Inter-compartment interaction in multi-impeller mixing: Part I. Experiments and multiple reference frame CFD

Abstract

CFD simulations of mixing in single-phase multi-Rushton stirred tanks based on the RANS methodology frequently show an over-prediction of the mixing time. This hints at an underprediction of the mass exchange between the compartments formed around the individual impellers. Some studies recommend tuning the turbulent Schmidt number to address this issue, but this appears to be an ad-hoc correction rather than physical adjustment, thereby compromising the predictive value of the method. In this work, we study the flow profile in between two Rushton impellers in stirred tank. The data hints at the presence of macroinstabilities, and a peak in turbulent kinetic energy in the region of convergent flow, which both may promote inter-compartment mass exchange. CFD studies using the steady-state multiple reference frame model (unsteady simulations are treated in part II) inherently fail to include the macro-instability, and underestimate the turbulent kinetic energy, thereby strongly overestimating mixing time. Furthermore, the results are highly mesh-sensitive, with increasing mesh density leading to a poorer prediction of the mixing time. Despite proper results for 1-impeller studies, we do not deem MRF-RANS models suitable for mixing studies in multi-impeller geometries.

Powder Injection Effect on Hot Metal Desulfurization Behavior in the Kanbara Reactor: a Transient 3D Coupled Numerical Model

“…The moving reference frame equations described the moving region containing the rotating impeller, while the stationary region was defined by the equations of the stationary reference frame. [18][19][20][21][22]…”

Numerical Investigation on the Effects of Impeller Structures in Hot Metal Desulfurization Processes by Mechanical Stirring

“…Due to the existence of the rotating impeller, the computational domain was subdivided into: (i) moving and (ii) stationary regions, separated by the interface boundary. The moving reference frame equations described the moving region containing the rotating impeller, while the stationary region was defined by the equations of the stationary reference frame [23][24][25][26][27].…”

Coupled metabolic‐hydrodynamic modeling enabling rational scale‐up of industrial bioprocesses

“…The required time resolution has a big impact on overall computation time and depends on the timescale of the fastest relevant process dynamics. If flow transients such as impeller outflow fluctuations and bubble plume oscillations are to be included (Haringa, Vandewijer, & Mudde, , ), a 1–10 ms accuracy may be required. In any case, a time accuracy in the order of seconds or faster is required, as large‐scale gradients only occur if the timescales of metabolic dynamics are in the order of/faster than mixing.…”