Lagrangian marker particle trajectory and microconductivity measurements in a mixing tank

Abstract: Large regions of inhomogeneous mixing have been observed in industrial, bottom-sweeping impeller crystallizers. To investigate this phenomenon, we conducted experiments on a one-tenth volume model of this kind of mixing tank. Results are reported of Lagrangian marker particle (LMP) and microconductivity measurements using the model mixing tank with impeller tip Reynolds numbers of 25,000. Surprising structure is found in this high Reynolds number flow. Using the LMP trajectory data, we show the flow consists of a narrow region of rapidly moving, upward spiralling flow at the tank perimeter. This flow returns slowly through a vertical stack of tori and through a quiescent region centered on the impeller. These tori are concentric with the impeller and exist at loci of regions of shear found adjacent to the quiescent central region and the tank perimeter. Conditional analysis of the microconductivity signals reveals that large concentration fluctuations occur in the perimeter flow. In contrast, only small diffusive-like concentration fluctuations occur in the center of the tank. This segregation of regions of rapid transport in the perimeter flow from regions of micromixing in the quiescent region results in inhomogeneous mixing in the tank. The complexity of the flow is reflected in the large dynamical dimension () of the flow obtained from the calculation of the Kolmogorov entropy production rate. The return-time distribution was found to be composed of a superposition of two log-normal distributions. Period doubling phenomenon was also found in these distributions. [Copyright &y& Elsevier]