(248c) Flow and Shear In Agitated Vessels

Impellers are often described in qualitative terms using expressions such as "gentle", "low shear", high efficiency", "high shear", etc. Axial flow impellers, such as hydrofoils, are often considered gentle and low shear while radial flow impellers, such as disc turbines, are considered high shear. There is a class of bladeless impellers which are essentially discs or circular saw blades which operate at high rotational speeds and are used in "very high shear" applications.

Using the power and flow numbers, the energy input per unit mass of fluid E can be obtained by dividing power by the product of density and volumetric flow rate. Plotting E versus impeller to vessel diameter ratios shows that the impellers qualitatively said to be "high shear" have a higher value of E at the same power input per unit mass. Scaling-up at constant power per mass increases E for a given impeller. High shear impellers, having large E values also have greater turbulence intensity (ratio of fluctuating to mean velocity).

When droplet sizes generated by these impellers at the same mean power input per mass are considered, hydrofoils which are considered "low shear" produce a smaller droplet size than Rushton turbines which are "high shear". The reason for this is that the hydrofoils have a smaller turbulent length scale and, as a result, at a given mean power input per mass the local turbulence in the impeller region is more intense producing smaller droplets. But, the smaller length scale is a result of the shallow blade angle. This in turn results in a smaller power number which means that the hydrofoil must operate at a higher speed. At the same power per mass, the impeller with a higher tip speed produces a smaller droplet and vice versa (at the same tip speed, the impeller with a higher power input produces a smaller droplet).

Gross measures of the impeller characteristics, the Power and Flow numbers, can be used to quantify the flow and shear characteristics of impellers in agitated vessels but the fine details of the turbulence generated must also be considered in order to fully understand their performance.