耦合流动传热的聚合物结晶过程的多尺度模型与算法研究

Polymer crystallization is an important factor to affect the microstructure and to determine the mechanical properties of the products. Compared to the quiescent crystallization, crystallization occurred during the processing not only accelerates the crystallization rate, but also leads to different types of crystalline structures. Pervious work is mostly concentrated on the traditional modeling and simulation of quiescent crystallization with heat transfer which neglects the effect between the fluid flow and crystallization. In additional, these results only predict the parameters in the macroscopic level (e.g., crystallinity rate, relative crystallinity) and can not reflect the microstructure details (e.g., crystal morphology, mean size of crystals) which in turn can not explore the mechanical properties of the products. Hence, in this program we will introduce the multi-scale method to the study of polymer crystallization during processing. Through considering the effect of fluid flow and crystallization, we want to build up a multi-scale model which couples the morphology evolution in the microscopic level and the phase change in the macroscopic level. The model in the microscope contains the nucleation and growth of crystalline structures while the model in the macroscope contains the mass, momentum, energy transport for non-isothermal, viscoelastic flow with phase change. According to this multi-scale model, we will first develop the pixel coloring method in the microscopic level and the finite volume method in the macroscopic level respectively, and then build up a micro-macro hybrid numerical method. By using the multi-scale model and multi-scale algorithm, we will predict the crystal morphology development, explore the effect of processing conditions (e.g., thermal condition, flow condition) on the microstructure (e.g., type of crystalline structures, crystalline size) and determine the relationship between crystal morphology and the mechanical properties of the products. Results of this program are hoping to give more insight into the microscopic details of crystallization and thus be more helpful for industrial application.

聚合物的结晶过程是影响其微观结构并决定制品性能的关键因素。成型过程中，聚合物受到复杂温度及流场作用，其结晶行为表现出静态结晶与流动诱导结晶共存、微观结构演化与宏观流动传热相耦合的特点。由于目前的工作大多仅停留在静态结晶的宏观模拟上，尚未考虑流动效应，且只能预测宏观信息，无法获得微观结晶形态并为分析制品性能所用。为此，本项目拟将多尺度方法引入其中，通过考虑温度、流场与结晶的相互作用，建立耦合微观结晶形态演化与宏观流体流动传热的多尺度模型，并构建微观像素着色法与宏观有限体积法相耦合的多尺度算法，从而实现结晶过程多尺度模拟；进而基于模拟结果，研究宏观加工条件、微观结晶形态、制品宏观性能间的影响规律。本项目将解决相关文献中忽略流动效应以及结晶形态无法预测的问题。研究成果将为聚合物相变多尺度模型的建立以及微观-宏观算法的研究提供理论与技术支持，为深入探索聚合物的工艺-结构-性能之间的关系提供途径。

本项目的背景：聚合物在成型过程中，受到复杂温度及流场作用，其结晶行为表现出静态结晶与流动诱导结晶共存、微观结构演化与宏观流动传热相耦合的特点。但目前的工作大多停留在静态结晶的宏观模拟上，尚未考虑流动效应，且只能预测宏观信息，无法获得微观结晶形态并为分析制品性能所用。. 本项目的主要研究内容：运用多尺度方法研究聚合物成型中的结晶过程，建立耦合微观结晶形态演化与宏观流体流动传热的多尺度模型；并构建微观随机模拟法与宏观有限体积法相耦合的多尺度方法，探索宏观成型条件对微观结晶形态的影响规律及微观结晶形态对制品宏观性能的影响规律。. 本项目的重要研究结果：构建了微观球晶、串晶的成核-生长-碰撞模型，利用Monte Carlo等随机模拟法，实现了简单温度及流场下的球晶、串晶的二维、三维形态演化模拟。建立了宏观温度、流场与微观结晶形态相耦合的微观-宏观模型，构建了宏观有限体积法和微观Monte Carlo等随机方法相结合的多尺度算法，实现了Couette流场和管道流场中的多尺度模拟，获得了剪切速率、剪切时间、流体温度等宏观成型条件对微观结晶形态的影响规律及对宏观制品性能的影响规律。. 本项目的科学意义：本项目解决了相关文献中忽略流动效应以及结晶形态无法预测的问题。研究成果为聚合物相变多尺度模型的建立以及微观-宏观算法的研究提供理论与技术支持，为深入探索聚合物的工艺-结构-性能之间的关系提供途径。