基於三維流場模擬之同向雙螺桿混合效率預測

Abstract

本研究主要以數值模擬探討典型運輸和捏合元件在雙螺桿押出機內部流場特徵以及其混合效率。本研究在等溫和不可壓縮流的假設下，以Cross-WLF黏度修正模型描述高分子的流變性質，使用暫態三維模型求解連續和動量方程式，以描述雙螺桿流場的混煉行為。本研究首先使用STAR-CCM+軟體進行雙螺桿流場數值模擬計算，獲得流場剪切率、速度和壓力等物理量分佈，接著基於流場特性以四階Runge-Kutta方法預測虛擬粒子的軌跡。本研究以上述方法預測三種質量流率（10、20、40 kg/hr）和三種螺桿轉速（75、150、300 rpm）的雙螺桿元件混合能力。計算結果顯示，剪切率受螺桿轉速的影響大，而壓降則受流量影響較大。與捏合螺塊相比，在相同條件下運輸螺塊具有更大的壓降效果。轉速對於分散性影響較大，低轉速螺塊有更好的分散混合效率。相較於運輸螺塊，同條件下的捏合螺塊的分散混合效率較好。此外，高轉速捏合螺塊與低轉速運輸雙螺塊則具有較好的分配效率。

This research is to study the flow properties and mixing characteristics of a typical conveying and kneading element in a twin screw extruder via a numerical approach. An unsteady, three-dimensional model is implemented in this study under the assumptions of an isothermal and incompressible flow. By solving the continuity and momentum equation, physical quantities such as strain rate, velocity components and pressure, are obtained. The modified Cross-WLF model is employed to describe the rheological properties of the polymer adopted in this study. The commercial software, STAR-CCM+ is used for the numerical simulation of the twin screw extrusion flow. A Lagrangian description of particle tracing method is developed to predict the trajectory of pseudo particles of the twin screw extrusion flow based on a fourth-order Runge-Kutta method. Three mass flowrate, 10 kg/hr, 20 kg/hr, 40 kg/hr, as well as three screw speed, 75 rpm, 150 rpm, 300 rpm, are adopted to investigate the influence on the both screw elements. Numerical results show that the strain rate in the flow is mainly affected by the screw speed, while the pressure drop is substantially influenced by the mass flowrate. Under the same operation condition, the conveying element has a larger pressure drop when compared to the kneading one. Screw speed has a dominant effect on enhancing dispersion, where reducing the screw speed leads to a better dispersibility. The kneading element has a better dispersive mixing efficiency than the conveying counterpart under the same operation condition. However, the kneading element at high rotational speed and the conveying element at low rotational speed tend to deliver a better distributive mixing performance.