单螺杆挤出过程中界面拉伸脉冲效应

PULSE EFFECT OF INTERFACE STRETCH IN SINGLE SCREW EXTRUSION

基于速度涡量法,推导出单螺杆挤出机内三维流动无因次控制方程,提出一种求解非牛顿流的新方法。采用交错网格对挤出机内的幂律流体流动进行有限体积方法数值模拟,得到了螺槽内的三维速度分布。借助4阶Runge-Kutta方法对流体质点运动轨迹进行数值积分追踪,实现了螺槽内的层流混合过程界面形变的捕捉。发现界面演化过程中螺槽下部不同初始位置及取向的示踪剂界面拉伸总体表现出线性增长规律,但伴随着周期性脉冲效应,越靠近螺槽底部脉冲的幅度越大。当界面取向沿螺槽宽度方向时,表现为等幅正脉冲,周期与流体沿螺槽前进一个螺距周期相同。当界面取向沿螺槽高度方向时,脉冲幅度出现了逐级放大现象。研究结果表明,稳态挤出的混合过程存在着非稳态效应。

The dimensionless control equations of three dimensional flow in single screw extruder is developed in terms of velocity and vertorcity, and then a new method for solving the Non-newtionian flow based on the finite volume method is presented. A staggered grid arrangement is used in which the dependent variables are located at different mesh points in the computational domain. The simulation concerns the incompressible fluid obeying power law properties and the application of finite volume method to the geometrical configuration of channel with aspect ratio equal up to 13. The deformation of the tracer is numerically integrated by the fourth order Runge-Kutta scheme. The evolutions of tracers are captured when time is forward. The interface stretchs of the tracers from different initial positions with different orientations reveal that growth of interfaces fall on the whole onto the linear relations accompanied by periodie pulse effect. The closer the tracer approachs the channel walls, the more obvious the pulse effect is. The positive pulse with the same amplitude appears when the tracer orientation is along the direction of the channel width, which has the same periods as that the tracer move forward one pitch. On the contrary, the pulse amplitude become larger and larger with time when the orientation is along the direction of the channel height. The result shows that non-steady effect occurs in the mixing process during the course of the steady extrusion.