Numerical analysis of particle mixing characteristics in a single helical ribbon agitator using DEM simulation

Numerical analysis of three-dimensional motion of particles in a single hel ical ribbon agitator was carried out by means of the Discrete Element Metho d (DEM). To validate the computed results experiments were carried out with a cold scale model of 0.3 m inside diameter. Circulation time of particles in the agitator and the horizontal particle velocity distribution in the c ore region predicted by the present simulation agreed well with those obtai ned by experiments. Based on DEM simulation, the particle circulation and m ixing characteristics in the agitator vessel were investigated in detail. V ertical mixing of particles was found rather poor during upward and downwar d flows through the blade and core regions, respectively. However, in the c ore region, particles flow in the manner of funnel flow. Additionally, in t he blade region the vertical velocity decreases toward the wall and reduces to zero near the boundary between the blade and core regions. Particles we re carried up to the bed surface by the helical ribbon flow down into the c enter of the core, and those not carried completely to the surface flow int o the outer periphery of the core. The effect of bed height on circulation and mixing was analyzed numerically in terms of tracer particle concentrati on response as well as particle velocity and angular velocity distributions , which are quite difficult to obtain by experiment. The bed height was fou nd to be the most important factor for mixing and circulation. When the bed height is appropriate, particles circulate rather regularly. If the bed he ight is lower than the blade top, particles mix rapidly because of the high fall of particles from the blade top. On the other hand, if the bed height is higher than the blade top, the velocity of particles in the bed surface region is much lower than that of the other region and accordingly a semi- stagnant eddy is formed. (C) 2000 Elsevier Science S.A. All rights reserved .