中文摘要：

采用Phan-Thien-Tanner本构方程，建立Y型包覆挤出模型，通过有限元数值模拟聚苯乙烯、聚丙烯的三维等温高黏弹流动过程，对比不同进料组合条件下流体在传统和气辅流动中的速率场、剪切速率分布。结果表明：传统共挤出受不同进料方式的影响，挤出物胀大主要由其各向速率场量分布不匀造成，而气辅共挤出的挤出物胀大不受其影响，且胀大均基本得到消除；气辅共挤出时，其x轴、瑚方向的速率均几乎为零，没有二次流动，且z轴向的速率稳定，说明气辅共挤出能消除挤出物膨胀现象。

英文摘要：

The three-dimensional isothermal viscoelastic processes of polystyrene and polypropylene were simulated by the finite element method based on Y-shaped co-extrusion model which was established by the Phan-Thien-Tanner constitutive equation. The distributions of velocity field and shear rate of polymer with different feedstock were compared in conventional and gas-assisted co-extrusion. The results show that the die swell in conventional co-extrusion affected by feeding method, is mainly caused by the uneven distribution of velocity field in every direction, while the die swell in gas-assisted co-extrusion is not affected and almost eliminated. Both of the velocity ofx and y are zero in the process of gas-assisted co-extrusion, therefore there is no secondary flow, furthermore, the flow rate is uniform at the direction of z, which indicates that the die swell can be eliminated in gas-assisted profile co-extrusion.