毛细管中聚乙烯熔体流动的数值模拟与实验

Numerical Simulation and Experiment of Polyethylene Melt Flow in Capillary

分别采用2种微分型黏弹性本构模型——DCPP模型和S-MDCPP模型模拟了线型低密度聚乙烯熔体在毛细管中的流动情况,并与实验结果作比较。为了保证数值计算的稳定性,采用离散的弹性黏性应力分裂方法(DEVSS)/迎风流线方法(SU)在动量方程中引入椭圆型算子,并抑制高Weissenberg数下的对流占优情况。一种改进的FIC方法被引入构造出压力稳定型分步算法来实现速度、压力、应力的等低阶线性插值计算黏弹性流动问题。结果表明,2种模型预测的挤出胀大比、速度、压力、应力及主链拉伸程度分布吻合较好,S-MDCPP模型所采用的等低阶线性单元插值方式和压力稳定的迭代分步算法是可靠的;另外,还采用2种模型预测了不同剪切速率下线型低密度聚乙烯熔体的挤出胀大比和挤出压力,并与实验结果作比较,发现在低剪切速率的时候,吻合程度较好。

Two differential viscoelastic constitutive models,DCPP model and S-MDCPP model,were used to simulate the flow of linear low-density polyethylene melts in capillary rheometer,and compared with experimental results.In order to ensure the stability of numerical calculations,the discrete viscoelastic stress splitting method(DEVSS)/streamline-upwind method(SU)were used to introduce an elliptic operator into the momentum equation and suppress the convection dominance for high Weissenberg numbers.An improved finite incremental calculus method(FIC)was introduced to reconstruct the mass conservation equation,so as to apply the pressure stabilized iterative fractional step scheme to solve the velocity,pressure and stress by using the equal low-order interpolation elements.The results show that the extrusion swell ratio,velocity,pressure,stress and backbone stretch distributions predicted by the DCPP and S-MDCPP models agree very well,and the pressure stabilized iterative fractional step scheme and the equal low-order interpolation elements adopted for the S-MDCPP model are reliable.Moreover,two models were also used to predict the extrusion swell ratio and extrusion pressure of linear low-density polyethylene melts at different shear rates.And compared with the experimental results,it is found that at low shear rate,they have the good agreement.