表面粗糙度对干燥细颗粒流动行为的介尺度调控机制研究

As an important industrial raw material, fine granular powders (the particle size is less than 100 μm) have been widely used in a variety of industrial processes. The cohesive force, which has taken a more important role in the interaction between fine powders, has a significant impact on the flow behavior of fine powders in various industrial processes (such as pulverized coal combustion and gasification). For fine dry powders, the dominant cohesive force is van der Waals force. Thus, the flow characteristic of fine dry powders is closely related to van der Waals force. The van der Waals force depends largely on the surface property of powders, especially the surface roughness. So it is possible to achieve the mesoscale, accurate regulation on the flow behavior of fine powders by regulating the surface roughness quantitatively. Our research is on the fine dry powders with asperities. The main objective of our work is to reveal the flow behaviors of fine dry powders with different particle sizes and surface roughnesses, to determine the size, shape, periodicity, evolution, and formation mechanism of flow features (such as aggregates, clusters), to put forward a new model applied to predict the van der Waals force between rough spherical particles, to discuss the mesoscale regulation mechanism of nanoscale asperity on the flow behaviors, to propose a general criterion for evaluating the flow features of fine powders.

作为重要的工业原料，细颗粒(粒径不超过100 μm)常见于多种工业生产过程中。由于细颗粒间粘性力的地位越发突出，将显著影响许多工业过程(如粉煤燃烧与气化过程)中的颗粒流动行为。对于干燥的细颗粒，粘性力以范德华力为主，故干燥细颗粒的流动特性将与范德华力密切相关。而颗粒的表面性质，特别是表面粗糙度对范德华力起着决定性作用，因此通过定量控制颗粒表面的粗糙程度有望实现对细颗粒流动行为的介尺度精确调控。本项目以表面粗糙的干燥细颗粒为研究对象，目的在于揭示不同粒径及粗糙程度的干燥细颗粒的流动行为，确定流动特征(如颗粒团聚体、团簇)的尺寸、形状、周期性、演变历程及形成机理，建立预测粗糙球形颗粒间范德华力的计算模型，探讨纳米级表面粗糙度对颗粒流动行为的介尺度调控机制，提出具有普适性的细颗粒流动特征评判准则。

本课题以表面粗糙的干燥细颗粒为主要研究对象，采用氢氟酸蚀刻法对颗粒表面粗糙度进行了介尺度修饰和调控，并建立了粗糙球形颗粒间的范德华力计算模型。在小尺寸料仓中，考察了颗粒物性对细颗粒重力卸料特性及流动行为的影响，发现颗粒表面粗糙度是影响细颗粒卸料流率的重要因素，且其影响可通过范德华力体现在无量纲数Bog数中。通过修正颗粒堆积空隙率和Beverloo常数，提出了可准确预测小尺寸锥形料仓中干燥细颗粒卸料流率的关联式。此外，基于料仓出口直径与颗粒粒径之比D0/dp和Bog两个无量纲数，提出了一个判断颗粒流动状态的新准则。采用高速摄像仪研究了颗粒物性对颗粒流动状态的影响。结果表明，随着颗粒粒径的增大，细颗粒的卸料流动状态由结拱过渡至间歇流卸料再过渡至连续流卸料。改变颗粒的表面粗糙度能够实现颗粒卸料状态的转变。为了更接近工业应用中的实际情况，在放大的锥形料仓中，系统研究了颗粒表面粗糙度及料仓出口结构对重力卸料特性的影响，发现表面粗糙度通过改变颗粒间范德华力来调控卸料流率及流动状态，且范德华力和卸料流率随颗粒表面粗糙度呈非单调趋势变化，另外，锥形料仓添加出口管可显著提高颗粒卸料流率，甚至改变颗粒流动状态。基于Bulsara的卸料流率模型，同时引入颗粒表面粗糙度及出口管结构尺寸，提出了更为普适的卸料流率预测模型。采用高速摄像仪观察了细颗粒的流动状态，发现适当调控表面粗糙度或优化出口结构均可使颗粒从周期性间歇流转变为连续流，在间歇流和连续流之间存在过渡流动状态。