Chapter 2. Turbulence in Mixing Applications

Summary

Turbulence is central to much of liquid mixing technology, and all of the typical processes are dramatically affected by its presence. Without understanding the nature of turbulence, dealing with the interactions between turbulent fluctuations and mixing processes is almost impossible. In addition, time varying aspects of the flow can have a significant impact on the process results. Without an understanding of these basic physical phenomena, reliable predictions of performance can be difficult to achieve. Simple scale-up rules can be hopelessly inadequate. Unfortunately, the physics of turbulence still evades a general mechanistic description; and recirculation, strong geometric effects, and instabilities on several scales of motion further complicate the flow in a stirred tank. This chapter focuses on providing a physical understanding both of turbulence and of the tools we use to understand its effects on process results. The role played by the scale-up of turbulent motions is first applied to reactor design to help clarify the interaction for the reader. The second section digs deeper into the description of turbulence, considering the various time and length scales involved in the description of turbulent flow, the scaling arguments which are used for engineering estimates, and how these estimates are related to the flow field. The information lost in the time averages and scaling arguments is revisited from the perspective of experimental and theoretical approximations of the flow in section three. Finally, the mathematical approach to the problem, the modeling of turbulence, is discussed in the last section.