chapter 6, a comprehensive unified model is developed for melt blowing process and electrospinning process. This model involves the simulation of conservation law of mass and charge as well as momentum balances. All kinds of forces imparted on the fiber element are summarized into three kinds of forces, namely external force, internal force and bending restoring force.This model can predict the fiber diameter, fiber vibration amplitude and fiber trajectory. This model also gives a method to compare the whipping dynamic of these two spinning processes. The results show that although the aerodynamic force is one or two orders larger than the electric force on each fiber element, the drawing ratio in melt blowing process is less than that in electrospinning, and fiber whipping in the melt blowing process is not as significant as in electrospinning. The main reason is that the Coulomb force in electrospinning always has the function to sustain and increase the bending instability; while in the melt blowing process, whether the aerodynamic force increases the bending instability or not depends on its value and direction at relevant fiber element.Conclusions and outlooks were presented in Chapter 7. Main research findings and insufficiencies of this dissertation as well as further research points involved in this field were described one by one.
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