Understanding the Impact of Particle Density on Mixing Behavior for Contra-Rotating, Baffle Free Impellers

In mixing systems, when both impellers are rotating in the same direction, the mixing efficiency is very poor, mainly due to the bulk flow and poor mass transfer. To counter this, baffles are placed in the mixing tank; and while this eliminates the vortex, the particles can hit or even get stuck behind the baffles, resulting in data that is not representative of the experiment. In response, the impellers have been configured so the two rotate in opposite directions. This configurations both eliminates the vortex and the need for baffles. This objective of this research is to experimentally and numerically prove that contra-rotating, baffle-free impellers produce the highest mixing efficiency, at the lowest torque. This research specifically deals with solid-liquid mixing systems, and there are a multitude of industrial applications, such as drug compounding for pharmaceutical companies, as well as food and drink processing. Several cases will be studied by changing the direction of impeller rotation (co-rotating or contra-rotating) and impeller pumping direction (up or down). For each trial, five-hundred particles with differing specific gravities will be introduced two ways: experimentally, through a series of trials; and computationally, through a series of simulations, verifying the parameters of the experiment. In conclusion, the both-down case produced the lowest mixing efficiency, regardless of configuration. For one particle, the co-rotating impellers with baffles produced the most efficient mixing. The contra-rotating impellers, specifically inward flow, however, produced the highest mixing efficiency for the other two particles tested. This case also produced the highest torque.