(536b) The Effect of Mixer Properties and Fill Level On Granular Flow in a Bladed Mixer

Many industrial processes involving granular materials employ the use of mixers with mechanical agitators that provide shear, induce flow and encourage mixing. The appeal of these mixers comes from their ability to handle a wide variety of solids ranging from free flowing to cohesive powders and even pastes. Mixer characteristics and operating conditions determine degree of mixing and the shear profile achieved in such devices. Although these parameters play an important role, currently it is difficult - if not impossible - to predict granular behavior in mechanically agitated beds when these attributes are changed.

The discrete element method was used to study the effect of mixer properties and fill level on granular flow in a bladed mixer. Simulation velocity profiles were found to be in good agreement with experimental profiles obtained by using the Particle Image Velocimetry technique. For fill levels just covering the span of the blades, a 3-dimensional recirculation zone develops in front of the blades which promotes vertical and radial mixing. Increasing fill level reduces the size of the recirculation zone, decreases bed dilation and hinders particle diffusivities. However, above a critical fill level, the behavior of the particles within the span of the blade is found to be invariant of fill level. Cylinder wall friction is shown to significantly influence granular behavior in bladed mixers. Blade position along the vertical axis is shown to influence flow patterns, granular temperature and stress. The effect of increasing the mixer diameter at a constant particle diameter was also studied. When the mixer diameter is larger than a critical size such that wall effects are minimized, the observed granular behavior follows simple scaling relations. Particle velocities and diffusivities scale linearly with mixer size and blade speed.