Prediction of impeller torque in high shear powder mixers

Abstract

An investigation was made of the factors that determine the impeller torque of vertical axis high speed mixers containing granular solids of low cohesion, the experimental material being sand. The diameter of the mixer bowls, which were constructed of stainless steel, ranged from 0.13 to 0.30 m . Disc impellers with both smooth and grooved surfaces were used. Two and three blade flat impellers were used with heights in the range 3– 12 mm and bevel angles ranging from 11° to 90°. The study was supported by the application of positron emission particle tracking (PEPT) to investigate the flow of the material in the mixer. A dimensional analysis was made of the data. The effects of the mass of powder, M, and the bowl radius, R, could be satisfactorily represented by the dimensionless torque group, T/ MgR. In the case of disc impellers, the dimensionless torque was independent of impeller rotational speed. For the blade impellers, the dimensionless torque was found to be a function of the impeller Froude number and a dimensionless blade height. A powder mechanics analysis was made of the flow of material in the mixer fitted with both disc and blade impellers. The flow of the powder was modelled as ‘rigid’ body rotation and both frictional and inertial interactions with the impeller were accounted for. The analysis provides a first order representation of the effects of scale, mass fill, impeller rotational speed, blade height and blade bevel angle on the torque. The assumptions made in the model are critically discussed.