低密度聚乙烯熔体毛细管挤出的数值模拟

Numerical Simulations of Capillary Extrusion of Low Density Polyethylene Melt

借助2种微分型黏弹性本构模型DCPP模型和S-MDCPP模型来描述支化高分子熔体的复杂流变行为,并采用离散的弹性黏性应力分裂方法(DEVSS)/迎风流线方法(SU)解决黏弹性流体流动过程中的对流占优问题以及缺少椭圆算子的问题,进而用基于有限增量微积分(FIC)方法的压力稳定型分步算法求解质量守恒方程、动量守恒方程,对低密度聚乙烯(PE-LD)熔体在毛细管中的流动情况以及挤出胀大过程进行模拟,并把模拟结果和实验结果进行比较。结果表明,在低剪切速率时,模型预测的挤出胀大比和壁面剪切应力与实验结果比较接近;2种模型预测的速度、应力以及主链拉伸的吻合程度较好,说明2种模型均能较好地预测PE-LD熔体在毛细管中的复杂流变行为;同时表明计算S-MDCPP模型时所采用的算法是可靠的。

With the help of two differential viscoelasticity constitutive models,i.e.DCPP model and S-MDCPP model,the complex rheological behavior of branched polymer melts was described,and the problems of convection dominance and the lack of elliptic operators in the process of viscoelastic flow were solved by means of the discrete viscoelastic stress splitting method and upwind streamline method.A pressure-stabilized iterative fractional step scheme based on the finite incremental calculus method was used to solve the mass and momentum conservation equations.The flow behaviors of PE-LD melt in the capillary and extrusion swell were simulated,and the simulation and experimental results were com-pared.The results indicated the extrusion-swelling ratio and wall shear stress predicted by the model at low shear rate were close to the experimental results.The velocity,stress and backbone stretch predicted by the two models were also in good agreement with the experimental results.This indicates that the two models can well predict the complex rheo-logical behavior of PE-LD melt in capillary and the algorithm is reliable to be used to resolve the S-MDCPP model.