Discrete characterization of cohesion in gas–solid flows

Despite the fact that a sizable portion of gas–solid flows are cohesive in nature, the mechanics of cohesive flowing gas–particle systems is still poorly understood and manipulation/control of the flow variables is still largely done on a trial-and-error basis. While recent advances have been made in our understanding of liquid-induced cohesion at the macroscopic level, in general, it is still not possible to directly connect this macroscopic understanding of cohesion with a microscopic picture of the particle properties and interaction forces. In fact, conventional theories make no attempt to distinguish between these modes of cohesion, despite clear qualitative differences (lubrication forces in wet systems and electrostatic repulsion are two good examples). In this work, we introduce a discrete characterization tool for gas–solid flow of wet (cohesive) granular material—the Granular Capillary Number (Cag). The utility of this tool, which is a ratio of the capillary force to the drag force, is computationally tested over a range of cohesive strengths in two prototypical applications of gas–solid flows. It is shown that rescaling our results in this way yields a collapse of data for varying surface tensions and fluidization velocities, and that a clear transition from free-flowing to cohesive behavior occurs at a distinct value of Cag.