(98b) Effect of Shear Gap Width on Flow and Power Draw in an Inline Rotor-Stator Mixer

Rotor-stator mixers are close tolerance devices with a narrow “shear gap width” or clearance between the rotor tip and the inner stator surface. It is often recommended that they be scaled on nominal shear rate (ratio of rotor tip speed to shear gap width) but this is often inadequate. Pumping rate is a factor and in turbulent flow energy dissipation rates are highest in the mixing layers and strong stator slot jets. These are created when fluid leaving the rotor abruptly changes direction as it impinges on the leading edge of the stator slot and is forced through it. The effect of shear gap width on rotor-stator process performance is not well studied.

Three-dimensional computational fluid dynamics (CFD) simulation was performed for water in turbulent flow to investigate the effect of shear gap width on the flow field, pumping rate and power draw of a Silverson L4R bench scale inline rotor-stator mixer equipped with a square hole stator head. Sliding mesh simulations of the Reynolds-Averaged Navier-Stokes (RANS) equations were performed using Fluent with the realizable k-ε turbulence model and enhanced wall functions. The power draw was determined from the rotor torque and the pumping capacity was calculated from the velocity exiting the stator slots.

The stator dimensions were held constant (inner diameter = 31.3 mm). Several shear gap widths were investigated ranging from the standard shear gap width w = 0.2 mm to the widest one with w = 1.7 mm. It was found that increasing the shear gap width caused a small decrease in pumping capacity. However, the mill head with the standard shear gap draws the most power and power draw decreases significantly with increasing shear gap width. Flow field features such as mean velocity, turbulent kinetic energy and energy dissipation rate are significantly influenced by shear gap width, particularly the extent to which the stator jets penetrate the volute. Examining these quantities provides physical insight into the flow and power results. Motivation for the study will be presented and use of the results will be illustrated by application to the analysis of crystal wet milling experiments performed in these devices.