Hydrodynamic difference between inline and batch operation of a rotor‐stator mixer head ‐ A CFD approach

Abstract

Rotor‐stator mixers (RSMs) can be operated in either batch or inline mode. When operating a rotor‐stator geometry in batch mode, it typically experiences an order of magnitude higher volumetric flow through the stator than in inline mode. This is expected to cause differences in the flow and turbulence in the rotor‐stator region. This study uses computational fluid dynamics (CFD) to study the hydrodynamic differences in and near the stator hole as a function of volumetric flow rates between those experienced in inline and batch modes of operation. It is concluded that both radial flow profiles and turbulent kinetic energy across a range of rotor speeds and flow rates can be described by a velocity ratio: average tangential fluid velocity in the stator hole divided by the rotor tip speed. Moreover, the position where dissipation of turbulent kinetic energy takes place—and hence the effective region of dispersion or mixing—differs between the two modes of operation. The relative importance of the two regions can be described in terms of the velocity ratio and the transition can be predicted based on the relative power input due to rotational and pumping power of the mixer. This study provides a starting point for understanding differences between emulsification efficiency between inline and batch modes of operation with relevance for both equipment design and process scale‐up.

Rotor-Stator Mixers: From Batch to Continuous Mode of Operation—A Review

“…Since transitioning from batch RSM to a continuous mode RSM decreases N Q , the velocity profile in the rotor-stator head will undergo a substantial change. This change is not merely a scaling due to the reduction in flowrate but a shift into a fundamentally different turbulent flow [54]. Most notably the position of the highest local dissipation rate of TKE shifts from the turbulent jet formed downstream of the slot in the batch RSM flow number, to the rotor-stator clearance for the continuous RSM flow numbers [54].…”

“…where d is the slot diameter and A tot is the total flow-through area of the stator, has a small but significant effect on the resulting drop size (when tip-speed is kept constant). When keeping the geometry constant, it can be shown that Re Q is proportional to the product between flow number and Reynolds number [54]:…”

“…Unfortunately, no such experimental studies have been reported. However, a recent CFD investigation might be used to shed some light on the situation [54]. The study [54] reports flow fields obtained with CFD for a continuous mode RSM run at different flowrates and rotor speeds.…”

“…However, a recent CFD investigation might be used to shed some light on the situation [54]. The study [54] reports flow fields obtained with CFD for a continuous mode RSM run at different flowrates and rotor speeds. The lower flowrates correspond to those generally obtained for continuous mode of operation (N Q = 0.007) and the higher to those obtained in batch mode of operation (N Q = 0.082).…”

“…The radial velocity profiles in the stator holes were compared and it was found that neither Re Q nor N Q Re Q were appropriate scaling laws, in contrast to what was suggested in Section 6.1. Instead, it was found that both radial and tangential velocities (appropriately scaled with rotor speed) were determined by the flow number [54]. This has an important implication on the difference between the two modes of operation.…”