Department of Chemical and Biological Engineering, University of Colorado Boulder, Boulder, Colorado 80309, USA.
Phys Rev Lett. 2018 Dec 7;121(23):238001. doi: 10.1103/PhysRevLett.121.238001.
Clustering is often presumed to lead to enhanced agglomeration between cohesive grains due to the reduced relative velocities of particles within a cluster. Our discrete-particle simulations on gravity-driven, gas-solid flows of cohesive grains exhibit the opposite trend, revealing a new mechanism we coin "cluster-induced deagglomeration." Specifically, we examine relatively dilute gas-solid flows and isolate agglomerates of cohesive origin from overall heterogeneities in the system, i.e., agglomerates of cohesive origin and clusters of hydrodynamic origin. We observe enhanced clustering with an increasing system size (as is the norm for noncohesive systems) as well as reduced agglomeration. The reduced agglomeration is traced to the increased collisional impact velocities of particles at the surface of a cluster; i.e., higher levels of clustering lead to larger relative velocities between the clustered and nonclustered regions, thereby serving as an additional source of granular temperature. This physical picture is further evidenced by a theoretical model based on a balance between the generation and breakage rates of agglomerates. Finally, cluster-induced deagglomeration also provides an explanation for a surprising saturation of agglomeration levels in gravity-driven, gas-solid systems with increasing levels of cohesion, as opposed to the monotonically increasing behavior seen in free-evolving or driven granular systems in the absence of gravity. Namely, higher cohesion leads to more energy dissipation, which is associated with competing effects: enhanced agglomeration and enhanced clustering, the latter of which results in more cluster-induced deagglomeration.
团聚体通常被认为会由于团聚体内颗粒的相对速度降低而导致团聚体之间的团聚增强。然而，我们在粘性颗粒的重力驱动气固流动的离散颗粒模拟中观察到了相反的趋势，揭示了一种我们称之为“团聚体诱导解团聚”的新机制。具体来说，我们研究了相对稀松的气固流动，并从系统的整体非均匀性中分离出粘性团聚体，即粘性团聚体和由流体动力引起的团聚体。我们观察到随着系统尺寸的增加（这是非粘性系统的正常现象）团聚体增加，团聚体减少。团聚体减少可归因于团聚体表面颗粒的碰撞冲击速度增加；也就是说，更高水平的团聚体导致团聚体和非团聚体区域之间的相对速度增加，从而成为颗粒温度的另一个来源。这一物理图像还得到了一个基于团聚体生成和破碎速率平衡的理论模型的进一步证实。最后，团聚体诱导的解团聚也解释了在具有增加的粘性的重力驱动气固系统中，团聚体水平令人惊讶地饱和的原因，而在没有重力的情况下自由演化或驱动的颗粒系统中观察到的则是单调增加的行为。即，更高的粘性会导致更多的能量耗散，这与竞争效应有关：团聚体增强和团聚体增强，后者会导致更多的团聚体诱导解团聚。
