(301c) Effective Transitional Flow Blending

Effective Transitional Flow Blending

In many industrial applications, effective blending in the transitional flow regime is very important to accomplishing process objectives (such as proper reaction kinetic, product uniformity and heat transfer).  The ability to effectively blend fluids at lower power and torque inputs offers initial (capital) and longer term (power and maintenance) savings.  Many different impeller configurations have been used in the past to accomplish this type of mixing.

Blending in transitional flow has been accomplished in a number of different ways historically.  This includes the use of helical ribbons and conventional impellers including straight blade turbines, pitch blade turbines and hydrofoil high efficiency impellers.  Although they may provide the best performance in laminar and low transitional operation, helical ribbons are typically not used for transitional blending due to their high cost.  To approximate the axial flow pattern and movement throughout the entire tank volume provided by a helical ribbon, multiple open impellers are typically used in transitional operation.  The design for effective blending using this approach is a function of fluid rheology and Reynolds number.  The number of impellers required to mix the entire vessel increases with both a decrease in Reynolds number and an increase in shear rate sensitivity of a material.  The impeller D/T increases with both a decrease in Reynolds number and an increase in shear rate sensitivity of a material. 

This presentation will introduce a new transitional flow impeller that provides significant improvements over existing designs.   A comparison to existing impeller technologies will made   over the Reynolds number range for transitional flows (Reynolds numbers between roughly 100 and 1000).   A range of D/Ts will also be examined.  Power numbers and blend times will be reported.  In addition, a map of where different impeller technologies are optimal will be presented.