(61b) DEM Simulation of the Mixture of Dry and Wet Particles inside Rotary Mixer

Industrial particle coating process often involves spraying
the coating fluid on the top layer of moving particles inside mixer. It takes
time on the order of several minutes or more for all particles to pass through
the “spray zone”.  During this transient
time the bulk of the granular material inside the mixer contains the mixture of
dry particles and sticky wet particles with amount of wet particles
continuously increasing while the amount of dry particles is continuously
decreasing. The surface frictional properties of dry and wet particles can be
very different. We developed a numerical model based on the Discrete Element
Method (DEM) to simulate this dry-to-wet transition during coating.  In this model every dry particle is replaced
by wet particle if it is inside the cone-shape spray zone. The model is
implemented within commercial software STAR-CCM+ version 11.04 developed by
CD-adapco. We tested the model in several mixer geometries and analyzed the
bulk properties of the solid phase as a function of time. For the simplest
geometry case, a rotating drum, we obtained the expected change in dynamic
angle of repose in the cases of all-dry and all-wet particles and, in terms of the
flow pattern, repose angle, and the frequency of avalanching, we obtained
results for all-wet particles comparable with published experimental results
[1]. Figure below shows the side-view of dry (grey color) and wet (blue color)
particles inside the rotating drum for increasing values of time.

We believe that our model is useful for both better
understanding of the bulk flow during dry-to-wet transition and its ability to
differentiate different mixer designs in industrial processes where the
difference in frictional properties between wet and dry particle cannot be
neglected.

[1] P. Y. Liu, R. Y. Yang and A. B. Yu. Dynamics of wet
particles in rotating drums: Effect of liquid surface tension. Physics of
Fluids, 23, 013304, (2011)