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 helical ribbon agitator was carried out by means of the Discrete Element Method (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 core region predicted by the present simulation agreed well with those obtained by experiments. Based on DEM simulation, the particle circulation and mixing characteristics in the agitator vessel were investigated in detail. Vertical mixing of particles was found rather poor during upward and downward flows through the blade and core regions, respectively. However, in the core region, particles flow in the manner of funnel flow. Additionally, in the blade region the vertical velocity decreases toward the wall and reduces to zero near the boundary between the blade and core regions. Particles were carried up to the bed surface by the helical ribbon flow down into the center of the core, and those not carried completely to the surface flow into the outer periphery of the core. The effect of bed height on circulation and mixing was analyzed numerically in terms of tracer particle concentration response as well as particle velocity and angular velocity distributions, which are quite difficult to obtain by experiment. The bed height was found to be the most important factor for mixing and circulation. When the bed height is appropriate, particles circulate rather regularly. If the bed height 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.