平行三螺杆挤出机聚合物混沌混合的拉格朗日拟序结构分析

从拉格朗日拟序结构的新视角,对三螺杆挤出机二维流场的动力学特性进行有限元分析。采用4阶Runge-Kutta法计算粒子的运动轨迹,验证了时间步长对计算结果精确度的影响。利用有限时间李雅普诺夫指数(FTLE),Poincaré截面和粒子可视化技术分析了三螺杆挤出机流场的混沌混合机理。结果表明,拉伸和折叠作用促进了流体粒子的物质交换。由于在啮合区存在双曲不动点,混合效率较高,啮合区是流场混沌混合的主要区域;在紧靠螺杆壁区域,物质线的闭合导致流体输运受阻,近螺杆区混合效果较差。通过与双螺杆挤出机进行拉格朗日拟序结构(LCS)结构的比较发现,由于啮合区的增加,强化了挤出机流场的混沌混合;通过对数拉伸值的比较,三螺杆挤出机的混合能力优于双螺杆挤出机。

Lagrangian Coherent Structures Analysis of Chaotic Mixing in Parallel Arranged Tri-screw Extruder

From a new viewpoint of Lagrangian coherent structure,the finite element analysis of dynamic characteristics was used to analyze the two-dimensional flow field of a tri-screw extruder.The trajectory of particles was calculated by using the fourth order Runge-Kutta scheme to validate the time step of the simulation method.Finite-time Lyapunov exponent(FTLE),Poincarésection and particle visualization technology were adopted to analyze the flow chaotic mixing mechanism in the tri-screw extruder.The results show that stretching and folding actions promote the material exchange of fluid particles.In nip region,which is the main area of chaotic mixing,the mixing efficiency is high due to the existence of hyperbolic fixed point.In the near wall region,the mixing effect is poor due to the fluid transposition is hampered by the closed material line.Compared with the Lagrangian coherent structure(LCS)of twin screw extruder flow field,the tri-screw extruder has more meshing arear to strengthen the chaotic mixing of the extruder.In the end,the logarithmic stretching was used to analyze the mixing ability,which demonstrate the mixing ability in the triscrew extruder is better than that of the twin screw extruder.