CFD Prediction of Flow and Homogenization in a Stirred Vessel: Part II Vessel with Three and Four Impellers

Abstract

This article deals with CFD simulations of flow inside stirred vessels equipped with three and four radial or axial impellers mounted on the same shaft. A comparison was made between simulated data and experiments for one-and two-impeller systems and was presented in Part I [1]. The effect of the lowest impeller off-bottom clearance, number of impellers used, and impeller type on the tracer distribution was studied. The simulations were mainly focused on the grid size and type and the analysis of the concentration curves in each impeller section. The predicted velocity fields, power and pumping numbers, concentration curves, and mixing times were validated with experimental data. The simulation results show the significant influence of the grid density on the velocity profiles and power and pumping numbers in contrast to the low impact on the concentration curves. A better prediction of the concentration curves was reached when radial impellers were used; the mixing times were generally over-predicted.

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

“…Montante et al required a strongly lower turbulent Schmidt number, Sc t = 0.1 (Montante et al, 2005), to yield agreement with experiments in a 4 impeller tank; similar observations were made in other studies (Delafosse et al, 2014;. Moštěk et al (2005) did not tune Sc t , and reported an approximately 20% overestimation in  95 compared to their measurements with 3 impellers (40% compared to Jahoda and Machon (1994)). They found decent agreement in 4 impeller systems compared to their own measurements (they did over-estimate  95 compared to Jahoda and Machon (1994)).…”

“…This requires that CFD simulations sufficiently capture the true mixing behavior, which gave rise to a significant body of validation literature. A review of literature focusing on single phase flows with Rushton turbines (Section 2) shows that Reynolds Averaged Navier Stokes (RANS) simulations yield decent results for single-impeller geometries (Coroneo et al, 2011), but appear to consistently overestimate mixing times in multiimpeller geometries (Montante et al, 2005;Moštěk et al, 2005;Kukuková et al, 2005;Jahoda et al, 2007); this over-estimation may hold for other impeller types at large spacing (Montante et al, 2005). Large Eddy simulations (LES) appear to adequately capture mixing behavior, but the required computation time prohibits routine application.…”

Multi‐impeller mixing performance prediction in stirred tanks using mean age theory approach

“…For 2-PBT configurations, the MAT approach and the FTT both predict around 10% deviations from experimental data. Equation (1) with a ¼ 1:67 yields 47.0, a within 5% deviation to experiment for 2PBT C 2 ¼ 0:18; and 47.4 for 2PBT C 2 ¼ 0:23, which means an overestimation of 30%, this is expected since the accuracy of this equation has not been confirmed for configurations with large impeller spacing.…”

“…9 The second category of mixing performance studies involves experimental correlations. Rodgers et al 10 proposed Equation (1) as the correlation of 95% dimensionless mixing time N rps θ 95 for dishedbottom stirred tanks with a Cowles disc, a six-blade Rushton turbine, and a four-blade pitched-blade turbine. The N rps is impeller revolution per second, and α represents the ratio of the dished-base height to the tank diameter.…”

“…RANS simulations with SLM techniques (without deactivating the flow field solver) yield decent results for single impeller geometries, but overestimated mixing times have been reported for multi-impeller systems. 1,15,17 This overestimation, associated with the underestimation of mass exchange due to macro-instability in the compartment region, increases with increasing impeller spacing. 18 In contrast, LES is able to resolve large-scale eddies, and the small scales can be modeled using subgrid scale models.…”