Eliminating Segregation in Free-Surface Flows of Particles

Abstract

By introducing periodic flow inversions, we show both experimentally and computationally that forcing with a value above a critical frequency can effectively eliminate both density and size segregation. The critical frequency is related to the inverse of the characteristic time of segregation and is shown to scale with the shear rate of the particle flow. This observation could lead to new designs for a vast array of particle processing applications and suggests a new way for researchers to think about segregation problems.

Granular mixing and segregation in zigzag chute flow

“…We develop an approximate theory applicable to S systems that is analogous to that developed by Shi et al [14] for D systems, and show that it matches both computational results in a chute flow and experimental results in a "rock and rotate" tube. Experimentally, a mixed result is obtained for all size ratios when a short length of tube is used; however, a segregated result is obtained for a sufficiently long length of tube.…”

“…In our related paper that included theory for density segregation [14], we defined an experiment as yielding a mixed result if the equilibrium I s value calculated from the 'rock and rotate' trial differed (was smaller than) that of the 'control' trial. This strategy was (and is) used in order to eliminate any variation in mixing rates within this apparatus that might arise from differences in the flow lengths as we increase the tube sizes.…”

“…This can be understood as follows. We first verified [14] that the flow down the inclined plane is essentially linear (using metallic particles and "streak line" measurements [15]). This suggest that the shear rate can be simply expressed asγ = 2Ū /H, whereŪ is the average streamwise flow velocity and H is the radial height of the rod.…”

“…In this work, we propose a model similar to that of Shi et al [14] based on a segregation velocity expression that can be used to derive a critical frequency prediction for S system segregation (as opposed to D-systems). A range of experiments and DEM simulations are carried out to validate the critical forcing frequency model.…”

“…In a complementary approach, Shi et al [14] proposed to adapt a fluid-mixing technique -specifically, the use of periodic flow perturbations -to thwart segregation in a material-independent way. In particular, they showed that periodic flow inversions either manually (in a chute) or via selective baffle placment (in tumbler-type mixer) can serve as a general method for eliminating segregation in free-surface flows for either density-driven (D systems) or size-driven (S systems) segregation.…”