非球形颗粒在粗糙壁面槽道湍流中的行为研究

Understanding the dynamics of inertial particles with anisotropic shapes in a turbulent carrier fluid over a solid wall is of fundamental interest and a key in making advances in many industrial processes, such as, pulp fibres in papermaking, bio-mass particles combustion in a chamber, the particle suspensions in fluidized bed reactor, to name a few. In reality, very few wall surfaces associated with turbulent flow can be regarded as perfectly smooth. Most of previous studies in past decades were focused on turbulent suspensions over a smooth wall, while the role of surface roughness in the anisotropic particle-laden turbulent flow, as an important but less noticed research topic, has never been explored up to now. The major objective of this project is to achieve the fundamental understanding of the collective dynamics of anisotropic particles rotation, orientation, clustering, deposition and particle-fluid interaction in a turbulent flow with different types of wall roughness through a variety of different accurate computer experiments - direct numerical simulations (DNSs). The underlying physics of this complex two-phase flow system discovered in this project is expected to deliver an innovative control approach of the anisotropic particle suspensions in the wall-bounded turbulence, such as particle orientation and deposition rate, by adjusting the particle shape, inertia and wall configurations, which would advance industrial process with complex flow system and accordingly benefit the society and industry. Furthermore, the findings of this project would be helpful for developing a predictive model tool for the wide natural and industrial applications.

研究和理解具有复杂形状的惯性颗粒在壁面湍流中的运动力学对解决工业过程中的问题具有重要意义，比如造纸业中木纤维在纸浆中的朝向，生物质颗粒在燃烧室中的分布，以及流化床反应器里颗粒的运动等。现实情况里，很少有颗粒具有规则的球形，并且很少有流体流过的壁面是完全平滑的，而过去几十年里为了模型和分析的简化，大部分研究关注于球形颗粒在光滑壁面湍流中的运动和聚集。本研究项目的主要目的是运用直接数值模拟技术研究在不同种粗糙壁面的条件下不同形状的惯性颗粒如何在湍流中转动，平动，取向，团聚，沉降以及颗粒和流体的相互作用。理解这种复杂两相流的流动机理，通过调节颗粒的形状，惯性，以及壁面粗糙类型，可以建立一种新型的对异形颗粒群体运动，团聚和沉降的控制方法，更好设计和改进工业中的多相流动系统。此外，通过对流动机理的深入理解，有助于开发出应用于自然和工程问题的预测模型。

本项目关注颗粒两相流领域的重要基础问题，对光滑和粗糙壁面条件下不同形状的惯性颗粒如何在壁湍流中转动、平动、取向、团聚、沉降以及颗粒和流体的相互作用开展了研究。在本项目支持，申请人系统地研究了近壁湍流流动特性对非球形颗粒行为的作用规律，发现颗粒的倾向性取向对颗粒转动模式有更强的影响，而壁湍流的平均剪切起次要作用。其次，在复杂形状颗粒与壁湍流的相互作用方面，分析了全分辨颗粒对壁湍流的调制作用，发现相比球形颗粒，大尺寸扁平颗粒会抑制减弱展向和壁面法向的流体湍流脉动速度，并且近壁面的条带间距较无颗粒流动有所增加。对粗糙壁湍流颗粒两相流进行了数值模拟，对比光滑壁湍流结果，结果表明粗糙壁面导致湍流中惯性较大的颗粒会向槽道中部聚集，而槽道中部的颗粒流向平均速度小于流体速度但其速度脉动大于湍流脉动。此外，粗糙元对粗糙壁湍流中非球形颗粒的倾向性取向有抑制作用。通过全分辨计算和理论模型，研究了椭球颗粒受到的流体惯性力矩与形状和雷诺数的关系，为建立重力沉降过程中颗粒力矩模型提供了数据。